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Temporal evolution and spatial variation of the boundary layer over complex terrain
PII: S1352–2310(97)00193–3
Atmospheric Environment Vol. 32, No. 7, pp. 1179–1194, 1998 ? 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1352–2310/98 $19.00 + 0.00
TEMPORAL EVOLUTION AND SPATIAL VARIATION OF THE BOUNDARY LAYER OVER COMPLEX TERRAIN N. KALTHOFF,*; † H.-J. BINDER,† M. KOSSMANN,† R. VOGTLIN,† U. CORSMEIER,† F. FIEDLER,† and H. SCHLAGER‡
† Institut fur Meteorologie und Klimaforschung, Universitat Karlsruhe=Forschungszentrum Karlsruhe, Germany; and ‡ Deutsche Forschungsanstalt fur Luft und Raumfahrt, Oberpfa enhofen, Germany (First received 20 May 1995 and in nal form 21 October 1995. Published April 1998) Abstract— The temporal and spatial evolution of the convective boundary-layer has a strong e ect on air pollutant dispersion, especially under inhomogeneous conditions with varying orography and=or land use. During the TRACT experiment the evolution of boundary-layer height in space and time was measured by radiosondes and aircraft during three special observation periods (SOPs). The observations show that (a) The boundary-layer height follows the height of the underlying orography. The correlation between boundarylayer height and orography holds best in the morning hours, but decreases slightly during the afternoon. This result con rms with previous observations on a smaller scale, which show that the PBL top con rms to the underlying orography until the boundary-layer depth exceeds the characteristic scale of the obstacle. (b) Coincident with the spatial variation of the boundary-layer height, a strong spatial variation of air pollutants can be observed. (c) On two of the three SOPs surface fog-covered parts of the TRACT area (the upper Rhine valley) until noon. Due to the reduced surface heating the boundary-layer height in the foggy areas was much lower than in the clear areas. ? 1998 Elsevier Science Ltd. All rights reserved Key word index: Mixed layer, air pollutants, orography.
1. INTRODUCTION
The vertical mixing of air pollutants strongly depends on the depth and strati cation of the planetary boundary-layer (PBL). During daytime, the growing unstable boundary-layer is marked by a capping inversion. Due to the in uence of inversion height on the diffusion of air pollutants, the inversion statistics (e.g. Hentschel and Leidreiter, 1960; Gutsche and Lefevre, 1981) as well as the correlation of air pollutants and meteorological conditions (Noack, 1963) has often been investigated. The growth of the boundary-layer, and the accompanying vertical mixing of the air pollutants, depends on surface conditions like sensible heat
ux, large-scale vertical motion, horizontal advection and entrainment at the boundary-layer top (e.g. Ball, 1960; Stull, 1973; Tennekes, 1973). While under horizontally homogeneous conditions the boundary-layer growth is mainly understood, under non-homogeneous conditions the boundary-layer growth may be in uenced by additional e ects (e.g. secondary circulations in valleys) and may di er from the evolution over at terrain (Whiteman, 1982; Bader et al., 1987). Analyses of the spatial and temporal variations of inversions characteristics in complex terrain have been presented by several investigators. Dayan et al. (1988) * Author to whom correspondence should be addressed.
found that subsidence inversion heights are almost uniform above sea level, while convective inversions are mainly in uenced by the underlying orography. However, the top of the mixed layer tends to rise at a slower rate than the rise in the topographical relief. Kuwagata et al. (1990) observed an increase of the convective boundary-layer (CBL) above sea level from the coastal regions toward the inland areas. The increase is caused by the higher surface heating and due to the higher ground level over the inland than over the coastal areas. Bader et al. (1987) and McElroy and Smith (1991) studied the behaviour of the mixed layer above complex terrain. The latter studied the boundary-layer evolution in conjunction with a sea breeze and found differences in the boundary-layer thickness of hundreds of meters within only a few kilometers. Observations by Fiedler (1989) for the area of the Rhine valley indicated that the CBL during the morning hours follows the shape of the orography, while at noon, when the CBL is fully developed, the inversion height is constant with respect to sea level, i.e. independent from the orography. The area of the TRansport of Air pollutants over Complex Terrain (TRACT) experiment is a complex terrain region consisting of valleys (e.g. the Rhine valley) and mountainous regions like the Black Forest, the Vosges and the Swabian Alb (Fig. 1). Thus, effects like the trapping of air pollutants within valleys
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Fig. 1. Position of the di erent sounding systems: The solid lines mark the aircraft ight legs and the encircled numbers mark the positions of the sounding systems. C = constant level balloon, L = lidar, RO = radiosonde, RS = rass, S = sodar, F = tethered sonde.
The boundary layer over complex terrain
in the case of low level or nocturnal surface inversions and air pollutant transport from the convective boundary-layer into the free atmosphere in the case of convection at the mountain slopes may be of importance for the area under consideration. Especially, the Rhine valley is a region of enhanced industrial activities where the emission of air pollutants is higher than in the surroundings. Because of these high emissions, the analysis of surface as well as CBL capping inversions in this area has been studied previously by several other investigators. Surface inversion layers in the mountainous terrain often lead to the decoupling of the local and synopticscale ow elds. Especially in the upper Rhine valley, the decoupling of the upper and lower ow systems under stable strati cation leads to strong windshear phenomena (e.g. Dammann, 1960; Kaltho and Vogel, 1992; Corsmeier and Walk, 1993) and significantly in uences the horizontal transport of air pollutants. Even under neutral conditions the in uence of the Rhine valley topography on the overlying ow eld has been clearly demonstrated (Fiedler and Prenosil, 1980). An analysis of the inversion heights for the Karlsruhe=Stuttgart area has been presented by Kleiss (1963), revealing that nocturnal inversions in the autumn are more frequent than in the annual mean. Havlik (1970) and Mayer (1972) have analysed surface inversion characteristics in the area of the upper Rhine valley (Freiburg and at Karlsruhe, respectively). Both emphasize the signi cance of “inversion phenomena” for this area. Investigations on the convective boundary-layer for this area are less frequent. Fiedler (1983) found that the ow eld in the Rhine valley rises along with the mixed-layer depth. Besides the orographic e ects, studied by Vogtlin (1992) for the Black forest, spatially varying surface uxes due to land use (the mountainous regions mainly are covered with forests, while the land use type of the Rhine valley is grass land or cultivated land) may lead to di erent boundary-layer heights and therefore to di erent mixing of the air pollutants. The present study focuses on the temporal evolution and spatial variation of the convective boundary-layer on the meso- scale. This has been done for the 2nd and 3rd special observation period (SOP) using data from 17 radiosonde stations and two aircraft. The paper is divided into three parts. First an overview of the synoptic situation is given. Then the measuring systems (radiosonde stations, aircraft ight pattern) are described. Finally, the temporal evolution of PBL-growth is compared at valley and mountain sites and the spatial variation of the PBL within the whole TRACT area is investigated. 2. THE SYNOPTIC SITUATION DURING THE SOPs
The main objective of the TRACT experiment was to study the transport of air pollutants under convec-
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tive conditions. The special observing periods were consequently scheduled for daytime periods where the TRACT area was in uenced by high-pressure systems. Because afternoon ights were not available for the rst SOP, we focus our analyses on the PBL-evolution during the 2nd and 3rd SOPs. 2.1. 2nd SOP The synoptic situation during the second SOP (16=17 September) was dominated by an eastward moving high-pressure system. The centre was located over the English Channel (Fig. 2a), on the 16th, but had moved to Denmark on the 17th. This led to northwesterly winds in front of the high-pressure centre and to easterly winds when the centre was situated to the north of the TRACT area. The turning of the wind direction occurred in the late evening of the 16th (22 CEST) when the pressure began to fall in the TRACT area. Within 24 h from the 16th to the 17th, the surface pressure decreased by 2 hPa. The northern part of the TRACT area was a ected by a weak cold front moving in from the north, and was accompanied by stratocumulus clouds. This cold front led to weak large-scale rising motions to the north of the TRACT area at 14 CEST, while subsiding motions dominated to the south (Fig. 2b). The linear interpolation from the 850 hPa level down to the top of the boundary-layer (≈1 km) resulted in a subsidence of w = −0:38 cm s−1 (for ! = 1 dPa s−1 at 700 hPa) in the southern part of the TRACT area. The 700 hPa data were calculated by model of the German Weather Service (DWD). On the 17th, fog covered parts of the upper Rhine valley and remained at some sites until noon. 2.2. 3rd SOP The synoptic conditions during the third SOP (21=22 September) were dominated by a high-pressure system with its axis directed from Scandinavia to Italy (Fig. 3a). An eastward-moving low-pressure system over the Atlantic led to southerly winds and the advection of warm and moist air in the TRACT area. Thus fog and haze were found in the upper Rhine valley until noon of the 21st (Fig. 4). These foggy conditions in the upper Rhine valley are a well-known phenomenon, which in the area between Freiburg in the south and Hagenau in the north are more frequent than on the surrounding mountains and in the northern part of the Rhine valley (Paul and Wahl, 1993). Because of this synoptic condition, subsidence dominated in the TRACT area (Fig. 3b), ranging from w = −0:63 cm s−1 in the area of Strasbourg to w = −0:13 cm s−1 in the area of Mannheim at 1 km above ground. Small upward vertical motions at the southeastern edge of the TRACT area arose from the cold front at the Alps. On the second day of this SOP the high-pressure system moved eastwards. From 21 to 22 June the surface pressure decreased by about 5 hPa=24 h. On the evening of the 22nd thunderstorms appeared
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Fig. 2. Synoptic charts of 16 September at 14 CEST. (a) 850 hPa level, according to the Europaischer Wetterbericht of the DWD. Solid lines are geopotential heights in gpdm. (b) Vertical wind eld at 700 hPa as calculated from the !- eld of the DWD. The solid lines are the vertical wind eld isolines in cm s−1 .
