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Kuo-Nan Liou and Szu-Cheng S. Ou

Abstract

We have developed a thermodynamic model for the determination of the temperature profile based on the balance of radiative and turbulent fluxes. In the context of a one-dimensional case, we show that the temperature field is governed by a first-order differential-integro equation involving temperature to the fourth power. In conjunction with the temperature profile determination, parameterization programs for the transfer of broadband thermal infrared and solar fluxes in inhomogeneous atmospheres are developed. In addition, the vertical transport is parameterized using a first-order closure scheme with the eddy thermal diffusion coefficients derived either from known theories or from available data. By virtue of an efficient perturbation technique devised for the solution of the thermal equilibrium temperature, we show that the simulated temperatures compare well with those of a standard atmosphere employing climatological water vapor, ozone and cloud profiles. Simulations using the steady-state one-dimensional model also reveal that high clouds will produce warming everywhere in the atmosphere whereas middle and low clouds will generate cooling as noted by several previous investigators, Moreover, extension of the model to a two-dimensional case is accomplished by incorporating empirical data for the horizontal heat transport. Simulated temperature profiles for the tropics (0–30°) and midlatitude (30–60°) compare closely with the climatological data except in the vicinity of the tropopause. For the subarctic case (60–90°), simulated temperatures are colder than the climatological values within about 5 K in the troposphere. Finally, two-dimensional heat budget analyses reveal that there is a slight gain of energy in the tropical region of the Northern Hemisphere.

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Kuo-Nan Liou and Szu-Cheng S. Ou

Abstract

Parameterization of the transfer of infrared fluxes is developed for an atmosphere containing nonblack and semi-transparent clouds. The concept of parameterization makes use of the predetermined broadband cloud emissivity, transmissivity and reflectivity as functions of the cloud liquid water/ice content. In conjunction with the cloud radiative properties, broadband emissivities for water vapor, carbon dioxide and ozone are derived from band parameters in which the effects of pressure and temperature on absorption are accounted for in the path length based on a number of physical adjustments. Infrared cooling rates computed from the parameterized broadband model for clear and cirrus cloudy atmospheres show close agreement with those obtained from a more sophisticated band-by-band model with accuracy within about 0.2°C day−1.

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Szu-Cheng S. Ou and Kuo-Nan Liou

Abstract

Based on the thermodynamic energy balance between radiation and vertical plus horizontal dynamic transports, a two-dimensional radiation-turbulence climate model is developed. This model consists of a broadband solar and IR radiation transfer scheme previously presented by the authors and vertical and horizontal dynamic eddy transports utilizing the elementary turbulent theory. In the model, three kinds of feedback mechanisms are considered: the humidity feedback via the constant relative humidity assumption, the ice-albedo feedback via a preliminary correlation between the surface albedo and the surface temperature and the dynamic transport feedback through the parameterization of eddy transports and the prescribed mean wind field. A standard temperature field, which differs from the climatological data by no more than 0.1°C, is first obtained by solving the coupled thermodynamic and surface flux budget equations using climatological distributions of H2O, CO2, O3, surface albedo and cloud properties. The model-derived atmospheric radiation budget, surface energy balance and horizontal transport patterns compare reasonably well with available observational data. Further validation of the model includes sensitivity studies on the effects of doubling of CO2 and a 2% increase in the solar constant. The temperature changes relative to the standard field on these experiments agree closely with those presented by Manabe and Wetherald utilizing a general circulation model. To investigate the two-dimensional cirrus–radiation interaction, a relationship between the cirrus IR emissivity and solar reflectance (and transmittance) is established based on the parameterization equations. On the basis of a number of experiments involving various couplings and feedbacks, we find that 1) the humidity and albedo feedbacks are most active in the tropics and arctic area, respectively, 2) the dynamic transport is a negative feedback in the equatorial and polar regions but a positive one in midlatitudes and 3) the relative importance of each feedback depends only slightly on the radiative properties of cirrus. Finally, we demonstrate that slight variations in the cirrus IR emissivity lead to significant temperature perturbations in the arctic surface and tropical troposphere.

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Ming-Dah Chou, Guoliang Ji, Kuo-Nan Liou, and Szu-Cheng S. Ou

Abstract

The difficulties encountered in the derivation of surface radiation budget in arid regions are studied using the surface and satellite data measured during the preliminary field experiment for the Land-Atmosphere Interactions Experiment conducted at the Heihe River basin in western China. The surface radiation is derived by coupling theoretical radiative calculations with satellite cloud retrievals. Comparisons with the surface measurements of solar and thermal IR fluxes show that a large error in the computed surface fluxes occurs in some cases. The error is attributable to the lack of aerosol data, the uncertainty in cloud retrievals, and the time difference between the surface and satellite measurements.

For cloud-free cases, the modeled downward solar fluxes are systematically larger than the measured fluxes. The major cause of the error appears to be the failure to include aerosols in the calculations. The error is particularly law in the afternoon hours when the ground temperature is very high (>50°C) and the atmosphere dust content is large due to an unstable boundary layer. We find that the error can be reduced and that a good agreement between the computed and measured surface solar fluxes can be obtained by using an aerosol single-scattering albedo of 0.5 and an optical thickness of '0.2 in the afternoon hours. Nevertheless, the reason for the strong absorption of solar radiation in the atmosphere remains unclear.

For all the cases studied when both surface and satellite data are available, the mean errors are 4.3 and −4.7 W m−2 for the net downward surface solar flux and the downward surface IR flux, respectively. The rms errors are 17.4 and 22.1 W m−2 for the respective surface fluxes. The relatively law errors found in the cases with small cloud amounts can be explained by the fact that aerosols are often misinterpreted as clouds in the cloud retrievals.

Results of this study reemphasize the importance of aerosols in surface radiation calculations. Because the diurnal variation of ground temperature is very large in the and region, reliable calculations of surface IR radiation require high temporal resolution for temperature measurements. Aerosol and ground-temperature retrievals from satellite data should be the highest priority in the computations of surface radiation budget over arid regions.

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