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V. Ramaswamy and V. Ramanathan

Abstract

Radiative transfer calculations employing observed values of the ice crystal size distribution demonstrate that the absorption of solar radiation by cirrus clouds can make a significant contribution to the diabatic heating of the upper troposphere. The effects due to this absorption on the upper tropospheric (100–300 mb) thermal profile are investigated in a general circulation model (GCM) with interactive clouds; guided by observations, two experiments are performed assuming vastly different vertical profiles of the ice water density. Solar heating rates within the extensive cirrus decks associated with monsoon and other convective clouds reach values of 1.5 K day−1. Thus, cirrus solar heating can be an important source for east-west asymmetries in the tropical diabatic heating. Furthermore, because of the latitudinal gradients in the solar insolation, cirrus solar absorption can also influence the meridional beating gradients within the upper troposphere.

In spite of the significant east-west asymmetries in the imposed cirrus solar heating, the change in the GCM tropical temperatures is nearly zonally uniform. The magnitude of the zonal mean tropical temperature changes in the GCM (up to 5°K at P ≈ 165 mb) indicate that lack of cirrus solar heating may be one reason for the cold bias of the GCMS. Furthermore, the shortwave beating can also account for the observed lapse rate stabilization in the upper troposphere.

In addition to the solar effect, the longwave radiative effects of cirrus can also be important but their sign and magnitude are very sensitive to the vertical distribution of clouds. Cirrus longwave heating rates can range from large negative values (cooling) when overlying optically thick clouds (for example, in “deep” extended systems with base below the upper troposphere) to large positive values (heating) for “anvil” type cirrus located in the upper troposphere and with no other clouds below. For the overcast portions of the tropics, if “anvil” type cirri are the only clouds of significance in the upper troposphere, the longwave heating would be the dominant radiative component and this effect becomes more pronounced with increasing altitude of cloud location. Hence, for the tropical zone as a whole, the sign and magnitude of the longwave effect depends on the relative composition of the “deep” and “anvil” clouds. Radiation model calculations that employ climatological values of the vertical distribution of clouds yield a longwave heating effect for the cirrus with the magnitude being comparable to the solar effect.

Thus, our results suggest a significant role for the cirrus radiative effects in maintaining the zonal mean thermal structure of the upper troposphere. This inference should be contrasted with the notion that the steep positive gradient in the tropical upper-troposphere potential temperatures is maintained by the latent heat released in penetrating cumulus towers.

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J. Li and V. Ramaswamy

Abstract

No abstract available.

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J. Li and V. Ramaswamy

Abstract

This paper presents a four-stream extension of the δ-Eddington approximation by considering the higher-order spherical harmonic expansion in radiative intensity. By using the orthogonality relation of the spherical harmonic functions, the derivation of the solution is fairly straightforward. Calculations show that the δ-four-stream spherical harmonic expansion approximation can reduce the errors in reflection, transmission, and absorption substantially in comparison with the δ-Eddington approximation. For the conservative scattering case, the error of the new model is generally less than 1% for optical thickness greater than unity except for gracing incident solar beam. For nonconservative scattering cases (single scattering albedo ω=0.9), the error is less than 5% for optical thickness greater than unity, in contrast to errors of up to 20% or more under the δ-Eddington approximation. This model can also predict the azimuthally averaged intensity to a good degree of accuracy. The computational time for this model is not as intensive as for the rigorous numerical methods, owing to the analytical form of the derived solution.

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Andrew Detwiler and V. Ramaswamy

Abstract

Results from one-dimensional cirrus cloud model simulations in the absence of upward velocities are used to show that the growth/sublimation of the ice particles in the cloud, and the fact that they are falling, can be important factors in determining the net heating rate in the air through which these clouds settle. The vertical profiles of the heating rate inside the cloud are stretched as a result of the settling of the cloud. Results for clouds at various altitudes in both midlatitude and tropical atmospheres are compared.

