Monthly Weather Review VOLUME 99, NUMBER 5 MAY 1971 UDC 551.589:651.613.1:561.511.33:556.1(1W) SIMULATION OF CLIMATE BY A GLOBAL GENERAL CIRCULATION MODEL 1. Hydrologic Cycle and Heat Balance J. LEITH HOLLOWAY, JR., and SYUKURO MANABE Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, N.J. ABSTRACT The primitive equations of motion in spherical coordinates are integrated with respect to time on global grids with mean horizontal resolutions of 500 and 250 km. There are nine levels in the models from 80 m to 28 km above the ground. The models have realistic continents with smoothed topography and an ocean surface with February water temperatures prescribed. The insolation is for a Northern Hemisphere winter. In addition to wind, temperature, pressure, and water vapor, the models simulate precipitation, evaporation, soil moisture, snow depth, and runoff. The models were run long enough beyond a state of quasi-equilibrium for meaningful statistics to be obtained. Time means of meteorological and hydrological quantities computed by the models compare favorably with observed climatic means. For example, the thermal structure of the model atmosphere is very similar to that of the actual atmosphere except in the Northern Hemisphere stratosphere; and the simulated distributions of the major arid regions over continents and the distributions of the rain belts, both in the Tropics and in middle latitudes, are success- fully simulated by the models described in this paper. The increase in the horizontal computational resolution improved the distributions of mean surface pressure and precipitation rate in particular. CONTENTS 1. INTRODUCTION 335 The first objective of general circulation modeling 336 336 experiments is the simulation of climate on a continental, 337 hemispheric, or global scale. Once this initial goal has been 338 achieved to a reasonable degree of faithfulness to reality, 338 the more interesting and enlightening goal can be pursued, 339 which is the use of these models for studying the physical 340 342 mechanisms that create and maintain climate. This is 343 accomplished by controlled experiments in which some 343 of the defining parameters of the model are changed while 344 others are kept constant. In this way, meteorology may 346 become an empirical science in a manner similar to the 348 field of experimental chemistry where different substances 349 355 are mixed in a laboratory to learn about various chemical 355 reactions. Modifications can be made to the model that 357 could never even be considered as remotely feasible in 360 nature [e.g., the removal of mountain ranges (Mintx 1968 360 and Kasahara and Washington 1969) or the elimination of 361 362 ocean currents (Manabe and Bryan 1969)I. Furthermore, Appendix A-computational techniques- - - - - - - - - - - - - - - - - - 363 proposed methods for climate modification can be tested A. Time integration ______________________________363 in atmospheric models without the tremendous expense B. Revised pressure gradient formulation--- - - - - - - - - - 363 C. Revised vertical pressure-velocity equation- - - - - - - 363 and risk involved in testing them in the red atmosphere. D. Shortcomings of the Kurihara grid-..---_---_----- 364 Some ideas can be disposed of as useless even without the Appendix B-some results from the high-resolution model- - 364 cost of research into for making their applica- Appendix C-derivation of the smoothed topography- - - - - - 367 tion inexpensive enough to be feasible. Finally, theories Acknowledgments- __ - - - - - - - - - - - - - - _ - - - - - - - - - - - - 369 References- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 369 involving inadvertent modification of weather by human 335 Unauthenticated | Downloaded 09/23/21 08:10 PM UTC 3 36 MONTHLY WEATHER REVIEW VOl. 99, No. 5 activity can be tested in general circulation models with ture search, we are presenting in the next section a greater certainty than by means of statistical studies of moderately detailed description of the current models. atmospheric data spanning the period over which the In the remainder of this paper, we shall describe the amount of the particular human activity changed results obtained from time integrations of these models significantly. with respect to simulating the global distributions of cli- Following the pioneering simulations of basic features mate and hydrologic elements. Since it is not reasonable to of the general circulation of the atmosphere by Phillips cover all aspects of our results in one paper, special (1956) and Smagorinsky (1963), various attempts have emphasis is placed here on the discussion of the balance of been made to simulate the global distribution of climate. heat and water in the earth-atmosphere system of