The Thermodynamic Structure of the South Indian Monsoon

One of the rainiest areas in the South Asian monsoon region is the foothills of the western Ghats, a small mountain range running near and parallel to the west coast of India. The foothills of these mountains, and the eastern Arabian Sea just offshore, receive more than 250 cm in June-September, more than twice that of much of the rest of India. The processes controlling the spatial distribution of rainfall in this region are still poorly understood, in part because we lack detailed observations for hypothesis testing and model verification.

In June 2016, the joint UK-India INCOMPASS (Interaction of Convective Organisation with Monsoon Precipitation, Atmosphere, Surface, and Sea) project conducted an aircraft-based field campaign from Bangalore, in south India, in order to observe this key part of the monsoon system. Six flights were conducted over ten days following the same path from the rain shadow of Bangalore, over the western Ghats to the open Arabian Sea. These flights captured the zonal contrasts and how they changed as the synoptic environment evolved during the campaign. We also sampleds the diurnal cycle, with three flights during the day, two in the evening, and one at night.

We present an overview of these observations, and we explain the following features: a) a persistent drying of the low-level onshore monsoon flow before it reaches land, which occurs regardless of the details of the flow or the rainfall regime; and b) a standing gravity wave response to flow over orography. We also show that over the course of the campaign, the magnitude of rainfall offshore is controlled by two factors: the blocking of the flow and mid-tropospheric humidity.

Finally, we show comparisons between aircraft observations and model-based products, including ERA-Interim reanalysis, Unified Model analysis, and a high-resolution convection-permitting simulation.