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## Abstract

A steady-state quasi-geostrophic two-layer model for the quasi-stationary perturbations of the Northern Hemisphere midwinter conditions is used to study the response of the model to stimulated anomalies at the earth's surface. Three anomalous surface conditions are simulated: 1) a cold pool of water in the Pacific Ocean, 2) a vegetation cover over the Sahara Desert and 3) removal of a climatologically observed snow cover over Tibet. The cold pool of water in the Pacific Ocean is simulated by inserting a −3°C perturbation in temperature at a level below the seasonal variation of the thermocline. An anomalous vegetation cover over the Sahara Desert is simulated by using appropriate values for surface albede and water availability for evaporation. The climatologically observed snow cover over Tibet in the control experiment is removed in the anomaly experiment. These changes affect the heat fluxes at the earth's surface and hence heating in the atmosphere.

The anomalously cold subsurface temperature cools the surface by about -2.4°C. This change in the surface temperature produces a cooling of 0.2°C day^{−1} in the atmosphere and decreases the 500 mb temperature by 0.4°C. The vegetation over the Sahara Desert cools the earth's surface by 0.5−1°C. Heating in the atmosphere is increased by 0.2−0.3°C day^{−1} due to the release of latent heat by excessive precipitation resulting from increased evaporation at the surface. The temperature at 500 mb increases by 0.4°C downstream from the anomaly. Removal of snow over Tibet results in an increase in surface temperature of the region by 1.5−2°C. Atmospheric heating increases by 0.3°C day^{−1}. The temperature at 500 mb increases by 0.5°C. In all cases, the heating (cooling) in the atmosphere produces a low (high) in the lower troposphere and a high (low) in the upper troposphere downstream from the anomaly. In spite of the simplicity of the model, the results of the experiments appear to be physically meaningful.

## Abstract

A steady-state quasi-geostrophic two-layer model for the quasi-stationary perturbations of the Northern Hemisphere midwinter conditions is used to study the response of the model to stimulated anomalies at the earth's surface. Three anomalous surface conditions are simulated: 1) a cold pool of water in the Pacific Ocean, 2) a vegetation cover over the Sahara Desert and 3) removal of a climatologically observed snow cover over Tibet. The cold pool of water in the Pacific Ocean is simulated by inserting a −3°C perturbation in temperature at a level below the seasonal variation of the thermocline. An anomalous vegetation cover over the Sahara Desert is simulated by using appropriate values for surface albede and water availability for evaporation. The climatologically observed snow cover over Tibet in the control experiment is removed in the anomaly experiment. These changes affect the heat fluxes at the earth's surface and hence heating in the atmosphere.

The anomalously cold subsurface temperature cools the surface by about -2.4°C. This change in the surface temperature produces a cooling of 0.2°C day^{−1} in the atmosphere and decreases the 500 mb temperature by 0.4°C. The vegetation over the Sahara Desert cools the earth's surface by 0.5−1°C. Heating in the atmosphere is increased by 0.2−0.3°C day^{−1} due to the release of latent heat by excessive precipitation resulting from increased evaporation at the surface. The temperature at 500 mb increases by 0.4°C downstream from the anomaly. Removal of snow over Tibet results in an increase in surface temperature of the region by 1.5−2°C. Atmospheric heating increases by 0.3°C day^{−1}. The temperature at 500 mb increases by 0.5°C. In all cases, the heating (cooling) in the atmosphere produces a low (high) in the lower troposphere and a high (low) in the upper troposphere downstream from the anomaly. In spite of the simplicity of the model, the results of the experiments appear to be physically meaningful.

## Abstract

Using parameterized forms of time-averaged heat fluxes at the interface between the earth and the atmosphere, the normal temperature at the earth's surface is determined by satisfying the heat balance condition. The calculations for the normal surface temperature and the heat fluxes are made for January and July conditions for every 5° latitude and longitude grid points over the entire globe. The computed results are in good agreement with the observations. Sensitivity of the temperature calculations to the changes in the parameter values is discussed.

