Impact of satellite-retrieved atmospheric temperature profiles assimilation on Asian summer monsoon 2010 simulation

Abstract

The impact of assimilation of satellite-retrieved atmospheric temperature profiles data in simulating mean monsoon circulation and rainfall of summer monsoon 2010 is examined in this study using the regional climate model, Weather Research and Forecasting model (WRF). Two experiments are performed; the first one is the control experiment (WRF-CTL) where no assimilation is carried out, and the second one is similar to the first one, but satellite-retrieved atmospheric temperature profiles are assimilated using a four-dimensional data assimilation method (WRF-AIRS). Mean monsoon features such as the low level jet, monsoon trough, tropical easterly jet, meridional pressure gradient and the spatial distribution of rainfall are better simulated in WRF-AIRS. Correlation coefficients between the observed and WRF-AIRS (WRF-CTL) daily zonal wind shear, meridional wind shear and rainfall indices over the Indian summer monsoon region are 0.98, 0.96 and 0.67 (0.32, 0.35 and 0.23), respectively. The zonal and meridional wind indices over the western Pacific and East Asia are 0.9 and 0.8 (0.6 and 0.5), respectively. Spatial distribution of rainfall displays double ITCZ like rainfall pattern over the tropical Indian Ocean in WRF-CTL, whereas WRF-AIRS display a single ITCZ pattern, which is similar to the observed one. The temporal evolution of the vertical structure of temperature (associated with rainfall activity over the monsoon core region) shows warming in the midtroposphere to upper troposphere (by 0.5 to 1.5 °C) and cooling in the midtroposphere to lower troposphere (by 0.5 to 1.0 °C). This atmospheric temperature distribution associated with rainfall is well simulated by WRF-AIRS. Under heavy rainfall conditions, WRF-AIRS produces strong vertical motion consistent with the observations but is absent (or weak) in WRF-CTL. This study deduces that the assimilation of temperature profiles in the regional climate model can significantly improve the dynamical and thermodynamical features of monsoon by representing the vertical distribution of temperature more realistically. Our analysis reiterates that the Asian summer monsoon circulation is mainly controlled by thermal forcing. Our study suggests that it is essential to improve the existing parameterization schemes for better simulation of summer monsoon.