Interpolated soundings at SURFRAD stations

We wanted profiles of temperature and moisture at SURFRAD stations because those data are needed to initialize radiative transfer models. However, because of strict selection criteria, most SURFRAD stations are located far from rawinsonde sites. The exception is Desert Rock, which is collocated with an operational rawinsonde station that is part of the national network. In the SURFRAD sounding files, the sounding for Desert Rock is the actual sonde data except that it has been normalized vertically to 25-mb surfaces to be compatible with the interpolated soundings.


Software has been developed to interpolate soundings from the North American operational rawinsonde network to the locations of the SURFRAD stations. Operational soundings are accessed from the U.S., southern Canada, northern Mexico, and the Caribbean, a total of about 100 stations. If fewer than 40 rawinsonde stations report for a particular synoptic time, no interpolation to the SURFRAD stations is performed.

In the first step, parameters from the operational soundings are interpolated vertically to common pressure levels at 25 mb increments, beginning at 1000 mb and ending at 100 mb. The surface value is always retained as the first entry of the sounding. Using the actual position of the balloon in the horizontal interpolation at a particular level improves the quality of the interpolated soundings. Therefore, balloon displacements from the launch site are computed for each 25-mb level. The new positions are converted to a latitude and longitude and reported at each line in the 25-mb data file. The 25-mb soundings are used as input to the horizontal interpolation program.

For stability in the horizontal interpolations at the lowest-levels, all of the national network soundings are extrapolated down through the surface to 1000 mb using the low-level lapse rate and a constant RH assumption. Below-surface heights are computed hydrostatically using the extrapolated temperatures and dew-point temperatures. Winds are assumed constant from the surface downward.

Horizontal interpolations to the SURFRAD stations are made at each 25 mb surface and for ground level. The method used is the Analytic Approximation scheme of Caracena (1987). It is an improved version of the Barnes weighted sum scheme (Barnes, 1964). Two parameters control the manner in which weighted sum methods work, the weight function, and the number of passes of successive removal of residuals. The Analytic Approximation and Barnes schemes both use a Gaussian weighting function. In this application the width of the Gaussian weights are defined by a 400 km scale length. This is the distance from an observation where its weight falls to 1/e. 400 km was chosen because it is close to the mean separation of the rawinsonde stations. The Analytic Approximation is designed to mathematically apply, in one processing step, n passes of successively removing residuals. In this implementation, four passes are used. Sensitivity tests on these controlling parameters revealed that 4-passes with a 400 km scale length were an optimal balance between computing efficiency and accuracy.

Like all mathematical approximations, the quality of the output is linked to the quality of the input. To mitigate some known problems with rawinsonde data, several data quality control steps are employed before the horizontal interpolation is performed. The interpolated soundings are checked each day by comparing them to the nearest real sounding. If problems are perceived, the bad interpolated soundings are deleted. This check has been performed regularly starting in February of 2000, when the near real time automated processing of the soundings began. Interpolations for the period before February, 2000, have not been checked. Therefore, please consider these data provisional.

File description

A file of interpolated soundings for all SURFRAD sites is generated for each synoptic time that operational soundings are taken (00 and 12 UTC). File names are of the form, e. g., Each of these files has one header record which contains the number of soundings to follow (variable name ng in the program linked below), the number of lines per sounding (num_levels), the date and time in the format dd-mmm-yyyy_hh:mm:ss.dd, the number of passes carried out in the weighted sum interpolation (npass), and the scale length in kilometers (xl) used to define the width of the Gaussian weights. In the interpolated sounding files before 1997, npass and xl may be missing from the initial header record. In those cases, four passes and a scale length of 400 km were used.

The file header is followed by the soundings. Each sounding has a header of its own containing the station name, latitude, longitude, and elevation above sea level in meters. The convention used is that north latitude and east longitude are positive, and south latitude. and west longitude are negative.

All interpolated soundings have 38 levels, so num_levels should always be 38. The first level is always the surface, and it is followed by 37 upper levels at 25 mb increments from 1000 mb to 100 mb. Below-surface levels are filled with missing values (-999.0). Data on each line includes pressure (mb), geopotential height (m), temperature (C), dew point temperature(C), u component of the wind (m/s) and v component of the wind (m/s).

There are 7 soundings per file. Six correspond to the SURFRAD stations and the seventh to the Central Facility of the ARM Southern Great Plains site.


Barnes, S. L., 1964: A technique for maximizing details in numerical weather maps analysis. J. Appl. Meteor., 3, 396-409.

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.

Click the following link for a Fortran program that will read the interpolated sounding files:

Program read_int_sonde.f