The objective of this NATO Advanced Research Workshop was to discuss our current understanding of the role of clouds in climate and chemistry. The range of topics dis cussed during the workshop included: modeling of clouds in GCMs; observations of the cloud micro physical properties; the water vapor cycle; troposphere-stratosphere exchange; role of in-cloud transport in tropospheric ozone; regulation of current and paleo climate by clouds; and anthropogenic sulfate aerosols and modification of cloud properties. The essence of the discussions is captured in the accompanying summary by the rapporteurs and the chapters by some of the speakers. The underlying message is that significant progress has been made, resulting in exciting new developments in our perception of the role of clouds in the global system . .. The tropical convective-cirrus cloud systems emerge as a major influence on the climate system. Micro physical processes, such as the rate of precipitation and re evaporation of ice particles, seem to regulate the large scale vertical distribution of water vapor which is the dominant greenhouse gas. Water vapor data collected during the Central Equatorial Pacific Experiment (CEPEX), document the large moistening effect of deep convection on scales of thousands of kilometers. A major chemical finding in the same region was the observation of extremely low ozone 8 volume mixing ratios of less than 10- in the entire troposphere of the central equatorial Pacific extending over a distance of about 2000 km. This finding establishes the Pacific as a major chemical sink region for tropospheric ozone.

Water is the most effective agent in the climate system to modulate energy transfer by radiative processes, through its exchanges of latent heat and within cascades of chemical processes. It is the source of all life on earth, and once convective clouds are formed, it enables large vertical transports of momentum, heat and various atmospheric constituents up to levels above the tropical tropopause. Water triggers very complex processes at the earth's continental surfaces and within the oceans. At last, water in its gaseous phase is the most important greenhouse-gas! Numerical modelling and measurements of the state of the present climate system needs a very thorough understanding of all these processes and their various interactions and forcings. This is a prerequisite for more substantial forecasts of future states in all scales of time, from days to centuries. Therefore, the management of the World Climate Research Programme established in 1988 the new programme GEWEX (Global Energy and Water Cycle Experiment). GEWEX is specifically defined to determine the energy and water transports in the fast components of the climate system with the presently available modelling and measurement means and to provide new capabilities for the future. Research in GEWEX must further develop methods to determine the influence of climatic anomalies on available water resources.

This volume covers aspects of numerical modeling of the atmosphere and climate from the microscales of turbulence to the very large scales associated with climate and climatic change. Each of the three major spatio-temporal scales of the atmosphere, namely, the microscale, the mesoscale, and the macroscale is addressed through a hierarchy of models. Results of model simulations are illustrated throughout the text, with many of these examples based on the author's original research work. For each type of model discussed here, the theoretical background, including governing equation sets, simplifying assumptions, and advantages and limits of the models, is provided. The topic of coupled, or nested, modeling systems as a promising approach to air pllution embedded in regional atmospheric flows, as well as to the regional atmospheric response to global climate forcings, is also addressed. An attempt is made throughout the book to highlight the highly interdisciplinary nature of atmospheric modeling, particularly in those sections dealing with climatic change issues.

Current climate models diverge in their assessment of global warming that will result from the anthropogenic increase in trace gases. This is because they differ in their representation of the hydrological cycle (water vapour, clouds, snow and sea ice, soil moisture) and because a direct validation in terms of sensitivity is not possible. Indirect methods and approaches are therefore necessary to verify the models efficiently. The book provides an overview on different validation approaches. The use of satellite data is particularly stressed.