## Abstract

Using parameterized forms of time-averaged heat fluxes at the interface between the earth and the atmosphere, the normal temperature at the earth's surface is determined by satisfying the heat balance condition. The calculations for the normal surface temperature and the heat fluxes are made for January and July conditions for every 5° latitude and longitude grid points over the entire globe. The computed results are in good agreement with the observations. Sensitivity of the temperature calculations to the changes in the parameter values is discussed.

## Abstract

The purpose of the study is to make a detailed investigation of mean meridional circulations forced by given eddy transports of heat and momentum and to describe the vertical variation of the energy conversions for the zonally averaged flow.

A nonhomogeneous second-order partial differential equation for the vertical *p*-velocity, *ω*, is obtained from the quasi-geostrophic vorticity and thermodynamic equations. The method of separation of variables is used to solve the zonally averaged form of this equation such that zonally averaged vertical *p*-velocity, *ω _{z}
*, is expressed as a series of Legendre polynomials. The boundary conditions used are that

*ω*is zero at the top of the atmosphere and that at the surface it is equal to that value of

_{z}*ω*which is produced by the topography of the earth. After the solution for

_{z}*ω*is obtained, the mean meridional velocity is determined from the zonally averaged continuity equation.

_{z}The diabatic heating in the meridional plane is estimated from the zonally averaged steady-state thermodynamic equation. Computations of the zonal available potential energy and the conversion from zonal available potential energy to zonal kinetic energy are made using the distributions of diabatic heating, the vertical *p*-velocity and the temperature in the meridional plane.

The general conclusions which can be drawn on the basis of the calculations are:

(1) Three-cell meridional circulations are produced by the eddy transport processes in the atmosphere.

(2) The eddy transport of momentum is twice as effective as the eddy transport of heat in forcing the meridional circulations.

(3) The influence of the planetary scale motion on the circulation is predominant during winter whereas that of the baroclinically unstable waves dominates the forcing mechanism during the other seasons.

(4) The seasonal variation of the meridional circulations shows that the circulation cells move toward the pole and undergo a decrease in their intensity from winter to summer.

(5) The net diabatic heating in the meridional plane is positive south of 40°N. and negative north of that latitude during winter months. In the upper troposphere, the heating decreases gradually with height in the region of net heating whereas the cooling decreases sharply in the region of net cooling.

(6) The generation of zonal available potential energy is maximum in the lower troposphere, decreases sharply with height, and becomes negative in the lower stratosphere.

(7) The conversion from zonal available potential energy to zonal kinetic energy is positive in the lower troposphere and negative in the upper troposphere.

## Abstract

The purpose of the study is to make a detailed investigation of mean meridional circulations forced by given eddy transports of heat and momentum and to describe the vertical variation of the energy conversions for the zonally averaged flow.

A nonhomogeneous second-order partial differential equation for the vertical *p*-velocity, *ω*, is obtained from the quasi-geostrophic vorticity and thermodynamic equations. The method of separation of variables is used to solve the zonally averaged form of this equation such that zonally averaged vertical *p*-velocity, *ω _{z}
*, is expressed as a series of Legendre polynomials. The boundary conditions used are that

*ω*is zero at the top of the atmosphere and that at the surface it is equal to that value of

_{z}*ω*which is produced by the topography of the earth. After the solution for

_{z}*ω*is obtained, the mean meridional velocity is determined from the zonally averaged continuity equation.

_{z}The diabatic heating in the meridional plane is estimated from the zonally averaged steady-state thermodynamic equation. Computations of the zonal available potential energy and the conversion from zonal available potential energy to zonal kinetic energy are made using the distributions of diabatic heating, the vertical *p*-velocity and the temperature in the meridional plane.

The general conclusions which can be drawn on the basis of the calculations are:

(1) Three-cell meridional circulations are produced by the eddy transport processes in the atmosphere.

(2) The eddy transport of momentum is twice as effective as the eddy transport of heat in forcing the meridional circulations.

(3) The influence of the planetary scale motion on the circulation is predominant during winter whereas that of the baroclinically unstable waves dominates the forcing mechanism during the other seasons.

(4) The seasonal variation of the meridional circulations shows that the circulation cells move toward the pole and undergo a decrease in their intensity from winter to summer.

(5) The net diabatic heating in the meridional plane is positive south of 40°N. and negative north of that latitude during winter months. In the upper troposphere, the heating decreases gradually with height in the region of net heating whereas the cooling decreases sharply in the region of net cooling.

