Indian Ocean Convection as seen by the S-PolKa radar in DYNAMO

A feature of the CINDY2011/DYNAMO/AMIE campaign was the deployment of a multiple-radar "supersite" on Addu Atoll to document as completely as possible the cloud population involved in the initiation of the MJO. As part of the supersite, the NCAR S-PolKa radar system made simultaneous Ka-band and S-band measurements with matched beamwidths to determine aspects of air motions, rainfall, convective and stratiform components of precipitating cloud systems, heights of convection, microphysical characteristics of precipitation particles, morphology of non-precipitating clouds, cold pool behavior and convective triggering in the boundary layer, and humidity in the moist layer surrounding clouds. Preliminary results showing observations of these aspects of Indian Ocean convection during suppressed and active phases of the MJO will be presented. Highlights seen by S-PolKa include the detection of moist layers in clear air, diurnal changes in the character of the boundary layer in suppressed and active periods, nonprecipitating boundary layer clouds, patterns of triggering deep convection along lines of nonprecipitating clouds and cold pool boundaries, the different behavior of convection in sheared an unsheared environments, the excitement of convection inside mesoscale systems with stratiform regions in contrast to convection in dry environments, the characteristics of nonprecipitating anvil clouds, and the vertical distribution of different types of ice particles and liquid hydrometeors in convective and stratiform regions. The ability to see boundary layer features and nonprecipitating cloud elements with the radar allows evaluation of the complete evolution from cloud formation to deep precipitating convection and mesoscale organization. The triggering by cold pools and along cloud lines in the boundary layer shows that apparently random convection is really being ordered by previous convection. The S-band data show upscale growth to mesoscale organization and how convective properties change depending on whether or not the convection is connected with a stratiform region. Combining the S-PolKa dataset with satellite data over the Indian Ocean shows how the convective and mesoscale features seen with S-PolKa are often actually part of larger subsynoptic scale organizations such as long lines and giant rings of convection on the scale of 1000 km or more. Detailed information on the hydrometeor type provided by the dual-polarimetric capability of S-PolKa indicates the physical mechanisms of precipitation growth in the warm (liquid phase) and cold (ice phase) parts of both convective and stratiform precipitation regions. The occurrence of graupel is indicated by these data, and when combined with lightning data provides useful information on the intensity of convective cells. The S-PolKa observations compared with DOE ARM vertically pointing radar data obtained near the S-PolKa site makes possible further interpretations of the S-PolKa data on nonprecipitating low clouds and anvil clouds of larger convective and mesoscale systems. The highly sheared environment over the Indian Ocean produces many anvils, which are part of the process of moistening the upper levels of the atmosphere in this region. Combining S-PolKa datasets with analysis of sounding data is revealing how the form of Indian Ocean convection relates to both the humidity and large-scale wind shear. The strong shear that predominates over the Indian Ocean (westerly wind and low levels, easterly components aloft) is generally unfavorable to convection organizing upscale. Soundings over the experimental array show that variations in the shear and humidity are connected at least partially with equatorial waves, and preliminary combined sounding and radar results suggest that larger mesoscale systems are favored when the shear decreases.