Comparison of Mesoscale Convective Systems in a Seasonal Convection-Permitting Simulation With Observations Over the Indian Monsoon Zone

The largest type of deep convection, mesoscale convective systems (MCSs), regulate changes in the hydrological cycle and large-scale tropical circulation. During the Indian summer monsoon (June-September), synoptic-scale systems move across the monsoon zone, causing MCSs to form frequently. MCSs cause widespread and heavy rain throughout the monsoon zone. Past MCS studies in India used either observations or simulation in a short period or with case studies approach. Studies on structure and evolution of MCSs highlighting the organization of convection over the monsoon zone are lacking.

In this study, a 4-month, convection-permitting simulation is conducted over the Indian monsoon zone using the Weather Research Forecast (WRF) model with 4-km grid spacing and two microphysics parameterizations and is compared with observations to evaluate composite MCS characteristics and microphysics sensitivities. We first apply a cloud-tracking algorithm to two high-resolution observation data sets, NASA global merged infrared brightness temperature (IR Tb) and GPM IMERG surface precipitation to identify and track individual MCS events during monsoon. Ground-based S-band radar observations are used to examine the 3-D structures of storms embedded within the tracked MCSs and analyze evolution of convective, stratiform and anvil components of the MCSs. A similar cloud-tracking algorithm is then applied to WRF simulated data (radar reflectivity, IR Tb and precipitation) to identify and track MCS in model simulation. As a result, the observed and simulated MCSs are consistently identified and tracked, making it possible to compare WRF MCS population statistics with observed MCSs.

Results show that the properties of MCS including composite evolution, and frequency distribution are reasonably captured by the two simulations with some noticeable differences. In general, the Thompson simulation produces better agreement with observations for convective area and precipitation amount, MCS propagation speed but exhibits underestimation of stratiform area. The composite evolution of simulated MCS cloud and precipitation structures showed a gradual increase from convective initiation to around the first half of the MCSs lifetime, which was consistent with observations. The MCS eccentricity reaches to minimum value at maximum horizontal extent, indicating a quasi-circular shape of MCS. We observed that PDF of MCS precipitation intensity largely agrees well with observations. The highest altitude reached by intense convective cores (30-dBZ echo-tops) is 8 km, but the model significantly underestimates it. The detailed comparison of multiple aspects of MCSs (e.g., initiation, size, intensity, lifetime, propagation) and embedded storms (e.g., convective-stratiform areas) and associated precipitation between the simulation and observations for one monsoon season will be discussed.