- Email: [email protected]
Wind corners in the 70–100 km altitude range as observed at andenes (69° latitude)
J,,~,urnui,,fArmmphfr~cund Terrairiul Printed m Great Britam
Phyiu,
Vol. 52,No.
0021-9169.90%3.00+ .OO Pergamon Prer5 plc
lO/li,pp.YY5-1015,1990
Wind corners in the 70-100 km altitude range as observed at Andenes (69’ latitude) H.-U. * Max-Planck-lnstitut
WIDDEL*
and
U.
VON
ZAHN?
fiir Aeronomie, D-341 I Katlenburg-Lindau, F.R.G. ; TPhysikalisches Universitlt Bonn, Nussallce 12, D-5300 Bonn 1. F.R.G. (Received in$nal,form
22 Ma)
Institut
der
1990)
the altitude range 70-100 km, high-resolution wind profiles have been measured during the 1988 at Andenes (69’N). We report on the wind corners observed in these profiles and compare their properties with those of wind corners seen in the winter of 1983-1984 and autumn 1987. Five of the main results are as follows. (1) The occurrence rates for wind corners in general are similar in summer and in winter. The database for autumn (only 7 flights) was too small to draw any firm conclusions. A strong wind corner was seen, roughly, on every third experiment, both in summer and in winter. (2) The results obtained on the temporal occurrence of wind corners suggest that wind corners seem to have no preference to appear at certain hours of the day. (3) Wind corners tend to appear at preferred heights which are higher in summer than in winter. The spacing between these preferred heights is about 5 km in summer and about 3-3.5 km in winter. (4) In strong wind corners the sense of rotation of the wind direction is positive in summer and negative in winter (with positive being defined as a rotation of the wind direction from northward towards eastward with increasing altitude). (5) At altitudes below 90 km wind corners tend to occur at or close to atmospheric layers having Ri z 0.25. Abstract-In
summers of
1987 and
motion to changes in wind speed and wind direction with height. The most important parameter which determines this descent velocity is the mass-over-area ratio, m/A, of the sensor. If employed above about 75 km this ratio should be of the order of some grams per square metre. Such low values cannot be realized technically for solid targets as inflated spheres and parachutes (e.g. the much used ROBINSPHERE has a m/A z 200 g/m’). Here the solid target is replaced by a cloud of electric dipoles (‘chaff’). The dipoles arc realized as thin foils cut from appropriate material with their length chosen to be resonant to the radar’s operational wavelength. Flat plates (strips) optimize the mass-over-arearatio for a given material and have the advantage that their drag in the mesosphere is higher than that of cyclinders and, further, rather importantly, have much less tcndcncy to form lumps during deployment which seriously spoil the measurements. Foils fall in the mesosphere with their longitudinal axis orientated perpendicular to the direction of fall and to the drift in the wind. and their flight behaviour can be described satisfactorily by a simple approximation (WIDIXL, 1987) which shows that the m/A ratio of the foils has to be matched to the height range covered with the mcasurcmcnts. This was the reason why different kinds of foil chaff were flown in the various field campaigns under consideration here. All chaff clc-
I. INTROIXJCTION corners were recognized for the first time in results of foil cloud experiments performed in the mesosphere over Andenes (69”N) during the winter 1983. 1984 (VON ZAHN and WIDDEL, 1985 ; VON ZAHN et al., 1985). Wind corners are characterized by a simultaneous occurrence of two features: (1) a very strong directional shear dcp/dz (>5”/lOOm) over a height interval which is much smaller than 1 km and (2) a local minimum of the total wind speed in this shear. The detection of wind corners and their resolution in detail requires precise wind measurements of high spatial resolution. We achieve this goal by tracking the free-fall trajectories of small clouds of foils by a precision radar. The aim of this paper is to give an overview on wind corner observations obtained at 69 N latitude in the summers 1987 and 1988 and to compare these with what was seen in winter 1983 -1984 and in autumn 1987. Wind
2. THE
FOIL
CLOUD
EXPERIMENT
Tracers for wind measurements released into the atmosphere at some height and tracked by radar from the ground should have a descent velocity well below 100 m/s to obtain a good response of the sensor 995
H.-U.
996
WIDDELand U. VON ZAHN
ments were cut from aluminized polyester foils and had the same length (24mm), but differed by their width and thickness; the latter determines the m/A ratio. Table 1 gives details on the types of foils used in the four field campaigns. The thickness of the foils ranges from 10 pm down to 1 pm, the m/A ratio from 13.6 g/m* to 1.7 g/m’ and the optimum altitiude range between 6G80 km and 9GlOO km. The 1 pm foil clouds come close to the limit in height up to which chaff as a tool for measurement of wind can be sensibly used at all (estimates of this height range between 105 and IOSkm, and this depends upon season and latitude). The limit is set by descent velocity of the tracer, which should not be much higher than about lOOm/s in order to keep corrections for lag between measured and true wind velocity within acceptable limits and to obtain a resolution of height for the horizontal wind which is better than 1 km. Table 1 further indicates the number of foils used in each experiment and shows that this number was successively reduced from campaign to campaign. It turned out that the foils spread out much less during descent than previously assumed. A tracking radar draws its information not from the whole extension of a chaff cloud, but from a rather small volume only. This volume is a cut-out from a spherical shell around the radar and the thickness of this shell is given by the depth of the range gate which was about 190 m, and the outer contour is set by the width of the antenna beam of the radar which was I” half-power-width. For the measurement of range-target the range gate is divided into subgates which slightly overlap and have a special waveform for weighting purposes. Range is measured by comparing the echo amplitudes seen in these two subgates. Simultaneously these two subgatcs control the angular position measurements by gating the output of the four antenna beam receiver outputs. For solid targets like an aircraft or a falling sphere the accuracy of the range measurement is of order of metres ( + 2 m in our case). ‘Soft’ targets like a chaff cloud often deliver a strongly fluctuating return
Table
1. Parameters
Thickness (pm) Width (mm) m/A (g/m’) Altitude range (km) MAP/WINE MAC/SINE MAC/EPSILON SODIUM 88
signal and some amount of averaging and smoothing of the raw data is then required for all three coordinates. The height range over which such smoothing is required depends upon the rate at which the position measurements are recorded at the radar. Everything which lies outside of this tracking volume defined by range gate and beam width is ignored by the radar. The ideal case would be just to fill this tracking volume with an adequate number of foils. The tracking radar used on all campaigns was an RCA MPS 36. The position update records were increased from 10 (winter 1983-1984) to 50 position records/s in summer 1987 and in the two campaigns which followed to the benefit of a more detailed analysis. The height resolution of the wind measurements was 50 m and the r.m.s. error in wind was of the order of 0.5 m/s over about 90% of the height range covered. The 10 and 2.5 pm chaff experiments were launched by ‘Stretched Super Loki Dart’ meteorological rockets. Subsequently, the decrease in the number of foils compared to that flown in the winter 1983-1984 campaign allowed us to use a standard version of the ‘Dart’ vehicle which has a smaller diameter, and this paid off in greater ceiling heights. The 1.5 and 1 pm chaff were carried aloft by Viper 3A Dart vehicles.
3. NUMBER OF EXPERIMENTS AND HEIGHT RANGE COVERED This section gives an overview (in historical order) on what was obtained in the four campaigns discussed in this paper. All campaigns took place at the Andraya Rocket Range, Norway (69’ N, 16 ‘E). 3.1. Cumpuign MAP/ WINE (winter 1983-1984) The overall scientific aims of the MAP/WINE campaign have been summarized by VON ZAHN (1987). Fourteen foil cloud experiments with 2.5 pm foils yielded data in the height range of interest here, and two payloads with 10 pm foils were also flown then.
of the foils used. number of experiments per experiment IO 9 13.6 60~ 82 2 5
2.5 9 3.4 70 92 16 17 4
1.5 I 2.35 75 -96
3 3
I
in each campaign,
Number
and number
of foils per experiment
7 1.7 80 100
7
of foils
17,OO(f23,000 10,000 10,000 4000-6000
991
Wind corners in the 70-100 km altitude range MAC/SINE
(b)
(a)
100
0‘
‘“7
MAP/WINE
DEC 84/FEB
ii-i=13km
85
LIGHT
CHAFF
1 I
JUNE/JULY
,z=lS.S
1
87 LIGHT
km
CHAFF
h
jSO%
/ 92 i
I
“;~“p,\yy-y8
68’hJiII
I
MORNINGNOON
EVENING
2
6
10
NUMBER
OF
OBSERVATIONS
14
NUMBER OF OBSERVATIONS
Fig. 1.Individual and total height coverage by foil chaff experiments with 2.5 pm chaff in winter [(a), left] and summer [(b). right]. The figures at the bottom of each bar indicate the height intervals which were covered by individual experiments. The unusually frequent break-up at 72km in the winter data is an artefact because tracking of the foil cloud had to be abandoned in winter prematurely on some occasions due to the launch schedule of other experiments. The dotted lines in the summer data indicate the additional contribution to the height coverage by five flights with 10pm chaff.
The height range covered with measurements in Fig. la.
is shown
The overall scientific aims of the MAC/SINE campaign have been summarized by THRANE (1990). There were 22 successful foil cloud experiments. Seventeen launches were performed with 2.5 pm foils and five launches with 10pm foils. The height range covered with measurements is shown in Fig. Ib. The Loss of information from 2.5 pm chaff experiments at heights below about 80 km in summer was more than compensated by the results obtained from the experiments which used 10 pm foils, as is shown in Fig. 1b (dotted lines) and, by this, the two sets of data (winter and summer) became comparable. Figure 2 shows the distribution of the launches over the day in this campaign. 3.3. Campaign EPSILON
(autumn 1981)
The overall scientific aims of the EPSILON campaign have been summarized by THRANE (1990). Technical problems with the ‘Stretched Super Loki’ motors limited both the number of launches and the ceiling height of flights with 2.5pm foils. Four experiments
were successful. The l.Sjlrn foils were launched on Viper 3A vehicles and three flights yieided good data. Though the database for autumn conditions is rather small, some conclusions could still be drawn on the occurrence of wind corners as will be shown later. 3.4. Campaign SODIUM
88 (summer 1988)
This campaign was designed to study the dynamics of the upper atmosphere during the formation, and in close vicinity, of ‘sudden sodium layers (SSL)’ fvoN ZAHN et al., 1987). SSLs are characterized by a rapid increase of sodium density with time (typically 5 min) in a thin layer (typically 1 km thickness) located between about 90 and 110 km. SSLs may last from about 10 min up to some hours. The data collected at Andsya on the occurrence of SSLs suggest that SSLs seem to occur preferably (or even only) between 2000 and 0200LT (VON ZAHN et al., 1989). Hence, all 10 experiments of this campaign were iaunched between 2130 and 0103 LT. Out of the total of 10 experiments, six were launched while a SSL was developing. Interest focused on what was happening in the height range between 90 and 100 km. For this reason, 1.5 and 1 pm chaff were used which was specifically designed for this altitude range.
998
H.-U.
