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Baijun Tian
and
V. Ramanathan

Abstract

In this paper diagnostic estimates of cloud radiative forcing (CRF) and clear-sky radiation budget at the surface and in the atmosphere, based on satellite-observed radiation budget at the top of the atmosphere (TOA) and empirical parameterizations derived from radiation models and field observations, are presented. This analysis is restricted to the tropical Pacific. High clouds over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), and the warm pool (WP) exert a positive CRF of about 70 W m−2 within the atmosphere and a negative CRF of about −70 W m−2 at the surface, although with a negligible net CRF at the TOA. On the other hand, low clouds over the eastern subtropical Pacific and the equatorial cold tongue exert a negative CRF of about −20 W m−2 at the surface as well as in the atmosphere. The spatial gradients of the clear-sky radiation budget at the surface and in the atmosphere are small. In particular, it is shown that the clear-sky radiative cooling in the atmosphere is larger over the ITCZ, the SPCZ, and the WP, when compared with that over the subtropics and the cold tongue. Next, based on these diagnostic estimates and available surface turbulent heat flux data, the role of atmospheric CRF in the large-scale atmospheric moist static energy (MSE) transport is quantified. It is found that the atmospheric CRF provides the major energy source for balancing the divergence of MSE transport (from the ITCZ, the SPCZ, and the WP to the subtropics and the cold tongue) by the large-scale atmospheric circulation. On the other hand, the clear-sky radiative flux convergence and the surface turbulent heat fluxes have just the reverse spatial pattern and hence cannot satisfy the large-scale atmospheric MSE transport requirements.

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Baijun Tian
and
V. Ramanathan

Abstract

A simple moist model for the large-scale tropical atmospheric circulation is constructed by combining the simple models of Gill and Neelin and Held. The model describes the first baroclinic mode of the moist troposphere with variable “gross moist stability” in response to given thermodynamic forcing from surface evaporation and atmospheric cloud radiative forcing (CRF), which is a measure of the radiative effects of clouds in the atmospheric radiative heating. When the present model is forced solely by the observed atmospheric CRF, quantitatively reasonable Hadley and Walker circulations are obtained, such as the trades, the ascending branches in the intertropical convergence zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), as well as the descending branches in the cold tongue and subtropics. However, when the model is forced only by the observed surface evaporation, the Walker circulation totally disappears, and the Hadley circulation reverses. These results indicate that, in the context of a moist dynamic model, the spatial variations of atmospheric CRF are more important in terms of driving and maintaining the Hadley and Walker circulations than the spatial variation of surface evaporation.

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V. L. Galinsky
and
V. Ramanathan

Abstract

The solution of the three-dimensional radiative transfer equation in weakly horizontally inhomogeneous medium has been obtained in the diffusion approximation using the expansion of the three-dimensional delta-Eddington approximation. The solution approach, referred as the gradient correction (GC) method, expands the horizontal fluxes and the source function in terms of the horizontal gradient of the extinction coefficient and/or the cloud-top boundary. In the transfer equation, only the zeroth- and first-order gradient terms are retained and hence the following limitations apply. First, the length of the horizontal variations of optical properties of the medium should be large in comparison to the mean radiative transport length. Second, the ratio of the vertical to horizontal scales should be small enough so that fluxes from boundaries may be neglected.

Since there are no restrictions to the amplitude of the optical properties variations, this method may even be applicable to a medium with strong horizontal variations of optical properties, as long as scales of the variations are large enough in comparison to the radiative transport length. The analytical solutions are in excellent agreement with the more accurate numerical solutions. The solution also shows the solar zenith angle dependence of the albedo, similar to that observed in analyses of satellite imagery.

The GC approach may be useful as a fast and computationally inexpensive method both for the correction of the independent pixel approximation used for extraction of cloud fields from satellite imagery and possibly for the calculation of the radiation fluxes in climate models.

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V. Ramanathan
and
K. R. Saha

Abstract

A primitive equation, limited-area, barotropic model with east-west cyclic boundary conditions is used to predict the movement of “Western Disturbances” in the Asian subtropics in two cases. Forecast and verification charts up to 72 hr and relevant error statistics are presented. Results are encouraging enough for the study of more such cases, and for the application of the model for low-latitude monsoon depressions and tropical cyclones.

