2. ATMOSPHERIC CIRCULATION 



Modeling the Global Atmospheric Circulation 



An understanding of the structure 

 and variability of the global atmos- 

 pheric circulation requires a knowl- 

 edge of: 



1. The quality and quantity of 

 radiation coming from the sun. 



2. The atmospheric constituents — 

 not only the massive ones, but 

 also such thermodynamically 

 active components as water va- 

 por, carbon dioxide, ozone, and 

 clouds as well as other partic- 

 ulates. Furthermore, one must 

 understand the process by 

 which these constituents react 

 with the circulations and their 

 radiative properties — i.e., ab- 

 sorption, transmission, scatter- 

 ing, and reflection. 



3. The processes by which the 

 atmosphere interacts with its 

 lower boundary in the trans- 

 mission of momentum, heat, 

 and water substance over land 

 as well as sea surfaces. The 

 behavior of the atmosphere 

 cannot be considered independ- 

 ent of its lower boundary be- 

 yond a few days. In turn, the 

 lower boundary can react sig- 

 nificantly. Even the surface 

 layers of the oceans have im- 

 portant reaction times of less 

 than a week, while the deeper 

 ocean comes into play over 

 longer periods. Hence, the 

 evolution of the atmospheric 

 circulation over long periods 

 requires consideration of a dy- 

 namical system whose lower 

 boundary is below the earth's 

 surface. 



4. The interactions of the large- 

 scale motions of the atmos- 

 phere with the variety of 



smaller-scale motions normally 

 present. If these smaller scales 

 have energy sources of their 

 own, as is the case in the at- 

 mosphere, the nature of the 

 interactions will be consider- 

 ably complicated. 



In principle, mathematical models 

 embodying precise statements of the 

 component physical elements and 

 their interactions provide the means 

 for numerically simulating the nat- 

 ural evolution of the large-scale at- 

 mosphere and its constituents. Suc- 

 cessful modeling would have potential 

 applications in a number of areas: 

 long-range forecasting; determination 

 of the large-scale, long-term disper- 

 sion of man-made pollutants; the 

 interaction of these pollutants in in- 

 advertently altering climate; the in- 

 fluence of intentionally tampering 

 with boundary conditions to arti- 

 ficially modify the climate equilib- 

 rium. No doubt there are a variety 

 of other applications of a simulation 

 capability to problems that may not 

 yet be evident. 



Current Status 



Efforts to model the large-scale 

 atmosphere and to simulate its be- 

 havior numerically began more than 

 twenty years ago. As additional re- 

 search groups and institutions in the 

 United States and elsewhere became 

 involved, steady advances in model 

 sophistication followed. These came 

 from refinements in numerical meth- 

 ods as well as from improved formu- 

 lations of the component processes. 



Today's multi-level models account 

 for a variety of interacting influences 

 and processes: large-scale topographic 



variations; thermal differences be- 

 tween continents and oceans; varia- 

 tions in roughness characteristics; 

 radiative transfer as a function of an 

 arbitrary distribution of radiatively 

 active constituents; large-scale phase 

 changes of water substance in the 

 precipitation process; interactions 

 with small-scale, convectively un- 

 stable motions; the thermal conse- 

 quences of variable water storage in 

 the soil; and the consequences of 

 snow-covered surfaces on the heat 

 balance. More recently, combined 

 models have taken into account the 

 mutual interaction of the atmosphere 

 and ocean, including the formation 

 and transport of sea-ice. 



Although many of these elements 

 are rather crudely formulated as cogs 

 in the total model, it has been pos- 

 sible to simulate with increasing detail 

 the characteristics of the observed 

 climate — not only the global wind 

 system and temperature distribution 

 from the earth's surface to the mid- 

 stratosphere, but also the precipita- 

 tion regimes and their role in forming 

 the deserts and major river basins 

 of the world. Attention is beginning 

 to be given to the simulation of 

 climatic response to the annual radia- 

 tion cycle. 



Detailed analyses of such simula- 

 tions in terms of the flow and trans- 

 formation of energy from the primary 

 solar source to the ultimate viscous 

 sink show encouragingly good agree- 

 ment with corresponding analyses of 

 observed atmospheric data. Such 

 models have also been applied to 

 observationally specified atmospheric 

 states in tests of transient predict- 

 ability. Even within the severe limita- 

 tions of the models, the data, and the 

 computational inadequacies, it has 

 been possible to simulate and verify 



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