WIDDEL and
U. VON ZAHN
01 02 03 OL 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 UT
Fig. 2. Distribution of chaff experiments launched in campaign MAC/SINE 1987 over the day. (a) Number of launches. Blank : 2.5 pm chaff; hatched : 10 pm chaff; filled : flight on which the only strong wind corner event (defined by a directional shear dq/dz > 40“/lOOm and a minimum horizontal velocity IL.,,~ in shear < 10 m/s) was seen between 80 and 81 km. (b) Observation of wind corners. Hatched : strong wind corner events
4. STATISTICS
In the following sections we provide statistical information on temporal and spatial properties of wind corners. In Tables 2-4 all wind corner events seen in the summer and autumn campaigns are listed, together with their characteristics. A similar list for the events of the winter campaign was published by VON ZAHN et ~11. (1985). We point out the similarities and differences of wind corners occurring in summer and winter, though on an admittedly limited statistical basis. 4.1. Seasonal and temporal occurrence
qf wind corners
Tables 2-4 show that in general wind corner events are seen in summer, autumn and winter. A more detailed analysis concerning the frequency of occurrence in summer and winter yields differences between these seasons, but also similarities : Fig. 3, which presents the results listed in Table 2 in graphical form, shows that the number of ‘weak’ wind corners, defined by a directional shear S with S”/lOOm 9 S d lo”/ IOOm, is much smaller in summer than in winter. If one confines the comparison to strong shears with S > 40”/100m and relates the number of these events to the total number of flights it turns out that this relative frequency of occurrence is almost equal for both seasons : roughly, on every third experiment a strong wind corner was seen. Concerning a potential dependence of the frequency of occurrence of wind corners on local time we have to note that many of our experiments concentrated close to either noon or late-evening hours. Wind corners were observed in those periods as well
as other local times. So far we do not see a significant preference of wind corners to occur at certain hours of the day. This is quite different from. for example, the behaviour of SSLs. A further difference between summer and winter was that the observation on more than one wind corner per flight is an infrequent event in summer : in summer 1987 it was seen in 4 flights out of 19 (see Table 2) ; in summer 1988 it was seen in only 1 flight out of 10 (see Table 4). The relevant statistics for winter read as follows (VON ZAHN et al., 1985) : total number of flights = 18 ; of them, 3 flights had 2 wind corners, 5 flights had 3 wind corners, 3 flights had 4 wind corners, 2 flights had 5 wind corners and 3 flights had 6 wind corners, which means that on 16 flights (about 89% of all flights) more than one wind corner was seen. 4.2. Prgfbred
hei,qhts and sense qfdirectional
shear in
wind corners
Clear differences between summer and winter were recorded for the heights at which wind corners appeared and the sense of directional shear of the wind in the wind corner: In summer as in winter, wind corners tend to appear at certain preferred heights; both these heights are different in the two seasons : Fig. 4a shows the heights at which wind corners were seen in winter 1983-1984 (VON ZAHN et al., 1985) and Fig. 4b shows the height of the wind corners listed in summer 1987 (see Table 2). In summer no wind corner was seen at heights below 80 km, despite the fact that the height range between 76 and 80 km was covered by a number of
SC24
SC25 SC26 SC21 SC28
I306
2030
2117 2201 2230 2254
15 July 1987
15 July 1987
SC23
SC17 SC18 SC21
0852 0943 1203
14 July 1987
SC14 SC15 SC16
2215 2334 2349
SC12
1336
1 July 1987
SC11
1244
1987
26June
SC04 SC05 SC07 SC08
SC09
1120 1153 1240 1333
1987
24June
Flight code
1127
Time (UT)
Date
Launch
88.8 83.6 89.0 88.7 87.2 87.0
91.9 85.9
93.2 95.0 85.2
80.9 92.5 80.4
91.9 87.2 92.8 86.1 84.2
87.3 93.2 90.6 91.9
Altitude of wind corner (km)
21 16 22 145 54 33
13 23
14 44 15
26 64 17
6 27 IO 82 11
6 33 16 14
Directional shear (deg./tOOm)
Table 2. List of wind corners
100 30 45 180 140 135
135 70
150 175 140
135 170 130
45 90 110 170 95
70 110 105 140
Total angle of change in direction (deg.)
seen in campaign
W-SES ENE-NNE W--SW NW-SE NW-E NW-E
WNW-E NE SE
SEPWN W SE- NW SW-E
WNW-E NW-E WNW-ENE
NW-N NE-SE SSE-NE NWN-SE N-E
NPENE SSE-NE SES-NEN SWS-E
Change of wind direction (from-to)
MAC/SINE
+ + +
_
+ +
SODIUM
19 34 19 2 I 10
salvo
salvo
Turbulence/gravity
salvo
Chaff salvo No. 3
EISCAT
-
1
Chaff salvo No. 2
Chaff salvo No.
Remarks
8 IO
12 7 15
4 15
36 7 27 9 25
+ +
_
52 12 28 32
+ _
(m/s)
Horizontal velocity in wind corner
1987
Sense of change
summer
6
2, B B 1
%
2
P
z
a 8 ;.
< 5’ a 8
1987
21 October
2028 2135 2210 2222 2256
2220 2313
0003
13 July 1988
14 July 1988
Time (UT)
1430
2321
1138
1332
-
Time (UT)
1988
Date --24June
Launch
1987
1987
I November
1987
15 October
Launch
17 October
Date
~-
NC10
NC08 NC09
NC01 NC02 NC03 NC04 NC05
Flight code
EC09
EC08
EC05
EC04
Flight code
..-
88.X
91.9 99.7 90.2
93.6 92.6 92.3 91.7 90.8
(km)
Altitude of wind corner
62
17 4 44
7 18 62 34 35
110
60 95
50
90
180
105 140 180
90 130 200 165 165
WSW-N
W-E
NNW-E SSW-NWW W-E
WNW-NNW W-NE WPESE NW-E WNW-E
Change of wind direction (from--to)
88 summer
SWPNNW
WNW-N SE-NW
NW-NNW
1987
+
f + +
+ + + + +
Sense of change
+
+ +
-
+
Sense of change ~._
1988
October
Change of wind direction (from-to)
EPSILON
SODIUM
Total angle of change in direction (deg.)
seen in campaign
Directional shear (deg./l00 m)
Table 4. Wind corners
44
10 55
94. I 86.6 79.0
13
15
in campaign
Total angle of change in direction (deg.)
observed
Directional shear (deg./ 100 m)
94.4
IS.7
Altitude of wind corner (km)
Table 3. List of wind corners
7
33 33 I1
42 25 9 13 12
Horizontal velocity in wind corner (m/s)
5
30 4
27
9
Horizontal velocity in wind corner (m/s)
Second salvo
First salvo
Remarks
1.5 pm chaff
1S nm chaff
Remarks
$
!Z? N
c
2:
Wind corners
in the 70-100 km altitude
60
50
2 z 40 > _ w
MAP/WINE WINTER 1~8318~ 18 EXPERIMENTS
MAC/SINE SUMMER 1987 22 EXPERIMENTS
2 30E Z = zoL IO0 SHEARIDEG/lOOml @
@
Fig. 3. Frequency of observation of wind corners during winter (a) and summer (b). Roughly on every third flight a wind corner was seen, which had a directional shear >40”/100m.
range
I001
individual experiments comparable to that launched during winter (this is shown in Fig. I), whereas in winter the 76-80 km altitude range appears to be the preferred height for wind corners. Figure I shows that this height range was adequately covered by the number of experiments in both seasons. In addition, it should be mentioned that the two wind corners listed for the height bin go--81 km were observed on only one day (1 July 1987) and on two consecutive flights launched about 1h 30 min apart in time. The spacing in height between preferred heights was significantly larger in summer than in winter : it was about 5-6 km in summer compared to 3-3.5 km in winter, and this is best seen when the wind corners are considered which are characterized not only by a strong directional shear dq/dz, but also by a low horizontal velocity in the wind corner (IV,,]< IO m/s) (hatched events in Fig. 4). For autumn 1987 (Fig. 4c) the number of successful flights remained quite small (7 flights only), and it is difficult to draw any firm conclusions from that small database. If we take the observations as they are, we saw one strong wind corner in the height bin 86-87 km (we found strong wind corners in this height bin in
, I I
I,
Winter 03104 (18FLlGHTS1
23 2;
;0
SUMMER 87 (22 FLIGHTS)
@ i
HEIGHT BIN [km1 Fig. 4. Heights at which wind corners were seen in winter (a), in summer 1987 (b), and in autumn 1987 (c). The winter data refer to the samples published in VON ZAHN et al. (1985), but results obtained below height bin 75-76 km were omitted. (No adequate height coverage there during summer.) Wind corner events in which the horizontal velocity IL’J was < lOm/s are marked by hatching.
1002
H.-U. W~DDEL and U.
our summer and winter data, too) and two strong wind corners in the height bins 78-79 and 75-16 km. These are levels in which we saw no wind corners in summer but in winter (cf. Fig. 4). In this context we might briefly discuss why we have not seen any wind corners at height levels below 80-81-km during summer. It was suggested that our chaff measurements did not extend to low-enough heights to see them. At present we have no database from chaff experiments which could rule out or prove that this suggestion is correct, and we must therefore leave this question open. The sense of rotation of wind direction in a wind corner shows characteristic differences between summer and winter: Fig. 5 shows the results for summer 1987. Defining ‘positive’ as the sense of change of wind direction from northwards towards eastwards with increasing altitude, in 15 wind corners the sense of directional shear was positive and in 8 corners negative (Fig. 5a). Considering wind corners characterized not only by a strong directional shear but also by a low horizontal velocity in the wind corner (lvt,l 6 lOm/s) too, it turns out that 11 of these wind corners had a positive sense of directional rotation (Fig. 5b). Taking these strong wind corners out and
SUMMER
1987
VON ZAHN
considering the rest, one fmds an almost even distribution between positive and negative rotation (Fig. SC). In winter 1983-1984 the scenario was reversed, at least for the strong events : 9 corners with strong directional shears had a positive and 19 had a negative directional change (VON ZAHN and WIDDEL, 1985). Considering all events, however, there was an about 1:l distribution in sense of rotation, not unlike the weak summer events. 5. DYNAMICALASPECTS A theoretical model of the atmospheric dynamics leading to the formation of wind corners is still pending, and hence this section is confined to our observations.
The results on wind corners listed in Tables 24 suggest that the directional shear and the horizontal wind velocity in a wind corner are in some way related to each other, and this was investigated more closely. In doing this, one has to take into account that three series of measurements exist which were launched in summer under very specific conditions. These were the flights SC 24-SC 28 on 15 July 1987 (Table 2) and all flights performed in summer 1988 (Table 4). These flights were considered separately, because they were launched when a sudden sodium layer (SSL event, cf. Section 3.4.) developed and for brevity, these flights are further referred to as ‘sodium flights’. The directional shear dq/dz observed in each wind corner is plotted against the minimum horizontal speed lr,,l in the shear in Fig. 6. The data points lie in a field having a negative slope, which is -0.83 for the summer 1987 measurements when the flights SC24SC28 are not considered (Fig. 6a) ; the slope is - 1.02 for all sodium flights, SC24-SC28 and summer 1988 (Fig. 6b). Figure 6c shows all summer (sodium flights included) and autumn data together. Evidently, the autumn data fit well into this picture, too. 5.2. Wind c’ornrrs and r.ertica/ uir rwlocitirs
HEIGHT
BIN [km1
Fig. 5. Sense of rotation of wind direction dqjdz in wind corners observed in summer 1987. Positive sense of rotation is defined as a turning of wind direction from north towards east with increasing height. (a) All wind corners. (b) Sense of rotation in strong wind corners defined by dq/dz 2 4tY / 100 m and a horizontal velocity in wind corner ll%Hl< lOm/s. In all of these strong wind corners the sense of rotation in wind direction was positive. (c) Fig. 5a, but strong shears taken out.
