Compiled Lidar Data Set
 

 

The fraction of cloud layers with lidar polarization delta<0.1 (indicating a predominantly liquid cloud layer) and the parameterized liquid fraction of the cloud condensate in the ECMWF model as functions of radiosonde-derived cloud-base temperature (Bretherton et al. 2000, Fig. 11).

 

The file lidardata.nc contains hourly averages of lidar polarization of clouds over the SHEBA ice camp. The vertical levels at which the data are stored are the same as in the radar reflectivity data set (115,205, 295,...,13075m).

 

Summary of all variables in lidardata.nc

netCDF VARIABLE NAME

DESCRIPTION

UNITS

nc{'time'}

Julian day (1997)

decimal day

nc{'observation_nr'}

number of 10-mins files used to compute hourly mean depolarization

[cnts]

nc{'cloud_fraction'}

cloud fraction (*)

[%]

nc{'altitude'}

cloud layer height

[m]

nc{'depol'}

depolarization at cloud layer height

depol*100

nc{'cnt_cldbase'}

number of 10-mins files that give a depolarization at cloud layer height

[cnts]

(*) The cloud fraction is computed by dividing the total number of 10-mins files for which at least one cloud layer was detected by the total number of 10-mins files in that hour.

The default value for all variables is -99.,

 

The file lidardata.nc can be downloaded by anonymous ftp: ftp eos.atmos.washington.edu, directory pub/roode (netCDF, 4.0 Mb)

 

Acknowledgements
 
The original data files were kindly provided by Janet Intrieri of ETL.

 

References
 
Bretherton, C. S., S. R. de Roode, C. Jakob, E. L Andreas, J. Intrieri, R. E. Moritz, and P. O. G. Persson, 2000: A comparison of the ECMWF forecast model with observations over the annual cycle at SHEBA. J. Geophys. Res., submitted 12/99, revised 5/00. Postscript version

Intrieri, J. M., M. D. Shupe, B. J. McCarty, and T. Uttal, 2000: Annual cycle of arctic cloud statistics from lidar and radar at SHEBA. J. Geophys. Res., submitted May 2000.

Stephan de Roode (roode@phys.uu.nl)
Chris Bretherton (breth@atmos.washington.edu)

University of Washington
Dept of Atmospheric Sciences
Box 351640
Seattle, WA 98195-1640 USA

 

Example from original data file
 
In the original file 'CLD.1a.1998-05-18_00:00:00.avge' we have the following data:

1998 05 18 00 00 00 1
3109 8968 5572242.500000 0.818917 0.318917
1998 05 18 00 10 00 1
4693 7862 1749188864.000000 0.213290 0.255454
1998 05 18 00 20 00 2
89 119 339192064.000000 0.026210 0.026285
4364 7922 1771164672.000000 0.214793 0.250544
1998 05 18 00 30 00 1
3706 8759 11044701.000000 0.232749 0.210265
1998 05 18 00 40 00 1
3557 8669 3067987.750000 0.309619 0.309619
1998 05 18 00 50 00 2
59 119 981184064.000000 0.032045 0.030738
3557 8759 3262011.000000 0.357380 0.357380

These numbers represent:
year month day hour minutes seconds nr_of_cloud_layers
cloud_base cloud_top x1 x2 depolarization

x1 and x2 are not stored in lidardata.nc.

 

How these data are stored in lidardata.nc
 

If these data are read by the matlab routine testlidar.m, we get the following result:

>>testlidar (98051800,98051800)

cbcnt =
Columns 1 through 12
2 1 2 1 1 1 -99 -99 -99 -99 -99 -99
Columns 13 through 24
-99 -99 -99 -99 -99 -99 -99 -99 -99 -99 -99 -99

depol =
Columns 1 through 12
3 32 33 21 25 26 -99 -99 -99 -99 -99 -99
Columns 13 through 24
-99 -99 -99 -99 -99 -99 -99 -99 -99 -99 -99 -99

alt =
Columns 1 through 6
115 3085 3535 3715 4345 4705
Columns 7 through 12
-99 -99 -99 -99 -99 -99
Columns 13 through 18
-99 -99 -99 -99 -99 -99
Columns 19 through 24
-99 -99 -99 -99 -99 -99

