Observations and simulations of multiscale convective systems in the south Asian summer monsoon

Yi-Chi Wang, Purdue University

Abstract

Multiscale convective systems are found to be ubiquitous in the tropical climate systems such as the south Asian summer monsoon. Involved with a wide range of scales, multiscale convective systems are beyond the synoptic observation network, and present a serious challenge for operational and climate global models because of the inevitable scale truncation used in cumulus parametrization. This dissertation studies multiscale convective systems in the south Asian summer monsoon with satellite observations and regional mesoscale modeling. The observational part of this study utilizes the TRMM 3B42 multisatellite rain estimate (40°S-40°N, 3 hourly, 0.25x0.25 degree resolution) to characterize the diurnal signals and the long-range dependence of the tropical rainfall. The long-range dependence within the rainfall time series implies relations between multiple time scales, suggesting the existence of the multiscale convective systems. Geospatial patterns of the long-range dependence are found to be complementary with patterns of the diurnal rainfall cycle. Prominent long-range dependence and weak diurnal rainfall amplitude are found over the open oceans, suggesting oceanic rainfall variability is likely to attribute to precipitating processes over a wide range of scales, rather to a single-scale process. The modeling study of this dissertation focuses on the multiscale convective systems and associated scale interactions in the south Asian summer monsoon. The Weather Research and Forecasting model (WRF) is used to simulate a case of a deep monsoon depression (MD) occurred over the Bay of Bengal during August 1–6, 2006. The impacts of moist convection on the MD are studied with a physics-based ensemble. It is found that the ability of convection schemes (including cumulus and microphysics schemes) to effectively trigger and maintain deep convection of the convective southwest quadrant of the MD is essential for better MD hindcasts. Cloud-system-resolving modeling is found to be a promising approach that resolves realistic core structures and the coupling between convection and MD circulations. WRF experiments are also conducted to study the biases induced by a hypothesis on cloud-radiative forcing in the south Asian summer monsoon for August 2006. Compensating biases due to the hypothesis include the enhancement of the convective activities around the monsoon trough, the prolonging of the MD life cycle, and the broadening of the MD size and anvil clouds. Results presented in this study demonstrate the significance of representations for moist convection and associated scale interactions for predictions of multiscale convective systems and the south Asian summer monsoon.

Degree

Ph.D.

Advisors

Harshvardhan, Purdue University, Wen-wen Tung, Purdue University.

Subject Area

Atmospheric sciences

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