If one assumes that wind corners are the result of some kind of wave interaction, one would expect that a relation between vertical air motions and wind corners exists. Vertical air motions can be derived from foil cloud experiments if temperature and air density profiles are available which have a good resolution in height and which are measured by independent techniques as close as possible in time to the launch of the foil cloud
Wmd corners
in the 70-100 km altitude range
1003
4
86
& l
x
0
rw x
)I
.&ox” '0 !ALL DATA: - x SINE 1987 cl SOOIUM JUNE/JULY 1988 . EPSILON OCTOBER 1987
_
1
10
100
1
10
100
I-1
10
94 c
C’ I 199.7 km,
100
HORIZONTAL VELOCITY Iv,l[mlsl
Fig. 6. Relation between directional shear dq,/dz and minimum horizontal wind velocity IL.~,(in wind corner. From left to right : (a) all data obtained in summer 1987 (campaign MAC/SINE) except those of 15 July 1987 ; (b) all flights performed under conditions of a developing sudden sodium event; crosses : 15July 1987 ; circles : summer 1988 (campaign SODIUM 88) ; (c) all data; SINE 1987 (crosses), EPSILON 1987 (filled circles) and SODIUM 88 (open circles). The results point towards an inverse relation between
dq/dz and 10~1in wind corners.
experiment. Temperature and air density profiles were derived from inflatable falling sphere experiments performed in the MAC/SINE campaign by the University of Bonn (L~~BKEN et al., 1990). In eight cases the launch of the falling sphere was considered to be close enough in time to the launch of the chaff experiment to allow a comparison to be made. The result of this comparison is given in Fig. 7ax, which shows the horizontal wind speed 1z.,,[, the vertical air velocity M’and the directional shear S. The height at which the wind corner was seen is indicated by a dashed line, and it appears that wind corners are related more to the gradients in vertical air velocity than to relative extrema in vertical air velocity. Figure 8 shows this in more detail: in four cases the height at which the wind corner was seen coincided with the height at which dw/dz was negative and had a (relative) maximum ; in two cases, the vertical velocity gradient dn,/dz was positive; one case remained undecided. The attempt, however, to relate dM,/dz to the directional shear dq/dz, to the total change of direction p or to the minimum velocity (cHI in wind corner had no result. This is shown in Fig. 9, in which the vertical velocity gradient dbr/dz is plotted against total change in direction q and against the directional shear dq/dz.
5.3. Wind corner patterns A convenient and informative way to present results on radar tracking of a target is the projection of the target’s trajectory onto the Earth’s surface (later
referred to as ‘radar ground plot’). It can be read like a map (+X: eastward, + Y: northward). The radar points are plotted at equidistant times and therefore the horizontal speed of the foil cloud can be visualized quickly: clustering of points shows low wind speed, spread-out points indicate high wind speed. Also, the retardation of the wind speed in wind corners is easily recognizable. Altitudes are printed out in this plot at regular intervals of time (e.g. at each full minute), which gives indirect information on the descent velocity of the cloud, too. On 15 July 1987 five rockets carrying chaff cxperiments were lauched in sequence when a SSL developed overhead the Andoya Rocket Range. The SSLs were detected by a ground-based sodium lidar (HANSEN and VON ZAHN, 1990). A fairly unusual, spiky pattern was seen (Fig. 10, left panels) of a kind which was not seen in winter. The question of whether this pattern was just accidental or a regular feature was answered by the results of two series of launches performed under similar conditions (developing SSL) in summer 1988. The same, spiky pattern was observed then also in the two salvoes as Fig. IO (right panels) and Fig. 11 show. There was another similarity between these three salvoes in that the wind corner descended steadily with time. Notably, the phase velocity of this descent was fairly uniform (average : 0.5 m/s) in ail three cases, as Fig. 12 shows. The downward phase velocities seem to indicate upward motion of energy. These similarities suggest that the same mechanism leading to
1004
H.-U. WIUDEL and U. VON ZAH~V
wind corners was at work
when these three salvoes
were launched. A certain hint of this is given by the
results of a spectral analysis of the vertical profiles of the scalar horizontal wind 1~1~~1 = (u’+z?) I.” which was made for flights SC24-SC28 on 15 July 1987 (Wu and WIDDEL, 1989). It turned out that the spectral slopes for the individual flights were very close to - 3 ; the average was - 3 f0.1 and this is the slope proposed by SMITH et al. (1987) for a saturated gravity wave spectrum. We conclude from this that in these series of measurements saturated gravity waves may have been involved in the formation of the wind corners. 5.4. Wind corners and Richardson numbers The strong wind shears observed in wind corners are suggestive of wind corners being regions of turbulence. For the winter 1983-1984 observations THRANE et al. (1987) found, however, that only about
I
100
,
I
,
50% of all wind corners contained turbulence and 50% were without it. During the MAC/SINE and the EPSILON campaigns turbulence parameters were derived from data obtained by several high-resolution in-situ measurements of air density, ion density, and electron density. These experiments and their results are described in the paper by BLIX et al. (1990), and references therein. We calculated Richardson numbers (Ri) using temperature and air density profiles obtained from inflatable falling sphere experiments during the MAC,’ SINE campaign (L~BKEN et al.. 1990) launched close in time to the launch of our foil experiments. We searched for altitude regions with Ri < 0.25. as the latter value is commonly taken to indicate that turbulence might possibly be present there. Temperatures derived from falling sphere experiments are given at 1 km altitude intervals and a temperature profile was obtained by a polynominal fit of
SC 07: 24. JUNE 1987, 12:40 UT / I I
100
80
80 0
20
40
Horizontal
60
80
velocity
-8-4 Vertical
0
0
velocity
Directional
Wm/sl
~Hb/Sl
SC 25: 15. JULY 1987,
21:17
45
shear
S[deg/lOO m] UT
80
80 0 30 60 90 Horizontal velocity
-8-4 Vertical
Wblsl
0 velocity
WWsl Fig. la
0 Directional
90 shear
S[deg/Ioom]
Wmd corners
in the 70-100 km altitude
SC 04: 24. JUNE
1987,
11:20
7
1005
range
UT
1 ----_
-__
__
90
_ r
80 0
20
40
60
Horizontal
80
_4
_8
0
Vertical velocity
velocity
*If b/s1
Directional
shear
S[deg/lOO m]
w&l
SC 09: 26. JUNE
45
0
1987, II:27
UT
100
100
3 ?L UJ 7 90 3 4
90
80
80 0
30
60
Horizontal
-12
velocity
-8
0
Vertical velocity
wb/sl SC
-4
45
Directional S[deg/lOO
Wb/sl 11:
26.
JUNE
1987,
12:44
shear m]
UT
80 0
30
60
Horizontal
90
velocity
WI+1
12
8
-4
0
Vertical velocity
w[+l Fig. 7b.
4
0
45 Directional S[deg/lOO
90 shear m]
H.-U.
1006
WII)DEL and
U. VON ZAHN
SC 28: 15. JULY
1987, 22:54
UT
80
80 0
30
60
Horizontal
-8
velocity
-4
0
Vertical velocity
v&/s1
Wlmisl SC 27: 15. JULY
0
90
Directional
shear
SIdeg/lOOm]
1987, 22:30 IJT
100
90
90
80
30
0
30
Horizontal
60
-8-4
velocity
0
Verticai velocity
~Hlm/sl
Wb/s] SC 26: 15. JULY
Fig. 7a--c. Horizontal velocity 1~~1,vertical wind corner (dashed horizontal line). In all velocity IL.“/ a minimum at the height where the height at which the vertical air
45 Directional
90 shear
S[deg/lOO m] 1987, 22:Ol UT
air velocity u’, directional shear of wind dq:dz and position of cases, the directional shear has a maximum and the horizontal the wind corner is seen, but this height does not coincide with velocity M’(centre panels) has a (relative) extremum.
in the 70-100 km altitude
Wind corners
range
80
VERTICAL
VELOCITY
GRADIENT
[m/s/100 m]
Fig. 8. The position of the wind corner is at a height where the vertical air velocity gradient dw/dz has a (relative) extremum or is close to that. In five out of the eight cases this gradient is negative.
i -I-
3
X
N
0
0-v
5 :
z E
2 9 ;;
-2-
x
0
v
!G Y
-3
0
I
I
I
I
20
40
60
80
I
100
1
120
I
140
I
160
180
TOTAL CHANGE IN DIRECTION[DEGIW DIRECTIONAL SHEAR IDEG/lOO ml(xJ
Fig. 9. Vertical velocity shear dw/dz plotted against total change in direction of wind in wind corner, cp (filled circles) and directional shear of wind dq/dz (crosses) for the flights shown in Figs 7 and 8. No clear relation between these parameters is seen.
H.-U. 15. July 21.20 - 21.27
ZAHN
24. June
:,*“’
i
VON
21.38
- 21.41
UT
‘.. 89.8
d ~:1.._+38.0
~“-03.2 ? ‘-Breakup
J .f , B-59.3
22.13
84.1
$-
88
+3.6
.:- zy34.5 ‘.,. :J
U.
85.4 .r.. 4,
93.1:: 83.9
and
87
-c
UT
W~DDEL
- 22.16
Breakup
:?
