Monsoon low-pressure systems over India: a climatology study and high-resolution climate sensitivity simulations

Monsoon low-pressure systems (LPS) are thought to be one of the most rain-bearing synoptic scale systems that develop during the Indian monsoon. Because of the large socioeconomic impact these cyclonic storms may have on the Indian community, it is important to understand their dynamical and thermodynamic structure. A cyclone tracking algorithm has been used to detect the monsoon LPS in the ERA-Interim reanalysis for the time period 1979-2010. By using both the ERA-Interim precipitation and the Indian Meteorological Department (IMD) daily gridded rainfall, the LPS that generated an extreme rainfall event, defined by the 99.5 percentile, is picked out, and analyzed. Composite and statistical analysis has been preformed on different ERA-Interim parameters that has been chosen based on a simple description of the precipitation. The general structure of the LPS is seen in the different composite, with a pronounced cold core at lower levels and warm core aloft. The results from the composite also suggest that it is the latent heat release that induces the strong upward motion, and is thus important for driving the monsoon LPS. By performing a stepwise linear regression we show that we can describe a large part of the precipitation variability within a monsoon LPS, with only a few meteorological parameters. The sensitivity to a change in one of the meteorological parameters is also investigated, and the precipitation shows largest sensitivity to a change in the temperature at 950hPa, and to the vertical velocity and specific humidity at 750hPa. Next, 10 of the monsoon low-pressure systems detected by the tracking algorithm, is picked out and simulated with the high resolution Weather Research and Forecasting (WRF) model. The horizontal resolution is 4km. For each case, 3 different simulations are performed: one control run, where the initial and lateral boundary conditions are from the ERA-Interim reanalysis; and two perturbed runs. The method for generating the perturbed runs are done in the same way as for the surrogate climate change simulations presented in previous studies. Then the sea surface temperature (SST), atmospheric temperature (on pressure levels), surface and soil temperature has been increased uniformly with 2K and 4K. The aim of these surrogate climate change simulations is to investigate how sensitive the precipitation associated with the monsoon LPS is to a changing climate, and preliminary results suggest that the precipitation is changing with the atmospheric temperature according to the Clausius-Clapeyron relation.