Measurements of the radiative and microphysical properties of stratocumulus over the South Atlantic and around the British Isles

•gI'~ IOSPHEt{I(' I~.ESEAHCH ELSEVIER

Atmospheric Research 34 (1994) 27-41

Measurements of the radiative and microphysical properties of stratocumulus over the South Atlantic and around the British Isles J o n a t h a n P. T a y l o r U.K. Meteorological Office, Meteorological Research Flight, DRA (Aerospace), Farnborough, Hants GU14 6TD, UK (Received April 21, 1993; accepted after revision November 25, 1993 )

Abstract

Measurements of the microphysical and radiative properties of stratocumulus clouds are presented for two flights, one around the British Isles and the other in the South Atlantic. The microphysical properties of the two sheets are totally different, in the South Atlantic case cloud droplet effective radii reached a maximum of 13.2/~m whereas in the British Isles flight effective radius reached a maximum of 6.5/tm. A multi-wavelength scheme for the retrieval of optical depth and effective radius is presented and tested against these two cloud fields. In both cases agreement between remotely retrieved effective radius and in situ measurements is good. The broad band radiative properties of the cloud sheets are measured and their reflectance, transmittance and absorptance compared.

1. Introduction

There are many regions in the world which have large semi- permanent stratocumulus or stratus decks. Such low, warm, liquid water clouds have a considerable impact on the earth's radiation budget. It has been recognised that the representation of clouds and their impact on the radiation budget is central to the ability to predict climate in numerical models. The radiative properties of stratiform water clouds can be represented in radiation codes by parametrizing the extinction coefficient, single scattering albedo and asymmetry factor in terms of the liquid water content and cloud droplet effective radius (Slingo and Schrecker, 1982 ), the effective radius of a cloud drop0169-8095/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-8095 ( 93 ) E0093-E

28

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

let distribution being given by the following relation:

re =

fn(r)r3dr fn(r)r2dr

(1)

where r is the droplet radius and n (r) the concentration of droplets of radius r. In the new Unified Model developed by the United Kingdom Meteorological Ofrice the effective radius of all liquid water clouds is assumed to be 7 #m (Ingram, 1990). It has been shown in a sensitivity study (Slingo, 1990) that, all other parameters remaining constant, a decrease in cloud droplet effective radius in a climate model from l0/~m to between 7.9 and 8.6/Lm was enough to offset the "greenhouse" warming due to a doubling of carbon dioxide. As yet little data have been published on the variability of effective radius of stratocumulus cloud droplets around the world. Most results to date come from measurements offthe west coast of California (Albrecht et al., 1988) and other northern latitude regions such as northern Europe (Foot, 1988). In this paper measurements of the radiative and microphysical properties of two quite different cloud sheets are presented, one around the British Isles the other in the South Atlantic. A scheme for remotely retrieving effective radius is tested on the two cloud cases. Broad band radiometer measurements of the two cloud fields are also presented. These observations show that the variability in microphysical properties of stratocumulus clouds as a function of geographical location could be large (although it is noted that only two cases are discussed here) and therefore there is a need to build up a better picture of global variations, ideally by remote sensing using satellites, in order for their representation in numerical models to be improved. A further description of the geographical variability of cloud microphysical properties is given by Taylor and McHaffie ( 1993 ). During November 1991 the Meteorological Research Hight (MRF) C 130 made a series of flights off the west coast of Africa at latitudes near 10 °S as part of the First ATSR (Along Track Scanning Radiometer) Tropical Experiment, FATE. Several flights were made over the extensive stratocumulus sheets found to the south east of Ascension Island (8 ° S, 14.5 ° W ) at this time of year. Climatological data suggest that in this region of the South Atlantic (SA), during the period September to November, stratocumulus and stratus is present for 80-90% of the time, (based on 1952-1981 data), and that when cloud is present its coverage is between 80 and 95% (i.e. the cloud sheet has very few breaks). This, combined with the probability that airmasses in this region are likely to have had a large sea track and hence would be maritime and relatively unpolluted, made this an interesting area to study the radiative and microphysical properties of maritime stratocumulus to contrast with stratocumulus characteristics around the British Isles (aI). In this report data from one flight on 11 November 1991 in the South Atlantic

J.P. Taylor/Atmospheric Research 34 (1994) 27--41

29

( 10.5 °S, 12°W) will be compared with that from a flight around the British Isles on 28 June 1990 (51°N, 7°W).