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Fig. 3. Same as Fig. 2 but for 21 September at 14 CEST.
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Fig. 4. Fog distribution (encircled black dots) for 21 September at 11 CEST.
in France associated with an eastward-moving lowpressure system.
3. THE MEASURING SYSTEMS
Radiosondes, tethersondes, sodar-, lidar- and rass-systems were installed to determine inversion heights. Within the TRACT area, 17 radiosonde stations are available (Fig. 1), delivering temperature, humidity, wind speed and wind direction pro les. The radiosonde sites were selected to insure that the edges of the TRACT area were well-covered. Radiosondes were also concentrated in the Rhine valley from Mann-
heim (3) in the north to Lorrach (20) in the south to resolve the complex ow eld in the Rhine valley, known from previous investigations (e.g. Vogel et al., 1987; Adrian, 1993; Corsmeier and Walk, 1993). In order to analyze the evolution of the mixed layer and accompanying small-scale processes at the transition zone from the Rhine valley to the top of the Black Forest, i.e. perpendicular to the Rhine valley, an additional line of sounding systems was installed. Between the Rhine in the west to Sasbach (9) at the foot of the Black Forest and to Musbach (13) at the rear, tethersondes, radiosonde and sodar systems were in use (Fig. 1). The results from this line experiment are given elsewhere (Komann et al., 1998).
The boundary layer over complex terrain
Radiosonde ascent frequencies di er between one ascent per day (e.g. Mannheim) and 16 ascents per day at intervals of 1 12 h during the SOPs (e.g. Bruchsal). The standard interval during the SOPs is 3 h. Within the TRACT experiment, a radiosonde intercomparison was performed to determine di erences between the speci c radiosonde systems (Kolle and Kaltho , 1993). Besides the radiosondes, data from DO228aircraft are available performing curtain ights (zigzag pattern) at two boxes (northern and southern box) within the TRACT area (Fig. 1), temperature, humidity, wind speed, wind direction and the concentration of various chemical species (Corsmeier and Zimmermann, 1993). The curtain ights cover heights between ≈200 m and ≈2000 m above ground, where the upper
ight level was intended to be above the top of the PBL. These ights were normally twice a day (morning ight from about 8 –11 CEST, afternoon ight from about 13 –17 CEST) and provide an overview of the spatial variation of the mixed layer. An overview of the entire TRACT experiment including aircraft activities has been provided by Fiedler and Zimmermann (1993).
4. THE CONVECTIVE BOUNDARY LAYER
4.1. CBL-height de nition Several de nitions are given in the literature for the height of the convective boundary-layer or the mixed layer (ML). Most commonly, the top of the mixed layer is de ned as that height where (i) the pro le of the sensible heat ux reaches a minimum, (ii) surface borne air parcels are in equilibrium with the surrounding air and (iii) the base of the rst capping inversion is detected. Further, additional de nitions are often given in terms of the detectable signal of a measuring system, such as sodar (maximum of the temperature structure parameter CT2 ) or lidar (negative gradient in the backscatter pro le). Slight differences in mixing layer height can be expected because of the e ects of the de nitions and measuring systems (Coulter, 1979; Pul et al., 1994). Concerning the mixed or convective boundary-layer depth zi (zi indicates the boundary-layer height above ground (m AGL)) and mixed or convective boundary-layer height Zi (Zi indicates the boundary-layer height above sea level (m MSL)) we assume that the top is identical with the average base of the overlying stable layer (Stull, 1988), i.e. the capping inversion of the CBL. This de nition has been chosen because this height can be detected from radiosonde and aircraft temperature pro les. An initial analysis of inversion height over hilly terrain (Vogtlin, 1992) found that the in uence of an orographic obstacle on the inversion height depends on the scale of the obstacle and on inversion height itself. Referring to this observation, we here introduce a dimensionless number A, characterizing the state of the
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inversion, i.e. terrain following or not, given by √ A=
HL S = zi zi
(1)
where H de nes the characteristic height of the obstacle above the surrounding area, and L the characteristic length of the obstacle. S, therefore, represents the scale of the obstacle. Analysing data from the KONVEGS-experiment (Vogtlin, 1992) gives values of about A = 2:2 for those cases when the inversion follows the shape of the orography, and A = 0:6 for the cases when the in uence of the orograhy can be neglected. Therefore, the change between the two states occurs for A ≈ 1, when the boundary-layer depth is approximately equal to the scale of the orographic obstacle. The characteristic scale of obstacles in the KONVEGS-experiment, estimated from the topographic charts, was S = 0:55 km (H = 0:2 km and L = 1:5 km), and one purpose of the present investigation is to determine if this assumption holds for obstacles of di erent scales. The typical scale of the mountains in the TRACT area is H ≈ 0:5 km and L ≈ 40 km, so that S ≈ 4:5 km. If this scaling argument is valid, the boundary-layer top should conform to the underlying orography until the boundary-layer depth exceeds 4.5 km height. 4.2. The temporal evolution of the CBL at di erent sites The temporal evolution of the boundary-layer at different sites depends on the local surface conditions, the large-scale vertical motions, the stability and the entrainment e ects at the inversion zone. Further, over non-homogeneous or complex terrain, secondary circulation systems and enhanced advection e ects are expected to be of importance for the evolution of the mixed layer. Within the TRACT area, the Rhine valley with the surrounding mountains is one area where strong di erences in PBL evolution can be expected (e.g. Fiedler, 1983). In order to analyse the evolution of the PBL in and outside of the Rhine valley the radiosonde stations Oberbronn (274 m MSL) and Musbach (694 m MSL) have been selected. Figure 5 shows the temperature-, humidity- and wind pro les from Oberbronn (a valley site) and Musbach (a mountain site) for 16 September. On this nearly cloud-free day, the boundary-layer height Zi at Oberbronn reached 1225 m MSL (i.e. a boundary-layer depth of zi = 975 m) at 13 CEST, with well-mixed temperature- and humidity pro les. This gives a growth rate of about 4 cm s−1 which is about one order greater than the mean subsidence on this day (w ≈ 0:38 cm s−1). At this time the potential temperature in the mixed layer is about 20◦ C, increasing to the maximum value of 22◦ C at 16 CEST. The potential temperature is referenced to 1000 hPa. At Musbach, 420 m higher than Oberbronn, the boundary-layer height at 13 CEST reached Zi = 1550 m MSL (i.e. a boundary-layer depth
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Fig. 5. Temporal evolution of potential temperature, speci c humidity, wind speed and wind direction on 16 September at (a) Oberbronn (5), 274 m MSL in the Rhine valley, and (b) Musbach (13), 694 m MSL. The solid line indicates the boundary layer depth at 13 CEST.
