RN-33 : Atmospheric general circulation models

Abstract

Hydrological cycle encompasses in itself two major processes the atmosphere phase and the ocean-land surface phase. Mathematical and conceptual modelling of these two processes independently have been accomplished for a long time now combined models for the representation of the hydrological processes and atmospheric general circulation have been in the process of development since the last 15 years.

Precipitation is the most important climatic element for hydrological processes. The General Circulation Models (GCMs) simulate precipitation as a result of large scale convective motion. Several atmospheric GCMs have been integrated in the10 to 15 years and have shown themselves capable of simulating not only the average seasonal oscillation but also inter annual variability.

Some GCMs have also been made operational by India Meteorological Department and Indian Institute of Technology, Delhi for prediction of weather from 2 to 7 days in advance. Attempts have also been going on for introducing further sophistication into combined Atmospheric General Circulation Land surface process models.

The atmospheric general circulation is a largest and most persistent scale of motion and serves to transport the hot air from the equatorial region .toward poles and to maintain return flow of cold air from polar to tropical latitudes.

The GCMs synthesize the large scale distribution of the climate by application of the physical laws governing the atmospheric structure and behaviour. With realistic boundary conditions, the GCMs were successful in reproducing the continental - scale features of the observed distribution of temperature and pressure and other circulation patterns. Some of the GCMs developed for tropical regions were able to simulate the seasonal shifts of surface temperatures and precipitation which accompany the monsoons.

A review of the available general circulation models is carried out in this note with a view to identify such models which have the necessary space-time structure and requisite output so that they could be coupled with suitable hydrological models.

A number of regional and general circulation models made operational in India with various constraints and forcing introduced into them are reviewed in particular.