Dear D. Singh,

There are three main conservation laws in Numerical Weather Prediction (NWP) models:

- The law of conservation of mass implies that no air mass is created or destroyed in the process of weather forecasting. Hence air can be moved around, and in particular accumulate in one area, but then there must be less air somewhere else. Conversely, not all regions on the planet can suddenly experience cyclogenesis (dropping air pressure). The same law applies to other geochemical species such as water, carbon, sulfur, etc., though specific sources and sinks may need to be accounted for to reflect transformations and processes that affect the total amounts available in the atmosphere.

- The law of conservation of energy is crucial in physics in general and in NWP in particular. Here also, energy can't be created of destroyed: it can only be transformed from one form into another form. If air is moving up a slope in a mountainous area, then some of the kinetic energy is transformed into potential energy (the wind speed must slow down). Or whenever solar radiation is absorbed in an air layer, it must cause a change of temperature (heating), etc.

- The laws of conservation of linear and angular momentum are essential in predicting the evolution of wind systems and storms. Both quantities are subject to the laws of motion (in fluids), and constrain how weather systems evolve.

These constraints are essential to prevent the appearance of unrealistic solutions. You may want to study the book by

James R. Holton (2004) 'An Introduction to Dynamic Meteorology', originally published as Volume 88 in the International Geophysics Series of Academic Press, and now made freely available on the Web by Elsevier at

http://www.sciencelib.net/files/Holton%20-%20Introduction%20to%20Dynamic%20Meteorology%204e%20%28Elsevier%2C%202004%29.pdf

and in particular Chapter 2, to get further information on this topic. Another excellent book on this matter, though perhaps harder to get hold of, is the monograph by

Edward N. Lorenz (1967) 'The Nature and Theory of the General Circulation of the Atmosphere', World Meteorological Organization (WMO), Geneva, Switzerland.

You will find a scan of the first 24 pages of that book here:

http://eaps4.mit.edu/research/Lorenz/General_Circ_WMO_1967_Part1.pdf

Best regards, Michel.

Dear Dr. Singh,

The most concise description of the problem is in the book by professor Lynch: The Emergence of Numerical Weather Prediction: Richardson Dream. The pertinent information is at

http://mathsci.ucd.ie/~plynch/Dream/Dream.html

I'm sure that you will enjoy this excellent and highly informative book.

Best Regards,

Janusz

I think you can read very basic and useful information on the following website

http://www.meted.ucar.edu/nwp/model_structure/print.htm