C-
towards
---
West
towards
West
Fig. 10. Projections of the trajectories of chaff clouds onto the Earth’s surface (radar ground plots) of flights which show the development of an unusual pattern of a wind corner (centre panel). These plots are to be read like a map (top: North) and cover an area of 16.5 x 10 km’. Heights are printed out at each full minute UT rounded to the next 1OOm and unsmoothed radar raw data are shown. Left panels: I5 July
1987; right panels: 24 June 1988.
these data. The fluctuations in the vertical profile of the horizontal wind 1~~1= (u2+a2) “* were reduced by running means over a certain height interval, and we choose 300m in a first test and 1 km in a second. When comparing the results of these two test runs, we found that the main features of the Ri profiles were the same in both tests, but the numerical value of Ri and the heights at which minimum Ri were seen differed slightly by some 100 m, and a sample for this is given in Fig. 14a-c which will be discussed later. In the following we present the results obtained when the horizontal wind loHI was smoothed over a height interval of 300m. We assume that smoothing the horizontal wind over a height interval of 300m,
which is about six times the height resolution of the wind measurement (50 m), is sufficient to remove random fluctuations and matches the height resolution of the temperature measurements obtained from falling sphere experiments. Because temperatures derived from falling sphere experiments are somewhat uncertain for heights above about 90 km. we confined our comparison between Richardson number profiles and wind corners to altitudes lower than 90 km. What was obtained is shown in Fig. 13a-c. In four cases (Fig. 13a) the height interval in which the Richardson number was rather close or even slightly smaller than 0.25 centred around the altitude where the wind corner was found. We have to note, however,
Wind corners 13. July
-
in the 70 100 km altitude
88
towards West
Fig. 11. Same as Fig. 10, but for the second salvo of campaign SODIUM 88, 1 pm chaff. Area covered by each plot: 20 x 20 km’. A spiky pattern is seen in the centre panel again and the wiggles in the trajectories are believed to be caused by billows or vortex plaids.
range
1009
that three of these four samples were obtained from a series of flights launched in close sequence on the same day. In five other cases (Fig. 13b,c) the Richardson number for the altitude at which the wind corner was seen was larger than 0.25 (of the order of Ri = l), but Ri z l/4 or even smaller was approached at an altitude lOO-500m lower than that of the wind corner. The extension in height of these regions varied between a few 100 m up to about a little over 1 km in two cases, and the latter are shown separately in Fig. 13c, which presents the results obtained on two consecutive flights launched 52min apart in time. Both the height at which the wind corner was seen and the height at which the Richardson number assumes its minimum descent with time, and this allows an estimate on the phase velocity of descent which is about -0.6 m/s for the wind corner and about - 0.9 m/s for the height at which the minimum Richardson number is seen. Both values compare favourdbty with phase velocities derived from the descent of MST echoes (KLOSTERME~ER,1989, personal communication). Two interesting facts showed up when the Ri (z) profile and the height vs time z(t) plot of the chaff descent were compared with the z(t) variation of MST radar echoes. The SOUSY MST radar which was used for comparisons between chaff data and MST echo pattern was operated almost continuously throughout the summer 1987 campaign (MAC/SINE) by the Max-Planck-Institut fur Aeronomie (e.g. CZECHOWSKY et al., 1984; ROSTER. 1989, personal communication). This instrument was located about 6 km southwest of the rocket launch site. This radar collected its data in 300m height bins for 11s of each minute in the vertical position of its antenna beam. This means a certain smear-out of data in height and for a comparison between MST radar data and results obtained by the chaff experiment we used Richardson number profiles in which the horizontal wind was averaged over 1 km in order to get a better match of the chaff data to the height resolution of the MST radar. At heights below about 86 km there was, in general, a one-to-one correspondence between appearance of MST radar echoes and altitudes for which the in-situ experiments indicated Ri < 0.25 (a comparison in detail is difficult because the height and time resolution of the MST radar is much less than that of the foil cloud measurement). This was clearly not the case for heights greater than 86 km. Figure 14 gives examples for this. The first sample (Fig. 14a) shows no MST radar echoes at the height of 89 km where the Richardson number is smaller than 0.25. Yet, echoes appear 5min later at a height roughly 1.5km higher than the height of the wind corner. A
H.-U.
1010
WIDDEL and
total of 44 min later (sample of Fig. 14b) only weak MST echoes are seen at the altitude and time of the foil cloud measurement. These echoes, however, become stronger about 556min later. Another 53min later (Fig. 14~) all three heights coincide : that of the wind corner, that of the region with Ri d 0.25, and that of the MST radar echoes. The ground distance between the location of the MST radar and the point over which the wind corner was observed was 15-25 km away almost due North of the MST radar. Therefore it seems plausible to assume that the MST radar echoes seen in these cases at heights above 87-88 km were caused by frozen-in turbulence drifting in the wind, as was suggested by ROOSTER and KLOSTERMEYER (1987). The fact that these three foil clouds had very ‘clean’ radar trajectories (cf. Fig. 10) must raise serious doubts, however, about the prevalence of strong turbulence in just those wind shears. This intriguing observation obviously needs further study. A second topic of interest concerns the final decay of the foil cloud. If one identifies strong MST radar echoes with regions of strong turbulence, then one would predict that foil clouds might decay preferentially at altitudes with strong MST echoes. Surprisingly, this was not the case : with one exception all foil clouds were destroyed below the bottom of the height region in which MST radar echoes were seen. The foil clouds pass the region of strong MST echoes relatively unaffected and putting all observations together (height vs time plot, change of relative radar return cross-section, the width of the radar return echo at the radar’s gate and the projection of the trajectory
U. VOX ZAHN
onto the Earth’s surface) the conclusion is that the clouds were spread out and destroyed in all cases by rather violent, ‘organized’ air motions which obviously did not contain the 3m structures the MST radar requires for echoes.
6. SUMMARY
Our results concerning wind corners identified by foil experiments performed at 69” latitude are summarized as follows : 1. Our observations in summer and in winter indicate that the occurrence rates for wind corners are similar for both seasons. of occurrence of strong 2. The relative frequency wind corners defined by a directional shear larger than 40-/IO0 m was found to be equal in summer and in winter: roughly, on every third experiment a strong wind corner was seen. suggest that wind corners have 3. The observations no preference to appear at certain hours of the day. 4. Wind corners tend to appear at certain preferred heights but these heights are different in winter and in summer. They are higher in summer than in winter and the spacing between these heights is larger in summer (about 5 km) than in winter (3 3.5 km). The limited database for autumn suggests that the preferred heights and their spacings between them are similar to those seen in winter. 5. In strong wind corners the sense of rotation of the
92
\‘\\ -0.50
1E 90 ‘N I- 09 Z+ w = 88
0 0
.
-0.L6 m/s
I
0
mls
Cl
\
86 85-l , 20
15 JULY 1987 SC24/25/26/27/28
I
I
21
22
1 I
23 21
UT
Fig. 12. Phase velocities
13 JULY 1988 NC819110
24 JUNE 1988 NC2/314/5
I
22
I ,
23 22
I
of wind corner
24
I
01
UT
UT
of descent
,
23
for all three sodium
event salvoes.
Wind corners in the 70-100 km altitude range
1011
t
r i 103 Fig. I3a.
1”3
--.-_‘_L
.,,,
Wind corners
in the 7&100 km altitude
S-Cl2
S-c!1
ZG-JUNE-198713236UT
~-3UNE-l~l~4~UT
10-z
10-l
100 101 RICHARDSONNO.
range
102
103
10U
101
RICHARDSONNO.
Fig. 13~. Fig. 13a+c. Wind corners and Richardson numbers. (a) Height profiles of Richardson numbers. Position of wind corner marked by dashed horizontal line. In four cases a small Richardson number close to 0.25 was seen at the height of the wind corner. (b) Cases in which the Richardson number is larger than Ri = 0.25 (of the order of 1) at the height of the wind corner but smaller Richardson numbers are found at heights between about IOOm and about 1km lower than that of the wind corner. (c) Two consecutive flights on which the Richardson number is much smaller than 0.25 over a height interval of the order of 1km, and both the height of the wind corner and the height at which the Richardson number has its minimum descend with time. The phase velocity of descent is of the order of - 0.6 m/s for the wind corner and -0.9m/s for the height at which the minimum Richardson number was seen.
1014
H.-U.
WIDDEL
and
U. VON ZAHN
15. JULY
1987
a
0.1 Richardson
I
2120
10 Number,
Ri
Richardson
Number.
Ri
Richardson
Number,
RI
2125
2130
Time (U.T)
‘I’ime (U.T)
Time (L1.T)
Fig. 14. Correspondence between Richardson number profile, the height at which a wind corner was seen (altitude of wind corner marked by a dashed line) and MST radar echoes with height vs time nlot of chaff trajectory superimposed (right pa&l). Strength of MST radar echoes coded in rel&e scale. la) No MST radar echoes at the height where Ri < I/4 and that of the wind corner are seen. (b) Weak MST radar echoes are present and become strong minutes later. (c) Coincidence between MST radar echoes, small Richardson number and wind corner. Note that in all three ~nses the clouds are destroyed at altitudes well below the heights at which strong MST radar echoes are present (set text).
Wind corners in the 70- 100 km altitude range wind direction is positive in summer and negative in winter. Wind corners appear at an altitude where the gradient in vertical air velocity dw/dz has a relative extremum. In two cases out of eight, this gradient was positive and in five case negative. At altitudes below 90 km wind corners tend to occur at or close to atmospheric layers having Ri z 0.25. No clear correspondence between MST radar echoes and wind corners was found for heights above about 87 km.
1015
Ackno~~~iedgements-R. Siebenmorgen (University of Bonn) and Dr Y.-F. Wu (on leave from Academia Sinica, Beijing) assisted in launch support, data acquisition and data processing. Dr P. Czechowsky (Max-Planck-lnstitut fiir Aeronomie, Katlenburg-Lindau) provided us with the raw results of the SOUSY MST radar. Crews of the Andwrya Rocket Range and of DLR supervised the preparation and launch of the rockets. Radar tracking of the foil clouds was performed by the radar crew of DLR-Moraba (OberpfaFfenhofen). All this co-operation and these services are gratefully acknowledged, as are the work and efforts of two referees. This work was supported by the Bundesministerium fiir Forschung und Technologie, Bonn, by grants 01 OE 86033 and 01 OE 86046.
REFERENCES
BLIXT. A., THKANEE. V., FKITTSD. C., ZAHNU. VON,LUBKENF.-J., HILLERTW., BLOOD S. P., MITCHELL J. D., KOKING. A. and PAKHOMOV S. V. CZECHOWSKY P., RCSTEKJ.
and SCHMIDT G. HANSENG. and ZAHNU. VON L~~BKEN F.-J.. ZAHN U. VON, MANSON A., MEEK
C.,
HOPPE U.-P., SCHMIDLIW F. J., STEGMAN J., M~!RTAGH D. P., ROSTER R.. ScHMi%x G.. WIUDEL H.-U. and ESPYP. ROSTER R. and KLOSTERMEYER J.
SMITHS. A.. FRITTSD. C. and ZANDTT. E. VAN THRAKE E. V. THRANE E. V., BL.IXT. A., HALL C., HANSEN T. L.. ZAF~N U. VOX MEYEK W., CZECHOWSKK P., SCHMIDT G., WIDDEL H.-U. and NEUMANN A. WIDDEL H.-U. Wu Y.-F. and WIDDEL H.-U. ZAHN
L’. VOK
ZAHX U. VOX,GATEENP. VON DER and HANSEN G. ZAHN U. VOX, GOLDBERG R. A., STEGMAN J.
and WITT G. U. VON.MEYEK W. and
ZAHN
ZAHN U. VON and
WIDDEL H.-U.
WI~DELH.-U.
1990
J. atmm.
1984 1990 1990
Ado. Space Res. 4(4), 41. J. atmos. terr. Phys. 52, 585. J. atmos. terr. Phys. 52,955.
1987 1987 1990 1987
J. J. J. J.
1987 1989 1987 1987 1989
J. afmox. terr. J. utmos. terr. J. atmos. few. Geophys. Rrs. Planet. Spaw
1985
Proceedings of’ the 7th ESA PAC Symposium on European Rocker and Balloon Ptqrammes and Relayed Research (p. 61), ESA SP-229. ESAIESTEC, Noord-
198.5
Geophyx
atmos. atmos. atmos. atmos.
terr. Phys. 52, 835.
terr. terr. terr. terr.
Phys. Phys. Phys. Phys.
49, 44, 52, 49,
143. 1404. 8 15. 751.