2. Instrumentation The U K C 130 aircraft is extensively equipped for the study of atmospheric processes. The radiation instrumentation flown during the FATE detachment included the Multi-Channel Radiometer, MCR, (Rawlins and Foot, 1990) which measures radiances in 14 narrow spectral bands between 0.55 and 12/tin. The MCR is mounted in a pod on the port wing of the aircraft and can view in the zenith, nadir and at angles out to sixty degrees from the nadir. Internal calibration targets can also be viewed for calibration of the thermal infrared channels. The field of view of the optical system is 1.5 °. The incident radiation is split into four streams, two thermal and two solar and viewed by four detectors. Each detector has a rotating filter wheel with four filters. One complete revolution of a filter wheel takes four seconds. Broad band hemisphere viewing Eppley pyranometers and pyrgeometers were also flown on the C 130 to measure upwelling and downwelling fluxes in the wavelength ranges: 0.3-3.0 pm, from now on referred to as clear flux, and 0.7 -3.0 #m, from now on referred to as red flux. The visible flux 0.3-0.7/zm can also be calculated as the difference between the clear and red fluxes. Aerosol concentrations were measured using a Particle Measuring Systems (PMS) Passive Cavity Aerosol Spectrometer Probe (PCASP) which measures aerosol in the size range 0.3-3.0 #m. Cloud droplet size and concentration were measured using a PMS Forward Scattering Spectrometer Probe (FSSP) for droplet radii from 0.5-25/zm. The FSSP was calibrated with glass beads of known size and the estimated accuracy of the measured radius is _+ 1 #m. The liquid water content of the clouds was obtained from measurements made by a Johnson Williams hot wire probe. Cloud condensation nuclei (CCN) were also measured using an MRF-developed thermal diffusion chamber. The use of thermal diffusion chambers for CCN counting is discussed by Saxena and Kassner ( 1970 ).

3. South Atlantic flight on 11 November 1991 This flight was over an extensive sheet of stratocumulus to the south east of Ascension Island. Fig. 1 shows profiles of the Johnson Williams cloud liquid water content and FSSP-measured cloud droplet effective radius. The cloud base was at 0.6 km and the tops at 1.18 km. The descent rate of the aircraft during this profile was ~2.5 m-s-~ with a horizontal speed of ~ 100 m-s -~. The descent through the cloud took approximately 230 seconds which corresponds to a horizontal distance of 23 km. The effective radius at cloud top was a maximum of 13.2/tm. A run below the cloud showed the mean PCASP aerosol concentration to be 125 cm-3; the CCN concentration below the cloud as a function of super-

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

30 1500 S F

tF ~r o o f it l oe sc

u

r

nT h r ouu g hl

u

r

s

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 LWC (g.kg -I)

"~

~ Eff

~

~

Rodius

1 0 (p,m)

15

Fig. 1. SA flight 11 November 1991. Profiles of (a) liquid water content, and (b) cloud droplet effective radius. 10:14-10:23 GMT.

A146 CCN Concentrotions 1000

.

•

•

i

.

.

.

,

•

.

.

J

.

.

.

i

.

.

J 100 u

§ o Fa

10

1

0.0

012

0.4 Supersaturation

0.6 (3)

'018

Fig. 2. SA flight 11 November 1991, CCN concentration versus supersaturation during a run just below cloud base. 10:29 GMT.

saturation is shown in Fig. 2. The CCN concentrations in this airmass do not exceed 200 cm-3, in continental air CCN concentrations up to 1000 cm-3 are not uncommon. The long sea track of this airmass combined with low CCN and PCASP concentrations indicate that this airmass is maritime. There is evidence in the profiles of Fig. 1 that lower in the cloud ( 6 0 0 - 7 0 0 m) the aircraft passed through the remains of some convective cloud that had intruded into the stratocumulus deck; the effective radius increases slightly and there is a small increase in liquid water content. A straight and level run at 1.1 km, in the tops of the stratocumulus, Fig. 3, showed variations in liquid water content from 0.4 to 1. l g-kg-1 and variations in droplet concentration from 50 to 180 cm -3. An interesting feature along this run is the sudden step change in cloud properties where we observe an increase in droplet concentration and decrease in effective radius. The adiabatic liquid