(6) The generation of zonal available potential energy is maximum in the lower troposphere, decreases sharply with height, and becomes negative in the lower stratosphere.

(7) The conversion from zonal available potential energy to zonal kinetic energy is positive in the lower troposphere and negative in the upper troposphere.

## Abstract

A zonal-average, annual-mean, statistical-dynamical climate model governing two domains (the atmosphere and a subsurface medium consisting of either ice or “swamp”), and including the dynamics of mean poloidal motions and the hydrologic cycle as well as the ice-albedo feedback, is integrated numerically as a function of the solar constant. The adiabatic effect of the mean poloidal motion is to cool the system in the region of the ascending branch of the Ferrel cell (thereby promoting an advance of the equilibrium ice extent in this region) and to warm the system in the region of the descending branch (hence posing a “barrier” to the ice advance in this region). This latter barrier effect is amplified as the solar constant is reduced because the subtropical descending motion increases in magnitude as the ice advances. The hydrologic non-adiabatic consequences of the mean poloidal motions tend to offset these adiabatic consequences to some degree. In general, the release of latent heat in middle and high latitudes associated with the poleward flux of water vapor reduces the equilibrium ice advance that would otherwise occur due to reductions in the solar constant.

## Abstract

A zonal-average, annual-mean, statistical-dynamical climate model governing two domains (the atmosphere and a subsurface medium consisting of either ice or “swamp”), and including the dynamics of mean poloidal motions and the hydrologic cycle as well as the ice-albedo feedback, is integrated numerically as a function of the solar constant. The adiabatic effect of the mean poloidal motion is to cool the system in the region of the ascending branch of the Ferrel cell (thereby promoting an advance of the equilibrium ice extent in this region) and to warm the system in the region of the descending branch (hence posing a “barrier” to the ice advance in this region). This latter barrier effect is amplified as the solar constant is reduced because the subtropical descending motion increases in magnitude as the ice advances. The hydrologic non-adiabatic consequences of the mean poloidal motions tend to offset these adiabatic consequences to some degree. In general, the release of latent heat in middle and high latitudes associated with the poleward flux of water vapor reduces the equilibrium ice advance that would otherwise occur due to reductions in the solar constant.

## Abstract

State-of-the-art general circulation models have deficiencies in simulating the observed amplitude and phase of the mean patterns of circulation and precipitation over the Asian monsoon region. They are also deficient in simulating the observed seasonal variations of circulation and precipitation averaged over the monsoon region. To improve these simulations, the National Centers for Environmental Prediction regional Eta Model is nested in the Center for Ocean–Land–Atmosphere Studies (COLA) GCM. The Eta Model is a gridpoint model with a horizontal resolution of 80 km and 38 layers in the vertical. The Eta Model domain (30°–140°E, 30°S–50°N) covers the Asian monsoon region, which includes the Indian, Chinese, and Southeast Asian monsoons. The COLA GCM is a sigma coordinate spectral model with rhomboidal truncation at 40 waves and 18 vertical levels.

The Eta Model is nested in the GCM such that its lateral boundary conditions and initial conditions are derived from the GCM simulations. The nested model is used to simulate the summer monsoons of 1987, an El Niño year, and 1988, a La Niña year, prescribing the seasonally varying sea surface temperature of the respective years. The model was integrated from mid-April to the end of September. Three separate runs were made for each year with atmospheric initial conditions for 14, 15, and 16 April, respectively.

The ensemble means of the three simulations for 1988 were calculated for the GCM and the Eta Model. Comparison of the results with observations shows that the amplitude and phase of the mean monsoon circulation and precipitation patterns, as well as the seasonal variations of areally averaged circulation parameters and precipitation, simulated by the Eta Model are closer to observations than the GCM simulations. The Eta Model simulation for 1987 showed deficient summer rainfall in northern and peninsular India and the Indonesian region, and enhanced rainfall in southeast China, Burma, and the sub-Himalayan region compared to the 1988 simulations. These results are in agreement with observations. The phase and amplitude of the variability in the 2 yr simulated by the Eta Model were closer to observations than the GCM simulations over both India and southeast China. Variability in the simulations due to changes in the initial conditions was smaller in the Eta Model than in the GCM.

