The cloud population and onset of the Madden‐Julian Oscillation over the Indian Ocean during DYNAMO‐AMIE

Abstract Variability of the cloud population in the central equatorial Indian Ocean was observed in the context of the Madden‐Julian Oscillation (MJO) during the Dynamics of the Madden‐Julian Oscillation (DYNAMO) and Atmospheric Radiation Measurement Madden‐Julian Investigation Experiment (AMIE) field campaigns. Radar observations from the polarimetric S‐band radar on Addu Atoll in the Maldives characterize the types of convective and stratiform radar echoes and the heights their 20 dBZ contours reach. To gain insight into the relationship between clouds and humidification of the troposphere leading up to and during an active MJO event, the work relates variability of the observed precipitation structure to that of tropospheric humidity and upper level zonal wind. The variability in stratiform precipitation areas dominates variability in the nature of precipitating convection associated with the MJO. Areal coverage of precipitating radar echo, convective echo top height, and tropospheric humidity above 850 hPa rapidly increase over ~3–7 days near MJO onset. This rate of increase is substantially faster than the 10–20 days needed for buildup of moisture prior to MJO onset as hypothesized by the “discharge‐recharge” hypothesis. Convective echoes become more common during the days prior to MJO onset, and the increased convection occurs before low‐tropospheric moistening. The upper troposphere rapidly moistens as the first widespread stratiform region passes over an area. Thus, clouds likely play a role in tropospheric humidification. Whether increased low‐tropospheric humidity causes vertical growth of convection has not yet been determined.