Phys. 49, 723. P&s. 51, 991. P&s. 49, 607. Lett. 14, 76. Sci. 37, 657.
wijk, The Netherlands. Res. Lett. 12, 673.
Phyiu,
Vol. 52,No.
0021-9169.90%3.00+ .OO Pergamon Prer5 plc
lO/li,pp.YY5-1015,1990
Wind corners in the 70-100 km altitude range as observed at Andenes (69’ latitude) H.-U. * Max-Planck-lnstitut
WIDDEL*
and
U.
VON
ZAHN?
fiir Aeronomie, D-341 I Katlenburg-Lindau, F.R.G. ; TPhysikalisches Universitlt Bonn, Nussallce 12, D-5300 Bonn 1. F.R.G. (Received in$nal,form
22 Ma)
Institut
der
1990)
the altitude range 70-100 km, high-resolution wind profiles have been measured during the 1988 at Andenes (69’N). We report on the wind corners observed in these profiles and compare their properties with those of wind corners seen in the winter of 1983-1984 and autumn 1987. Five of the main results are as follows. (1) The occurrence rates for wind corners in general are similar in summer and in winter. The database for autumn (only 7 flights) was too small to draw any firm conclusions. A strong wind corner was seen, roughly, on every third experiment, both in summer and in winter. (2) The results obtained on the temporal occurrence of wind corners suggest that wind corners seem to have no preference to appear at certain hours of the day. (3) Wind corners tend to appear at preferred heights which are higher in summer than in winter. The spacing between these preferred heights is about 5 km in summer and about 3-3.5 km in winter. (4) In strong wind corners the sense of rotation of the wind direction is positive in summer and negative in winter (with positive being defined as a rotation of the wind direction from northward towards eastward with increasing altitude). (5) At altitudes below 90 km wind corners tend to occur at or close to atmospheric layers having Ri z 0.25. Abstract-In
summers of
1987 and
motion to changes in wind speed and wind direction with height. The most important parameter which determines this descent velocity is the mass-over-area ratio, m/A, of the sensor. If employed above about 75 km this ratio should be of the order of some grams per square metre. Such low values cannot be realized technically for solid targets as inflated spheres and parachutes (e.g. the much used ROBINSPHERE has a m/A z 200 g/m’). Here the solid target is replaced by a cloud of electric dipoles (‘chaff’). The dipoles arc realized as thin foils cut from appropriate material with their length chosen to be resonant to the radar’s operational wavelength. Flat plates (strips) optimize the mass-over-arearatio for a given material and have the advantage that their drag in the mesosphere is higher than that of cyclinders and, further, rather importantly, have much less tcndcncy to form lumps during deployment which seriously spoil the measurements. Foils fall in the mesosphere with their longitudinal axis orientated perpendicular to the direction of fall and to the drift in the wind. and their flight behaviour can be described satisfactorily by a simple approximation (WIDIXL, 1987) which shows that the m/A ratio of the foils has to be matched to the height range covered with the mcasurcmcnts. This was the reason why different kinds of foil chaff were flown in the various field campaigns under consideration here. All chaff clc-
I. INTROIXJCTION corners were recognized for the first time in results of foil cloud experiments performed in the mesosphere over Andenes (69”N) during the winter 1983. 1984 (VON ZAHN and WIDDEL, 1985 ; VON ZAHN et al., 1985). Wind corners are characterized by a simultaneous occurrence of two features: (1) a very strong directional shear dcp/dz (>5”/lOOm) over a height interval which is much smaller than 1 km and (2) a local minimum of the total wind speed in this shear. The detection of wind corners and their resolution in detail requires precise wind measurements of high spatial resolution. We achieve this goal by tracking the free-fall trajectories of small clouds of foils by a precision radar. The aim of this paper is to give an overview on wind corner observations obtained at 69 N latitude in the summers 1987 and 1988 and to compare these with what was seen in winter 1983 -1984 and in autumn 1987. Wind
2. THE
FOIL
CLOUD
EXPERIMENT
Tracers for wind measurements released into the atmosphere at some height and tracked by radar from the ground should have a descent velocity well below 100 m/s to obtain a good response of the sensor 995
H.-U.
996
WIDDELand U. VON ZAHN
ments were cut from aluminized polyester foils and had the same length (24mm), but differed by their width and thickness; the latter determines the m/A ratio. Table 1 gives details on the types of foils used in the four field campaigns. The thickness of the foils ranges from 10 pm down to 1 pm, the m/A ratio from 13.6 g/m* to 1.7 g/m’ and the optimum altitiude range between 6G80 km and 9GlOO km. The 1 pm foil clouds come close to the limit in height up to which chaff as a tool for measurement of wind can be sensibly used at all (estimates of this height range between 105 and IOSkm, and this depends upon season and latitude). The limit is set by descent velocity of the tracer, which should not be much higher than about lOOm/s in order to keep corrections for lag between measured and true wind velocity within acceptable limits and to obtain a resolution of height for the horizontal wind which is better than 1 km. Table 1 further indicates the number of foils used in each experiment and shows that this number was successively reduced from campaign to campaign. It turned out that the foils spread out much less during descent than previously assumed. A tracking radar draws its information not from the whole extension of a chaff cloud, but from a rather small volume only. This volume is a cut-out from a spherical shell around the radar and the thickness of this shell is given by the depth of the range gate which was about 190 m, and the outer contour is set by the width of the antenna beam of the radar which was I” half-power-width. For the measurement of range-target the range gate is divided into subgates which slightly overlap and have a special waveform for weighting purposes. Range is measured by comparing the echo amplitudes seen in these two subgates. Simultaneously these two subgatcs control the angular position measurements by gating the output of the four antenna beam receiver outputs. For solid targets like an aircraft or a falling sphere the accuracy of the range measurement is of order of metres ( + 2 m in our case). ‘Soft’ targets like a chaff cloud often deliver a strongly fluctuating return
Table
1. Parameters
Thickness (pm) Width (mm) m/A (g/m’) Altitude range (km) MAP/WINE MAC/SINE MAC/EPSILON SODIUM 88
signal and some amount of averaging and smoothing of the raw data is then required for all three coordinates. The height range over which such smoothing is required depends upon the rate at which the position measurements are recorded at the radar. Everything which lies outside of this tracking volume defined by range gate and beam width is ignored by the radar. The ideal case would be just to fill this tracking volume with an adequate number of foils. The tracking radar used on all campaigns was an RCA MPS 36. The position update records were increased from 10 (winter 1983-1984) to 50 position records/s in summer 1987 and in the two campaigns which followed to the benefit of a more detailed analysis. The height resolution of the wind measurements was 50 m and the r.m.s. error in wind was of the order of 0.5 m/s over about 90% of the height range covered. The 10 and 2.5 pm chaff experiments were launched by ‘Stretched Super Loki Dart’ meteorological rockets. Subsequently, the decrease in the number of foils compared to that flown in the winter 1983-1984 campaign allowed us to use a standard version of the ‘Dart’ vehicle which has a smaller diameter, and this paid off in greater ceiling heights. The 1.5 and 1 pm chaff were carried aloft by Viper 3A Dart vehicles.
3. NUMBER OF EXPERIMENTS AND HEIGHT RANGE COVERED This section gives an overview (in historical order) on what was obtained in the four campaigns discussed in this paper. All campaigns took place at the Andraya Rocket Range, Norway (69’ N, 16 ‘E). 3.1. Cumpuign MAP/ WINE (winter 1983-1984) The overall scientific aims of the MAP/WINE campaign have been summarized by VON ZAHN (1987). Fourteen foil cloud experiments with 2.5 pm foils yielded data in the height range of interest here, and two payloads with 10 pm foils were also flown then.
of the foils used. number of experiments per experiment IO 9 13.6 60~ 82 2 5
2.5 9 3.4 70 92 16 17 4
1.5 I 2.35 75 -96
3 3
I
in each campaign,
Number
and number
of foils per experiment
7 1.7 80 100
7
of foils
17,OO(f23,000 10,000 10,000 4000-6000
991
Wind corners in the 70-100 km altitude range MAC/SINE
(b)
(a)
100
0‘
‘“7
MAP/WINE
DEC 84/FEB
ii-i=13km
85
LIGHT
CHAFF
1 I
JUNE/JULY
,z=lS.S
1
87 LIGHT
km
CHAFF
h
jSO%
/ 92 i
I
“;~“p,\yy-y8
68’hJiII
I
MORNINGNOON
EVENING
2
6
10
NUMBER
OF
OBSERVATIONS
14
NUMBER OF OBSERVATIONS
Fig. 1.Individual and total height coverage by foil chaff experiments with 2.5 pm chaff in winter [(a), left] and summer [(b). right]. The figures at the bottom of each bar indicate the height intervals which were covered by individual experiments. The unusually frequent break-up at 72km in the winter data is an artefact because tracking of the foil cloud had to be abandoned in winter prematurely on some occasions due to the launch schedule of other experiments. The dotted lines in the summer data indicate the additional contribution to the height coverage by five flights with 10pm chaff.
The height range covered with measurements in Fig. la.
is shown
The overall scientific aims of the MAC/SINE campaign have been summarized by THRANE (1990). There were 22 successful foil cloud experiments. Seventeen launches were performed with 2.5 pm foils and five launches with 10pm foils. The height range covered with measurements is shown in Fig. Ib. The Loss of information from 2.5 pm chaff experiments at heights below about 80 km in summer was more than compensated by the results obtained from the experiments which used 10 pm foils, as is shown in Fig. 1b (dotted lines) and, by this, the two sets of data (winter and summer) became comparable. Figure 2 shows the distribution of the launches over the day in this campaign. 3.3. Campaign EPSILON
(autumn 1981)
The overall scientific aims of the EPSILON campaign have been summarized by THRANE (1990). Technical problems with the ‘Stretched Super Loki’ motors limited both the number of launches and the ceiling height of flights with 2.5pm foils. Four experiments
were successful. The l.Sjlrn foils were launched on Viper 3A vehicles and three flights yieided good data. Though the database for autumn conditions is rather small, some conclusions could still be drawn on the occurrence of wind corners as will be shown later. 3.4. Campaign SODIUM
88 (summer 1988)
This campaign was designed to study the dynamics of the upper atmosphere during the formation, and in close vicinity, of ‘sudden sodium layers (SSL)’ fvoN ZAHN et al., 1987). SSLs are characterized by a rapid increase of sodium density with time (typically 5 min) in a thin layer (typically 1 km thickness) located between about 90 and 110 km. SSLs may last from about 10 min up to some hours. The data collected at Andsya on the occurrence of SSLs suggest that SSLs seem to occur preferably (or even only) between 2000 and 0200LT (VON ZAHN et al., 1989). Hence, all 10 experiments of this campaign were iaunched between 2130 and 0103 LT. Out of the total of 10 experiments, six were launched while a SSL was developing. Interest focused on what was happening in the height range between 90 and 100 km. For this reason, 1.5 and 1 pm chaff were used which was specifically designed for this altitude range.
998
H.-U.