 

Explanation
 
In CNT_CLDBASE 10-mins files a cloud layer was given at ALT m and the mean depolarization at this altitude is POL.
In 2 10-mins files a cloud layer was given at 115 m and the mean depolarization at this altitude is 3.
In 1 10-mins files a cloud layer was given at 3085 m and the mean depolarization at this altitude is 32.
In 2 10-mins files a cloud layer was given at 3535 m and the mean depolarization at this altitude is 33.
In 1 10-mins files a cloud layer was given at 3715 m and the mean depolarization at this altitude is 21.
In 1 10-mins files a cloud layer was given at 4345 m and the mean depolarization at this altitude is 25.
In 1 10-mins files a cloud layer was given at 4705 m and the mean depolarization at this altitude is 26.

Note that the cloud layer was given is the average of the minimum and maximum bin size, so a cloud layer was given of 3715 m means that the average cloud base is located somewhere between 3670 and 3760 m (height interval is 90 m). However, a detected cloud layer anywhere between 0<=alt<=159.99 is set to alt = 115 m (the first height interval).

The default value is -99. The number of records is set to the maximum number of cloud layers (24) detected during the whole SHEBA period.

 

testlidar.m, matlab routine to read lidardata.nc
 
function testlidar(yymmddhh1, yymmddhh2)

%convert date to program variables
yymmddhh0 = 1997110100;
hh0 = mod(yymmddhh0,100);
dd0 = (mod(yymmddhh0,10000) - hh0)/100;
mm0 = (mod(yymmddhh0,1000000) - 100*dd0 - hh0)/10000;
yy0 = (mod(yymmddhh0,100000000) - 10000*mm0 - 100*dd0 - hh0)/1000000;
Jdd0 = 365*(yy0 - 97) + Jmmstart(mm0) + dd0 + hh0/24;

hh1 = mod(yymmddhh1,100);
dd1 = (mod(yymmddhh1,10000) - hh1)/100;
mm1 = (mod(yymmddhh1,1000000) - 100*dd1 - hh1)/10000;
yy1 = (mod(yymmddhh1,100000000) - 10000*mm1 - 100*dd1 - hh1)/1000000;
Jdd1 = 365*(yy1 - 97) + Jmmstart(mm1) + dd1 + hh1/24;
rec1 = round(24*(Jdd1-Jdd0)) + 1;

hh2 = mod(yymmddhh2,100);
dd2 = (mod(yymmddhh2,10000) - hh2)/100;
mm2 = (mod(yymmddhh2,1000000) - 100*dd2 - hh2)/10000;
yy2 = (mod(yymmddhh2,100000000) - 10000*mm2 - 100*dd2 - hh2)/1000000;
Jdd2 = 365*(yy2 - 97) + Jmmstart(mm2) + dd2 + hh2/24;
rec2 = round(24*(Jdd2-Jdd0)) + 1;

timedim = 6715;
layers=1:24;

if rec2 > timedim
rec2 = timedim; %no data available after timedim
end

nc = netcdf('lidardata.nc','nowrite');

description = nc.description(:)
author = nc.author (:)
date = nc.date (:)

timeunits = nc{'time'}.units(:)
obsnunits = nc{'observation_nr'}.units(:) %number of observations (0-6)
cfunits = nc{'cloud_fraction'}.units(:)
altunits = nc{'altitude'}.units(:) %cloud layer height
polunits = nc{'depol'}.units(:)
cbunints = nc{'cnt_cldbase'}.units(:)

trec = rec2-rec1+1;
time (1:trec) = nc{'time'} (rec1:rec2)
obs (1:trec) = nc{'observation_nr'} (rec1:rec2)
cf (1:trec) = nc{'cloud_fraction'} (rec1:rec2)
alt (1:trec,layers) = nc{'altitude'} (rec1:rec2,layers)
depol (1:trec,layers) = nc{'depol'} (rec1:rec2,layers)
cbcnt (1:trec,layers) = nc{'cnt_cldbase'} (rec1:rec2,layers)

nc=close(nc);