New Technique for Retrieving Liquid Water Path over Land using Satellite Microwave Observations Page: 4 of 11
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Fourteenth ARM Science Team Meeting Proceedings, Albuquerque, New Mexico, March 22-26, 2005
PDP-based Retrieval Strategies
The PDP serves as the basis for both single- and dual-channel LWP retrieval algorithms. In both types
of retrieval algorithms, a priori knowledge of both Ts and PWV is assumed; these quantities may be
adequately predicted or estimated using reanalysis, NWP models, or other satellite remote sensing
products. Thus, determination of AF is the main obstacle to retrieving LWP from measured values of
ATB.
Single-channel PDP retrievals require ancillary data (e.g., visible/infrared satellite observations)
collocated with the satellite microwave observations to separate clear and cloudy scenes. Clear-sky
observations, where LWP may be assumed to be exactly zero, are processed using Eq. (1) to determine
AF. Cloudy scenes yield LWP retrievals by exploiting AF values retrieved during previous clear-sky
overpasses, and assume negligible temporal variability of Ag. (In the case study presented in Section 4,
clear/cloudy determinations are made on the basis of ground-based observations.)
Dual-channel PDP retrievals exploit the observed weak frequency dependence of Ag. Specifically, if AF
is assumed to be frequency independent, simultaneous measurements of ATB at 37 and 89 GHz may be
processed to directly retrieve LWP. Since no independent clear/cloudy determination is required, the
dual-channel strategy is much simpler to implement than the single-channel strategy. And, although it is
more restrictive than the single-channel method in terms of the assumed frequency dependence of Ag, it
requires no assumptions with regard to Ag temporal variability. For the ACRF SGP site, results of a
case study (presented in Section 4) indicated highly correlated values of Ag at 37 and 89 GHz.
For both single- and dual-channel PDP methods, data gridding suppresses the effects of random
instrumental error. Currently, we grid TB observations to a rectangular 0.25 by 0.250 latitude/longitude
grid before retrieving LWP.
Southern Great Plains Case Study
A time-series plot of LWP recorded from five microwave radiometer instruments in the SGP region
from November 28 to December 4, 2003, is shown in Figure 2. Microwave radiometer data from
instruments stationed at the SGP C1 (36.605N, 97.486W), B1 (38.305N, 97.301W), B4 (36.071N,
99.218W), B5 (35.688N, 95.856W), and B6 (34.985N, 97.522W) facilities were used in this study. As
indicated in Figure 2, liquid clouds were not detected over the SGP region from November 28 through
December 1, 2003. This clear period was followed by three days during which a passing cold front
produced liquid water-bearing clouds and precipitation. As indicated by NEXRAD radar and surface
meteorological data (including the microwave radiometer data wet-flag fraction), precipitation
associated with this front fell in the SGP region primarily between 0000 and 0900 Universal Time
Coordinates (UTC) on December 3. Surface temperatures throughout the SGP region were well above
freezing during the entire period from November 28 through December 4; any precipitation presumably
fell to the surface as rain.4
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Deeter, M.N. & Vivekanandan, J. New Technique for Retrieving Liquid Water Path over Land using Satellite Microwave Observations, article, March 18, 2005; United States. (https://digital.library.unt.edu/ark:/67531/metadc779165/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.