WIDDEL and
U. VON ZAHN
01 02 03 OL 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 UT
Fig. 2. Distribution of chaff experiments launched in campaign MAC/SINE 1987 over the day. (a) Number of launches. Blank : 2.5 pm chaff; hatched : 10 pm chaff; filled : flight on which the only strong wind corner event (defined by a directional shear dq/dz > 40“/lOOm and a minimum horizontal velocity IL.,,~ in shear < 10 m/s) was seen between 80 and 81 km. (b) Observation of wind corners. Hatched : strong wind corner events
4. STATISTICS
In the following sections we provide statistical information on temporal and spatial properties of wind corners. In Tables 2-4 all wind corner events seen in the summer and autumn campaigns are listed, together with their characteristics. A similar list for the events of the winter campaign was published by VON ZAHN et ~11. (1985). We point out the similarities and differences of wind corners occurring in summer and winter, though on an admittedly limited statistical basis. 4.1. Seasonal and temporal occurrence
qf wind corners
Tables 2-4 show that in general wind corner events are seen in summer, autumn and winter. A more detailed analysis concerning the frequency of occurrence in summer and winter yields differences between these seasons, but also similarities : Fig. 3, which presents the results listed in Table 2 in graphical form, shows that the number of ‘weak’ wind corners, defined by a directional shear S with S”/lOOm 9 S d lo”/ IOOm, is much smaller in summer than in winter. If one confines the comparison to strong shears with S > 40”/100m and relates the number of these events to the total number of flights it turns out that this relative frequency of occurrence is almost equal for both seasons : roughly, on every third experiment a strong wind corner was seen. Concerning a potential dependence of the frequency of occurrence of wind corners on local time we have to note that many of our experiments concentrated close to either noon or late-evening hours. Wind corners were observed in those periods as well
as other local times. So far we do not see a significant preference of wind corners to occur at certain hours of the day. This is quite different from. for example, the behaviour of SSLs. A further difference between summer and winter was that the observation on more than one wind corner per flight is an infrequent event in summer : in summer 1987 it was seen in 4 flights out of 19 (see Table 2) ; in summer 1988 it was seen in only 1 flight out of 10 (see Table 4). The relevant statistics for winter read as follows (VON ZAHN et al., 1985) : total number of flights = 18 ; of them, 3 flights had 2 wind corners, 5 flights had 3 wind corners, 3 flights had 4 wind corners, 2 flights had 5 wind corners and 3 flights had 6 wind corners, which means that on 16 flights (about 89% of all flights) more than one wind corner was seen. 4.2. Prgfbred
hei,qhts and sense qfdirectional
shear in
wind corners
Clear differences between summer and winter were recorded for the heights at which wind corners appeared and the sense of directional shear of the wind in the wind corner: In summer as in winter, wind corners tend to appear at certain preferred heights; both these heights are different in the two seasons : Fig. 4a shows the heights at which wind corners were seen in winter 1983-1984 (VON ZAHN et al., 1985) and Fig. 4b shows the height of the wind corners listed in summer 1987 (see Table 2). In summer no wind corner was seen at heights below 80 km, despite the fact that the height range between 76 and 80 km was covered by a number of
SC24
SC25 SC26 SC21 SC28
I306
2030
2117 2201 2230 2254
15 July 1987
15 July 1987
SC23
SC17 SC18 SC21
0852 0943 1203
14 July 1987
SC14 SC15 SC16
2215 2334 2349
SC12
1336
1 July 1987
SC11
1244
1987
26June
SC04 SC05 SC07 SC08
SC09
1120 1153 1240 1333
1987
24June
Flight code
1127
Time (UT)
Date
Launch
88.8 83.6 89.0 88.7 87.2 87.0
91.9 85.9
93.2 95.0 85.2
80.9 92.5 80.4
91.9 87.2 92.8 86.1 84.2
87.3 93.2 90.6 91.9
Altitude of wind corner (km)
21 16 22 145 54 33
13 23
14 44 15
26 64 17
6 27 IO 82 11
6 33 16 14
Directional shear (deg./tOOm)
Table 2. List of wind corners
100 30 45 180 140 135
135 70
150 175 140
135 170 130
45 90 110 170 95
70 110 105 140
Total angle of change in direction (deg.)
seen in campaign
W-SES ENE-NNE W--SW NW-SE NW-E NW-E
WNW-E NE SE
SEPWN W SE- NW SW-E
WNW-E NW-E WNW-ENE
NW-N NE-SE SSE-NE NWN-SE N-E
NPENE SSE-NE SES-NEN SWS-E
Change of wind direction (from-to)
MAC/SINE
+ + +
_
+ +
SODIUM
19 34 19 2 I 10
salvo
salvo
Turbulence/gravity
salvo
Chaff salvo No. 3
EISCAT
-
1
Chaff salvo No. 2
Chaff salvo No.
Remarks
8 IO
12 7 15
4 15
36 7 27 9 25
+ +
_
52 12 28 32
+ _
(m/s)
Horizontal velocity in wind corner
1987
Sense of change
summer
6
2, B B 1
%
2
P
z
a 8 ;.
< 5’ a 8
1987
21 October
2028 2135 2210 2222 2256
2220 2313
0003
13 July 1988
14 July 1988
Time (UT)
1430
2321
1138
1332
-
Time (UT)
1988
Date --24June
Launch
1987
1987
I November
1987
15 October
Launch
17 October
Date
~-
NC10
NC08 NC09
NC01 NC02 NC03 NC04 NC05
Flight code
EC09
EC08
EC05
EC04
Flight code
..-
88.X
91.9 99.7 90.2
93.6 92.6 92.3 91.7 90.8
(km)
Altitude of wind corner
62
17 4 44
7 18 62 34 35
110
60 95
50
90
180
105 140 180
90 130 200 165 165
WSW-N
W-E
NNW-E SSW-NWW W-E
WNW-NNW W-NE WPESE NW-E WNW-E
Change of wind direction (from--to)
88 summer
SWPNNW
WNW-N SE-NW
NW-NNW
1987
+
f + +
+ + + + +
Sense of change
+
+ +
-
+
Sense of change ~._
1988
October
Change of wind direction (from-to)
EPSILON
SODIUM
Total angle of change in direction (deg.)
seen in campaign
Directional shear (deg./l00 m)
Table 4. Wind corners
44
10 55
94. I 86.6 79.0
13
15
in campaign
Total angle of change in direction (deg.)
observed
Directional shear (deg./ 100 m)
94.4
IS.7
Altitude of wind corner (km)
Table 3. List of wind corners
7
33 33 I1
42 25 9 13 12
Horizontal velocity in wind corner (m/s)
5
30 4
27
9
Horizontal velocity in wind corner (m/s)
Second salvo
First salvo
Remarks
1.5 pm chaff
1S nm chaff
Remarks
$
!Z? N
c
2:
Wind corners
in the 70-100 km altitude
60
50
2 z 40 > _ w
MAP/WINE WINTER 1~8318~ 18 EXPERIMENTS
MAC/SINE SUMMER 1987 22 EXPERIMENTS
2 30E Z = zoL IO0 SHEARIDEG/lOOml @
@
Fig. 3. Frequency of observation of wind corners during winter (a) and summer (b). Roughly on every third flight a wind corner was seen, which had a directional shear >40”/100m.
range
I001
individual experiments comparable to that launched during winter (this is shown in Fig. I), whereas in winter the 76-80 km altitude range appears to be the preferred height for wind corners. Figure I shows that this height range was adequately covered by the number of experiments in both seasons. In addition, it should be mentioned that the two wind corners listed for the height bin go--81 km were observed on only one day (1 July 1987) and on two consecutive flights launched about 1h 30 min apart in time. The spacing in height between preferred heights was significantly larger in summer than in winter : it was about 5-6 km in summer compared to 3-3.5 km in winter, and this is best seen when the wind corners are considered which are characterized not only by a strong directional shear dq/dz, but also by a low horizontal velocity in the wind corner (IV,,]< IO m/s) (hatched events in Fig. 4). For autumn 1987 (Fig. 4c) the number of successful flights remained quite small (7 flights only), and it is difficult to draw any firm conclusions from that small database. If we take the observations as they are, we saw one strong wind corner in the height bin 86-87 km (we found strong wind corners in this height bin in
, I I
I,
Winter 03104 (18FLlGHTS1
23 2;
;0
SUMMER 87 (22 FLIGHTS)
@ i
HEIGHT BIN [km1 Fig. 4. Heights at which wind corners were seen in winter (a), in summer 1987 (b), and in autumn 1987 (c). The winter data refer to the samples published in VON ZAHN et al. (1985), but results obtained below height bin 75-76 km were omitted. (No adequate height coverage there during summer.) Wind corner events in which the horizontal velocity IL’J was < lOm/s are marked by hatching.
1002
H.-U. W~DDEL and U.
our summer and winter data, too) and two strong wind corners in the height bins 78-79 and 75-16 km. These are levels in which we saw no wind corners in summer but in winter (cf. Fig. 4). In this context we might briefly discuss why we have not seen any wind corners at height levels below 80-81-km during summer. It was suggested that our chaff measurements did not extend to low-enough heights to see them. At present we have no database from chaff experiments which could rule out or prove that this suggestion is correct, and we must therefore leave this question open. The sense of rotation of wind direction in a wind corner shows characteristic differences between summer and winter: Fig. 5 shows the results for summer 1987. Defining ‘positive’ as the sense of change of wind direction from northwards towards eastwards with increasing altitude, in 15 wind corners the sense of directional shear was positive and in 8 corners negative (Fig. 5a). Considering wind corners characterized not only by a strong directional shear but also by a low horizontal velocity in the wind corner (lvt,l 6 lOm/s) too, it turns out that 11 of these wind corners had a positive sense of directional rotation (Fig. 5b). Taking these strong wind corners out and
SUMMER
1987
VON ZAHN
considering the rest, one fmds an almost even distribution between positive and negative rotation (Fig. SC). In winter 1983-1984 the scenario was reversed, at least for the strong events : 9 corners with strong directional shears had a positive and 19 had a negative directional change (VON ZAHN and WIDDEL, 1985). Considering all events, however, there was an about 1:l distribution in sense of rotation, not unlike the weak summer events. 5. DYNAMICALASPECTS A theoretical model of the atmospheric dynamics leading to the formation of wind corners is still pending, and hence this section is confined to our observations.
The results on wind corners listed in Tables 24 suggest that the directional shear and the horizontal wind velocity in a wind corner are in some way related to each other, and this was investigated more closely. In doing this, one has to take into account that three series of measurements exist which were launched in summer under very specific conditions. These were the flights SC 24-SC 28 on 15 July 1987 (Table 2) and all flights performed in summer 1988 (Table 4). These flights were considered separately, because they were launched when a sudden sodium layer (SSL event, cf. Section 3.4.) developed and for brevity, these flights are further referred to as ‘sodium flights’. The directional shear dq/dz observed in each wind corner is plotted against the minimum horizontal speed lr,,l in the shear in Fig. 6. The data points lie in a field having a negative slope, which is -0.83 for the summer 1987 measurements when the flights SC24SC28 are not considered (Fig. 6a) ; the slope is - 1.02 for all sodium flights, SC24-SC28 and summer 1988 (Fig. 6b). Figure 6c shows all summer (sodium flights included) and autumn data together. Evidently, the autumn data fit well into this picture, too. 5.2. Wind c’ornrrs and r.ertica/ uir rwlocitirs
HEIGHT
BIN [km1
Fig. 5. Sense of rotation of wind direction dqjdz in wind corners observed in summer 1987. Positive sense of rotation is defined as a turning of wind direction from north towards east with increasing height. (a) All wind corners. (b) Sense of rotation in strong wind corners defined by dq/dz 2 4tY / 100 m and a horizontal velocity in wind corner ll%Hl< lOm/s. In all of these strong wind corners the sense of rotation in wind direction was positive. (c) Fig. 5a, but strong shears taken out.
