EPIC2001 Leg II

Stratocumulus Cloud Optical Properties

C. Fairall 6/3/03

Data from the EPIC2001 program have been processed to yield a file which abstracts information on cloud properties measured on the NOAA ship Ronald H. Brown.. The file cloud_opt_hds contains two header lines plus 4320 lines of data with 21 columns each at 5-min time resolution. The first 15 columns are essentially measurements and the last 4 are solar optical properties deduced from parameterizations of radiative transfer. The optical retrievals are done when solar zenith angle is less than 80 deg and when the vertical cloud fraction (determined from ceilometer) exceeded 0.8. This yields 1596 optical property values, but the entire data time series is included (day and night).

Retrievals were done by specifying solar transmission coefficient and solar zenith angle. The Dong method was used to obtain effective radius, optical thickness, and the number of cloud droplets

alb=0.05;

ref=2.49*wm/100+10.25*trm*(1-alb^3)-.25*mu+20.28*wm/100.*trm*(1-alb^3)-3.14*wm/100.*mu;

log(na)=(12.66-sqrt(4*0.84*(ref-5.69)))/2/.84;

tau=1.5*wm./ref;

where wm is the liquid water path, mu the cosine of the solar zenith angle, trm the transmission coefficient, na the number of droplets per cm^3, and tau the optical thickness.

We also retrieved values for tau using the Stephens parameterization. Because the Dong parameterization uses lwp, it yields a strong correlation between lwp and tau. Typical tau values are 9.5 for Dong and 8.8 for Stephens. Typical reff values were 12.7 micron for Dong and 13.2 micron for Stephens. One other amusing point, Dong recommended using his 98 result rather than an earlier version. The later version with alb=0.00 gives smaller values for reff.

Note, Dong does not strongly recommend this method for getting Ndrop. Instead, he suggests a parameterization where cloud thickness is specified.

nnx=.75/pi./(1e-6*ref).^3./clodthk(ii).*wm/1e6*exp(3*.38^2)/1e6;

I don't have the full time series of cloud top height, but I checked this for the one-week period we were on station at 25 S latitude. This yielded values for Ndrop about half that of the other method. I think this is a more robust estimate, so you might consider dividing the values of Ndrop in the data file by 2.0. A typical value for nnx for this period was 50 cm^-3 while na was about 105 cm^-3.

Data columns

1	Julian day GMT (start of hour)

2	cloud fraction

3	16th percentile cloud base (m)

4	50th percentile (median) cloud base (m)

5	84th percentile cloud base (m)

6	Precipitable water vapor (cm)

7	Liquid water path (g/m^2)

8	Latitude (deg)

9	Longitude (deg)

10	Downward solar flux

11	Clear sky model (Iqbal) Downward solar flux (W/m^2

12	Downward IR flux

13	Clear sky model (Fairall) Downward IR flux (W/m^2)

14	Air temperature at 15 m (C)

15	Air specific humidity at 15 m (g/kg)

16	Solar zenith angle (deg)

17	Transmission coefficient

18	Effective radius(micron), Dong

19	Number of cloud droplets (#/cm^3

20	Optical thickness, Dong

21	Optical thickness, Stephens