Two-day Convective Disturbances in the Equatorial Indian Ocean

Abstract:

Quasi two-day convective disturbances were observed in the Madden-Julian Oscillation (MJO) convectively active period in the equatorial Indian Ocean during the Dynamics of the MJO (DYNAMO) field campaign in 2011.

The initial focus of the study is on seven significant precipitating events at Gan in October having two-days periodicity identified using TRMM 3B42(V7) rainfall data. In this study, gridded observations, TRMM rainfall and Meteosat-7 IR brightness temperature datasets were analyzed, the time-longitude diagrams and the composite analyses show that the two-day periodicity is related to westward propagating convection with propagation speed ~12m/s and zonal spatial scale ~2000km.

In order to examine the vertical structure of the two-day convective disturbances, high-vertical resolution upper-air sounding data and the combined KAZR/S-Pol radar data (only available at Gan Island, 0.69°S, 73.15°E) from DYNAMO were also used to construct composite fields over a 48-hour period centered at the maximum rain rate of these precipitating events.

The composited moisture, stability, temperature anomaly and cloud radiative effect reveal a distinct pattern of convective evolution – shallow convection to deep convection to stratiform precipitation – similar to that observed on longer time scales all the way up to that of the MJO. These results indicate several characteristics of two-day disturbances over the equatorial Indian Ocean, which can also be found in the western Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE).