Rain and Wet Hail Specific Attenuation Estimation for a 3D supercell case using RAMS model to simulate CASA X-band and WSR-88D S-band radar signals

A number of different algorithms are available for rain attenuation-correction using the differential propagation phase Фdp (either directly or as a constraint; e.g., Bringi et al 1990; Matrosov et al 2002; Testud et al 2000; Gorgucci et al 1996; Park et al 2005). But when rain is mixed with wet ice (which frequently occurs in convective storms), the Фdp-based methods will be inaccurate because wet ice (hail/graupel) does not have significant contribution on Фdp whereas it does contribute significantly to the X-band attenuation. While the attenuation due to the rain component of the mixture can be calculated using Фdp, the attenuation due to the wet ice component is very difficult to estimate because of the large variability in the size distribution and water coat thickness which gives rise to large family of k-Z relations (k is the specific attenuation, Z is the intrinsic reflectivity). In the past, the dual-wavelength (S/X-band) radar technique was developed primarily for hail detection but did need attenuation-correction of the X-band signal due to rain and wet ice along the path (e.g., Tuttle and Rinehart 1983).

A technique is developed and evaluated using the microphysics outputs from a Regional Atmospheric Modeling System (RAMS) 2-moment scheme supercell simulation described in Meyers et al (1997) to simulate the CASA and WSR-88D radar signals. The RAMS model has detailed microphysical information including the liquid fraction of hail and graupel (due to melting or wet growth aloft). A dual-frequency/dual-polarized radar emulator is used to simulate RHI or PPI scans from the RAMS gridded outputs.

This paper also presents a preliminary technique to separately estimate the X-band specific attenuation due to rain and wet ice along the path of the dual-polarized Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) X-band radar using the WSR-88D NEXRAD S-band as a reference signal.