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

31

Flight A146 Stroight ond Level Run Through Strotocumulus 1.2

1.0

0.6

o°1 12i

i

m

i

i

~ 9 ~200 ~100

~ 5o 1O0 Time from start of run

200

(Seconds)

300

Fig. 3. SA flight 11 November 1991, straight and level run. (a) Liquid water content, (b) cloud droplet effective radius, and (c) cloud droplet concentration. 10:49-10:54 GMT at a mean height of l160m.

water content calculated from the level of the underlying cumulus cloud base to the stratocumulus cloud tops is 1.48 g.kg-1, the adiabatic liquid water content across the depth of the stratocumulus alone is 0.86 g-kg-1. It can be seen therefore that the run shown in Fig. 3 was in a region of both uniform stratocumulus and cumulus intrusion, with the liquid water content observed in the region of stratocumulus modified by cumulus intrusion being very close to the corresponding adiabatic value. The variations in effective radius along the run in cloud tops, as measured by the FSSP, are considerably smaller, from around 10.2 to 11.2 #m.

4. British Isles flight on 28 June 1990

This flight was over a region of stratocumulus to the south west of the British Isles. Fig. 4 shows the liquid water content and effective radius during a profile. The stratocumulus during this flight was thinner than that found in the SA, with tops at 2.33 km and base at 2. I km. The descent rate during this profile was also ~ 2.5 m.s-1 which results in a horizontal distance of approximately 9 km being covered during the profile through the cloud layer. Fig. 5 shows the effective radius, liquid water content and droplet concentration for a run skimming through the cloud tops. During the run the droplet concentration remains relatively constant at between 60 and 80 c m - 3 and the effective radius changes from 6.5 to 5.5 am. The liquid water also decreases along the run from 0,2 to 0.14 g.kg-1. The regions where droplet concentration falls to zero are where the aircraft temporarily left the cloud. The liquid water content observed is always significantly less than the adiabatic liquid water content of the cloud, computed to be 0.41 g. kg-1.

32

J.P. Taylor / Atmospheric Research 34 (I 994) 27-41 Flight A 0 1 9 Profiles Through S t r o t o c u m u l u s 2400

2200

2OOO

1800 -0.05

0.00

0.05

0.10

0.15

0.20

0.25

LWC (9.kg "t)

2

4

6

Elf Radius (/~rn)

Fig. 4. BI flight 28 June 1990. Profiles of (a) liquid water content, and (b) cloud droplet effective radius. 13:01 - 13:07 GMT. Flight A 0 1 9 Straight and Level Run Through S t r o t o c u m u l u s 0.25 0.20 0.15 0.10 0.05 -J 0 . 0 0 101S

,-_140~--~:: 2

U U

"

"

,

,

LJ

I g

U

J

-

I L--JU

--

I

'E 120 ~'-

'°°t

I

so

o.

40

A 0

n 1O0

A

.

200

3C

Time from stort of run (Seconds)

Fig. 5. BI flight 28 June 1990, straight and level run. (a) Liquid water content, (b) cloud droplet effective radius, and (c) cloud droplet concentration. 13:33-13:38 G M T at a mean height o f 2270 m.

A run below the cloud showed PCASP aerosol concentrations of 130 cm-3. A high pressure region of 1020 hPa was centered over northern France during this flight and had been in that position for several days. The airmass in which this stratocumulus sheet was found had had quite a long sea track from the Biscay area. The PCASP aerosol concentrations show that this air is relatively clean, its trajectory suggesting it is now a maritime airmass although it was originally of continental origin. The CCN instrument was not operational on the C 130 at the time of this flight. The PCASP aerosol concentrations measured here are very similar to those found in the South Atlantic; however, the droplet concentrations and effective radii for the two flights are significantly different. The droplet con-

