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Sreerama M. Daggupaty and Dev R. Sikka

Abstract

This paper discusses the results obtained from a diagnostic study of a monsoon depression which formed in the northern part of the Bay of Bengal. The depression, while intensifying, progressed westward across India with a speed of about 5° longitude per day. The computed vertical velocity is in good agreement with the observed asymmetric distribution of rainfall around the depression. The presence of a low level of non- divergence (i.e., around 850 mb) is found to have a significant role in the dynamics of the monsoon depression.

The important result of the computed vorticity budget over the period of the intensification of the depression is the detection of a middle and upper tropospheric cyclonic vorticity depletion due to large-scale dynamics in the western sector of the depression. This result is rather unexpected because of the fact that the depression's observed cyclonic vorticity increases, not only in the lower troposphere but also in the middle and upper troposphere while progressing westward. It has been shown that the presence of deep convective cloud activity in the western sector provides the necessary process to compensate the negative vorticity tendency in the middle and upper troposphere. Through a simple parameterization it has been shown qualitatively that the transport of subgrid-scale vorticity by deep convective clouds in the western sector is significant.

This mechanism of vertical transport of extremely rich boundary. layer cyclonic vorticity by deep convective clouds is found to be essential for the intensification as well as for the westward movement of the monsoon depression.

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T. N. Krishnamurti, Anu Simon, Aype Thomas, Akhilesh Mishra, Dev Sikka, Dev Niyogi, Arindam Chakraborty, and Li Li

Abstract

This study addresses observational and modeling sensitivity on the march of the onset isochrones of the Indian summer monsoon. The first 25 days of the passage of the isochrones of monsoon onset is of great scientific interest. Surface and satellite-based datasets are used for high-resolution modeling of the impact of the motion of the onset isochrones from Kerala to New Delhi. These include the asymmetries across the isochrone such as soil moisture and its temporal variability, moistening of the dry soil to the immediate north of the isochrone by nonconvective anvil rains, and formation of newly forming cloud elements to the immediate north of the isochrone. The region immediately north of the isochrone is shown to carry a spread of buoyancy elements. As these new elements grow, they are continually being steered by the divergent circulations of the parent isochrone to the north and eventually to the northwest. CloudSat was extremely useful for identifying the asymmetric cloud structures across the isochrone. In the modeling sensitivity studies, the authors used a mesoscale Advanced Research Weather Research and Forecasting Model (ARW-WRF) to examine days 1–25 of forecasts of the onset isochrone. Prediction experiments were first modeled during normal, dry, and wet Indian monsoons using default values of model parameters. This study was extended to determine the effects of changes in soil moisture and nonconvective rain parameterizations (the parameters suggested by the satellite observations). These sensitivity experiments show that the motion of the isochrones from Kerala to New Delhi are very sensitive to the parameterization of soil moisture and nonconvective anvil rains immediately north of the isochrone.

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