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of zi = 850 m). At this time the mixed layer tempera- depth zi in the afternoon is the same in and outside ture was about 24◦ C, nally reaching the maximum the upper Rhine valley (Fig. 7). Such is the case on value of 26◦ C at 16 CEST. Thus, the mixed layer 16 September. • Under foggy weather conditions in the Rhine valtemperature is about 4◦ C higher above the mountain site than in the Rhine valley. This temperature di er- ley with clear-air conditions on the surrounding mounence indicates that the horizontal advection term may tains the convective boundary-layer develops totally become important for model calculations of boundary- di erently between the two sites (Fig. 8). Such is the layer growth at the mountain slopes. Comparing the case on 21 September. The spatial di erences can be boundary-layer at both sites we can state that the boundary-explainded by the lower net radiations before noon in layer depth zi is nearly the same at the two sites. The the Rhine valley (Fig. 9). scale of the Black Forest is H ≈ 0:5 km and L ≈ 40 km, resulting in a value of A ≈ 5. Hence, the assumption that the orography is a dominant parameter for A¿1 4.3. The spatial variation of the boundary layer holds for the scale of S = 4:5 km. height Resulting from the inhomogeneous spatial distribuThe spatial variation of the boundary-layer height tion of the boundary-layer height, horizontal gradients can be determined from aircraft data, which are preferof the air pollutants occur. To examine horizontal trans- able to radiosonde data due to the higher spatial resoport processes, the ow eld must be determined. The lution of the curtain ights. The lowest aircraft ight heterogeneity of ow conditions within the boundary- level, however, is approximately 200 m or more above layer over complex terrain can be seen from the pro- ground, so that surface inversions or low-lying inver les of the wind speed and wind direction. Winds in sions cannot be detected. Therefore, when the CBL the PBL at Oberbronn at 13 CEST turn from north- depth is of interest, the use of aircraft data is restricted westerly at the surface to westerly at the boundary- to ights around noon when the convective boundary layer top, while in Musbach at 13 CEST the winds turn layer is fully developed. Here, aircraft data are prefrom northeasterly in the surface layer to southwesterly sented for the 2nd and 3rd SOP, when afternoon ights in the mixed layer. Above the PBL west-north-westerly were available. winds exist at both sites. In Fig. 10 the inversion height is presented on 16 A totally di erent evolution of the boundary-layer September for the south box ight path. At this time at both sites can be observed on 21 September. Sur- the CBL can be regarded as fully developed. Neverface heating of the boundary-layer within parts of the theless, a strong dependence of inversion height on Rhine valley is suppressed due to a shallow fog layer orography can be found. While over the valleys, like (Fig. 4), while at the surrounding mountains with clear the Rhine valley, the CBL grows up to about 1000 m air conditions the boundary-layer at 13 CEST attained MSL, the CBL reaches 1600 m MSL over the mounZi = 1900 m MSL or zi = 1200 m AGL (Fig. 6b for the tains (e.g. the Black Forest and Jura). These inversite of Musbach). At the same time the boundary-layer sion heights agree well with the heights estimated from height at Oberbronn only reached Zi =675 m MSL or the radiosonde data (1050 m MSL at O enburg in the zi = 400 m AGL (Fig. 6a). At this time the mixed layer Rhine valley and 1550 m MSL at Musbach in the Black temperature was 18◦ C, i.e. 6◦ C lower than at Musbach. Forest at 13 CEST). Only in one region can a difAgain, this means that the horizontal temperature gra- ference from this general (terrain-following) behaviour dient between the valley and the mountain is high and of the inversion height be detected, namely between that horizontal advection at the slope area will be im- the Black Forest and the Swabian Alb. A very high portant for boundary-layer height modelling. boundary layer of about 2000 m MSL can be found in The wind direction at Oberbronn at this time within this region, which is even higher than the mixed layer the boundary-layer is southeast turning to southwest- over the mountains. This high mixed layer may be exerly above the PBL. At Musbach the wind turns at plained by the surface conditions in this area. High 400 m AGL from southeast to southwest. Thus, the surface temperatures and accompanying high sensible main di erence between Musbach and Oberbronn is heat uxes are estimated in this region for this day that the wind sheer at Musbach is not correlated with (Wenzel, 1994). While over the Black Forest a daily PBL-top, as valid for Oberbronn. mean sensible heat ux of about 40 W m−2 is given, In the autumn, fog often occurs in the Rhine val- about 70 W m−2 can be found between the Black Forley when clear-air conditions occur over the surround- est and the Swabian Alb, an area covered mainly with ing mountains (Paul and Wahl, 1993). Therefore, the grass. boundary-layer evolution during both days and at both Coincident with the spatial variation of the boundsites can be regarded as typical for this region. ary layer height, a strong spatial variation of humidSummarizing the data from both days two cases can ity can be observed (Fig. 10). In those regions where be separated: the CBL is high, the humidity is mixed deep into the • During clear-air conditions with weak mean subsidence (w = 0:1 @zi ) the convective boundary-layer @t
atmosphere, e.g. over the Black Forest, the Jura and the Swiss Lower Alps. Especially, the increase of the boundary layer height between the Black Forest and
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Fig. 6. Same as Fig. 5, but for 21 September.
The boundary layer over complex terrain
Fig. 7. Pro les of potential temperature at 7 and 13 CEST on a cross-section through the Rhine valley running through Oberbronn (5), O enburg (14) and Musbach (13) on 16 September. The solid line marks the top of the boundary layer at about 13 CEST, the dashed line marks the top of the residual layer at about 7 CEST.
Fig. 8. Same as Fig. 7, but for 21 September.
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the Swabian Alb coincides with high humidity values up to about 2000 m MSL over this region. Figure 11 contains pro les of potential temperature, speci c humidity and the chemical species NOx and O3 over (a) a characteristic mountain and (b) a valley site (see Fig. 10). Here the role of the inversion as the barrier for vertical mixing becomes obvious. The inversion is not only correlated with a decrease of the humidity but also with the decrease of NOx and O3 concentrations. NOx drops from about 3 ppbv in the mixed layer to about 1.5 ppbv at the top of the ML, i.e. at 1400 m MSL at the Swiss Lower Alps (Fig. 11a) and at 900 m MSL at Sundgau (Fig. 11b). In order to analyse the in uence of orography on boundary-layer height and hence on the spatial variation over hilly terrain, calculations have been made using the linear regression: Zi = a0 + a1 h
Fig. 9. Components of the energy balance at Sasbach (9) in the Rhine valley (top) and at the Hornisgrinde (12) in the Black Forest (bottom) on 21 September. Shown are the net radiation Qo , the latent heat ux Vo , the sensible heat ux Ho and the soil heat ux Bo .
(2)
where h de nes the height of the orography, and a0 and a1 are the regression coecients. In this formulas a1 ≈ 1 indicates that the boundary-layer depth is the same at every site (if RMS is small), so that the inversion height follows the orographical relief. On the other hand, a1 ≈ 0 indicates that the boundary layer height is the same at every site, with no correlation between the boundary layer height and the orography. Results are shown for two times on 16 September using both radiosonde and aircraft data (Fig. 12). The
Fig. 10. (Top) Inversion height in the south box on 16 September as determinded from the aircraft data. The solid line indicates the orography, the dashed line the ight pattern and the thin solid line the inversion height Zi . (Bottom) Isolines of the speci c humidity q along the south box on 16 September as determined from the aircraft data.
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Fig. 11 Pro les of potential temperature , speci c humidity q, and NOx and O3 concentrations at a mountain site (a) and a valley site (b) as measured on 16 September in the south box. The pro ling sites are indicated in Fig. 10. The dotted lines indicate the minimum zmin and the maximum zmax of the orography.
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Fig. 12. Regression curves for the inversion height Zi on 16 September 11 (top) and 16 CEST (bottom). The crosses indicate radiosonde data, and the triangles indicate the aircraft data. RMS denotes the root mean square in m.
data show that in the morning (11 CEST) the boundary layer height Zi at all sites increases nearly with the rate of the orography (a1 = 0:80; RMS = 152 m). A similar behaviour of the PBL-evolution over complex terrain has been modelled by Lieman and Alpert (1993). During the course of the day, however, the in uence of orography decreases (e.g. at 16 CEST when a0 is 1045 m and a1 = 0:42). This agrees with results from Dayan et al. (1988), who found that on the crosssection from the sea up to the mountains the inversion layer Zi rises slower than the rise of the orographical relief, i.e. the boundary layer depth zi over the mountains is smaller than over the low-level terrain. Figure 13 depicts the CBL-height during the third SOP from the south box ight in the afternoon of 21 September. As described above, on this day a fog layer covered parts of the TRACT area, especially in the upper Rhine valley until noon (Fig. 4). The in uence of this fog layer became apparent in the evolution of the convective boundary layer. Good correlation between the inversion height and the orography is restricted to the Swiss Lower Alps, the Jura and the Black Forest, where the CBL-height is low. Very high inversion heights again can be found east of the Black Forest. This behaviour is similar to that observed in the second SOP and is again probably caused by the higher surface uxes in this region. Figure 14 shows the regression curves for 21 September. In the morning at 11 CEST a good correlation occurred between the boundary-layer height Zi
Fig. 13. Same as Fig. 10, but for 21 September.