J.P. Taylor I Atmospheric Research 34 (1994) 27-41

33

centrations observed during the BI flight (60 c m - 3 ) , were nearly always less than those observed in the SA flight ( 50-180 c m - 3). This suggests that the fraction of aerosol that could act as CCN in the SA was higher than that in the BI so activating more drops. These two stratocumulus sheets represent the extremes in terms of cloud microphysics. In one case we have high liquid water content and large effective radii, in the other we have low liquid water content and low effective radius. The cloud sheets are also physically quite different, the BI sheet has tops at 2.33 km and is 230 m thick whereas the SA sheet has tops at 1.18 km and is thicker at 580 m. Since the microphysical properties of these two stratocumulus sheets are so different it is worth studying their radiative properties both in narrow spectral bands and in terms of broad band fluxes.

5. Narrow spectral band measurements In order to study the radiative properties of the cloud sheets, runs were made over the tops of the same area of cloud that was sampled in situ. Fig. 6 shows the reflectance of the SA cloud sheet (due to instrument problems there is a data break of 20 seconds in the middle of the run), and Fig. 7, that of the BI sheet, at 0.55, 1.25, 1.55 and 2.26 #m. The solar zenith angles for the BI and SA flights are 31.8 ° and 26.2 °, respectively. The cloud sheet in the BI case is optically thinner than the SA case and the reflectivity of the sheet is lower at all four wavelengths than the optically thick SA case. It should also be noted that the reflectance of the BI cloud case is almost the same for all four wavelengths whereas for the tropical cloud it is a strong function of wavelength. This suggests that in the case of optically thinner cloud the changes in reflectance as a function of wavelength become imperceptible and any scheme which A146

South

Atlantic 0.55/.~m

II ue

I

I

0.6

0.4 " " - . ' "

........... ..........,....... ....... ............... j ~

;1..,................... .. . . . . . . . . . . . . .

.;. ....

I

-

'-

:

i

i

I

i I

0.2

O.C 0

. . . . . . . . . . . . . . 50 1 O0

2.26/~m

I . 150

I

.

.

~ . . . . . . . . . 200 250

300

Time f r o m s t o r t of run ( S e c o n d s )

Fig. 6. SA flight 11 November 1991. Reflectance of stratocumulus at specified MCR wavelengths• 10:42-10:47 GMT.

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

34

A019 British Isles 1.0 0.8 _ _ _

c o

~.

0.55/~m

........

2.26/.~m

1.25/.L m

.....

1.55#m

0.6

0.4

Ax

0.2

-

0.0

"

,

~

,

':~

, :

,

~'~

,

0

,

Time

from

start

of run

~i," 400

200

(Seconds)

Fig. 7. BI flight 28 June 1990. Reflectance of stratocumulus at specified MCR wavelengths. 13:1013:15 GMT.

uses reflectance to derive effective radius or optical depth will be less accurate and may indeed break down. For the SA flight the variation in reflectance along a run is largest for the lowest wavelengths which suggests that variations in liquid water path are greater than variations in effective radius, as was observed in situ. However, for the BI flight the large variations in reflectance are common to all wavelengths. In this optically thinner cloud the variation of reflectance with changes in droplet size is relatively small, both at shorter wavelengths such as 0.55 #m and longer wavelengths, 2.26 #m (fig. 2 of Nakajima and King, 1990). The large variations in reflectance observed during this run are therefore also likely to be due primarily to variations of liquid water path along the run. The radiative properties of a plane parallel homogeneous stratocumulus cloud sheet can be characterised in terms of the cloud droplet effective radius, re, and the cloud optical depth, z (Slingo and Schrecker, 1982 ). Using the MCR on the C 130 it is possible to retrieve these two properties of a stratocumulus sheet (Rawlins and Foot, 1990; Taylor, 1992 ). Other retrieval schemes are discussed in the work of Nakajima and King (1990) and Minnis et al ( 1992 ). The retrieval scheme works by measuring the reflectivity of the cloud sheet in two narrow wavelength bands. The reflectivity in a spectral region of weak liquid water absorption is relatively insensitive to changes in re but increases monotonically with increasing z. The reflectivity in a spectral region of strong liquid water absorption, on the other hand, is highly dependent on re and less so on the optical depth. An initial guess at the effective radius (in this scheme 7 #m ) is made and the reflectance measurement made at a wavelength of weak liquid water absorption (e.g. 1.25/zm) is used to retrieve an optical depth for the cloud. This optical depth is then combined with the reflectance measurement from a second wavelength (e.g. 2.01 #m or 2.26 #m), in a region of strong liquid water absorption to retrieve an effective radius. The scheme is relatively insensitive to which combination of channels, e.g. 1.25 #m and 2.01 #m or 1.25 #m and 2.26 #m, are used.

