Polarimetric radar observations of nonprecipitating echo during DYNAMO/AMIE

During the DYNAMO/AMIE field campaign in 2011, the NCAR S-PolKa radar observed precipitating systems ranging from shallow isolated convection to mesoscale convective systems. The sensitivity of this radar also allowed for data to be collected on nonprecipitating clouds and boundary layer features, including detailed polarimetric observations of these phenomena. These observations contribute to and expand upon previous studies of early echo associated with deep continental convection, shallow cumuli over Florida, and trade wind cumulus in the Caribbean in which they described mantle echoes associated with Bragg scattering. In addition to using reflectivity and differential reflectivity to describe this early echo, as in those previous studies, the full suite of polarimetric variables, including linear depolarization ratio and correlation coefficient, are used to identify unique polarimetric signatures associated with shallow nonprecipitating clouds over the tropical Indian Ocean. S-PolKa RHIs provided increased vertical resolution for this analysis, including focused scans over a vertically pointing Ka-band radar (KAZR), which operated as part of the DOE component of the project. Data from KAZR, including multi-wavelength retrievals, provided additional information for this analysis of early echo, while S-PolKa placed the KAZR observations within a broader spatial context, as well as providing polarimetric information. As part of this broader view, S-PolKa detected the nonprecipitating shallow clouds organizing along parallel lines within the boundary layer. Once precipitating, cold pools produced by showers were tracked as bounded echo holes at the lowest elevation scans. After the sun set, polarimetric variables became increasingly useful for identifying and tracking the fine lines associated with these outflow boundaries, although their values varied considerably, even between fine lines within the same scan. This topic will be further elaborated upon within the context of biological scatterers. Combined with discussions of early echo, this study highlights the wealth of information available from the highly sensitive polarimetric S-band radar in this tropical oceanic environment.