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on orography decreases somewhat during the course of the day. In agreement with the results from earlier experiments, conducted on a smaller scale topography, the in uence of orography is a dominating parameter for A¿1, i.e. when the characteristic scale of the orography is larger than the boundary layer depth. The strong dependence of CBL height on the orography, however, is reduced for areas covered with fog. Here the boundary-layer depth is much lower than in the other areas. As the capping inversion layer marks the top of the vertical mixing, the spatial variation of the depth of the CBL causes strong horizontal gradients of moisture and air pollutants. The purpose of this paper was to analyse CBL growth in the TRACT area. The spatial variation of the boundary-layer height results from superposed in uences of the orography, varying surface conditions (i.e. di erent surface heat uxes resulting from di erent land use and soil moisture), mean vertical motion and entrainment and advection processes. The individual in uences can hardly be separated from the observations only. Therefore, in the next step, PBL growth models will be applied and simulations will be compared with the observed data for the di erent sites, to quantify the contribution of the di erent processes. Fig. 14. Same as Fig. 12, but for 21 September.
and the orography (a1 = 1). This was unexpected because a fog layer was present in the Rhine valley, with reduced sensible heat uxes. The reason for the good correlation is that the growing inversion in the areas under clear air conditions had just reached the top of the fog layer in the foggy areas (about 300 m MSL). In the afternoon at 16 CEST a value of a1 = −0:02 and a high RMS-value reveal the decreasing in uence of the orography on this day. The great scatter probably is mainly caused by the spatial di erences in surface heating due to the fog in various areas. 5. CONCLUSIONS
Radiosonde and aircraft data of two special observation periods have been analysed to study the temporal and spatial evolution of the boundary layer during the TRACT campaign. During these days high-pressure systems dominated with nearly cloudless sky. However, on 17 and 21 September fog builds in the early morning hours and remains until noon in parts of the TRACT area (especially in the upper part of the Rhine valley). Fog is a frequently appearing phenomenon for this region in the autumn, so that the spatial di erences of CBL evolution during these days can be regarded as typical for this region. A dependence of CBL growth on orography is clearly in evidence and can be observed best during the morning hours when the boundary-layer strongly follows the shape of the orography. This dependence
Acknowledgement—We would like to thank the “Landesanstalt fur Umweltschutz”, the “Deutscher Wetterdienst”, “Meteo France”, the “Schweizerische Meteorologische Anstalt”, the “Geophysikalischer Dienst der Bundeswehr”, the TRACT participants for performing radiosonde ascents and O. Kolle for preparing parts of the radiosonde data. The contribution to the EUROTRAC subproject TRACT was funded by the Bundesministerium fur Forschung und Technologie (Germany) under contract number 07EU7344. The TRACT aircraft programme was supported by the Community of European Countries (CEC), Brussels.
REFERENCES
Adrian, G. (1993) Wake ows in the upper Rhine valley. Contributions to Atmospheric Physics 65, 147–156. Bader, D. C., McKee, T. B. and Tripoli, G. J. (1987) Mesoscale boundary layer evolution over complex terrain. part I: numerical simulation of the diurnal cycle. Journal of the Atmospheric Sciences 44, 2823–2838. Ball, F. K. (1960) Control of inversion height by surface heating. Quarterly Journal of the Royal Meteorological Society 86, 983– 994. Corsmeier, U. and Walk, O. (1993) Bericht u ber die Intensivmephase Winterliche Hochdrucklage (Intensivmephase in der nordlichen REGIO). REKLIP Arbeitsbericht D-002, 90 pp. Corsmeier, U. and Zimmermann, H. (1993) Aircraft measurements in the TRACT main area—an overview EUROTRAC Annual Report 1992, Part 2, Section II, 50 – 63. Coulter, R. L. (1979) A comparison of three methods for measuring mixing-layer height. Journal of Applied Meteorology 18, 1495–1499. Dammann, W. (1960) Die Windverhaltnisse im Rhein-MainGebiet: Eine Studie zur dynamischen Klimatologie der Mittelgebirge. Erdkunde 14, 10 –29.
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Dayan, U., Shenhav, R. and Graber, M. (1988) The spatial and temporal behavior of the mixed layer in Israel. Journal of Applied Meteorology 27, 1382–1394. Fiedler, F. (1983) Einige Charakteristika der Stromung im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 4, 113–123. Fiedler, F. (1989) personal communication. Fiedler, F. and Prenosil, T. (1980) Das MESOKLIPExperiment, Mesoklaliges Klimaprogramm im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 1, 107. Fiedler, F. and Zimmermann, H. (1993) General report of TRACT—an overview. EUROTRAC Annual Report 1992, Part 2, Section II, 3–36. Gutsche, A. and Lefevre, Ch. (1981) Statistik der maximalen Mischungsschichthohe nach Radiosondenmessungen an den aerologischen Stationen des Deutschen Wetterdienstes im Zeitraum 1957–1973. Berichte des Deutschen Wetterdienstes 154, 49. Havlik, D. (1970) Inversionswetterlagen im sudlichen Oberrheingebiet. Meteorologische Rundschau 23, 129–134. Hentschel, G. and Leidreiter, W. (1960) Die Hau gkeit von Inversionen im bodennahen Raum (15–76 m u ber Grund) in Abhangigkeit von Jahreszeit, Tageszeit und Windrichtung. Angewandte Meteorologie 3, 353–362. Kaltho , N. and Vogel, B. (1992) Counter-current and channelling e ect under stable strati cation in the area of Karlsruhe. Theoretical and Applied Climatology 45, 113–126. Kleiss, M. (1963) Inversionen in der unteren Troposphare im Raum Karlsruhe-Stuttgart. Berichte des Deutschen Wetterdienstes 90, 44. Kolle, O. and Kaltho , N. (1993) Intercomparison of radiosonde data during the TRACT campaign, Summer 1992. EUROTRAC Annual Report 1992 Part 2, Section II, 142–150. Komann, M., Vogtlin, R., Corsmeier, U., Vogel, B., Fiedler, F., Binder, H.-J., Kaltho , N. and Beyrich, F. (1998) Aspects of the convective boundary layer structure over complex terrain. Atmospheric Environment 32, 1323–1348. Kuwagata, T., Masuko, N., Sumioka, M. and Kondo, J. (1990) The daytime PBL heating process over complex
terrain in central Japan under fair and calm weather condtions part II: regional heat budget, convective boundary layer and surface moisture availability. Journal of the Meteorological Society of Japan 68, 639 – 650. Liemann, R. and Alpert, P. (1993) Investigation of the planetary boundary layer height variations over complex terrain. Boundary-Layer Meteorology 62, 129–142. Mayer, H. (1972) Inversionen in der bodennahen Atmosphare u ber Karlsruhe. Meteorologische Rundschau 25, 153–161. McElroy, L. and Smith, T. B. (1991) Lidar description of mixing-layer thickness characteristics in a complex terrain=coastal environment. Journal of Applied Meteorology 30, 585–597. Noack, R. (1963) Untersuchungen u ber Zusammenhange zwischen Luftverunreinigungen und meteorologischen Faktoren. Angewandte Meteorologie 4, 299–303. Paul, P. and Wahl, L. (1993) Parametre brouillard. REKLIP Jahresbericht 1992, 55 – 61. Pul, W. A. J. van, Holtslag, A. A. M. and Swart, D. P. J. (1994) A comparison of ABL heights inferred routinely from lidar and radiosondes at noontime. Boundary-Layer Meteorology 68, 173–191. Stull, R. B. (1973) Inversion rise model based on penetrative convection. Journal of the Atmospheric Sciences 30, 1092–1099. Stull, R. B. (1988) An Introduction to Boundary Layer Meteorology. Atmospheric Sciences Library, Kluwer Academic Publishers, Dordrecht, 666 pp. Tennekes, H. (1973) A model for the dynamics of the inversion above a convective boundary layer. Journal of the Atmospheric Sciences 30, 558–567. Vogtlin, R. (1992) Die tageszeitliche Entwicklung der konvektiven Grenzschicht u ber hugeligem Gelande. Diplomarbeit, Meteorological Institute of the University of Karlsruhe, 113 pp. Vogel, B., Adrian, G. and Fiedler, F. (1987) MESOKLIPAnalysen der meteorologischen Beobachtungen von mesoskaligen Phanomenen im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 7, 369. Wenzel, A. (1994) personal communication. Whiteman, C. D. (1982) Breakup of temperature inversions in deep mountain valleys: part I. observations. Journal of Applied Meteorology 21, 270–288.