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

35

A thorough description of the scheme is given by Rawlins and Foot (1990) and an improvement to the scheme, which removed the occurrence of anomalous absorption often cited by other authors, is given by Taylor ( 1992 ). The optical depth, retrieved at 1.25 am, and the effective radius, retrieved at 2.26 #m are shown for the SA flight in Fig. 8, and for the BI flight in Fig. 9. The mean in situ effective radius, as measured by the FSSP, is shown as a dotted line. In the SA case the run in cloud was flown at an altitude of 1160 m very near the cloud top. In the BI flight the run in cloud was flown at 2270 m a distance of approximately 60 m from the top of the cloud measured in the profile; however, the in situ data show regions where the liquid water content dropped to zero as the aircraft left the cloud. This suggests that the cloud top in the BI case was inhomogeneous and varied along the run by at least 60 m. In both cases the retrieved effective radius is in good agreement with that measured in situ with the Fll ht A146 Straight and Level Run Above Stratocumulus 2O

'6

p

t

i

I:

(o)

~

I:

10

g s

:

2OO

~

150

~

100

(b)

.......

FSSP M e a n r,

I

Ill

8- 50

i too 200 Time from start of run (Seconds)

300

Fig. 8. SA flight 11 November 1991. (a) Retrieval of effective radius at 2.26 #m, and (b) retrieval of optical depth at 1.25 #m. 10:42-10:47 GMT.

Flight A019 Straight and Level Run Above Stratocumulus

~

8

~

4

~

~ o

2~ 110 r-

....... .~

FSSP M ~ n ro +

6

u

4

~

2 0 0

1 O0 200 Time from start of run (Seconds)

300

Fig. 9. BI flight 28 June 1990. (a) Retrieval of effective radius at 2.01 pm, and (b) retrieval of optical depth at 1.25/Lm. 13:10-13:15 GMT.

36

J.P. Taylor/AtmosphericResearch 34 (1994) 27-41

FSSP (Figs. 3 and 5 ). The retrieved effective radius is therefore indicative of that to be found near cloud top. The optical depth of the SA stratocumulus field is very variable ranging from 100 to 40. Calculations of the optical depth based on the adiabatic liquid water content and cloud thickness suggest a maximum optical depth of 100 for the case of cumulus intruding into stratocumulus. When considering the adiabatic liquid water content over the depth of the stratocumulus alone a maximum optical depth of 65 is predicted. Comparing with Fig. 3 it can be seen that the large optical depths, around 100, correspond to regions in the stratocumulus which have been modified by suspected cumulus intrusion where the liquid water content is at a local maximum. However, the liquid water contents measured in the cumulus regions are lower than the adiabatic values so the optical depth should still be lower than retrieved. The remotely retrieved optical depths in the region of cumulus intrusion are slightly larger (by around 25%) than those that would be predicted. The anomalously large estimated optical depths may arise from the inherent sensitivity of reflectance to changes in optical depth at these large optical depths. Also the retrieved optical depth of the stratocumulus may be being enhanced by the non-uniformity of the cumulus tops, which is an area of study being followed by the author. The MCR views in the nadir and so may be susceptible to these effects; however, the problems of enhanced reflectance would be expected to be larger for oblique views. This effect may therefore be of more consequence when considering the development of a satellite retrieval scheme. These results show that the effects of cloud inhomogeneity on its reflectance can be quite large particularly at non-absorbing wavelengths such as 1.25/~m. At 2.26/tm, an absorbing wavelength, inhomogeneities do not have as large an effect on cloud reflectance hence anomalously low effective radii are not retrieved. Generally the optical depth of the SA case is an order of magnitude greater than the BI case. There is considerable variation in the optical depth for the SA flight whilst the effective radius only changes slowly from around 11 jzm at the beginning of the run to 12/tm in the middle of the run, then back to 10 jzm at the end. This shows that in this case the effective radius is largely independent of the optical depth which varies as the liquid water content (with some modification by cloud inhomogeneity), as seen in situ. The retrieval of optical depth for the flight around the BI shows considerably less variation, ranging from a minimum of 4 to a maximum of 9. The effective radius for the BI flight shows more variation along the run with sudden changes of the order of 1.5/~m occurring within a few seconds. The retrieval of re at the very low optical depths found in the BI flight is less satisfactory for a number of reasons, including more significant departures from the plane parallel assumption; also as we move to very low optical depths there arises the problem of multiple values of effective radius that can give the same reflectance, (Nakajima and King, 1990).

