Tropospheric humidification and cloud microphysical structure observed over the Indian Ocean during AMIE

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Description

The interaction between the tropical cloud population and the large-scale environment of the central-equatorial Indian Ocean is investigated in the context of the Madden-Julian Oscillation (MJO) during the 2011–12 ARM MJO Investigation Experiment (AMIE) field campaign. Rawinsonde measurements taken every three hours at the ARM Mobile Facility (AMF) deployment on Gan Island provide an extremely detailed view of the variability in tropospheric humidity that occurs on 2–6 day timescales, as well as the longer intraseasonal (about 30 days) timescale associated with the MJO. Combining rawinsonde data with observations from the National Center for Atmospheric Research’s (NCAR) S-PolKa scanning precipitation radar shows that the heights of convective echo tops rise rapidly at about the same time that the depth of an anomalously moist layer in the mid-troposphere increases rapidly. The near simultaneous build-up of clouds and humidity occurs over a period of 3 to 6 days just prior to or at the onset of an MJO-scale convective outbreak, suggesting that pre-humidification of the troposphere over timescales of more than a week is not necessary for a deep convective phase of an MJO to form. Analysis of the upper-tropospheric wind field using the rawinsonde measurements and reanalysis strongly suggest that propagation of an equatorial Kelvin wave into the central Indian Ocean region played a decisive role in cloud development by making the upper-level environment conducive to active convection, which in turn, enhanced the environmental humidity. Data from S-PolKa, combined with ARM vertically pointing Ka-band ARM zenith radar (KAZR) observations, allow for the microphysical characteristics of non-precipitating and precipitating clouds to be investigated, providing a context to describe the feedback on the surrounding environment through tropospheric moistening, heating, and radiative feedbacks.