## Abstract

State-of-the-art general circulation models have deficiencies in simulating the observed amplitude and phase of the mean patterns of circulation and precipitation over the Asian monsoon region. They are also deficient in simulating the observed seasonal variations of circulation and precipitation averaged over the monsoon region. To improve these simulations, the National Centers for Environmental Prediction regional Eta Model is nested in the Center for Ocean–Land–Atmosphere Studies (COLA) GCM. The Eta Model is a gridpoint model with a horizontal resolution of 80 km and 38 layers in the vertical. The Eta Model domain (30°–140°E, 30°S–50°N) covers the Asian monsoon region, which includes the Indian, Chinese, and Southeast Asian monsoons. The COLA GCM is a sigma coordinate spectral model with rhomboidal truncation at 40 waves and 18 vertical levels.

The Eta Model is nested in the GCM such that its lateral boundary conditions and initial conditions are derived from the GCM simulations. The nested model is used to simulate the summer monsoons of 1987, an El Niño year, and 1988, a La Niña year, prescribing the seasonally varying sea surface temperature of the respective years. The model was integrated from mid-April to the end of September. Three separate runs were made for each year with atmospheric initial conditions for 14, 15, and 16 April, respectively.

The ensemble means of the three simulations for 1988 were calculated for the GCM and the Eta Model. Comparison of the results with observations shows that the amplitude and phase of the mean monsoon circulation and precipitation patterns, as well as the seasonal variations of areally averaged circulation parameters and precipitation, simulated by the Eta Model are closer to observations than the GCM simulations. The Eta Model simulation for 1987 showed deficient summer rainfall in northern and peninsular India and the Indonesian region, and enhanced rainfall in southeast China, Burma, and the sub-Himalayan region compared to the 1988 simulations. These results are in agreement with observations. The phase and amplitude of the variability in the 2 yr simulated by the Eta Model were closer to observations than the GCM simulations over both India and southeast China. Variability in the simulations due to changes in the initial conditions was smaller in the Eta Model than in the GCM.

## Abstract

To simulate the onset and intraseasonal variability of summer monsoons, the National Centers for Environmental Prediction Eta Model (80 km, L38) is nested in the Center for Ocean–Land–Atmosphere Studies GCM (R40, L18). The region of the Eta Model is (30°S–50°N and 30°–140°E), which includes the Indian, Chinese, and Southeast Asian monsoons. The summer monsoons of 1987 and 1988 are simulated by integrating the nested model from mid-April to the end of September, prescribing the seasonal variations of SST of the respective years. The summer monsoons of 1987 and 1988 were extreme. In 1987, an El Niño year, the Indian monsoon rainfall was far below normal but over southeast China the rainfall exceeded normal. In contrast, in 1988, a La Niña year, Indian monsoon rainfall was far above normal but the rainfall over southeast China was below normal.

The Eta Model was able to simulate the typical observed features of the monsoon onset, that is, an abrupt increase in the precipitation rate as well as in the strength of the circulation. The simulated onset dates for 1987 and 1988 were in good agreement with observations. The Eta Model was also able to simulate the observed circulation features of the break and active periods during these two years. To investigate the contrasting characteristics of the Indian and the Chinese monsoons, for these two years the following hypothesis, largely based on observational evidence, is verified. There are two preferred locations of ITCZ: one over the warm waters of the equatorial Indian Ocean and the other over the heated continent in the vicinity of the seasonal monsoon trough. There is a northward migration of the convective precipitation bands from the equatorial ITCZ to the continental ITCZ with the timescale of a few weeks. There exists an inverse relationship between the strength of the two ITCZs. During an El Niño year, sea level pressure over the Indian subcontinent and over the Maritime Continent increases. Consequently, the ITCZ over the Indian subcontinent and over the Maritime Continent weakens and the ITCZ over the equatorial Indian Ocean, Southeast Asia, and southeast China strengthens. The Eta Model simulated circulations are in support of the hypothesis. The simulations also show that there is a northward migration of convective precipitation bands from the equatorial ITCZ to the continental ITCZ.

