| Abstract: |
In general, the validation of microwave remote sensing retrievals of geophysical parameters, using in-situ surface truth, has been challenging for most applications. Typically, the remote sensing retrievals are instantaneous spatial averages over antenna instantaneous fields of view (IFOV) of at least a few Km’s. For comparison purposes, the surface truths are temporal averages of corresponding in-situ observations over equivalent time scales to match the remote sensor resolution. Statistical relations such as Taylors “frozen turbulence” hypothesis for given geophysical parameters usually describe these spatial/temporal relationships.
Whereas, most rain propagation field experiments involve active microwave techniques measuring signal amplitude along an instrumented (rain gauges) horizontal propagation path, this experiment is somewhat unique in that it involves the collection and analysis of downwelling atmospheric brightness temperatures, using a zenith-viewing microwave radiometer, to infer rain extinction coefficients during intense rainfall. Because the volume distribution of rain is spatially heterogeneous and temporally random, the usual statistical approach (described above) is not satisfactory. Rather, this measurement uses the NOAA WSR-88D NEXRAD measurements of rain reflectivity during volume scans and in-situ measurements of rain rate and drop size distribution (DSD) to determine the volumetric distribution of rain within the microwave radiometer antenna beam, for the purposes of estimating the rain propagation extinction coefficients during tropical convective rain events.
This paper provides a detailed description of the analysis approach, which involves procedures that accommodate the spatial/temporal mismatch between the experimental data sets; namely: radar, radiometer, rain gauges and disdrometer. Fundamental to this analysis is the construction of a dynamic 3-D rain volume that is tuned to match the NEXRAD rain reflectivity volume scans and the associated measured rain rate time series for in-situ instruments (disdrometer and rain gauges). The rain extinction (absorption and scattering coefficient) is inferred using a maximum likelihood estimation procedure, by comparing the time series of noise emission (brightness temperature) captured by the zenith-viewing C-band radiometer, with the theoretical radiative transfer model (RTM) calculations using the dynamic rain observations. Results are presented for several cases of thunderstorm squall lines that rapidly propagated over the test site.
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