GIST National Weather Service High-Resolution Upper-Air Dataset
1.0 General Description
This is one of the upper air sounding datasets developed for the
GEWEX Continental-scale International Project (GCIP) Integrated Systems
Test (GIST) conducted from 01 April to 31 August 1994. The GIST
domain extends from approximately 91W to 107W longitude and 31N to
40N latitude. Included in this dataset are 13 National Weather Service
(NWS) sounding stations in the GIST domain. The soundings were
typically released at 00 and 12 UTC, however, several stations had
additional launches on an as requested basis, particularly Norman OK
(OUN). The final dataset consists of 6-sec vertical resolution files.
2.0 Detailed Data Description
2.0.1 National Weather Service High-Resolution Sounding Algorithms
The detailed description of NWS sounding collection and
instrumentation is located in section 2.1 below
2.1 Data Remarks
The NWS soundings during GIST were comprised of two types:
Space Data radiosondes (Amarillo, El Paso, Midland, and Stephenville TX
and Albuquerque NM) and VIZ radiosondes (Denver CO, Dodge City KS,
Little Rock AR, Longview TX, Monett MO, Norman OK, Topeka KS,
Albuquerque NM, and Stephenville TX). Albuquerque NM and
Stephenville TX both used Space Data radiosondes from 01 April to 31
May 1994. From 01 June to 31 August 1994 they used the VIZ
radiosondes.
OFPS applied a correction to the Space Data radiosonde relative
humidity calculations. The NWS Micro-ART sounding system, in the
resistance ratio and RH calculations, used, instead of the observed
temperature, the observed temperature divided by 100. The OFPS. applied
correction was to rederive the resistance ratio, using the observed
temperature divided by 100 and the observed RH, by iteration using
the so termed "1A" and "1B" coefficients. Now with the new calculated
resistance ratio, the observed temperature and using the "1A" coefficients
only, the new RH was calculated.
OFPS also applied a correction to the VIZ radiosonde RH
calculations. Using the observed temperature and RH, the resistance ratio
was rederived by iteration using both the "1A" and "1B" coefficients.
With the calculated resistance ratio, the observed temperature
and using the "1A" coefficients only, the new RH was calculated.
In both of the radiosonde types, the use of the raw 6-sec
resolution elevation and azimuth angle data to derive the winds sometimes
led to large oscillations in wind speed, due to the presence of
oscillations in the elevation angle data, particularly at low elevation
angles. The general approach to correct this problem was to remove the
outlier radiosonde position data before computing the wind components.
For both the azimuth and elevation angles from 360 sec to the end of the
sounding, a ninth order polynomial was fit to the curve. The residuals
were calculated and compared to the observed values. The outliers of the
residuals were then removed.
Then to help correct the more extensive problems at low elevation
angles within 10 degrees of the limiting angles (LA) some additional
smoothing was applied. If the elevation angle was between (LA + 7.5)
and (LA + 10), the new elevation angle was computed with a 2 min linear
fit. If the elevation angle was between (LA + 5) and (LA + 7.5), the new
elevation angle was computed with a 3 min linear fit. If the elevation
angle was less than (LA + 5), the new elevation angle was computed with
a 4 min linear fit. If the number of observations with low elevation
angles was greater than 20% of the total number of observations for the
sounding no frequency smoothing occurred.
Then, for the elevation angle only, a finite Fourier analysis
was performed on the residuals. Periods from 90-190 sec were removed and
those below 30 sec were flattened.
Finally, a 2 min second order polynomial was then fit to the
position to derive the u and v wind components, except for the beginning
and end minute (or 1.5 minutes if over 50 mb) which used a 3 min fit. If
there were less than 15% of the total number of points, not counting the
beginning or end of the flight, on one side of the point for which the wind
value was being computed, a linear fit was used.
3.0 Quality Control Processing
This dataset underwent a two-stage QC process. First, the dataset
underwent internal consistency checks. This included two types of checks,
"reasonable" limit checks on all parameters and rate-of-change checks on
temperature, pressure and ascension rate. Second, each sounding was
visually examined to verify those parameters that are too variable for
automatic checks (wind speed, wind direction and moisture). This stage
of the QC process also allows for a verification of the QC flags generated
by the automatic checks.
4.0 Reference
Micro-ART Observation and Rework Programs Technical
Document, National Weather Service, National Oceanic and
Atmospheric Administration, Washington, D.C., March 1991.
Williams, S.F., C.G. Wade, and C. Morel, 1993: A comparison of
high resolution radiosonde winds: 6-second Micro-ART winds
versus 10-second CLASS LORAN winds. Preprints, Eighth
Symposium on Meteorological Observations and Instrumentation,
Anaheim, California, Amer. Meteor. Soc., 60-65.