6. Broad band spectral measurements

Broad band hemispherical fluxes, downwelling and upwelling, were measured

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

37

for both flights at levels above, within and below the cloud. The clear and red fluxes are measured directly and the visible fluxes calculated from their difference. Following the method ofRawlins ( 1989 ) the absorptance of the cloud layer is calculated as: (2)

A=.4-E

where A is the absorptance of the cloud layer, A is the directly measured absorptance and E is the net energy gained or lost through sides of cloud. In any practical application of Eq. (2), the term E will include both edge effects and sampling errors. Rawlins (1989) corrected for cloud edge effects and sampling errors by assuming that the divergence of visible radiation within the cloudy layer is very small. He also assumed the loss through edges to be approximately the same in different spectral regions, which is realistic as the scattering properties of clouds only change slowly with wavelength. The term E can therefore be equated to the apparent absorption at visible wavelengths, if the absorption at visible wavelengths (0.3-0.7/tm) can be assumed to be negligible. By calculating the absorptance of a cloud layer using Eq. (2) it is therefore possible to remove sampling errors and errors associated with in-homogeneous cloud fields and allows the direct comparison of the absorptance of different cloud sheets. Using run means of legs of at least 5 minutes endurance above and below both cloud sheets the results shown in Fig. 10 are obtained. The broad band data for the BI flight have been normalised to a common solar (a) British Isles Flight AO19

R = 24"3%

~x,x,x

~//

i = 100%

N

$ T = 68.9% (b) South Atlantic Flight A 146 R=69.6%

~k,XX

~///

$

1=100%

T = 25%

Fig. 10. Bulk radiative properties of the stratocumulus clouds for (a) the BI flight, and (b) the SA flight.

38

J.P. Taylor / Atmospheric Research 34 (1994) 2 7-41

zenith angle chosen to be 31.8 ° and in the SA flight have been normalised to 26.2 °. In Fig. 10, I is the incident radiation (100 units) and the albedo (R), absorptance (A), transmittance (T) and visible absorptance (E) are expressed as a percentage of the incident flux. The visible absorptance should be zero; any deviation from this is assumed to be due to sampling errors, i.e. the problems of assuming that the run below cloud is directly underneath the run above and that the same cloud is being observed. The BI flight was optically thinner and more inhomogeneous than the flight in the SA; this is self evident from the two values of E: 5.1% for the BI and 0.5% for the SA. This highlights the need for caution when analysing the radiative properties of optically thin or broken cloud fields. The measurement of solar absorption (0.3-3.0 #m) in clouds is difficult since it requires a small residual to be found from the subtraction of pyranometric observations made at different times and from different instruments. If the errors due to sampling and cloud edges associated with the inherent non-uniformity of clouds are not accounted for in the analysis of the observations then large errors in the perceived cloud absorption can occur. The absorptance in the SA cloud (4.9%) is almost three times as great as that of the cloud around the BI (1.7%). The magnitude of the term E, which under ideal conditions would be zero, gives some idea of the error in cloud absorptance which could be obtained if the problems of cloud inhomogeneity and sampling were ignored. Many authors in the past have noted anomalous absorption in their measurements and this may be due to the problems of cloud inhomogeneity, a review of these measurements is given by Stephens and Tsay (1990). Fig. 11 shows the clear (C), red (R) and visible (V) albedo for both flights and the ratio of red albedo to visible albedo (R/V). This figure shows that the variation in broad band albedo along a run for both flights is quite large and

1.0

:..,. ......

