Vaisala ceilometer file description

Within each daily zipped ceilometer file are four files:

MM_DD_YY.c39

MM_DD_YY.r39

MM_DD_YY.am

MM_DD_YY.pm

The first contains the time series (15-s) of lidar backscatter intensity in 255 30-m range gates. The second is the time series of cloud based height information (see below). The last two are .tif images of the first and second 12-hr time height cross sections of cloud base height and backscatter. The y-axis is range in km, but is off by a factor of two (top should be 8 km).

The file structure of the .c39 files is

MM_DD_YY.c39, where

MM = month (i.e., 01 = Jan.)

DD = day of the month

YY = year (i.e., 99 = 1999).

In these files there are 7 columns of data:

sky designation

0 = clear

1 = single cloud layer detected

2 = two cloud layers detected

3 = three cloud layers detected

4 = totally obscured conditions

5 = partially obscured conditions

height of lowest detected cloud base in meters

height of second detected cloud base in meters

height of third detected cloud base in meters

GMT hour

GMT minute

GTM second

"Totally obscured" conditions usually occur in fog or precipitation.

"Partially obscured" conditions occur when the ceilometer algorithm detects a

signal that does not fit the pattern of a "hard" cloud boundary. When the

sky designation is totally obscured, the value reported in the second column

is a calculation of vertical visibility. In these cases, the third column

reports the maximum height that a signal was detected. The totally obscured

data were included in the computation of cloud fraction statistics, but not of

cloud base statistics.

The MM_DD_YY.am or .pm files in the .zip archive

contain the data acquisition program screen display stored at the end of

each 12-hour period. These files may be imported into a word processor

(e.g., WordPerfct v9) for viewing.

The maximum range of the ceilometer is 7620 m, and the resolution in the

profiles is 30 m (100 ft). Other details on operation of the Vaisala CT25K

are given by White (NOAA Tech Memo. ERL ETL-281, NOAA Environmental

Technology Laboratory, 325 Broadway, Boulder, CO 80303, 1997).

Any questions regarding these data should be directed to

Allen White

awhite@etl.noaa.gov

(303) 497-5155