Atmospheric Environment Vol. 32, No. 7, pp. 1179–1194, 1998 ? 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1352–2310/98 $19.00 + 0.00
TEMPORAL EVOLUTION AND SPATIAL VARIATION OF THE BOUNDARY LAYER OVER COMPLEX TERRAIN N. KALTHOFF,*; † H.-J. BINDER,† M. KOSSMANN,† R. VOGTLIN,† U. CORSMEIER,† F. FIEDLER,† and H. SCHLAGER‡
† Institut fur Meteorologie und Klimaforschung, Universitat Karlsruhe=Forschungszentrum Karlsruhe, Germany; and ‡ Deutsche Forschungsanstalt fur Luft und Raumfahrt, Oberpfa enhofen, Germany (First received 20 May 1995 and in nal form 21 October 1995. Published April 1998) Abstract— The temporal and spatial evolution of the convective boundary-layer has a strong e ect on air pollutant dispersion, especially under inhomogeneous conditions with varying orography and=or land use. During the TRACT experiment the evolution of boundary-layer height in space and time was measured by radiosondes and aircraft during three special observation periods (SOPs). The observations show that (a) The boundary-layer height follows the height of the underlying orography. The correlation between boundarylayer height and orography holds best in the morning hours, but decreases slightly during the afternoon. This result con rms with previous observations on a smaller scale, which show that the PBL top con rms to the underlying orography until the boundary-layer depth exceeds the characteristic scale of the obstacle. (b) Coincident with the spatial variation of the boundary-layer height, a strong spatial variation of air pollutants can be observed. (c) On two of the three SOPs surface fog-covered parts of the TRACT area (the upper Rhine valley) until noon. Due to the reduced surface heating the boundary-layer height in the foggy areas was much lower than in the clear areas. ? 1998 Elsevier Science Ltd. All rights reserved Key word index: Mixed layer, air pollutants, orography.
1. INTRODUCTION
The vertical mixing of air pollutants strongly depends on the depth and strati cation of the planetary boundary-layer (PBL). During daytime, the growing unstable boundary-layer is marked by a capping inversion. Due to the in uence of inversion height on the diffusion of air pollutants, the inversion statistics (e.g. Hentschel and Leidreiter, 1960; Gutsche and Lefevre, 1981) as well as the correlation of air pollutants and meteorological conditions (Noack, 1963) has often been investigated. The growth of the boundary-layer, and the accompanying vertical mixing of the air pollutants, depends on surface conditions like sensible heat
ux, large-scale vertical motion, horizontal advection and entrainment at the boundary-layer top (e.g. Ball, 1960; Stull, 1973; Tennekes, 1973). While under horizontally homogeneous conditions the boundary-layer growth is mainly understood, under non-homogeneous conditions the boundary-layer growth may be in uenced by additional e ects (e.g. secondary circulations in valleys) and may di er from the evolution over at terrain (Whiteman, 1982; Bader et al., 1987). Analyses of the spatial and temporal variations of inversions characteristics in complex terrain have been presented by several investigators. Dayan et al. (1988) * Author to whom correspondence should be addressed.
found that subsidence inversion heights are almost uniform above sea level, while convective inversions are mainly in uenced by the underlying orography. However, the top of the mixed layer tends to rise at a slower rate than the rise in the topographical relief. Kuwagata et al. (1990) observed an increase of the convective boundary-layer (CBL) above sea level from the coastal regions toward the inland areas. The increase is caused by the higher surface heating and due to the higher ground level over the inland than over the coastal areas. Bader et al. (1987) and McElroy and Smith (1991) studied the behaviour of the mixed layer above complex terrain. The latter studied the boundary-layer evolution in conjunction with a sea breeze and found differences in the boundary-layer thickness of hundreds of meters within only a few kilometers. Observations by Fiedler (1989) for the area of the Rhine valley indicated that the CBL during the morning hours follows the shape of the orography, while at noon, when the CBL is fully developed, the inversion height is constant with respect to sea level, i.e. independent from the orography. The area of the TRansport of Air pollutants over Complex Terrain (TRACT) experiment is a complex terrain region consisting of valleys (e.g. the Rhine valley) and mountainous regions like the Black Forest, the Vosges and the Swabian Alb (Fig. 1). Thus, effects like the trapping of air pollutants within valleys
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Fig. 1. Position of the di erent sounding systems: The solid lines mark the aircraft ight legs and the encircled numbers mark the positions of the sounding systems. C = constant level balloon, L = lidar, RO = radiosonde, RS = rass, S = sodar, F = tethered sonde.
The boundary layer over complex terrain
in the case of low level or nocturnal surface inversions and air pollutant transport from the convective boundary-layer into the free atmosphere in the case of convection at the mountain slopes may be of importance for the area under consideration. Especially, the Rhine valley is a region of enhanced industrial activities where the emission of air pollutants is higher than in the surroundings. Because of these high emissions, the analysis of surface as well as CBL capping inversions in this area has been studied previously by several other investigators. Surface inversion layers in the mountainous terrain often lead to the decoupling of the local and synopticscale ow elds. Especially in the upper Rhine valley, the decoupling of the upper and lower ow systems under stable strati cation leads to strong windshear phenomena (e.g. Dammann, 1960; Kaltho and Vogel, 1992; Corsmeier and Walk, 1993) and significantly in uences the horizontal transport of air pollutants. Even under neutral conditions the in uence of the Rhine valley topography on the overlying ow eld has been clearly demonstrated (Fiedler and Prenosil, 1980). An analysis of the inversion heights for the Karlsruhe=Stuttgart area has been presented by Kleiss (1963), revealing that nocturnal inversions in the autumn are more frequent than in the annual mean. Havlik (1970) and Mayer (1972) have analysed surface inversion characteristics in the area of the upper Rhine valley (Freiburg and at Karlsruhe, respectively). Both emphasize the signi cance of “inversion phenomena” for this area. Investigations on the convective boundary-layer for this area are less frequent. Fiedler (1983) found that the ow eld in the Rhine valley rises along with the mixed-layer depth. Besides the orographic e ects, studied by Vogtlin (1992) for the Black forest, spatially varying surface uxes due to land use (the mountainous regions mainly are covered with forests, while the land use type of the Rhine valley is grass land or cultivated land) may lead to di erent boundary-layer heights and therefore to di erent mixing of the air pollutants. The present study focuses on the temporal evolution and spatial variation of the convective boundary-layer on the meso- scale. This has been done for the 2nd and 3rd special observation period (SOP) using data from 17 radiosonde stations and two aircraft. The paper is divided into three parts. First an overview of the synoptic situation is given. Then the measuring systems (radiosonde stations, aircraft ight pattern) are described. Finally, the temporal evolution of PBL-growth is compared at valley and mountain sites and the spatial variation of the PBL within the whole TRACT area is investigated. 2. THE SYNOPTIC SITUATION DURING THE SOPs
The main objective of the TRACT experiment was to study the transport of air pollutants under convec-
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tive conditions. The special observing periods were consequently scheduled for daytime periods where the TRACT area was in uenced by high-pressure systems. Because afternoon ights were not available for the rst SOP, we focus our analyses on the PBL-evolution during the 2nd and 3rd SOPs. 2.1. 2nd SOP The synoptic situation during the second SOP (16=17 September) was dominated by an eastward moving high-pressure system. The centre was located over the English Channel (Fig. 2a), on the 16th, but had moved to Denmark on the 17th. This led to northwesterly winds in front of the high-pressure centre and to easterly winds when the centre was situated to the north of the TRACT area. The turning of the wind direction occurred in the late evening of the 16th (22 CEST) when the pressure began to fall in the TRACT area. Within 24 h from the 16th to the 17th, the surface pressure decreased by 2 hPa. The northern part of the TRACT area was a ected by a weak cold front moving in from the north, and was accompanied by stratocumulus clouds. This cold front led to weak large-scale rising motions to the north of the TRACT area at 14 CEST, while subsiding motions dominated to the south (Fig. 2b). The linear interpolation from the 850 hPa level down to the top of the boundary-layer (≈1 km) resulted in a subsidence of w = −0:38 cm s−1 (for ! = 1 dPa s−1 at 700 hPa) in the southern part of the TRACT area. The 700 hPa data were calculated by model of the German Weather Service (DWD). On the 17th, fog covered parts of the upper Rhine valley and remained at some sites until noon. 2.2. 3rd SOP The synoptic conditions during the third SOP (21=22 September) were dominated by a high-pressure system with its axis directed from Scandinavia to Italy (Fig. 3a). An eastward-moving low-pressure system over the Atlantic led to southerly winds and the advection of warm and moist air in the TRACT area. Thus fog and haze were found in the upper Rhine valley until noon of the 21st (Fig. 4). These foggy conditions in the upper Rhine valley are a well-known phenomenon, which in the area between Freiburg in the south and Hagenau in the north are more frequent than on the surrounding mountains and in the northern part of the Rhine valley (Paul and Wahl, 1993). Because of this synoptic condition, subsidence dominated in the TRACT area (Fig. 3b), ranging from w = −0:63 cm s−1 in the area of Strasbourg to w = −0:13 cm s−1 in the area of Mannheim at 1 km above ground. Small upward vertical motions at the southeastern edge of the TRACT area arose from the cold front at the Alps. On the second day of this SOP the high-pressure system moved eastwards. From 21 to 22 June the surface pressure decreased by about 5 hPa=24 h. On the evening of the 22nd thunderstorms appeared
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Fig. 2. Synoptic charts of 16 September at 14 CEST. (a) 850 hPa level, according to the Europaischer Wetterbericht of the DWD. Solid lines are geopotential heights in gpdm. (b) Vertical wind eld at 700 hPa as calculated from the !- eld of the DWD. The solid lines are the vertical wind eld isolines in cm s−1 .
