A Numerical Experiment on the Interactions of Radiation, Clouds and Dynamic Processes in a General Circulation Model

Abstract

Physical interactions of radiation, cloud formation and dynamic processes are investigated utilizing a general circulation model designed for short and medium range prediction. The present model is a seven-level spectral model in the σ-coordinate system for the Northern Hemisphere with the generation of clouds in four σ layers through the predicted relative humidity field. Computations of the cloud liquid water content involving various cloud combinations for radiation transfer analyses are based on assumed particle size distributions assigned to cumulus, stratus, altostratus, cumulonimbus and cirrus. Parameterization of the thermal IR and solar flux transfer in a clear atmosphere utilizes the broadband emissivity (IR) and absorptivity (solar) concept for water vapor, ozone and carbon dioxide emission and absorption. Broadband solar flux reflection and transmission for various cloud types as functions of the liquid water content, solar zenith angle and average cloud temperature under a number of model conditions are parameterized in line with the gaseous broadband absorptivity. Moreover, high clouds are considered to be nonblack whose parameterized emissivity is expressed in terms of the vertical ice content. We have physically described and mathematically quantized the effects of radiative processes on the vertical velocity, temperature and cloud field (via the water vapor mixing ratio) by virtue of the fundamental equations governing these variables in the model. We demonstrate, through the predicted vertical velocity for the fifth day, that radiative flux exchanges in the tropics lead to a significant intensification of the Hadley circulation and that such processes also strengthen to some degree the meridional circulation. In addition, we show that the part of the vertical velocity produced by radiative processes represents a significant component in the tropical region, whereas it is only about 10–15% of the total in middle and high latitudes. Moreover, we also discover a consistent and characteristic pattern involving warm and cold advections of temperature associated with frontal activities and the part of the vertical velocity produced by radiative flux exchanges. In areas associated with high and low temperature advections radiative processes appear to enhance and suppress the upward motion, respectively. With verification from the satellite IR cloud picture we show that the present model performs quite realistically on the prediction of cloud cover. When radiative processes were not incorporated in the model, however, the cloud cover in the tropics is underestimated by as much as 50% due to the lack of the low cloud formation. Furthermore, we show that the present model with interactive radiative processes has improved the temperature prediction by about 1–2°C in middle latitudes and <1°C in the tropics. Statistical analyses show that this improvement is significant in low levels. Finally, we illustrate that the present model predicts accurately the mean 500 mb temperatures for nine consecutive days utilizing a data set covering the period from 7 to 15 June 1980.