## Abstract

To simulate the onset and intraseasonal variability of summer monsoons, the National Centers for Environmental Prediction Eta Model (80 km, L38) is nested in the Center for Ocean–Land–Atmosphere Studies GCM (R40, L18). The region of the Eta Model is (30°S–50°N and 30°–140°E), which includes the Indian, Chinese, and Southeast Asian monsoons. The summer monsoons of 1987 and 1988 are simulated by integrating the nested model from mid-April to the end of September, prescribing the seasonal variations of SST of the respective years. The summer monsoons of 1987 and 1988 were extreme. In 1987, an El Niño year, the Indian monsoon rainfall was far below normal but over southeast China the rainfall exceeded normal. In contrast, in 1988, a La Niña year, Indian monsoon rainfall was far above normal but the rainfall over southeast China was below normal.

The Eta Model was able to simulate the typical observed features of the monsoon onset, that is, an abrupt increase in the precipitation rate as well as in the strength of the circulation. The simulated onset dates for 1987 and 1988 were in good agreement with observations. The Eta Model was also able to simulate the observed circulation features of the break and active periods during these two years. To investigate the contrasting characteristics of the Indian and the Chinese monsoons, for these two years the following hypothesis, largely based on observational evidence, is verified. There are two preferred locations of ITCZ: one over the warm waters of the equatorial Indian Ocean and the other over the heated continent in the vicinity of the seasonal monsoon trough. There is a northward migration of the convective precipitation bands from the equatorial ITCZ to the continental ITCZ with the timescale of a few weeks. There exists an inverse relationship between the strength of the two ITCZs. During an El Niño year, sea level pressure over the Indian subcontinent and over the Maritime Continent increases. Consequently, the ITCZ over the Indian subcontinent and over the Maritime Continent weakens and the ITCZ over the equatorial Indian Ocean, Southeast Asia, and southeast China strengthens. The Eta Model simulated circulations are in support of the hypothesis. The simulations also show that there is a northward migration of convective precipitation bands from the equatorial ITCZ to the continental ITCZ.

## Abstract

The governing equations for stationary perturbations are derived by subtracting the equations for the mean zonally averaged flow from the equations for the time-averaged flow. The averaging period for the ensemble is chosen such that the terms involving the time derivatives can be neglected compared to the other terms in the equations. These equations contain the first moments (means and gradients of means) of stationary perturbations and the zonally averaged flow, second moments such as eddy transports of momentum and heat, heating due to diabatic processes, and frictional forces. The closure for the equations is sought by relating the second moments and frictional forces to the first moments, parameterizing the diabatic heating in terms of first moments and radiation-convection parameters, and evaluating the stationary perturbation from a quasi-geostrophic approximation. The equations are linearized by neglecting the products of the perturbation quantities and the products of perturbations and the mean meridional circulations. The vertical variation of the perturbation in the atmosphere is represented by a two-layer model. The zonally averaged variables are assumed to be uniform with latitude and prescribed from observations. In addition, we prescribe subsurface temperature, cloudiness and convection radiation parameters as well as the surface cover, such as ice, snow and vegetation. The simplified equations are solved by expanding the variables as a sum of spherical harmonics. The model equations yield solutions for stationary perturbations of the geopotential field at 250 and 750 mb; temperature, vertical velocity and diabatic heating at 500 mb; and the temperature of the earth's surface in the Northern Hemisphere for January conditions. The phase and the amplitude of the 250 mb geopotential field agree well with observations. The phase of the 750 mb geopotential field agrees favorably with observations but the amplitude is slightly smaller than observed. The amplitude of the temperature field at 500 mb agrees well with observations and is approximately in phase with the geopotential field. The average amplitude of the vertical velocity is about 2 mm s^{−1}. The rising motion is on the eastward side of the trough and the sinking motion on the westward side. The average amplitude of the heating field is equivalent to about 1K day^{−1} and qualitatively agrees with observations.

