Precipitation microstructure in different Madden–Julian Oscillation phases over Sumatra

• Precipitation during the inactive MJO phase is more continental in nature.
• The inactive phase has a deeper storm, a higher reflectivity aloft.
• More lightning and less stratiform characteristics during the inactive phase
• Higher concentration of medium and large-size drops during the inactive phase
• Larger reflectivity during the inactive phase, at the same rainfall rate

Intraseasonal variations of precipitation and its microstructure are investigated using measurements of the Equatorial Atmospheric Radar (EAR) facilities at Kototabang, west Sumatra, Indonesia (0.20°S, 100.32°E, 864 m above sea level). Raindrop size distribution (DSD) observations are obtained from a 2D-Video Disdrometer (2DVD) with a near continuous record of operation over eight consecutive years (2003–2010). Precipitation types are classified using 1.3-GHz wind profiler observation, and are partitioned according to active and inactive convective phases of Madden–Julian Oscillation (MJO). It is found that precipitation systems during the inactive phase are more continental in nature than those during the active phase. Cloud propagation from brightness temperature data indicates that Sumatra receives the rainfall mainly from maritime clouds during the active phase, while it is mainly from the continental clouds (land-based convection) during the inactive phase. Other remarkable differences between active and inactive phase precipitation systems are also observed from the vertical structure of precipitation. The precipitation during the inactive phase has deeper storms, a higher reflectivity aloft, more lightning activity and less stratiform characteristics, as compared to the active phase. Assessment of cloud effective radius of the Moderate Resolution Imaging Spectroradiometer (MODIS) data also shows a slight difference in the cloud droplet between the active and the inactive MJO phases. Different convective storms in different MJO phases lead to different DSD characteristics and Z–R relationships. The DSD during the inactive phase tends to have a higher concentration of medium and large-size drops than the active counterpart, consistent with the previous study during the first campaign of Coupling Processes in the Equatorial Atmosphere project. Although the DSD parameters and coefficient of Z–R relationships fall within the range of tropical maritime precipitation, mass-weighted mean diameter (Dm) for the deep convective rains during the inactive phase are somewhat larger than that for maritime and closer to the continental cluster. Therefore, continental-like DSDs are somewhat dominant during the inactive phase, consistent with the intraseasonal variation of precipitation structure. The causative processes of the observed difference in the DSD for the two phases have also been discussed with the help of satellite and radar data. Evaporation and updraft associated with the intense convection during the inactive phase seem to eliminate the small-sized drops from the spectra. Finally, radar reflectivity during the inactive phase is larger than that during the active MJO phase, at the same rainfall rate. This condition can limit the accuracy of radar-derived rainfall estimates for the tropics when applying a single Z–R relation to the two MJO phases, particularly for deep convective rains.