The structure and organization of clouds under suppressed conditions observed by S-band radar during DYNAMO

The NCAR S-PolKa dual-polarized Doppler S- and Ka-band radar captured three active periods of the Madden-Julian Oscillation (MJO) during the Dynamics of the MJO (DYNAMO) field project. These three multi-week periods were separated by periods of convectively suppressed conditions. While deep convection and widespread stratiform echo during the active periods were responsible for the majority of the precipitation, a goal of DYNAMO was to investigate the entire cloud population as it evolves from shallow, isolated convection to deep mesoscale systems since this evolution needs to be represented in models predicting the MJO. The S-PolKa radar is sensitive enough to detect non-precipitating clouds at both its wavelengths, and its data therefore can be used to describe the organization and structure of boundary layer clouds during the buildup phase of the MJO. The non-precipitating clouds are seen via Bragg scattering in which the boundaries of small cumulus clouds have signatures referred to as mantle echoes. The Bragg scattering also shows layers of strong water vapor gradient. The small non-precipitating clouds are often organized into lines parallel to the boundary layer wind or wind shear before they begin to precipitate. As soon as S-PolKa indicates the onset of precipitation from a small cloud, Bragg and Rayleigh echoes outline quasi-circular cold pools in the boundary layer surrounding the showers. At times, the cold pools intersect with each other and new convection tends to initiate at the intersection points. This study will document the non-precipitating cloud structures as seen by S-PolKa and describe the steps in the evolution of the boundary layer echoes from lines to cold pools, and hence to the development of the deeper MJO convective population.