The boundary layer over complex terrain
Fig. 3. Same as Fig. 2 but for 21 September at 14 CEST.
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Fig. 4. Fog distribution (encircled black dots) for 21 September at 11 CEST.
in France associated with an eastward-moving lowpressure system.
3. THE MEASURING SYSTEMS
Radiosondes, tethersondes, sodar-, lidar- and rass-systems were installed to determine inversion heights. Within the TRACT area, 17 radiosonde stations are available (Fig. 1), delivering temperature, humidity, wind speed and wind direction pro les. The radiosonde sites were selected to insure that the edges of the TRACT area were well-covered. Radiosondes were also concentrated in the Rhine valley from Mann-
heim (3) in the north to Lorrach (20) in the south to resolve the complex ow eld in the Rhine valley, known from previous investigations (e.g. Vogel et al., 1987; Adrian, 1993; Corsmeier and Walk, 1993). In order to analyze the evolution of the mixed layer and accompanying small-scale processes at the transition zone from the Rhine valley to the top of the Black Forest, i.e. perpendicular to the Rhine valley, an additional line of sounding systems was installed. Between the Rhine in the west to Sasbach (9) at the foot of the Black Forest and to Musbach (13) at the rear, tethersondes, radiosonde and sodar systems were in use (Fig. 1). The results from this line experiment are given elsewhere (Komann et al., 1998).
The boundary layer over complex terrain
Radiosonde ascent frequencies di er between one ascent per day (e.g. Mannheim) and 16 ascents per day at intervals of 1 12 h during the SOPs (e.g. Bruchsal). The standard interval during the SOPs is 3 h. Within the TRACT experiment, a radiosonde intercomparison was performed to determine di erences between the speci c radiosonde systems (Kolle and Kaltho , 1993). Besides the radiosondes, data from DO228aircraft are available performing curtain ights (zigzag pattern) at two boxes (northern and southern box) within the TRACT area (Fig. 1), temperature, humidity, wind speed, wind direction and the concentration of various chemical species (Corsmeier and Zimmermann, 1993). The curtain ights cover heights between ≈200 m and ≈2000 m above ground, where the upper
ight level was intended to be above the top of the PBL. These ights were normally twice a day (morning ight from about 8 –11 CEST, afternoon ight from about 13 –17 CEST) and provide an overview of the spatial variation of the mixed layer. An overview of the entire TRACT experiment including aircraft activities has been provided by Fiedler and Zimmermann (1993).
4. THE CONVECTIVE BOUNDARY LAYER
4.1. CBL-height de nition Several de nitions are given in the literature for the height of the convective boundary-layer or the mixed layer (ML). Most commonly, the top of the mixed layer is de ned as that height where (i) the pro le of the sensible heat ux reaches a minimum, (ii) surface borne air parcels are in equilibrium with the surrounding air and (iii) the base of the rst capping inversion is detected. Further, additional de nitions are often given in terms of the detectable signal of a measuring system, such as sodar (maximum of the temperature structure parameter CT2 ) or lidar (negative gradient in the backscatter pro le). Slight differences in mixing layer height can be expected because of the e ects of the de nitions and measuring systems (Coulter, 1979; Pul et al., 1994). Concerning the mixed or convective boundary-layer depth zi (zi indicates the boundary-layer height above ground (m AGL)) and mixed or convective boundary-layer height Zi (Zi indicates the boundary-layer height above sea level (m MSL)) we assume that the top is identical with the average base of the overlying stable layer (Stull, 1988), i.e. the capping inversion of the CBL. This de nition has been chosen because this height can be detected from radiosonde and aircraft temperature pro les. An initial analysis of inversion height over hilly terrain (Vogtlin, 1992) found that the in uence of an orographic obstacle on the inversion height depends on the scale of the obstacle and on inversion height itself. Referring to this observation, we here introduce a dimensionless number A, characterizing the state of the
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inversion, i.e. terrain following or not, given by √ A=
HL S = zi zi
(1)
where H de nes the characteristic height of the obstacle above the surrounding area, and L the characteristic length of the obstacle. S, therefore, represents the scale of the obstacle. Analysing data from the KONVEGS-experiment (Vogtlin, 1992) gives values of about A = 2:2 for those cases when the inversion follows the shape of the orography, and A = 0:6 for the cases when the in uence of the orograhy can be neglected. Therefore, the change between the two states occurs for A ≈ 1, when the boundary-layer depth is approximately equal to the scale of the orographic obstacle. The characteristic scale of obstacles in the KONVEGS-experiment, estimated from the topographic charts, was S = 0:55 km (H = 0:2 km and L = 1:5 km), and one purpose of the present investigation is to determine if this assumption holds for obstacles of di erent scales. The typical scale of the mountains in the TRACT area is H ≈ 0:5 km and L ≈ 40 km, so that S ≈ 4:5 km. If this scaling argument is valid, the boundary-layer top should conform to the underlying orography until the boundary-layer depth exceeds 4.5 km height. 4.2. The temporal evolution of the CBL at di erent sites The temporal evolution of the boundary-layer at different sites depends on the local surface conditions, the large-scale vertical motions, the stability and the entrainment e ects at the inversion zone. Further, over non-homogeneous or complex terrain, secondary circulation systems and enhanced advection e ects are expected to be of importance for the evolution of the mixed layer. Within the TRACT area, the Rhine valley with the surrounding mountains is one area where strong di erences in PBL evolution can be expected (e.g. Fiedler, 1983). In order to analyse the evolution of the PBL in and outside of the Rhine valley the radiosonde stations Oberbronn (274 m MSL) and Musbach (694 m MSL) have been selected. Figure 5 shows the temperature-, humidity- and wind pro les from Oberbronn (a valley site) and Musbach (a mountain site) for 16 September. On this nearly cloud-free day, the boundary-layer height Zi at Oberbronn reached 1225 m MSL (i.e. a boundary-layer depth of zi = 975 m) at 13 CEST, with well-mixed temperature- and humidity pro les. This gives a growth rate of about 4 cm s−1 which is about one order greater than the mean subsidence on this day (w ≈ 0:38 cm s−1). At this time the potential temperature in the mixed layer is about 20◦ C, increasing to the maximum value of 22◦ C at 16 CEST. The potential temperature is referenced to 1000 hPa. At Musbach, 420 m higher than Oberbronn, the boundary-layer height at 13 CEST reached Zi = 1550 m MSL (i.e. a boundary-layer depth
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Fig. 5. Temporal evolution of potential temperature, speci c humidity, wind speed and wind direction on 16 September at (a) Oberbronn (5), 274 m MSL in the Rhine valley, and (b) Musbach (13), 694 m MSL. The solid line indicates the boundary layer depth at 13 CEST.