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Yi Ming and V. Ramaswamy

Abstract

The equilibrium temperature and hydrological responses to the total aerosol effects (i.e., direct, semidirect, and indirect effects) are studied using a modified version of the Geophysical Fluid Dynamics Laboratory atmosphere general circulation model (AM2.1) coupled to a mixed layer ocean model. The treatment of aerosol–liquid cloud interactions and associated indirect effects is based upon a prognostic scheme of cloud droplet number concentration, with an explicit representation of cloud condensation nuclei activation involving sulfate, organic carbon, and sea salt aerosols. Increasing aerosols from preindustrial (1860) to present-day (1990) levels leads to a decrease of 1.9 K in the global annual mean surface temperature. The cooling is relatively strong over the Northern Hemisphere midlatitude land owing to the high aerosol burden there, while being amplified at high latitudes. When being subject to aerosols and radiatively active gases (i.e., well-mixed greenhouse gases and ozone) simultaneously, the model climate behaves nonlinearly; the simulated increase in surface temperature (0.55 K) is considerably less than the arithmetic sum of separate aerosol and gas effects (0.86 K). The thermal responses are accompanied by the nonlinear changes in cloud fields, which are amplified owing to the surface albedo feedback at high latitudes. The two effects completely offset each other in the Northern Hemisphere, while gas effect is dominant in the Southern Hemisphere. Both factors are crucial in shaping the regional responses. Interhemispheric asymmetry in aerosol-induced cooling yields a southward shift of the intertropical convergence zone, thus giving rise to a significant reduction in precipitation north of the equator, and an increase to the south. The simulations show that the change of precipitation in response to the simultaneous increases in aerosols and gases not only largely follows the same pattern as that for aerosols alone, but that it is also substantially strengthened in terms of magnitude south of 10°N. This is quite different from the damping expected from adding up individual responses, and further indicates the nonlinearity in the model’s hydrological response.

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V. Ramaswamy and A. Detwiler

Abstract

The important microphysical relationships determining the radiative properties and growth of ice crystals in stratiform cirrus clouds are investigated. A horizontally infinite cloud layer is modeled in the midlatitude upper troposphere. Optical properties of spheres of equal surface area are assumed to represent the scattering characteristics of nonspherical crystals, while the delta-Eddington approximation is used to solve the radiative transfer equations.

Classical expressions for ice particle growth and sublimation are coupled to those for radiative energy exchange in order to follow ice particle evolution within the cloud. The radiative properties of the clouds influence the balance among the cloud physical processes within the cloud. In the top 5 percent of optically thin clouds, the ice particle energy balance is essentially between latent and heat diffusion. In the case of clouds with large optical depths, the energy balance is between latent heat and radiation, i.e., radiative cooling enhances particle growth by vapor deposition. In the lower 5 percent of optically thin or thick clouds, latent heat and radiation are balanced by the diffusion of heat from the particle to the environment. Here, upwelling radiation enhances particle sublimation at cloud base. Environmental ice saturation ratio is the primary factor determining the energy balance during growth of ice crystals. When the ice saturation ratio is ∼1, crystal growth rates are small, and radiative heating/cooling exercises a strong influence. However, for ice saturation ratios more than a percentage above or below unity, radiative influences on growth rates of crystals with lengths less than 200 μm are negligible.

We have followed the one-dimensional temporal evolution of 1-km thick cirrus cloud layers subsiding in still air. Crystals at cloud top grow larger with time while those at cloud base sublimate as the cloud settles into dry air, with the vertical fall distance greater for larger initial crystal lengths. The temporal evolution of the cloud microphysical characteristics results in modification of the radiation fields, both within the cloud and at the cloud boundaries.

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Yi Ming and V. Ramaswamy

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This study investigates how anthropogenic aerosols, alone or in conjunction with radiatively active gases, affect the tropical circulation with an atmosphere/mixed layer–ocean general circulation model. Aerosol-induced cooling gives rise to a substantial increase in the overall strength of the tropical circulation, a robust outcome consistent with a thermodynamical scaling argument. Owing to the interhemispheric asymmetry in aerosol forcing, the zonal-mean (Hadley) and zonally asymmetrical components of the tropical circulation respond differently. The Hadley circulation weakens in the Northern Hemisphere but strengthens in the Southern Hemisphere. The resulting northward cross-equatorial moist static energy flux compensates partly for the aerosol radiative cooling in the Northern Hemisphere. In contrast, the less restricted zonally asymmetrical circulation does not show sensitivity to the spatial structure of aerosols and strengthens in both hemispheres. The results also point to the possible role of aerosols in driving the observed reduction in the equatorial sea level pressure gradient.

