Cloud organization and growth during the transition from suppressed to active MJO conditions

Abstract

AbstractDuring the Dynamics of the Madden‐Julian Oscillation/Atmospheric Radiation Measurement Madden‐Julian Oscillation (MJO) Investigation Experiment field experiment in the Indian Ocean, the National Center for Atmospheric Research dual‐polarimetric S‐ and Ka‐band radar (S‐PolKa) radar observed three active Madden‐Julian Oscillation (MJO) events. These events were separated by suppressed periods characterized by shallower, more isolated convection and relatively little rainfall. The sensitivity of S‐PolKa allowed investigation of the initiation and organization of both nonprecipitating and precipitating clouds. Early in the suppressed periods, shallow nonprecipitating clouds occurred in shear‐parallel lines along apparent boundary layer rolls during early morning. Once some of the clouds began to precipitate, small cold pools formed below the showers. By afternoon, the lines all but disappeared with nonprecipitating clouds instead forming along the edges of cold pools. All such convection was limited in depth early in suppressed periods. As the suppressed environment gained moisture, the nonprecipitating clouds were able to grow to larger size, with the deepest precipitating clouds occurring in clusters at intersections of cold pool boundaries by afternoon. Upscale growth into mesoscale convective systems was observed as the suppressed periods transitioned into active MJO phases, contributing to overnight precipitation during the later part of the suppressed period. This study demonstrates the need for models to accurately represent the organization and evolution of nonprecipitating clouds in association with boundary layer dynamics under suppressed conditions of the MJO, prior to the occurrence of precipitating clouds and their cold pools.