._o o

................ R/V SA R/V

0.8

I

BL

xl 1.0:

I

I

I

i 100

i 200

J 500

0.8

o

0.6

:~ 0 . 4

O.C 0

Time f r o m

400

s t a r t of r u n ( S e c o n d s )

Fig. 11. Albedo and albedo ratios of the stratocumulus clouds for both the BI and SA flight, where V= visible albedo, C = clear albedo, R = red albedo and R/V= red albedo/visible albedo.

J.P. Taylor/AtmosphericResearch 34 (1994) 27-41

39

comparable in magnitude to the variations in reflectance at the shorter wavelength MCR channels (0.55 a m and 1.25 am, Figs. 6 and 7). The ratio of red albedo to visible albedo was found, by Hignett (1987), to be relatively insensitive to variations in liquid water path, which is contrary to the work of DeVault and Katsaros ( 1983 ) who offer this ratio as a means of determining liquid water path. The reflectances measured at narrow wavelength bands with the MCR lead to the conclusion that for the SA flight the variations in liquid water were large as the changes in reflectance at low wavelengths were large. In Fig. 11 we see that there is little change in the albedo ratio for the SA flight suggesting that the ratio of red albedo to visible albedo has little variation with liquid water path, in support of Hignett's results. The variations in the albedo ratio for the BI flight are a combination of the cloud's low optical depth and increased inhomogeneity.

7. Conclusions

The two cases considered represent two extremes in the microphysics of stratocumulus. Both have similar aerosol concentrations below the cloud yet have very different microphysical properties. In the BI case the cloud had lower liquid water content and lower effective radius than the cloud in the South Atlantic. The difference in droplet concentration between the two flights suggests that the fraction of aerosol in the SA that can act as CCN is larger than that around the BI. Retrievals of cloud optical depth and effective radius have been carried out for both cases, the BI case suggesting that a lower bound in terms of optical depth may be reached for z< 5 where the retrieval scheme may no longer work. For both cases the retrieval of effective radius is in good agreement with in situ FSSP measurements, and the retrieved optical depths are in broad agreement with predicted values. The problems of enhanced reflectance have also been noted and will be of importance especially when oblique views are considered. These results give confidence in the possibility of using this scheme more widely in a future satellite retrieval but also highlight some of the problems likely to be encountered. Broad band measurements of albedo, reflectivity, transmissivity and absorptance have shown the SA cloud sheet to have a larger albedo and absorptance with a correspondingly lower transmittance than the BI case. A combination of narrow band and broad band measurements have thrown some light onto the causes of variability in reflectance and albedo and have shown that it does not appear possible to retrieve cloud liquid water path by using a ratio of the red albedo to the visible albedo. The results have shown that the microphysical and radiative properties of the two cloud sheets are very different. The warm moist air of the tropics results in the available liquid water being much larger than in mid-latitudes, consequently the optical depth increases. The range in effective radii seen in just these two cases, from 6 #m to 13.2/tm, is much larger than the estimated change in effective radius which would be required to offset global warming resulting from a doubling of carbon dioxide, and brings into serious doubt the validity of using a sin-

40

J.P. Taylor/Atmospheric Research 34 (1994) 27-41

gle prescribed effective radius in numerical models. The retrieval of effective radius using a multi-channel procedure at near infrared wavelengths has been shown to work over this range of droplet sizes. The use of measurements at 3.7/lm may allow a more accurate estimate of effective radius because of the very strong dependence of reflection to droplet sizes at this wavelength. Further work is being carried out to study the use of this wavelength. Translating this retrieval scheme to satellite use will have other problems such as sub-pixel clouds and the inherent problems of inhomogeneity.

Acknowledgments I would like to thank the scientists and Royal Air Force crew of the M R F without whose dedicated support this work would not have been possible. I would also like to thank Wing Commander Blake for allowing the Meteorological Research Flight C-130 to operate out of Ascension Island.

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