Introduction

Meteorology is a discipline concerned with observational earth sciences and theoretical physics. From a theoretical point of view, it has to deal with a turbulent fluid whose behaviour is governed by a complex set of nonlinear, partial differential equations, which model the atmosphere as thermo-hydrodynamical system obeying the laws of an ideal gas. As a branch of observational earth sciences, it has the task of providing an accurate knowledge of the state of the atmosphere, which can only be obtained through regular, simultaneous observations covering the whole globe from the earth surface to the upper atmosphere.

Progress on the observational side other than the surface was recently limited to a network of balloon sounding stations covering practically the whole globe, albeit sparsely. The data obtained through this network permitted the discovery of previously unknown characteristics of the atmospheric motions and provided for the first time a solid basis for work for the theoreticians. Since the introduction of mathematical models and high-speed computers,

Satellite Remote Sensing and GIS Applications in Agricultural Meteorology pp. 67-79

there has been a growing demand for adequately sampled (in space and time) and reliable observational data, since the forecast models are strongly dependent on assumed initial state of the atmosphere, as there are large areas without any conventional soundings of the atmosphere.

Before the advent of weather satellites the weathermen had been severely handicapped by having only a very limited knowledge of the state of the atmosphere at any given time. Even with the expansion of observational networks since the last world war, by various national meteorological services, the vast sparsely populated land areas of the globe and the large oceanic areas are virtually blank as far as conventional meteorological observations are concerned. Meteorological satellites have to a large extent has enabled to overcome this deficiency.

Satellite imagery is an invaluable source of information for operational forecasters. It is being used as (a) an analysis tool, especially to data sparse regions like the tropics; (b) direct aid to short period forecasts (6-12 hours ahead of cloud, rainfall, floods etc.; (c) input to numerical weather prediction models (NWP) for defining initial condition; (d) monitoring the model forecast. It also serves as a valuable indicator of dynamical and physical process at work providing the trained eye, some useful clues on atmosphere structure and its evolution. The sea-surface temperature, the sea surface/upper air winds from scatterometer/cloud motion vectors provide valuable input to numerical models. The rainfall from geostationary satellites, rain rate from microwave sensors, the OLR (Outgoing Longwave Radiation) from polar orbiting satellites are a few parameters which are frequently utilized in the initialization of the data for numerical weather prediction of monsoon.

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