Understanding the effects of aerosols on the microphysical characteristics of marine stratocumulus clouds, and the resulting influence on cloud radiative properties, is a primary goal of FIRE. The potential for observing variations of cloud characteristics that might be related to variations of available aerosols is studied. Some results from theoretical estimates of cloud reflectance are presented. Also presented are the results of comparisons between aircraft measured microphysical characteristics and satellite detected radiative properties of marine stratocumulus clouds. These results are extracted from Mineart where the analysis procedures and a full discussion of the observations are presented. Only a brief description of the procedures and the composite results are presented

Durkee, Philip A.

Mineart, Gary M.

English

Understanding the effects of aerosols on the microphysical characteristics of marine stratocumulus clouds, and the resulting influence on cloud radiative properties, is a primary goal of FIRE. The potential for observing variations of cloud characteristics that might be related to variations of available aerosols is studied. Some results from theoretical estimates of cloud reflectance are presented. Also presented are the results of comparisons between aircraft measured microphysical characteristics and satellite detected radiative properties of marine stratocumulus clouds. These results are extracted from Mineart where the analysis procedures and a full discussion of the observations are presented. Only a brief description of the procedures and the composite results are presented.Approved for public release; distribution is unlimited

Calhoun, Institutional Archive of the Naval Postgraduate School

Marine stratocumulus cloud characteristics from multichannel satellite measurements