## Abstract

The governing equations for stationary perturbations are derived by subtracting the equations for the mean zonally averaged flow from the equations for the time-averaged flow. The averaging period for the ensemble is chosen such that the terms involving the time derivatives can be neglected compared to the other terms in the equations. These equations contain the first moments (means and gradients of means) of stationary perturbations and the zonally averaged flow, second moments such as eddy transports of momentum and heat, heating due to diabatic processes, and frictional forces. The closure for the equations is sought by relating the second moments and frictional forces to the first moments, parameterizing the diabatic heating in terms of first moments and radiation-convection parameters, and evaluating the stationary perturbation from a quasi-geostrophic approximation. The equations are linearized by neglecting the products of the perturbation quantities and the products of perturbations and the mean meridional circulations. The vertical variation of the perturbation in the atmosphere is represented by a two-layer model. The zonally averaged variables are assumed to be uniform with latitude and prescribed from observations. In addition, we prescribe subsurface temperature, cloudiness and convection radiation parameters as well as the surface cover, such as ice, snow and vegetation. The simplified equations are solved by expanding the variables as a sum of spherical harmonics. The model equations yield solutions for stationary perturbations of the geopotential field at 250 and 750 mb; temperature, vertical velocity and diabatic heating at 500 mb; and the temperature of the earth's surface in the Northern Hemisphere for January conditions. The phase and the amplitude of the 250 mb geopotential field agree well with observations. The phase of the 750 mb geopotential field agrees favorably with observations but the amplitude is slightly smaller than observed. The amplitude of the temperature field at 500 mb agrees well with observations and is approximately in phase with the geopotential field. The average amplitude of the vertical velocity is about 2 mm s^{−1}. The rising motion is on the eastward side of the trough and the sinking motion on the westward side. The average amplitude of the heating field is equivalent to about 1K day^{−1} and qualitatively agrees with observations.

## Abstract

Outgoing longwave radiation (OLR) simulated by the GLAS general circulation model is compared with that derived from measurements made by polar orbiting satellites. The comparison is made for spatial and seasonal variations by expanding the spatial into surface spherical harmonics. The student's *t*-test is used to determine the statistical significance of differences between the simulated and observed fields.

The global mean and spatial standard deviation of the simulated fields are respectively about 20 W m^{−2} smaller and 10 W m^{−2} larger than the corresponding values of the observed field. The smaller value of the global mean is due to larger than observed cloud amount in the model. A major fraction of the variability in the simulated field is due to a sharper meridional gradient than is observed. The seasonal variations of the global mean and standard deviation of the simulated fields are in good agreement with observations. Correlation coefficients between the observed and simulated fields as a function of spatial scales show that the phase relationship for large spatial scales is very good for January and July but only fair for April and October. In the tropics, the differences between simulated and observed OLR means are not significantly different from zero, except over regions with deep convection (Asia, Amazon, Central Africa) where the model's convective clouds do not interact with radiation. The differences in the middle and high latitudes are highly significant, more so in the Southern Hemisphere than in the Northern Hemisphere.

## Abstract

Outgoing longwave radiation (OLR) simulated by the GLAS general circulation model is compared with that derived from measurements made by polar orbiting satellites. The comparison is made for spatial and seasonal variations by expanding the spatial into surface spherical harmonics. The student's *t*-test is used to determine the statistical significance of differences between the simulated and observed fields.

The global mean and spatial standard deviation of the simulated fields are respectively about 20 W m^{−2} smaller and 10 W m^{−2} larger than the corresponding values of the observed field. The smaller value of the global mean is due to larger than observed cloud amount in the model. A major fraction of the variability in the simulated field is due to a sharper meridional gradient than is observed. The seasonal variations of the global mean and standard deviation of the simulated fields are in good agreement with observations. Correlation coefficients between the observed and simulated fields as a function of spatial scales show that the phase relationship for large spatial scales is very good for January and July but only fair for April and October. In the tropics, the differences between simulated and observed OLR means are not significantly different from zero, except over regions with deep convection (Asia, Amazon, Central Africa) where the model's convective clouds do not interact with radiation. The differences in the middle and high latitudes are highly significant, more so in the Southern Hemisphere than in the Northern Hemisphere.

## Abstract

The barotropic vorticity equation and zonal kinetic energy equation are used to derive a formula expressing the transport of relative angular momentum by large-scale transient eddies in terms of other mean zonally averaged variables. To test the formula, computations are made using the observed winter and summer zonal average conditions. Generally good agreement is found between the calculated and observed momentum transport.

