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

Abstract

During 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.

Evolution of Precipitation Structure During the November DYNAMO MJO Event: Cloud‐Resolving Model Intercomparison and Cross Validation Using Radar Observations

“…Figures c and d are for the C‐band observations and model simulations using Revelle forcing. In all four panels, precipitation feature sizes increase with increasing echo‐top heights and surface rainfall rates, a trend also described for all three DYNAMO MJO events by Rowe and Houze (). The total numbers of features (shown in red dots in Figure ) tend to increase with feature size, too, although increases in feature numbers usually occur ahead of increases of feature sizes.…”

Upper Ocean Response to the Atmospheric Cold Pools Associated With the Madden‐Julian Oscillation

“…Recent studies suggest that atmospheric cold pools play an important role in organizing convection during the MJO development (e.g., Chen et al, 2016;de Szoeke et al, 2015de Szoeke & Edson, 2017;Feng et al, 2015;Rowe & Houze, 2015;Schlemmer & Hohenegger, 2014;Skyllingstad & de Szoeke, 2015). Atmospheric cold pools observed during the DYNAMO field campaign generated a substantial SST change through altering air-sea fluxes.…”

“…They form as convective downdrafts caused by evaporation of rainwater that originates from above the atmospheric boundary layer. Thus, the adequate representation of the multiscale interaction processes associated with cold pools in models may lead to the improvement of MJO prediction DeMott et al, 2015;Feng et al, 2015;Qian et al, 1998;Rowe & Houze, 2015;Schlemmer & Hohenegger, 2014;Seigel & van den Heever, 2011;Tompkins, 2001;Torri et al, 2015). Small cold pools sometimes are combined to form mesoscale features and typically last less than one day and span 10-200 km in diameter (Barnes & Garstang, 1982;Saxen & Rutledge, 1998;Young et al, 1995;Zipser, 1977;Zuidema et al, 2012Zuidema et al, , 2017.…”

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

“…Atmospheric cold pools over oceans are receiving attention for their ability to reorganize the mesoscale cloud distributions and potentially facilitate the transitions from high-to low-albedo shallow cloud cover. In the Tropics, the expansion of the spatial and height distribution of convection by cold pools may facilitate transitions from shallow to deep convective regimes, and therefore the eastward propagation of the Madden-Julian oscillation (MJO) into moistening environments (Rowe and Houze 2015;Feng et al 2015;Ruppert and Johnson 2015;Schlemmer and Hohenegger 2016;Hannah et al 2016;Ciesielski et al 2017).…”