NASA Technical Reports Server

Marine Stratocumulus Cloud Characteristics from Multichannel Satelllte Measurements 
PHILIP A. DURKEE AND GARY M. MINEART 
Department of Meteorology 
Naval Postgraduate School 
Monterey, California 93943 
INTRODUCTION 
Understanding the effects of aerosols on the microphysical characteristics of marine 
stratocumulus clouds, and the resulting influence on cloud radiative properties, is a primary 
goal of FIRE. The effect of aerosols on clouds and the impact on climate processes have 
recently been discussed by several authors (Twomey et al., 1984; Coakley et al., 1987, 
Charlson et al., 1987). Of particular concern in this presentation is the potential for 
observing variations of cloud characteristics that might be related to variations of available 
aerosols. Some results from theoretical estimates of cloud reflectance will be presented. We 
also present here the results of comparisons between aircraft-measured microphysical 
characteristics and satellite-detected radiative properties of marine stratocumulus clouds. 
These results are extracted from Mineart (1988) where the analysis procedures and a fu l l  
discussion of the observations are presented. Due to the space available, only a brief 
description of the procedures and the composite results will be presented. 
The satellite data used here are from the National Oceanic and Atmospheric 
Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) collected at 
the Scripps Satellite Oceanography Facility. The AVHRR channel I (0.63 pm), channel 3 
(3.7 pm), and channel 4 ( 1  I pm) data were used in the analysis. Cloud microphysical data 
were obtained by instruments on the NCAR Electra during MABL cloud-penetrating 
missions in support of the I F 0  from 29 June - 19 July 1988. 
RESULTS 
Theoretical Reflectance Estirnales 
Cloud reflectance can be estimated from cloud droplet distributions that represent 
anticipated conditions in marine stratocumulus clouds. Model cloud droplet distributions 
were generated using the modified gamma distribution after Deirmendjian (1969). Modal 
radii of 4 and 8 pm were used to show the effects of change in droplet size. The results 
presented here are for a constant liquid water content of 0.4 g m-’. Three distributions at 
each mode radius were chosen to illustrate the effects of distribution width. The droplet 
size distribution curves for the two mode radii are shown in Fig. 1. The optical properties 
of the cloud droplet distributions were calculated using Mie calculations after Wiscombe 
(1980). Reflectance was calculated using the delta-Eddington approximation from 
265 
https://ntrs.nasa.gov/search.jsp?R=19910001189 2020-03-19T20:30:02+00:00Z
Joseph et al. (1976). applied qt 45" solar zenith angle and cloud thickness ranging from 10 to 
750 meters. 
Fig. 2 illustrates the dependency of reflectance on cloud droplet size distribution, at 
a constant liquid water content (LWC) of 0.4 g m-', for 0.63 pm (AVHRR channel I 3) and 
3.7 pm wavelength (AVHRR channel 3). Reflectance decreases as droplet size increases for 
both wavelengths. 
Fig. 3 shows reflectance dependence on cloud thickness. Channel I reflectance 
increases sharply in the first few hundred meters and then asymptotically approaches a value 
of 1.0 with increasing cloud thickness. Channel 3 reflectance rises quickly at ctoud 
thickness below 0.1 km and then remains constant. Therefore, reflectance in channel 3 does 
not vary with cloud thickness once the cloud is greater than about 100 m thick. This is due 
to the moderate absorption by water droplets at 3.7 p m  wavelength. 
F I R  E I F 0  Aircra/t/Satellite Comparisons 
Mineart (1988) presents four case studies that show a consistent relationship between 
cloud microphysical characteristics and radiative properties. Here we present only the 
composite results. The results of comparisons between coincident aircraft and satellite 
observations are displayed in Figs. 4 and 5. 
The 3.7 pm (AVHRR channel 3) reflectance in Fig. 4 displays an excellent 
correlation with cloud droplet size. The outlying data point at 3 pm represents an 
observation of a cloud edge (about 80 m thick), where the decrease in cloud thickness 
dominates the drop size effect on reflectance. Fig. 4 also compares the channel 3 results 
with the model cloud reflectance data shown in Fig. 2. The variations closely match the 
expected values from simple theoretical estimates. Also, the shift toward model distribution 
D1 (broadest distribution) at large droplet sizes and the shift toward model distribution D2 
(narrow distribution) at the smaller droplet sizes is consistent with the shift in distribution 
shape observed in the aircraft measurements (not presented here, see Mineart, 1988). 
Fig. 5 relates 0.63 pm (AVHRR channel 1) reflectance to cloud thickness. Cloud 
thickness values are estimated from APN-159 radar altimeter and PMS-King LWC data. 
The vertical and horizontal bars indicate the 95% confidence intervals. The data show 
increasing channel 1 reflectance with increasing cloud thickness, which is consistent with the 
dependence shown in Fig. 3. Although channel 1 reflectance should vary as a function of 
droplet radius with constant cloud thickness and LWC, we have not analyzed enough cases to 
separate variations due to drop size from variations due to LWC and cloud thickness. 
CONCLUSIONS 
Relationships between cloud reflectance and cloud characteristics have been 
illustrated by comparing AVHRR satellite data and aircraft measurements. At  0.63 pm 
(AVHRR channel I )  reflectance variations relate strongly to cloud thickness. Also, The 
expected relationship of higher reflectances from smaller cloud droplet size spectra is 
confirmed for 3.7 pm wavelength (AVHRR channel 3). This dependence is closely 
approximated by model cloud reflectance estimates. Although not presented here, Mineart 
(1988) showed that a primary source of droplet size variations is related to 
continental/marine air mass differences. Continental air masses are generally have higher 
concentrations of.  aerosols, higher concentrations of cloud droplets, and a shift towards a 
smaller mean cloud droplet radius. The strong dependence of channel 3 reflectance on cloud 
droplet size distribution allows inference of cloud composition characteristics from satellite 
observations. 
266 
REFERENCES 
Charlson, R. J., J. E. Lovelock, M. 0. Andreae, and S. G. Warren, 1987: Oceanic 
phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655-661. 
Coakley, J. A., Jr., R. L. Bernstein and P. A. Durkee, 1987: Effect of ship-stack effluents 
on cloud reflectivity. Science, 237, 953- 1084. 
Deirmendjian, D., 1969 Electromagnetic scattering on spherical polydispersions. Elsevier, 
New York, 290pp. 
Joseph, J. H., W. J. Wiscombe and J. A. Weinman, 1976 The delta-Eddington 
approximation for radiative flux transfer. J. Atmos. Sei., 33, 2452-2459. 
Twomey, S., M. Piepgrass and T. L. Wolfe, 1984 An assessment of the impact of pollution 
on global cloud albedo. Tellus, 36B, 356-366. 
Wiscombe, W. J., 1980 Improved Mie scattering algorithms. Applied Optics, 19, 1505- 1509. 
FIGURES 
0.0 5.0 1o.n 15.0 20.0 25.0 
1 M ) l  US (h i  ICROhlETEIZS) 
FIG. 1. Model Cloud Droplet Distributions for a modal radius = 4 pm, 
LWC-0.4 g m-’. D1, D2, and D3 represent three modified gamma 
distributions of varying width. 
J.0 
267 
FIG. 2. Reflectance verses cloud thickness for solar zenith angle=45", modal 
radius=8 pm, LWC-0.4 g m-' and at the three modified gamma distributions 
(DI, D2, and D3). 
9 -. 
--.- - -  - - - - - - - -  - - - _  - - - -  - - - - - - - - - -  - _ _ _  - - - - - _ _ _ _  - - -  
- - - - - - - -  - - -  
- - - - - - - - _ _ _ _  D2 
- - -  D3 
D1 
- - - - _  - - - - - _ _  
- - - - - - - - - -  3.70 
. . . . . . . . . . . . . . . . . . . . . .  
----. 
- - - - - - - - - - _ _ _ _ _ _ _ _ _ _ _ _ _ _  
3 4 5 a 7 H 0 
1 1 ) I  c' !i 1 A1 1 ( ' I :I !.I 13'1: I ! ' 
FIG. 3. Reflectance verses droplet radius for 220.25 km, solar zenith 
angIe=45", LWC=O.4 g m-' and at the three modified gamma distributions 
(D1, D2, and D3). 
268 
D2 
D3 
D1 
0 
9 
w 
2- 
2- 
2 2- 
c g.. 
w 
cr. 
w 
cl 
C 
4 -  
40 
(1; z 
d 
2. 
z 2. 
u 2. - c
- 
a 
x 
I 
2=0.25Kh! EQI.WC=O.I 
I I 
1 
269 


Marine stratocumulus cloud characteristics from multichannel satellite measurements

Abstract

Similar works

Full text

Available Versions

Calhoun, Institutional Archive of the Naval Postgraduate School

NASA Technical Reports Server