Sensitivity studies on the structure of the monsoon desert heat flow.

Abstract

The sensitivity of the atmospheric response to large-scale summer continental sensible heat sources, such as those prevailing over the great deserts of the Afro-Asian landmass, is studied in this thesis. Understanding this sensitivity is essential to determine the role played by these heat sources in the behaviour of the Indian southwest monsoon. The present thesis is a theoretical study of this problem.

Chapter 1 discusses qualitatively the importance of such a study. Observed features over the centre of the Great Indian Desert (Thar), situated around 27°N and 70°E, are presented. Two main conclusions are drawn:

The sensible heating rate during summer over desert heat source regions is about 1.5°C per day, occurring mainly in the lower troposphere.

Lower pressure prevails over the lower tropospheric warm regions up to the 800 mb level, while higher pressure prevails above 800 mb during summer.

Some theoretical findings regarding the structural response of the atmosphere to such heating intensity and other parameters, such as the zonal mean wind, are presented in successive chapters.

Chapter 3 presents a barotropic wind model consisting of solutions for pressure perturbations and vertical velocity derived from basic linear steady-state quasi-geostrophic equations in the (x, y, z) system, where the vertical coordinate is ?? = ln ? ( ?? ?? / ?? ) . Solutions are obtained using multiple Fourier transforms. A heating profile with Gaussian distribution in latitude-longitude directions and an exponentially decaying function in the vertical is assumed. The rate of heating at the centre of the source is prescribed as 1.5°C per day.

Chapter 4 discusses numerical results of the model described in Chapter 3 for all sensitivity studies performed with mean winds and various heating profiles.

Chapter 5 introduces a baroclinic mean wind model in which vertical wind shear is incorporated by fitting u-profiles nearly coincident with observed profiles over the source. Solutions for pressure perturbations are given.

Chapter 6 analyzes results of the baroclinic mean wind model, considering three u-profiles with surface winds ?? = 0.2 , 1.0 , and 2.0 ? ?? ? ?? ? 1 . The vertical structure of the low is found to depend on vertical heating of the atmosphere over the source. The vertical structure, with a low extending from the surface to 800 mb and a high above it, is close to reality. Contrasts with the barotropic case are highlighted.

Chapter 7 concludes the thesis.