## Abstract

The barotropic vorticity equation and zonal kinetic energy equation are used to derive a formula expressing the transport of relative angular momentum by large-scale transient eddies in terms of other mean zonally averaged variables. To test the formula, computations are made using the observed winter and summer zonal average conditions. Generally good agreement is found between the calculated and observed momentum transport.

## Abstract

The roles played by the large-scale motion induced by vertical diffusion of heat from the lower boundary and condensational heating due to deep convection in maintaining the precipitation zones in the Tropics of an atmospheric general circulation model (GCM) are explored. A steady linearized version of the GCM is used to diagnose the wind forced by these processes. The wind field obtained from the linear model is combined with the time-mean moisture field from the GCM in order to determine the zonally asymmetric moisture flux convergence, which is the primary factor maintaining the zonally asymmetric precipitation distribution. The role of the other diabatic heating processes is explored as is the role of the orographic forcing in maintaining the precipitation distribution.

The vertically integrated moisture flux convergence forced by vertical diffusion of heat and condensational heating are found to be in phase over the ocean and 180 degrees out of phase over the land. Over the ocean, both of these forcings contribute to moisture flux convergence in the regions of largest precipitation. The moisture flux convergence forced by the vertical diffusion of heat tends to narrow the precipitation zones in the meridional direction over the ocean. Over the land, the condensational heating leads to moisture flux convergence in the regions of large precipitation, while the vertical diffusion of heat leads to moisture flux divergence. This indicates that the motions forced by the surface temperature provide a negative feedback on the precipitation. This feedback is apparently due to the relatively cool surface temperatures present in the regions of large precipitation over land. This locally cool surface temperature leads to a low-level divergent circulation from the cool region to warmer regions. Other forcing functions are found to play a minor role in the moisture flux convergence by the time-mean flow with the exception of the orographic forcing in some regions.

The lowest model sigma-level wind field over the tropical Pacific Ocean is examined. In general both the zonal and meridional wind fields are dominated by the response to convective condensational heating. Exceptions include the meridional wind in the western Pacific and the zonal wind along the equator. In these regions, the response to low-level temperature gradients is found to be nonnegligible in comparison with the response to convective condensational heating. The role of the orographic forcing is also significant along the coasts of the tropical continents and in the western Pacific.

## Abstract

The roles played by the large-scale motion induced by vertical diffusion of heat from the lower boundary and condensational heating due to deep convection in maintaining the precipitation zones in the Tropics of an atmospheric general circulation model (GCM) are explored. A steady linearized version of the GCM is used to diagnose the wind forced by these processes. The wind field obtained from the linear model is combined with the time-mean moisture field from the GCM in order to determine the zonally asymmetric moisture flux convergence, which is the primary factor maintaining the zonally asymmetric precipitation distribution. The role of the other diabatic heating processes is explored as is the role of the orographic forcing in maintaining the precipitation distribution.

The vertically integrated moisture flux convergence forced by vertical diffusion of heat and condensational heating are found to be in phase over the ocean and 180 degrees out of phase over the land. Over the ocean, both of these forcings contribute to moisture flux convergence in the regions of largest precipitation. The moisture flux convergence forced by the vertical diffusion of heat tends to narrow the precipitation zones in the meridional direction over the ocean. Over the land, the condensational heating leads to moisture flux convergence in the regions of large precipitation, while the vertical diffusion of heat leads to moisture flux divergence. This indicates that the motions forced by the surface temperature provide a negative feedback on the precipitation. This feedback is apparently due to the relatively cool surface temperatures present in the regions of large precipitation over land. This locally cool surface temperature leads to a low-level divergent circulation from the cool region to warmer regions. Other forcing functions are found to play a minor role in the moisture flux convergence by the time-mean flow with the exception of the orographic forcing in some regions.

The lowest model sigma-level wind field over the tropical Pacific Ocean is examined. In general both the zonal and meridional wind fields are dominated by the response to convective condensational heating. Exceptions include the meridional wind in the western Pacific and the zonal wind along the equator. In these regions, the response to low-level temperature gradients is found to be nonnegligible in comparison with the response to convective condensational heating. The role of the orographic forcing is also significant along the coasts of the tropical continents and in the western Pacific.