The boundary layer over complex terrain
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of zi = 850 m). At this time the mixed layer tempera- depth zi in the afternoon is the same in and outside ture was about 24◦ C, nally reaching the maximum the upper Rhine valley (Fig. 7). Such is the case on value of 26◦ C at 16 CEST. Thus, the mixed layer 16 September. • Under foggy weather conditions in the Rhine valtemperature is about 4◦ C higher above the mountain site than in the Rhine valley. This temperature di er- ley with clear-air conditions on the surrounding mounence indicates that the horizontal advection term may tains the convective boundary-layer develops totally become important for model calculations of boundary- di erently between the two sites (Fig. 8). Such is the layer growth at the mountain slopes. Comparing the case on 21 September. The spatial di erences can be boundary-layer at both sites we can state that the boundary-explainded by the lower net radiations before noon in layer depth zi is nearly the same at the two sites. The the Rhine valley (Fig. 9). scale of the Black Forest is H ≈ 0:5 km and L ≈ 40 km, resulting in a value of A ≈ 5. Hence, the assumption that the orography is a dominant parameter for A¿1 4.3. The spatial variation of the boundary layer holds for the scale of S = 4:5 km. height Resulting from the inhomogeneous spatial distribuThe spatial variation of the boundary-layer height tion of the boundary-layer height, horizontal gradients can be determined from aircraft data, which are preferof the air pollutants occur. To examine horizontal trans- able to radiosonde data due to the higher spatial resoport processes, the ow eld must be determined. The lution of the curtain ights. The lowest aircraft ight heterogeneity of ow conditions within the boundary- level, however, is approximately 200 m or more above layer over complex terrain can be seen from the pro- ground, so that surface inversions or low-lying inver les of the wind speed and wind direction. Winds in sions cannot be detected. Therefore, when the CBL the PBL at Oberbronn at 13 CEST turn from north- depth is of interest, the use of aircraft data is restricted westerly at the surface to westerly at the boundary- to ights around noon when the convective boundary layer top, while in Musbach at 13 CEST the winds turn layer is fully developed. Here, aircraft data are prefrom northeasterly in the surface layer to southwesterly sented for the 2nd and 3rd SOP, when afternoon ights in the mixed layer. Above the PBL west-north-westerly were available. winds exist at both sites. In Fig. 10 the inversion height is presented on 16 A totally di erent evolution of the boundary-layer September for the south box ight path. At this time at both sites can be observed on 21 September. Sur- the CBL can be regarded as fully developed. Neverface heating of the boundary-layer within parts of the theless, a strong dependence of inversion height on Rhine valley is suppressed due to a shallow fog layer orography can be found. While over the valleys, like (Fig. 4), while at the surrounding mountains with clear the Rhine valley, the CBL grows up to about 1000 m air conditions the boundary-layer at 13 CEST attained MSL, the CBL reaches 1600 m MSL over the mounZi = 1900 m MSL or zi = 1200 m AGL (Fig. 6b for the tains (e.g. the Black Forest and Jura). These inversite of Musbach). At the same time the boundary-layer sion heights agree well with the heights estimated from height at Oberbronn only reached Zi =675 m MSL or the radiosonde data (1050 m MSL at O enburg in the zi = 400 m AGL (Fig. 6a). At this time the mixed layer Rhine valley and 1550 m MSL at Musbach in the Black temperature was 18◦ C, i.e. 6◦ C lower than at Musbach. Forest at 13 CEST). Only in one region can a difAgain, this means that the horizontal temperature gra- ference from this general (terrain-following) behaviour dient between the valley and the mountain is high and of the inversion height be detected, namely between that horizontal advection at the slope area will be im- the Black Forest and the Swabian Alb. A very high portant for boundary-layer height modelling. boundary layer of about 2000 m MSL can be found in The wind direction at Oberbronn at this time within this region, which is even higher than the mixed layer the boundary-layer is southeast turning to southwest- over the mountains. This high mixed layer may be exerly above the PBL. At Musbach the wind turns at plained by the surface conditions in this area. High 400 m AGL from southeast to southwest. Thus, the surface temperatures and accompanying high sensible main di erence between Musbach and Oberbronn is heat uxes are estimated in this region for this day that the wind sheer at Musbach is not correlated with (Wenzel, 1994). While over the Black Forest a daily PBL-top, as valid for Oberbronn. mean sensible heat ux of about 40 W m−2 is given, In the autumn, fog often occurs in the Rhine val- about 70 W m−2 can be found between the Black Forley when clear-air conditions occur over the surround- est and the Swabian Alb, an area covered mainly with ing mountains (Paul and Wahl, 1993). Therefore, the grass. boundary-layer evolution during both days and at both Coincident with the spatial variation of the boundsites can be regarded as typical for this region. ary layer height, a strong spatial variation of humidSummarizing the data from both days two cases can ity can be observed (Fig. 10). In those regions where be separated: the CBL is high, the humidity is mixed deep into the • During clear-air conditions with weak mean subsidence (w = 0:1 @zi ) the convective boundary-layer @t
atmosphere, e.g. over the Black Forest, the Jura and the Swiss Lower Alps. Especially, the increase of the boundary layer height between the Black Forest and
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Fig. 6. Same as Fig. 5, but for 21 September.
The boundary layer over complex terrain
Fig. 7. Pro les of potential temperature at 7 and 13 CEST on a cross-section through the Rhine valley running through Oberbronn (5), O enburg (14) and Musbach (13) on 16 September. The solid line marks the top of the boundary layer at about 13 CEST, the dashed line marks the top of the residual layer at about 7 CEST.
Fig. 8. Same as Fig. 7, but for 21 September.
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the Swabian Alb coincides with high humidity values up to about 2000 m MSL over this region. Figure 11 contains pro les of potential temperature, speci c humidity and the chemical species NOx and O3 over (a) a characteristic mountain and (b) a valley site (see Fig. 10). Here the role of the inversion as the barrier for vertical mixing becomes obvious. The inversion is not only correlated with a decrease of the humidity but also with the decrease of NOx and O3 concentrations. NOx drops from about 3 ppbv in the mixed layer to about 1.5 ppbv at the top of the ML, i.e. at 1400 m MSL at the Swiss Lower Alps (Fig. 11a) and at 900 m MSL at Sundgau (Fig. 11b). In order to analyse the in uence of orography on boundary-layer height and hence on the spatial variation over hilly terrain, calculations have been made using the linear regression: Zi = a0 + a1 h
Fig. 9. Components of the energy balance at Sasbach (9) in the Rhine valley (top) and at the Hornisgrinde (12) in the Black Forest (bottom) on 21 September. Shown are the net radiation Qo , the latent heat ux Vo , the sensible heat ux Ho and the soil heat ux Bo .
(2)
where h de nes the height of the orography, and a0 and a1 are the regression coecients. In this formulas a1 ≈ 1 indicates that the boundary-layer depth is the same at every site (if RMS is small), so that the inversion height follows the orographical relief. On the other hand, a1 ≈ 0 indicates that the boundary layer height is the same at every site, with no correlation between the boundary layer height and the orography. Results are shown for two times on 16 September using both radiosonde and aircraft data (Fig. 12). The
Fig. 10. (Top) Inversion height in the south box on 16 September as determinded from the aircraft data. The solid line indicates the orography, the dashed line the ight pattern and the thin solid line the inversion height Zi . (Bottom) Isolines of the speci c humidity q along the south box on 16 September as determined from the aircraft data.
The boundary layer over complex terrain
Fig. 11 Pro les of potential temperature , speci c humidity q, and NOx and O3 concentrations at a mountain site (a) and a valley site (b) as measured on 16 September in the south box. The pro ling sites are indicated in Fig. 10. The dotted lines indicate the minimum zmin and the maximum zmax of the orography.
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Fig. 12. Regression curves for the inversion height Zi on 16 September 11 (top) and 16 CEST (bottom). The crosses indicate radiosonde data, and the triangles indicate the aircraft data. RMS denotes the root mean square in m.
data show that in the morning (11 CEST) the boundary layer height Zi at all sites increases nearly with the rate of the orography (a1 = 0:80; RMS = 152 m). A similar behaviour of the PBL-evolution over complex terrain has been modelled by Lieman and Alpert (1993). During the course of the day, however, the in uence of orography decreases (e.g. at 16 CEST when a0 is 1045 m and a1 = 0:42). This agrees with results from Dayan et al. (1988), who found that on the crosssection from the sea up to the mountains the inversion layer Zi rises slower than the rise of the orographical relief, i.e. the boundary layer depth zi over the mountains is smaller than over the low-level terrain. Figure 13 depicts the CBL-height during the third SOP from the south box ight in the afternoon of 21 September. As described above, on this day a fog layer covered parts of the TRACT area, especially in the upper Rhine valley until noon (Fig. 4). The in uence of this fog layer became apparent in the evolution of the convective boundary layer. Good correlation between the inversion height and the orography is restricted to the Swiss Lower Alps, the Jura and the Black Forest, where the CBL-height is low. Very high inversion heights again can be found east of the Black Forest. This behaviour is similar to that observed in the second SOP and is again probably caused by the higher surface uxes in this region. Figure 14 shows the regression curves for 21 September. In the morning at 11 CEST a good correlation occurred between the boundary-layer height Zi
Fig. 13. Same as Fig. 10, but for 21 September.
