Connecting Small-scale Humidity Variability to Dynamical Processes Governing Madden-Julian Oscillation Transitions

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This project studies the role of low-level atmospheric moisture in the development of convective clouds associated with the Madden-Julian Oscillation (MJO). The MJO is an envelope of organized convection that propagates from the Indian Ocean to the Pacific Ocean and constitutes the dominant form of intraseasonal precipitation variability in the tropics, and also has important consequences for US weather. The present work is a follow-on to the Dynamics of the MJO (DYNAMO) field campaign, which took place in the equatorial Indian Ocean in 2011-2012 and involved the deployment of radars and other observing assets on Gan Island in the Maldives. One hypothesis of the campaign, also pursued in this project, is that the deep, organized convective clouds associated with MJO initiation develop only when the atmospheric boundary layer, which contains most of the moisture in the atmospheric column, becomes sufficiently deep over a sufficiently large region. The primary task of the present work is to generate humidity datasets using radar and scanning radiometer data collected as part of the campaign, through techniques involving 1) the Bragg scattering of S-band radar signals in cloud-free air; and 2) a combination of radiometer data and the differential attenuation of radar returns from clouds at two wavelengths (S- and Ka-band in this case). The analysis of these humidity products will be complemented by numerical experiments carried out with the Weather Research and Forecasting (WRF) model.The work has broader scientific impacts due to the value new techniques for determining humidity, particularly techniques that can be applied in future field campaigns, as well as the value of the new humidity datasets for the research community. In addition, the work seeks a better understanding of the initiation of MJO, which would be of societal value to the extent that it results in better predictions of the MJO and its downstream consequences. In addition, the work has an educational component, including the use of the datasets in classroom teaching and the support of two graduate students.