Microphysical retrievals over stratiform rain using measurementsfrom an airborne dual-wavelength radar-radiometer
Meneghini, R.; Kumagai, H.; Wang, J.R.; Iguchi, T.; Kozu, T.
Geoscience and Remote Sensing, IEEE Transactions on
Volume 35, Issue 3, May 1997 Page(s):487 - 506
Digital Object Identifier 10.1109/36.581956
Summary:The need to understand the complementarity of the radar and
radiometer is important not only to the Tropical Rain Measuring Mission
(TRMM) program but to a growing number of multi-instrumented airborne
experiment that combine single or dual-frequency radars with
multichannel radiometers. The method of analysis used in this study
begins with the derivation of dual-wavelength radar equations for the
estimation of a two-parameter drop size distribution (DSD). Defining a
“storm model” as the set of parameters that characterize
snow density, cloud water, water vapor, and features of the melting
layer, then to each storm model there will usually correspond a set of
range-profiled drop size distributions that are approximate solutions of
the radar equations. To test these solutions, a radiative transfer model
is used to compute the brightness temperatures for the radiometric
frequencies of interest. A storm model or class of storm models is
considered optimum if it provides the best reproduction of the radar and
radiometer measurements. Tests of the method are made for stratiform
rain using simulated storm models as well as measured airborne data.
Preliminary results show that the best correspondence between the
measured and estimated radar profiles usually can be obtained by using a
moderate snow density (0.1-0.2 g/cm-3), the Maxwell-Garnett
mixing formula for partially melted hydrometeors (water matrix with snow
inclusions), and low to moderate values of the integrated cloud liquid
water (less than 1 kg/m-2). The storm-model parameters that
yield the best reproductions of the measured radar reflectivity factors
also provide brightness temperatures at 10 GHz that agree well with the
measurements. On the other hand, the correspondence between the measured
and modeled values usually worsens in going to the higher frequency
channels at 19 and 34 GHz. In searching for possible reasons for the
discrepancies, it is found that changes in the DSD parameter μ, the
radar constants, or the path-integrated attenuation can affect the high
frequency channels significantly. In particular, parameters that cause
only modest increases in the median mass diameter of the snow, and which
have a minor effect on the radar returns or the low frequency brightness
temperature, can produce a strong cooling of the 31 GHz brightness
temperature
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