These circulation changes have profound implications for the hydrological cycle. Aerosols alone make the subtropical dry zones in both hemispheres wetter, as the local hydrological response is controlled thermodynamically by atmospheric moisture content. The deep tropical rainfall undergoes a dynamically induced southward shift, a robust pattern consistent with the adjustments in the zonal-mean circulation and in the meridional moist static energy transport. Less certain is the magnitude of the shift. The nonlinearity exhibited by the combined hydrological response to aerosols and radiatively active gases is dynamical in nature.

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Yi Huang and V. Ramaswamy

Abstract

The variability and change occurring in the outgoing longwave radiation (OLR) spectrum are investigated by using simulations performed with a Geophysical Fluid Dynamics Laboratory coupled atmosphere–ocean–land general circulation model. First, the variability in unforced climate (natural variability) is simulated. Then, the change of OLR spectrum due to forced changes in climate is analyzed for a continuous 25-yr time series and for the difference between two time periods (1860s and 2000s). Spectrally resolved radiances have more pronounced and complex changes than broadband fluxes. In some spectral regions, the radiance change is dominated by just one controlling factor (e.g., the window region and CO2 band center radiances are controlled by surface and stratospheric temperatures, respectively) and well exceeds the natural variability. In some other spectral bands, the radiance change is influenced by multiple and often competing factors (e.g., the water vapor band radiance is influenced by both water vapor concentration and temperature) and, although still detectable against natural variability at certain frequencies, demands stringent requirements (drift less than 0.1 K decade−1 at spectral resolution no less than 1 cm−1) of observational platforms. The difference between clear-sky and all-sky radiances in the forced climate problem offers a measure of the change in the cloud radiative effect, but with a substantive dependence on the temperature lapse rate change. These results demonstrate that accurate and continuous observations of the OLR spectrum provide an advantageous means for monitoring the changes in the climate system and a stringent means for validating climate models.

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Petr Chylek and V. Ramaswamy

Abstract

We have derived a simple approximation for the emissivity and flux emissivity of water clouds inside the atmospheric window between 8 and 14 m. In our approximation the emissivity in the 8–11.5 m band is a function only of the cloud's liquid water content and cloud thickness. When compared with the exact radiative transfer calculations the broad-band flux emissivities (in the 8–11.5 m region) differ by less than 10%. At wavelengths > 11.5 m the emissivity is a function of the droplet size distribution as well. By considering a typical droplet size distribution for stratus, altostratus and cumulus clouds, we have shown that the effect of the size distribution on the broad-band flux emissivity in the 8–14 m band is about 35%. Our approximation should be useful for treatment of cloud infrared properties in climate models.

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C-T. Chen and V. Ramaswamy

Abstract

The sensitivity of the global climate to perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a static mixed layer ocean with fixed cloud distributions and incorporating a new broadband parameterization for cloud radiative properties. A series of GCM experiments involving globally uniform perturbations in cloud liquid water path or effective radius (albedo perturbations), along with one for a doubling of carbon dioxide (greenhouse perturbation), lead to the following results: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent (to ∼10%) of the sign, magnitude, and the spatial pattern of the forcings considered, thus revealing a linear and invariant nature of the model's global-mean response. 2) Although the total climate feedback is very similar in all the experiments, the strengths of the individual feedback mechanisms (e.g., water vapor, albedo) are different for positive and negative forcings. 3) Changes in moisture, tropospheric static stability, and sea ice extent govern the vertical and zonal patterns of the temperature response, with the spatial distribution of the response being quite different from that of the radiative forcing. 4) The zonal surface temperature response pattern, normalized with respect to the global mean, is different for experiments with positive and negative forcings, particularly in the polar regions of both hemispheres, due to differing changes in sea ice. 5) The change in the surface radiative fluxes is different for the carbon dioxide doubling and cloud liquid water path decrease experiments, even though both cases have the same radiative forcing and a similar global-mean surface temperature response; this leads to differences in the vigor of the hydrologic cycle (evaporation and precipitation rates) in these two experiments.

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