If one assumes that wind corners are the result of some kind of wave interaction, one would expect that a relation between vertical air motions and wind corners exists. Vertical air motions can be derived from foil cloud experiments if temperature and air density profiles are available which have a good resolution in height and which are measured by independent techniques as close as possible in time to the launch of the foil cloud
Wmd corners
in the 70-100 km altitude range
1003
4
86
& l
x
0
rw x
)I
.&ox” '0 !ALL DATA: - x SINE 1987 cl SOOIUM JUNE/JULY 1988 . EPSILON OCTOBER 1987
_
1
10
100
1
10
100
I-1
10
94 c
C’ I 199.7 km,
100
HORIZONTAL VELOCITY Iv,l[mlsl
Fig. 6. Relation between directional shear dq,/dz and minimum horizontal wind velocity IL.~,(in wind corner. From left to right : (a) all data obtained in summer 1987 (campaign MAC/SINE) except those of 15 July 1987 ; (b) all flights performed under conditions of a developing sudden sodium event; crosses : 15July 1987 ; circles : summer 1988 (campaign SODIUM 88) ; (c) all data; SINE 1987 (crosses), EPSILON 1987 (filled circles) and SODIUM 88 (open circles). The results point towards an inverse relation between
dq/dz and 10~1in wind corners.
experiment. Temperature and air density profiles were derived from inflatable falling sphere experiments performed in the MAC/SINE campaign by the University of Bonn (L~~BKEN et al., 1990). In eight cases the launch of the falling sphere was considered to be close enough in time to the launch of the chaff experiment to allow a comparison to be made. The result of this comparison is given in Fig. 7ax, which shows the horizontal wind speed 1z.,,[, the vertical air velocity M’and the directional shear S. The height at which the wind corner was seen is indicated by a dashed line, and it appears that wind corners are related more to the gradients in vertical air velocity than to relative extrema in vertical air velocity. Figure 8 shows this in more detail: in four cases the height at which the wind corner was seen coincided with the height at which dw/dz was negative and had a (relative) maximum ; in two cases, the vertical velocity gradient dn,/dz was positive; one case remained undecided. The attempt, however, to relate dM,/dz to the directional shear dq/dz, to the total change of direction p or to the minimum velocity (cHI in wind corner had no result. This is shown in Fig. 9, in which the vertical velocity gradient dbr/dz is plotted against total change in direction q and against the directional shear dq/dz.
5.3. Wind corner patterns A convenient and informative way to present results on radar tracking of a target is the projection of the target’s trajectory onto the Earth’s surface (later
referred to as ‘radar ground plot’). It can be read like a map (+X: eastward, + Y: northward). The radar points are plotted at equidistant times and therefore the horizontal speed of the foil cloud can be visualized quickly: clustering of points shows low wind speed, spread-out points indicate high wind speed. Also, the retardation of the wind speed in wind corners is easily recognizable. Altitudes are printed out in this plot at regular intervals of time (e.g. at each full minute), which gives indirect information on the descent velocity of the cloud, too. On 15 July 1987 five rockets carrying chaff cxperiments were lauched in sequence when a SSL developed overhead the Andoya Rocket Range. The SSLs were detected by a ground-based sodium lidar (HANSEN and VON ZAHN, 1990). A fairly unusual, spiky pattern was seen (Fig. 10, left panels) of a kind which was not seen in winter. The question of whether this pattern was just accidental or a regular feature was answered by the results of two series of launches performed under similar conditions (developing SSL) in summer 1988. The same, spiky pattern was observed then also in the two salvoes as Fig. IO (right panels) and Fig. 11 show. There was another similarity between these three salvoes in that the wind corner descended steadily with time. Notably, the phase velocity of this descent was fairly uniform (average : 0.5 m/s) in ail three cases, as Fig. 12 shows. The downward phase velocities seem to indicate upward motion of energy. These similarities suggest that the same mechanism leading to
1004
H.-U. WIUDEL and U. VON ZAH~V
wind corners was at work
when these three salvoes
were launched. A certain hint of this is given by the
results of a spectral analysis of the vertical profiles of the scalar horizontal wind 1~1~~1 = (u’+z?) I.” which was made for flights SC24-SC28 on 15 July 1987 (Wu and WIDDEL, 1989). It turned out that the spectral slopes for the individual flights were very close to - 3 ; the average was - 3 f0.1 and this is the slope proposed by SMITH et al. (1987) for a saturated gravity wave spectrum. We conclude from this that in these series of measurements saturated gravity waves may have been involved in the formation of the wind corners. 5.4. Wind corners and Richardson numbers The strong wind shears observed in wind corners are suggestive of wind corners being regions of turbulence. For the winter 1983-1984 observations THRANE et al. (1987) found, however, that only about
I
100
,
I
,
50% of all wind corners contained turbulence and 50% were without it. During the MAC/SINE and the EPSILON campaigns turbulence parameters were derived from data obtained by several high-resolution in-situ measurements of air density, ion density, and electron density. These experiments and their results are described in the paper by BLIX et al. (1990), and references therein. We calculated Richardson numbers (Ri) using temperature and air density profiles obtained from inflatable falling sphere experiments during the MAC,’ SINE campaign (L~BKEN et al.. 1990) launched close in time to the launch of our foil experiments. We searched for altitude regions with Ri < 0.25. as the latter value is commonly taken to indicate that turbulence might possibly be present there. Temperatures derived from falling sphere experiments are given at 1 km altitude intervals and a temperature profile was obtained by a polynominal fit of
SC 07: 24. JUNE 1987, 12:40 UT / I I
100
80
80 0
20
40
Horizontal
60
80
velocity
-8-4 Vertical
0
0
velocity
Directional
Wm/sl
~Hb/Sl
SC 25: 15. JULY 1987,
21:17
45
shear
S[deg/lOO m] UT
80
80 0 30 60 90 Horizontal velocity
-8-4 Vertical
Wblsl
0 velocity
WWsl Fig. la
0 Directional
90 shear
S[deg/Ioom]
Wmd corners
in the 70-100 km altitude
SC 04: 24. JUNE
1987,
11:20
7
1005
range
UT
1 ----_
-__
__
90
_ r
80 0
20
40
60
Horizontal
80
_4
_8
0
Vertical velocity
velocity
*If b/s1
Directional
shear
S[deg/lOO m]
w&l
SC 09: 26. JUNE
45
0
1987, II:27
UT
100
100
3 ?L UJ 7 90 3 4
90
80
80 0
30
60
Horizontal
-12
velocity
-8
0
Vertical velocity
wb/sl SC
-4
45
Directional S[deg/lOO
Wb/sl 11:
26.
JUNE
1987,
12:44
shear m]
UT
80 0
30
60
Horizontal
90
velocity
WI+1
12
8
-4
0
Vertical velocity
w[+l Fig. 7b.
4
0
45 Directional S[deg/lOO
90 shear m]
H.-U.
1006
WII)DEL and
U. VON ZAHN
SC 28: 15. JULY
1987, 22:54
UT
80
80 0
30
60
Horizontal
-8
velocity
-4
0
Vertical velocity
v&/s1
Wlmisl SC 27: 15. JULY
0
90
Directional
shear
SIdeg/lOOm]
1987, 22:30 IJT
100
90
90
80
30
0
30
Horizontal
60
-8-4
velocity
0
Verticai velocity
~Hlm/sl
Wb/s] SC 26: 15. JULY
Fig. 7a--c. Horizontal velocity 1~~1,vertical wind corner (dashed horizontal line). In all velocity IL.“/ a minimum at the height where the height at which the vertical air
45 Directional
90 shear
S[deg/lOO m] 1987, 22:Ol UT
air velocity u’, directional shear of wind dq:dz and position of cases, the directional shear has a maximum and the horizontal the wind corner is seen, but this height does not coincide with velocity M’(centre panels) has a (relative) extremum.
in the 70-100 km altitude
Wind corners
range
80
VERTICAL
VELOCITY
GRADIENT
[m/s/100 m]
Fig. 8. The position of the wind corner is at a height where the vertical air velocity gradient dw/dz has a (relative) extremum or is close to that. In five out of the eight cases this gradient is negative.
i -I-
3
X
N
0
0-v
5 :
z E
2 9 ;;
-2-
x
0
v
!G Y
-3
0
I
I
I
I
20
40
60
80
I
100
1
120
I
140
I
160
180
TOTAL CHANGE IN DIRECTION[DEGIW DIRECTIONAL SHEAR IDEG/lOO ml(xJ
Fig. 9. Vertical velocity shear dw/dz plotted against total change in direction of wind in wind corner, cp (filled circles) and directional shear of wind dq/dz (crosses) for the flights shown in Figs 7 and 8. No clear relation between these parameters is seen.
H.-U. 15. July 21.20 - 21.27
ZAHN
24. June
:,*“’
i
VON
21.38
- 21.41
UT
‘.. 89.8
d ~:1.._+38.0
~“-03.2 ? ‘-Breakup
J .f , B-59.3
22.13
84.1
$-
88
+3.6
.:- zy34.5 ‘.,. :J
U.
85.4 .r.. 4,
93.1:: 83.9
and
87
-c
UT
W~DDEL
- 22.16
Breakup
:?