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Thomas P. Charlock
and
V. Ramanathan

Abstract

A spectral general circulation model (GCM) is run for perpetual January with fixed sea surface temperature conditions. It has internally generated, variable cloud optical properties as well as variable cloud arm and heights. The cloud optics are calculated as functions of the cloud liquid water contents. The cloud liquid water contents are in turn generated by the model hydrological cycle. Model generated and satellite albedos are in rough agreement. An analysis of the cloud radiative forcing indicates that cloud albedo (cooling) effects overcome cloud infrared opacity (heating) effects in most regions, which is in accord with the inferences from satellite radiation budget measurements. Furthermore, both the computed and observed albedo of clouds decrease from low to high attitudes. When compared to a version of the model with fixed cloud optics, the model with variable cloud optics produces significantly different regional albedos especially over the tropics. The cloud droplet size distribution is also found to have a significant impact on the model albedos. The temperature of the tropical upper troposphere is somewhat sensitive to the microphysical characteristics of the model cirrus clouds.

The present study is an attempt to calculate the regional albedo of the planet more rigorously than has been done previously. Simplifying assumptions relating to cloud droplet size and lifetime must still be made. The model's results for the radiation budget are encouraging and it seems that the hydrological cycles of GCMs are sufficiently realistic to warrant a more physically based (than the one employed here) treatment of cloud microphysical and radiative processes.

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J. T. Kiehl
and
V. Ramanathan

Abstract

In the 12–18 μm spectral region, the CO2 bands are overlapped by the H2O pure rotational band and the H2O continuum band. The 12–18 μm H2O continuum absorption is neglected in most studies concerned with the climatic effects of increased CO2. In this study, we examine the role of H2O–CO2 overlap in detail. Specifically, the effect of the water vapor continuum in the 12–18 μm region on the radiative heating due to increased CO2 is investigated. It is found that although the longwave surface radiative heating due to increased CO2 is considerably reduced at low latitudes by H2O continuum absorption, where water vapor partial pressures are high, the radiative heating of the surface/troposphere system as a whole is minimally altered.

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V. Ramanathan
and
Robert E. Dickinson

Abstract

The role of ozone in the tropospheric-surface energy balance is discussed in the context of its latitudinally and seasonally varying modulation of solar and longwave energy fluxes. We analyze in detail the various radiative energy inputs to the stratosphere and the radiative fluxes from the stratosphere to the troposphere. To a very close approximation, on an annual and hemispherical mean, longwave emission from the stratosphere balances the absorbed radiant energy. The stratosphere absorbs about twice as much longwave radiation from the troposphere as it does solar radiation. About 20% of the longwave flux from the stratosphere to the troposphere is directly due to O2. A change in O3 concentrations perturbs the stratospheric and tropospheric-surface energy balances through a number of distinct mechanisms involving changes in solar and longwave fluxes and which are separated into direct effects due to the change of O3 and indirect effects due to the accompanying change of stratospheric temperature. The relative importance of average versus spatial varying changes in ozone is examined. We confirm the result of past analyses that, for a uniform change in ozone, the global average perturbation in radiative fluxes to the troposphere and surface is small since the perturbations in solar and infrared fluxes nearly cancel. However, this result probably severely underestimates the contribution of changes in the distribution of ozone to global climate. First, there generally are significant latitudinal and seasonal variations in the perturbation radiative fluxes to the troposphere. Second, vertical redistribution of Oe can produce larger perturbation fluxes to the troposphere than do uniform changes, and possibly of opposite sign. Third, most of the perturbation solar heating is deposited at or near the earth's surface, whereas much of the perturbation longwave fluxes are deposited in the upper troposphere with consequences for changes in the tropospheric lapse rates. Thus, it is difficult to evaluate the changes in tropospheric radiation needed to determine a change of climate due to a change of O3. Not even the net global average perturbation radiative fluxes to the troposphere can be calculated without knowing the change of the vertical ozone profile, and the vertical and latitudinal variations of the troposphere-surface perturbation heating rates are likely to be more important for climate change than the net global values.

