Numerical Simulation of the January and July Global Climate with a Two-Level Atmospheric Model

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  • 1 The Rand Corporation, Santa Monica, Calif. 90406
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Abstract

The global distributions of selected climatic variables simulated by numerical integration of a two-level atmospheric general circulation model for January and July are presented in comparison with the corresponding observed climatological fields. The model has reproduced the observed large-scale patterns of sea-level pressure, lower tropospheric temperature and circulation with reasonable accuracy, although there are a number of systematic errors. In particular, the intensity of the semipermanent low-pressure centers in January in the Northern Hemisphere is overestimated, and the 400 mb temperature is too high in the tropics. Accompanying these errors are overestimates of the meridional temperature gradient and zonal westerlies in the mid-latitudes of the winter hemisphere. Although their global patterns resemble those observed, systematic amplitude errors are also present in the simulations of precipitation and evaporation (both of which the model overestimates by nearly a factor of 2 in January and July), in the simulated mean meridional circulation (in which the strength of the Hadley cells is overestimated and that of the Ferrel cells underestimated in both summer and winter), and in the cloudiness (which is underestimated by nearly a factor of 2 in the summer hemisphere). These errors have resulted in the simulation of too vigorous a hydrologic cycle, and distortions in the total meridional transports of heat and moisture, especially in the subtropics of the summer hemisphere.

In spite of these shortcomings, the model has successfully simulated the principal features of the observed heat and energy balances, at least in the zonal average. The characteristic meridional structure of the observed evaporation-precipitation difference is reproduced (but with values that are slightly too negative), the interhemispheric gradient of the net radiation of the top of the atmosphere and at the surface is nearly correct (but with excessive net radiation in the higher latitudes of the summer hemisphere), and the observed meridional distribution of the net heating of the earth-atmosphere system is simulated with good overall accuracy (but is too high in high latitudes during summer and too low in the subtropics during winter).

Comparison of the separate January and July simulators shows that the model has reproduced the observed seasonal shifts of the heat and energy balance, as well as that of the principal climate variables themselves, such as the pressure, temperature, wind and precipitation. Except for the precipitation and the mean meridional circulation in the lower latitudes, these changes are generally comparable to those found from models with higher vertical resolution. With improved parameterizations of subgrid-scale processes, especially in the tropics, it is believed that the two-level model can be substantially improved, and will prove useful in the further simulation of climate and climatic change.

Abstract

The global distributions of selected climatic variables simulated by numerical integration of a two-level atmospheric general circulation model for January and July are presented in comparison with the corresponding observed climatological fields. The model has reproduced the observed large-scale patterns of sea-level pressure, lower tropospheric temperature and circulation with reasonable accuracy, although there are a number of systematic errors. In particular, the intensity of the semipermanent low-pressure centers in January in the Northern Hemisphere is overestimated, and the 400 mb temperature is too high in the tropics. Accompanying these errors are overestimates of the meridional temperature gradient and zonal westerlies in the mid-latitudes of the winter hemisphere. Although their global patterns resemble those observed, systematic amplitude errors are also present in the simulations of precipitation and evaporation (both of which the model overestimates by nearly a factor of 2 in January and July), in the simulated mean meridional circulation (in which the strength of the Hadley cells is overestimated and that of the Ferrel cells underestimated in both summer and winter), and in the cloudiness (which is underestimated by nearly a factor of 2 in the summer hemisphere). These errors have resulted in the simulation of too vigorous a hydrologic cycle, and distortions in the total meridional transports of heat and moisture, especially in the subtropics of the summer hemisphere.

In spite of these shortcomings, the model has successfully simulated the principal features of the observed heat and energy balances, at least in the zonal average. The characteristic meridional structure of the observed evaporation-precipitation difference is reproduced (but with values that are slightly too negative), the interhemispheric gradient of the net radiation of the top of the atmosphere and at the surface is nearly correct (but with excessive net radiation in the higher latitudes of the summer hemisphere), and the observed meridional distribution of the net heating of the earth-atmosphere system is simulated with good overall accuracy (but is too high in high latitudes during summer and too low in the subtropics during winter).

Comparison of the separate January and July simulators shows that the model has reproduced the observed seasonal shifts of the heat and energy balance, as well as that of the principal climate variables themselves, such as the pressure, temperature, wind and precipitation. Except for the precipitation and the mean meridional circulation in the lower latitudes, these changes are generally comparable to those found from models with higher vertical resolution. With improved parameterizations of subgrid-scale processes, especially in the tropics, it is believed that the two-level model can be substantially improved, and will prove useful in the further simulation of climate and climatic change.

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