SURFRAD sounding interpolation uses the analytic approximation technique of
Caracena (1987) to compute upper-air profiles of temperature, dew point,
geopotential height, and wind at SURFRAD stations and the central facility
of the ARM Southern Great Plains site.  To do this, it accesses a
file of North American soundings for a single time that have
have been interpolated to 25 mb levels and whose positions at those levels
have been corrected for balloon drift.  Below-surface values have been
extrapolated to 1000 mb for all soundings for stability of the low
levels' horizontal interpolation (objective analysis).

Interpolated soundings for the SURFRAD station locations, as well as
the DOE ARM site, are named with the convention yyyymmdd_hh.int,
e.g., 20060907_12.int for 7 Sept. 2006.  The .int file has one
header record at the top of the file, for example:

    9  38  7-sep-2006_12:00:00.00   4   400.00

It contains the number of interpolated soundings in the file (9),
the number of data lines per sounding (always 38), the date and time in
the dd-mmm-yyyy_hh:mm:ss.dd format, the number of passes used in the
objective analysis (4), and the scale length (km) used for the station
weighting function. The scale length is the distance where the
influence of the station in in weighted sum falls to 1/e.

The interpolated soundings follow.  Each sounding has one header
record followed by the interpolated sounding.  For example, the
interpolated sounding for the Bondville SURFRAD station follows
the file header record and looks like the following:

Bondville       40.060  -88.370  213

which includes the name of the station, its latitude,
its longitude, and its elevation above sea level in meters.

The interpolated sounding for Bondville follows.  The
first line is the surface and the levels above follow
at 25-mb increments.  The columns are:

Pressure(mb), height(m), temperature(C), dew point temperature(C), 
u wind component (m/s), and v wind component (m/s)

 994.09   213.14    12.58    11.70     0.02     0.12
-999.00  -999.00  -999.00  -999.00  -999.00  -999.00
 975.00   367.56    18.70    12.20     0.21     0.26
 950.00   589.37    20.55    11.11     0.36     0.38
 925.00   820.50    19.11     9.51     0.57     0.53
 900.00  1053.53    17.02     9.07     0.72     0.51
 875.00  1291.98    14.86    11.19     0.87     0.50
 850.00  1539.57    12.57     6.82     1.03     0.47
 .
 .
 .
 175.00 12913.67   -52.68   -67.84    14.27     3.40
 150.00 13910.54   -53.32   -72.67    12.71     1.83
 125.00 15065.49   -57.13   -77.07    10.18     0.58
 100.00 16472.68   -60.80   -80.06     7.13    -0.93

-999.00 is used for missing values.

All data entries are separated by at least one space so that
a free format read can be used.

The Bondville sounding is followed by the sounding for Fort Peck:

 Fort Peck       48.310 -105.100  634
 939.13   634.43    13.01     2.40     2.66    -3.42
-999.00  -999.00  -999.00  -999.00  -999.00  -999.00
-999.00  -999.00  -999.00  -999.00  -999.00  -999.00
-999.00  -999.00  -999.00  -999.00  -999.00  -999.00
 925.00   769.51    18.75     5.66     3.14    -2.00
 .
 .
 .
And so on until all 9 of the interpolated soundings are listed.


Reference

Caracena, F., 1987:  Analytic approximation of discrete field samples with
weighted sums and the gridless computation of field derivatives. 
J. Atmos. Sci., 44, 3753-3768.