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V. Ramanathan
and
W. L. Grose

Abstract

This paper examines the effects on the seasonal stratospheric circulation due to the following two longwave radiative transfer processes: 1) radiative coupling between the troposphere and lower stratosphere and 2) latitudinal and seasonal variation in the radiative response time h −1, where h is the Newtonian cooling coefficient. Two numerical experiments have been performed with a quasi-geostrophic nine-level global circulation model, in which one includes the two longwave processes mentioned above and the second experiment neglects both processes. The seasonal variations of the zonal temperatures and winds as simulated by the two experiments are compared to isolate the importance of the two longwave radiation processes.

According to the model results, the troposphere-lower stratosphere radiative coupling resulting from the exchange of longwave energy contributes about 5–10 K to the latitudinal and seasonal variation of tropopause and lower stratosphere temperatures. At the model tropopause, the O3 heating, both solar and longwave, reaches maximum values in the midlatitude region during winter and hence the O3 heating acts in conjunction with the dynamical beating processes to maintain the warm midlatitude belt in the lower stratosphere.

The large seasonal and latitudinal variations in h have significant effects on the latitudinal temperature gradients and on the seasonal variations of the temperature gradients in the middle and upper stratosphere. The results suggest that the effect of the seasonal and latitudinal variation of h is to cool the winter polar upper stratosphere and to enhance the winter and spring pole-to-equator temperature gradients.

We also examine the effect on the stratospheric circulation due to a perturbation in O3 concentration at the model tropopause. This experiment indicates that the sharp reversal of vertical temperature gradient at the equatorial tropopause may be due to the steep vertical gradient in the O3 solar and 9.6 µm beating above the tropopause.

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Tsing-Chang Chen
and
V. Ramanathan

Abstract

Two numerical experiments previously described by Ramanathan and Grose (1978) have been performed to test the climatological response of a three-dimensional stratospheric circulation model over an entire annual cycle to two different longwave radiation schemes in the stratosphere: 1) Experiment 1, a detailed radiative transfer model, and 2) Experiment 2, a simple Newtonian cooling (or heating) model. The energetics of these two experiments are analyzed in this study. It is found that the eddy energy variables-eddy kinetic energy KE ; eddy available potential energy AE , the generation of AE , G(AE ); and vertical propagation of eddy geopotential energy V E )-exhibit annual variations with maximum values in winter and minima in summer. On the other hand, the zonal energy variables-zonal kinetic energy KZ ; zonal available potential energy AZ ; and generation of AZ , G(AZ )-undergo semiannual variations with major maximum values in winter, minor maxima in summer and minimum values in spring and fall. Analysis of the energy cycle shows that V E ) is the primary energy source for energetics in the lower stratosphere, while V E ) and G(AZ ) are the energy sources for energetics in the upper stratosphere. The seasonal variations of eddy energy variables are under the control of V E ), while those of the zonal energy variables are vitally affected by the latitudinally differential diabatic heating, especially in the summer upper stratosphere.

The comparison study reveals clearly that the experiment that employes the detailed radiation model has significantly larger eddy kinetic energy and eddy available potential energy on the annual mean. Detailed analysis of the two experiments indicates the several ways in which the longwave radiation processes may affect the stratospheric energetics: 1) the temperature dependence of the Newtonian cooling coefficient h causes an increase of G(AZ ) and enhances the seasonal variation of G(AZ ) in the upper stratosphere; 2) the temperature dependence of h also augments the transmissivity of winter and spring season upper stratosphere to propagating planetary waves; and 3) the exchange of radiative energy between the troposphere and lower stratosphere alters the zonal wind profile during the winter season. The altered zonal wind profile facilitates the propagation of planetary waves within the lower stratosphere. Due to points 2) and 3) above, the magnitudes of eddy energies are much larger in Experiment 1, which employs the detailed radiation model.

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R. D. Cess
and
V. Ramanathan

Abstract

Cloud amount, as a climate feedback mechanism, encompasses separate and competing albedo and infrared feedbacks. In the present note we illustrate that the relative role of the infrared feedback mechanism cannot be determined from a model calculation, unless the model has the capability of predicting how the amounts of individual cloud layers change in relation to a change in total cloud cover.

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