Cloud structure and organization under suppressed conditions during DYNAMO/AMIE

Three multi-week active periods of the Madden-Julian Oscillation (MJO) were observed over the central Indian Ocean by the NCAR S-PolKa dual-polarized scanning Doppler radar and the ARM Ka-band KAZR vertically pointing Doppler radar during the 2011-2012 MJO field projects DYNAMO and AMIE. Between these periods, suppressed conditions were characterized by generally shallow convection that could be detected by the sensitive S-PolKa radar, with early echo exhibiting distinct polarimetric signatures. Using the Sband data, the organization and structure of the boundary layer clouds during this buildup phase of the MJO can therefore be described and related to the environmental conditions under suppressed conditions. The small nonprecipitating clouds often organized into lines parallel to the boundary layer wind shear before beginning to precipitate. Once precipitating, the cells send out cold pools triggering further nonprecipitating clouds, which can be traced by the radar. Thus, the boundary layer evolves from lines of clouds along the low-level shear to a boundary layer dominated by intersecting cold pools. The cloud lines at times exhibit distinctly positive differential reflectivity, which could be due to non-meteorological tracers such as birds or insects. The individual nonprecipitating clouds are shrouded by "mantle echoes" with distinct polarimetric signatures. The vertical reflectivity and vertical velocity structures of the nonprecipitating clouds are shown in detail by the vertically pointing KAZR. As the convective population matures, cold pool interactions (collisions) led to initiation of deeper convection, which generated wider and more robust cold pools, which generated more deep echo. This behavior of the boundary-layer, nonprecipitating convection has never been documented in a tropical oceanic environment and is an important step in the development of MJO convection. Relating these boundary-layer nonprecipitating clouds to the large-scale environment will be a key step in achieving the long-term goals of DYNAMO and AMIE, which aimed to development understanding of the development of the convective population in the MJO. Without an understanding of the nonprecipitating cloud development leading to the development of precipitating clouds, the desired understanding will be incomplete.