The spatiotemporal structure of precipitation in Indian monsoon depressions

Indian monsoon depressions are synoptic scale events typically spun up in the Bay of Bengal. They usually last 4–6 days, during which they propagate northwestward across the Indian subcontinent before dissipating over northwest India or Pakistan. They can have a significant effect on monsoon precipitation, particularly in primarily agrarian northern India, and therefore quantifying their structure and variability and evaluating these in NWP models and GCMs is of critical importance. In this study, satellite data from the CloudSat and recently concluded TRMM missions are used in conjunction with an independently evaluated tracking algorithm to form a three-dimensional composite image of cloud structure and precipitation within monsoon depressions. The composite comprises 34 depressions from the 1998–2014 TRMM mission and 12 from the 2007-present CloudSat mission, and is statistically robust enough to allow significant probing of the spatiotemporal characteristics of moisture and hydrometeor fields. Among the key results of this work are the discovery and characterisation of a bimodal, diurnal cycle in surface precipitation; the first picture of the structure of cloud type and density in depressions, showing that deep convection dominates south of the centre and prominent cirrus throughout; the first composite picture of vertical hydrometeor structure in depressions, showing significant precipitation for hundreds of kilometres outside the centre and well past the mid-troposphere; and novel discussion of drop size distributions (showing significant uniformity across the depression) and resulting latent heat profiles, showing average heating rates near the centre can reach 2K/hr.