C-
towards
---
West
towards
West
Fig. 10. Projections of the trajectories of chaff clouds onto the Earth’s surface (radar ground plots) of flights which show the development of an unusual pattern of a wind corner (centre panel). These plots are to be read like a map (top: North) and cover an area of 16.5 x 10 km’. Heights are printed out at each full minute UT rounded to the next 1OOm and unsmoothed radar raw data are shown. Left panels: I5 July
1987; right panels: 24 June 1988.
these data. The fluctuations in the vertical profile of the horizontal wind 1~~1= (u2+a2) “* were reduced by running means over a certain height interval, and we choose 300m in a first test and 1 km in a second. When comparing the results of these two test runs, we found that the main features of the Ri profiles were the same in both tests, but the numerical value of Ri and the heights at which minimum Ri were seen differed slightly by some 100 m, and a sample for this is given in Fig. 14a-c which will be discussed later. In the following we present the results obtained when the horizontal wind loHI was smoothed over a height interval of 300m. We assume that smoothing the horizontal wind over a height interval of 300m,
which is about six times the height resolution of the wind measurement (50 m), is sufficient to remove random fluctuations and matches the height resolution of the temperature measurements obtained from falling sphere experiments. Because temperatures derived from falling sphere experiments are somewhat uncertain for heights above about 90 km. we confined our comparison between Richardson number profiles and wind corners to altitudes lower than 90 km. What was obtained is shown in Fig. 13a-c. In four cases (Fig. 13a) the height interval in which the Richardson number was rather close or even slightly smaller than 0.25 centred around the altitude where the wind corner was found. We have to note, however,
Wind corners 13. July
-
in the 70 100 km altitude
88
towards West
Fig. 11. Same as Fig. 10, but for the second salvo of campaign SODIUM 88, 1 pm chaff. Area covered by each plot: 20 x 20 km’. A spiky pattern is seen in the centre panel again and the wiggles in the trajectories are believed to be caused by billows or vortex plaids.
range
1009
that three of these four samples were obtained from a series of flights launched in close sequence on the same day. In five other cases (Fig. 13b,c) the Richardson number for the altitude at which the wind corner was seen was larger than 0.25 (of the order of Ri = l), but Ri z l/4 or even smaller was approached at an altitude lOO-500m lower than that of the wind corner. The extension in height of these regions varied between a few 100 m up to about a little over 1 km in two cases, and the latter are shown separately in Fig. 13c, which presents the results obtained on two consecutive flights launched 52min apart in time. Both the height at which the wind corner was seen and the height at which the Richardson number assumes its minimum descent with time, and this allows an estimate on the phase velocity of descent which is about -0.6 m/s for the wind corner and about - 0.9 m/s for the height at which the minimum Richardson number is seen. Both values compare favourdbty with phase velocities derived from the descent of MST echoes (KLOSTERME~ER,1989, personal communication). Two interesting facts showed up when the Ri (z) profile and the height vs time z(t) plot of the chaff descent were compared with the z(t) variation of MST radar echoes. The SOUSY MST radar which was used for comparisons between chaff data and MST echo pattern was operated almost continuously throughout the summer 1987 campaign (MAC/SINE) by the Max-Planck-Institut fur Aeronomie (e.g. CZECHOWSKY et al., 1984; ROSTER. 1989, personal communication). This instrument was located about 6 km southwest of the rocket launch site. This radar collected its data in 300m height bins for 11s of each minute in the vertical position of its antenna beam. This means a certain smear-out of data in height and for a comparison between MST radar data and results obtained by the chaff experiment we used Richardson number profiles in which the horizontal wind was averaged over 1 km in order to get a better match of the chaff data to the height resolution of the MST radar. At heights below about 86 km there was, in general, a one-to-one correspondence between appearance of MST radar echoes and altitudes for which the in-situ experiments indicated Ri < 0.25 (a comparison in detail is difficult because the height and time resolution of the MST radar is much less than that of the foil cloud measurement). This was clearly not the case for heights greater than 86 km. Figure 14 gives examples for this. The first sample (Fig. 14a) shows no MST radar echoes at the height of 89 km where the Richardson number is smaller than 0.25. Yet, echoes appear 5min later at a height roughly 1.5km higher than the height of the wind corner. A
H.-U.
1010
WIDDEL and
total of 44 min later (sample of Fig. 14b) only weak MST echoes are seen at the altitude and time of the foil cloud measurement. These echoes, however, become stronger about 556min later. Another 53min later (Fig. 14~) all three heights coincide : that of the wind corner, that of the region with Ri d 0.25, and that of the MST radar echoes. The ground distance between the location of the MST radar and the point over which the wind corner was observed was 15-25 km away almost due North of the MST radar. Therefore it seems plausible to assume that the MST radar echoes seen in these cases at heights above 87-88 km were caused by frozen-in turbulence drifting in the wind, as was suggested by ROOSTER and KLOSTERMEYER (1987). The fact that these three foil clouds had very ‘clean’ radar trajectories (cf. Fig. 10) must raise serious doubts, however, about the prevalence of strong turbulence in just those wind shears. This intriguing observation obviously needs further study. A second topic of interest concerns the final decay of the foil cloud. If one identifies strong MST radar echoes with regions of strong turbulence, then one would predict that foil clouds might decay preferentially at altitudes with strong MST echoes. Surprisingly, this was not the case : with one exception all foil clouds were destroyed below the bottom of the height region in which MST radar echoes were seen. The foil clouds pass the region of strong MST echoes relatively unaffected and putting all observations together (height vs time plot, change of relative radar return cross-section, the width of the radar return echo at the radar’s gate and the projection of the trajectory
U. VOX ZAHN
onto the Earth’s surface) the conclusion is that the clouds were spread out and destroyed in all cases by rather violent, ‘organized’ air motions which obviously did not contain the 3m structures the MST radar requires for echoes.
6. SUMMARY
Our results concerning wind corners identified by foil experiments performed at 69” latitude are summarized as follows : 1. Our observations in summer and in winter indicate that the occurrence rates for wind corners are similar for both seasons. of occurrence of strong 2. The relative frequency wind corners defined by a directional shear larger than 40-/IO0 m was found to be equal in summer and in winter: roughly, on every third experiment a strong wind corner was seen. suggest that wind corners have 3. The observations no preference to appear at certain hours of the day. 4. Wind corners tend to appear at certain preferred heights but these heights are different in winter and in summer. They are higher in summer than in winter and the spacing between these heights is larger in summer (about 5 km) than in winter (3 3.5 km). The limited database for autumn suggests that the preferred heights and their spacings between them are similar to those seen in winter. 5. In strong wind corners the sense of rotation of the
92
\‘\\ -0.50
1E 90 ‘N I- 09 Z+ w = 88
0 0
.
-0.L6 m/s
I
0
mls
Cl
\
86 85-l , 20
15 JULY 1987 SC24/25/26/27/28
I
I
21
22
1 I
23 21
UT
Fig. 12. Phase velocities
13 JULY 1988 NC819110
24 JUNE 1988 NC2/314/5
I
22
I ,
23 22
I
of wind corner
24
I
01
UT
UT
of descent
,
23
for all three sodium
event salvoes.
Wind corners in the 70-100 km altitude range
1011
t
r i 103 Fig. I3a.
1”3
--.-_‘_L
.,,,
Wind corners
in the 7&100 km altitude
S-Cl2
S-c!1
ZG-JUNE-198713236UT
~-3UNE-l~l~4~UT
10-z
10-l
100 101 RICHARDSONNO.
range
102
103
10U
101
RICHARDSONNO.
Fig. 13~. Fig. 13a+c. Wind corners and Richardson numbers. (a) Height profiles of Richardson numbers. Position of wind corner marked by dashed horizontal line. In four cases a small Richardson number close to 0.25 was seen at the height of the wind corner. (b) Cases in which the Richardson number is larger than Ri = 0.25 (of the order of 1) at the height of the wind corner but smaller Richardson numbers are found at heights between about IOOm and about 1km lower than that of the wind corner. (c) Two consecutive flights on which the Richardson number is much smaller than 0.25 over a height interval of the order of 1km, and both the height of the wind corner and the height at which the Richardson number has its minimum descend with time. The phase velocity of descent is of the order of - 0.6 m/s for the wind corner and -0.9m/s for the height at which the minimum Richardson number was seen.
1014
H.-U.
WIDDEL
and
U. VON ZAHN
15. JULY
1987
a
0.1 Richardson
I
2120
10 Number,
Ri
Richardson
Number.
Ri
Richardson
Number,
RI
2125
2130
Time (U.T)
‘I’ime (U.T)
Time (L1.T)
Fig. 14. Correspondence between Richardson number profile, the height at which a wind corner was seen (altitude of wind corner marked by a dashed line) and MST radar echoes with height vs time nlot of chaff trajectory superimposed (right pa&l). Strength of MST radar echoes coded in rel&e scale. la) No MST radar echoes at the height where Ri < I/4 and that of the wind corner are seen. (b) Weak MST radar echoes are present and become strong minutes later. (c) Coincidence between MST radar echoes, small Richardson number and wind corner. Note that in all three ~nses the clouds are destroyed at altitudes well below the heights at which strong MST radar echoes are present (set text).
Wind corners in the 70- 100 km altitude range wind direction is positive in summer and negative in winter. Wind corners appear at an altitude where the gradient in vertical air velocity dw/dz has a relative extremum. In two cases out of eight, this gradient was positive and in five case negative. At altitudes below 90 km wind corners tend to occur at or close to atmospheric layers having Ri z 0.25. No clear correspondence between MST radar echoes and wind corners was found for heights above about 87 km.
1015
Ackno~~~iedgements-R. Siebenmorgen (University of Bonn) and Dr Y.-F. Wu (on leave from Academia Sinica, Beijing) assisted in launch support, data acquisition and data processing. Dr P. Czechowsky (Max-Planck-lnstitut fiir Aeronomie, Katlenburg-Lindau) provided us with the raw results of the SOUSY MST radar. Crews of the Andwrya Rocket Range and of DLR supervised the preparation and launch of the rockets. Radar tracking of the foil clouds was performed by the radar crew of DLR-Moraba (OberpfaFfenhofen). All this co-operation and these services are gratefully acknowledged, as are the work and efforts of two referees. This work was supported by the Bundesministerium fiir Forschung und Technologie, Bonn, by grants 01 OE 86033 and 01 OE 86046.
REFERENCES
BLIXT. A., THKANEE. V., FKITTSD. C., ZAHNU. VON,LUBKENF.-J., HILLERTW., BLOOD S. P., MITCHELL J. D., KOKING. A. and PAKHOMOV S. V. CZECHOWSKY P., RCSTEKJ.
and SCHMIDT G. HANSENG. and ZAHNU. VON L~~BKEN F.-J.. ZAHN U. VON, MANSON A., MEEK
C.,
HOPPE U.-P., SCHMIDLIW F. J., STEGMAN J., M~!RTAGH D. P., ROSTER R.. ScHMi%x G.. WIUDEL H.-U. and ESPYP. ROSTER R. and KLOSTERMEYER J.
SMITHS. A.. FRITTSD. C. and ZANDTT. E. VAN THRAKE E. V. THRANE E. V., BL.IXT. A., HALL C., HANSEN T. L.. ZAF~N U. VOX MEYEK W., CZECHOWSKK P., SCHMIDT G., WIDDEL H.-U. and NEUMANN A. WIDDEL H.-U. Wu Y.-F. and WIDDEL H.-U. ZAHN
L’. VOK
ZAHX U. VOX,GATEENP. VON DER and HANSEN G. ZAHN U. VOX, GOLDBERG R. A., STEGMAN J.
and WITT G. U. VON.MEYEK W. and
ZAHN
ZAHN U. VON and
WIDDEL H.-U.
WI~DELH.-U.
1990
J. atmm.
1984 1990 1990
Ado. Space Res. 4(4), 41. J. atmos. terr. Phys. 52, 585. J. atmos. terr. Phys. 52,955.
1987 1987 1990 1987
J. J. J. J.
1987 1989 1987 1987 1989
J. afmox. terr. J. utmos. terr. J. atmos. few. Geophys. Rrs. Planet. Spaw
1985
Proceedings of’ the 7th ESA PAC Symposium on European Rocker and Balloon Ptqrammes and Relayed Research (p. 61), ESA SP-229. ESAIESTEC, Noord-
198.5
Geophyx
atmos. atmos. atmos. atmos.
terr. Phys. 52, 835.
terr. terr. terr. terr.
Phys. Phys. Phys. Phys.
49, 44, 52, 49,
143. 1404. 8 15. 751.
Phys. 49, 723. P&s. 51, 991. P&s. 49, 607. Lett. 14, 76. Sci. 37, 657.
wijk, The Netherlands. Res. Lett. 12, 673.