the One important contribution toward this objective was model. The general circulation and energetics of disturb- made by Mintz and Arakawa (Mintz 1968) who con- ances in the model Tropics have already been discussed by structed a general circulation model of the atmosphere Manabe et al. (1970b). A detailed discussion of the model's with realistic topography and sea-surface temperatures flow field and the effects of mountains (Manabe and Hollo- as lower boundary conditions and successfully simulated way 1971) will be published. A synopsis of some of the the essential features of the global distribution of surface hydrologic results, as they pertain to climate modification, pressure. Their model, however, does not include a has been presented in Manabe and Holloway (1970a). hydrologic cycle that strongly controls the atmospheric Some of these results, applying to the future development circulation. On the other hand, Leith (1965) and Washing- of hydrologic models, were presented in Manabe and ton and Kasahara (1970) incorporated into their models Holloway (1970b). the effects of condensation, but these models do not take into consideration one of the most important climatic 2. DESCRIPTION OF THE MODEL processes (viz, the water balance of the earth's surface). Previous to this study, attempts have been made to A. EQUATIONS OF MOTION incorporate moist processes into general circulation models In deriving the equations of motion, we adopted the of the atmosphere at the Geophysical Fluid Dynamics so-called "u coordinate" system in which the pressure, Laboratory (GFDL) of NOAA. For example, Manabe et normalized by surface pressure, is chosen as the vertical al. (1965) constructed a hemispheric model in which the coordinate (Phillips 1957). Making the hydrostatic as- effects of both evaporation and precipitation were taken sumption, we may write the momentum equations on a into consideration. By using this model, they were success- spherical surface as ful in simulating some of the features of the hydrologic cycle such as the tropical rain belt, the subtropical dry a belt, and the rain belt of middle latitudes (see also Manabe -at (p*u)= --Ddu) +(f+yu) p*v and Smagorinsky 1967). Their model was,however, highly simplified, having neither land-sea contrast nor mountains. Instead, it had a flat moist surface with no heat capacity, which can be perhaps visualized as a very shallow swamp. and Following this preliminary study, Manabe (1969) incor- porated a computation scheme of ground hydrology into a general circulation model with idealized continents and oceanic areas in a limited domain. His success in simulat- ing some of the fundamental features of the hydrologic cycle, despite the extreme idealization of the geography and of the computation scheme of hydrology, encouraged where u and v are the eastward and northward components us to attempt this present study of the numerical simula- of the wind, respectively; p,, surface pressure; u, pressure tion of the hydrologic cycle on a global scale. normalized by surface pressure; T, temperature; 4, geo- For making a global atmospheric model, it is necessary potential height; e, latitude; A, longitude; a, radius of the to design a grid system that covers the whole globe. Such a earth;f, the Coriolis parameter; R, the gas constant for grid system was proposed by Kurihara (1965). Using this air; and and "F are the frictional forces due to subgrid system, Kurihara and Holloway (1967) successfully per- scale mixing in the horizontal and vertical directions, re- formed a numerical time integration of a global model spectively (see section 2E for details of these frictional without moist processes and laid the foundation for the terms). The three-dimensional divergence operator D3( ) present study. is defined by The models described in this paper, therefore, represent the most advanced stage in a series of atmospheric models (3) developed at GFDL during the last 14 yr. The evolution of these models can be traced by referring to some pre- where a denotes the individual change of normalized pres- of vious papers by the GFDL staff. For the benefit the sure u, and reader who does not wish to make such an extensive litera- 1 Smagorinsky 1963, Manabe and Strickler 1964, Smagorinsky et al. 1965, Manabe et al. 1965, Manabe and Wetherald 1967, Kurihara and Holloway 1967, Manabs 1969 Unauthenticated | Downloaded 09/23/21 08:10 PM UTC May 1971 j. Leith Holloway, jr. and Syukuro Manabe 337 The continuity equation may be written as -aP* =-03(1). at (5) Integration of eq (5) with respect to u yields the following prognostic equation for surface pressure: The vertical u and p velocities can be obtained from the diagnostic relations and FIGURE1.-Diagram of one octant of the low resolution (N24)