The boundary layer over complex terrain
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on orography decreases somewhat during the course of the day. In agreement with the results from earlier experiments, conducted on a smaller scale topography, the in uence of orography is a dominating parameter for A¿1, i.e. when the characteristic scale of the orography is larger than the boundary layer depth. The strong dependence of CBL height on the orography, however, is reduced for areas covered with fog. Here the boundary-layer depth is much lower than in the other areas. As the capping inversion layer marks the top of the vertical mixing, the spatial variation of the depth of the CBL causes strong horizontal gradients of moisture and air pollutants. The purpose of this paper was to analyse CBL growth in the TRACT area. The spatial variation of the boundary-layer height results from superposed in uences of the orography, varying surface conditions (i.e. di erent surface heat uxes resulting from di erent land use and soil moisture), mean vertical motion and entrainment and advection processes. The individual in uences can hardly be separated from the observations only. Therefore, in the next step, PBL growth models will be applied and simulations will be compared with the observed data for the di erent sites, to quantify the contribution of the di erent processes. Fig. 14. Same as Fig. 12, but for 21 September.
and the orography (a1 = 1). This was unexpected because a fog layer was present in the Rhine valley, with reduced sensible heat uxes. The reason for the good correlation is that the growing inversion in the areas under clear air conditions had just reached the top of the fog layer in the foggy areas (about 300 m MSL). In the afternoon at 16 CEST a value of a1 = −0:02 and a high RMS-value reveal the decreasing in uence of the orography on this day. The great scatter probably is mainly caused by the spatial di erences in surface heating due to the fog in various areas. 5. CONCLUSIONS
Radiosonde and aircraft data of two special observation periods have been analysed to study the temporal and spatial evolution of the boundary layer during the TRACT campaign. During these days high-pressure systems dominated with nearly cloudless sky. However, on 17 and 21 September fog builds in the early morning hours and remains until noon in parts of the TRACT area (especially in the upper part of the Rhine valley). Fog is a frequently appearing phenomenon for this region in the autumn, so that the spatial di erences of CBL evolution during these days can be regarded as typical for this region. A dependence of CBL growth on orography is clearly in evidence and can be observed best during the morning hours when the boundary-layer strongly follows the shape of the orography. This dependence
Acknowledgement—We would like to thank the “Landesanstalt fur Umweltschutz”, the “Deutscher Wetterdienst”, “Meteo France”, the “Schweizerische Meteorologische Anstalt”, the “Geophysikalischer Dienst der Bundeswehr”, the TRACT participants for performing radiosonde ascents and O. Kolle for preparing parts of the radiosonde data. The contribution to the EUROTRAC subproject TRACT was funded by the Bundesministerium fur Forschung und Technologie (Germany) under contract number 07EU7344. The TRACT aircraft programme was supported by the Community of European Countries (CEC), Brussels.
REFERENCES
Adrian, G. (1993) Wake ows in the upper Rhine valley. Contributions to Atmospheric Physics 65, 147–156. Bader, D. C., McKee, T. B. and Tripoli, G. J. (1987) Mesoscale boundary layer evolution over complex terrain. part I: numerical simulation of the diurnal cycle. Journal of the Atmospheric Sciences 44, 2823–2838. Ball, F. K. (1960) Control of inversion height by surface heating. Quarterly Journal of the Royal Meteorological Society 86, 983– 994. Corsmeier, U. and Walk, O. (1993) Bericht u ber die Intensivmephase Winterliche Hochdrucklage (Intensivmephase in der nordlichen REGIO). REKLIP Arbeitsbericht D-002, 90 pp. Corsmeier, U. and Zimmermann, H. (1993) Aircraft measurements in the TRACT main area—an overview EUROTRAC Annual Report 1992, Part 2, Section II, 50 – 63. Coulter, R. L. (1979) A comparison of three methods for measuring mixing-layer height. Journal of Applied Meteorology 18, 1495–1499. Dammann, W. (1960) Die Windverhaltnisse im Rhein-MainGebiet: Eine Studie zur dynamischen Klimatologie der Mittelgebirge. Erdkunde 14, 10 –29.
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Dayan, U., Shenhav, R. and Graber, M. (1988) The spatial and temporal behavior of the mixed layer in Israel. Journal of Applied Meteorology 27, 1382–1394. Fiedler, F. (1983) Einige Charakteristika der Stromung im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 4, 113–123. Fiedler, F. (1989) personal communication. Fiedler, F. and Prenosil, T. (1980) Das MESOKLIPExperiment, Mesoklaliges Klimaprogramm im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 1, 107. Fiedler, F. and Zimmermann, H. (1993) General report of TRACT—an overview. EUROTRAC Annual Report 1992, Part 2, Section II, 3–36. Gutsche, A. and Lefevre, Ch. (1981) Statistik der maximalen Mischungsschichthohe nach Radiosondenmessungen an den aerologischen Stationen des Deutschen Wetterdienstes im Zeitraum 1957–1973. Berichte des Deutschen Wetterdienstes 154, 49. Havlik, D. (1970) Inversionswetterlagen im sudlichen Oberrheingebiet. Meteorologische Rundschau 23, 129–134. Hentschel, G. and Leidreiter, W. (1960) Die Hau gkeit von Inversionen im bodennahen Raum (15–76 m u ber Grund) in Abhangigkeit von Jahreszeit, Tageszeit und Windrichtung. Angewandte Meteorologie 3, 353–362. Kaltho , N. and Vogel, B. (1992) Counter-current and channelling e ect under stable strati cation in the area of Karlsruhe. Theoretical and Applied Climatology 45, 113–126. Kleiss, M. (1963) Inversionen in der unteren Troposphare im Raum Karlsruhe-Stuttgart. Berichte des Deutschen Wetterdienstes 90, 44. Kolle, O. and Kaltho , N. (1993) Intercomparison of radiosonde data during the TRACT campaign, Summer 1992. EUROTRAC Annual Report 1992 Part 2, Section II, 142–150. Komann, M., Vogtlin, R., Corsmeier, U., Vogel, B., Fiedler, F., Binder, H.-J., Kaltho , N. and Beyrich, F. (1998) Aspects of the convective boundary layer structure over complex terrain. Atmospheric Environment 32, 1323–1348. Kuwagata, T., Masuko, N., Sumioka, M. and Kondo, J. (1990) The daytime PBL heating process over complex
terrain in central Japan under fair and calm weather condtions part II: regional heat budget, convective boundary layer and surface moisture availability. Journal of the Meteorological Society of Japan 68, 639 – 650. Liemann, R. and Alpert, P. (1993) Investigation of the planetary boundary layer height variations over complex terrain. Boundary-Layer Meteorology 62, 129–142. Mayer, H. (1972) Inversionen in der bodennahen Atmosphare u ber Karlsruhe. Meteorologische Rundschau 25, 153–161. McElroy, L. and Smith, T. B. (1991) Lidar description of mixing-layer thickness characteristics in a complex terrain=coastal environment. Journal of Applied Meteorology 30, 585–597. Noack, R. (1963) Untersuchungen u ber Zusammenhange zwischen Luftverunreinigungen und meteorologischen Faktoren. Angewandte Meteorologie 4, 299–303. Paul, P. and Wahl, L. (1993) Parametre brouillard. REKLIP Jahresbericht 1992, 55 – 61. Pul, W. A. J. van, Holtslag, A. A. M. and Swart, D. P. J. (1994) A comparison of ABL heights inferred routinely from lidar and radiosondes at noontime. Boundary-Layer Meteorology 68, 173–191. Stull, R. B. (1973) Inversion rise model based on penetrative convection. Journal of the Atmospheric Sciences 30, 1092–1099. Stull, R. B. (1988) An Introduction to Boundary Layer Meteorology. Atmospheric Sciences Library, Kluwer Academic Publishers, Dordrecht, 666 pp. Tennekes, H. (1973) A model for the dynamics of the inversion above a convective boundary layer. Journal of the Atmospheric Sciences 30, 558–567. Vogtlin, R. (1992) Die tageszeitliche Entwicklung der konvektiven Grenzschicht u ber hugeligem Gelande. Diplomarbeit, Meteorological Institute of the University of Karlsruhe, 113 pp. Vogel, B., Adrian, G. and Fiedler, F. (1987) MESOKLIPAnalysen der meteorologischen Beobachtungen von mesoskaligen Phanomenen im Oberrheingraben. Wissenschaftliche Berichte des Instituts fur Meteorologie und Klimaforschung der Universitat Karlsruhe 7, 369. Wenzel, A. (1994) personal communication. Whiteman, C. D. (1982) Breakup of temperature inversions in deep mountain valleys: part I. observations. Journal of Applied Meteorology 21, 270–288.