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Thomas P. Charlock

Abstract

Two versions of the LOWTRAN5 radiance code are used in a study of the earth's clear sky infrared radiation budget in the interval 30 cm−1 (333.3 μm) to 3530 cm−1 (2.8 μm). One version uses 5 cm−1 resolution and temperature dependent molecular absorption coefficients, and the second uses 20 cm−1 resolution and temperature independent molecular absorption coefficients. Both versions compare well with Nimbus 3 IRIS spectra, with some discrepancies at particular wavenumber intervals.

Up and downgoing fluxes, calculated as functions of latitude, are displayed for wavenumbers at which the principle absorbers are active. Most of the variation of the fluxes with latitude is found in the higher wavenumber intervals for both clear and cloudy skies. The main features of the wavenumber integrated cooling rates are explained with reference to calculations in more restricted wavenumber intervals. A tropical lower tropospheric cooling maximum is produced by water vapor continuum effects in the 760–1240 cm−1 window. A secondary upper tropospheric cooling maximum, with wide meridional extent, is produced by water vapor rotational lines between 30–430 cm−1. Water vapor lines throughout the terrestrial infrared spectrum prevent the upflux maximum from coinciding with the surface temperature maximum.

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Thomas P. Charlock

Abstract

Calculations made with a one-dimensional radiative-convective climate model show that the optical properties of clouds in solar wavelengths can be a stabilizing factor in climate change. Cloud reflectivity and absorption have been calculated as functions of cloud liquid water content using the parameterization of Liou and Wittman (1979). With the assumption that cloud liquid water content increases (decreases) as temperature and absolute humidity increase (decrease) during a climate perturbation, cloud reflectivity increases (decreases), damping the original perturbation by a few tens of percent. This probably should be regarded as an upper limit on the amount of negative feedback which changes in solar wavelength cloud optical properties (for fixed cloud area) can provide in a global climate model.

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Takashi Yamanouchi and Thomas P. Charlock

Abstract

Radiative fluxes at the top of the atmosphere (TOA) and the surface were compared at two Antarctic stations, Syowa and the South Pole, using Earth Radiation Budget Experiment (ERBE) data and surface observations. Fluxes at both sites were plotted against cloud amounts derived from surface synoptic observations. Throughout the year over the snow- and ice-covered Antarctic, cloud radiation was found to heat the surface and cool the atmosphere; cloud longwave (LW) effects were greater than cloud shortwave (SW) effects. Clouds have a negligible effect on the absorption of SW by the atmosphere in the interior, and clouds slightly increase the absorption of SW by the atmosphere along the coast. At the TOA, the LW cloud effect was heating along the coast in summer and winter, heating in the interior during summer, and slight cooling in the interior during winter. This unique TOA cloud LW cooling was due to the extremely low surface temperature in the interior during winter. At the TOA, clouds induced SW cooling in the interior and along the coast; sorting of pixel-scale ERBE data and surface cloud observations was needed to demonstrate this. The monthly averaged fluxes at the surface and TOA were compared, and the net radiative fluxes for the atmospheric column were estimated. The atmospheric column loses net radiant energy throughout the year with an asymmetrical seasonal variation. The loss of net radiant energy by the atmosphere is much larger than the loss by the surface.

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Thomas P. Charlock and V. Ramanathan

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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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Thomas P. Charlock and William L. Smith Jr.
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Thomas P. Charlock and William D. Sellers

Abstract

Three radiative-convective climate models are used to investigate the temperature changes caused by the presence of aerosol. One uses meridional heat transport (obtained from another model) and heat storage, in addition to solar and infrared radiation, to simulate the climatic effect of aerosols at selected latitude belts on a monthly, time-marching basis. A second neglects heat storage and calculates an annually averaged steady-state temperature distribution at a particular latitude belt. The third is the usual globally averaged radiative-convective model, which employs radiation as the only energy source/sink. A highly modified form of the adding radiative-transfer scheme, which splits incoming beams into either direct or diffuse streams, is used to calculate aerosol effects in solar wavelengths. The present atmospheric aerosol induces roughly comparable cooling in all three models.

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Zhonghai Jin, Thomas P. Charlock, and Ken Rutledge

Abstract

A coupled atmosphere–ocean radiative transfer model has been applied to analyze a full year of broadband solar irradiances (up and down) measured over an ocean site 25 km east of the coast of Virginia in the Atlantic. The coupled model treats absorption and scattering by layers for both the atmosphere and the ocean explicitly and consistently. Key input parameters for the model (aerosol optical depth, wind speed, and total precipitable water) are also from in situ measurements. Having more observations to specify properties of the atmosphere than of the ocean, better model–observation agreement is obtained for the downwelling irradiance, which depends primarily on the atmospheric optical properties, than for the upwelling irradiance, which depends heavily on the ocean optical properties. The mean model–observation differences for the ocean surface albedo are generally less than 0.01. However, the modeled upwelling irradiances and albedo over the ocean surface are mostly less than the observations for all seasons, implying that more scattering in the ocean needs to be included in the model calculations. Sensitivity tests indicate that the uncertainties in aerosol optical properties, chlorophyll concentration, wind speed, or foams are not the primary factors for the model–observation differences in the ocean surface albedo, whereas the scattering by air bubbles and/or by suspended materials have the potential to significantly reduce or eliminate the model–observation differences in the ocean surface reflection.

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Thomas P. Charlock and William D. Sellers

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No abstract available.

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Thomas P. Charlock and Timothy L. Alberta

Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison.

The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insolation is larger than the measurements for clear-sky and total-sky conditions.

This experiment has been developed to test retrievals of radiative fluxes and the associated forcings by clouds, aerosols, surface properties, and water vapor. Collaboration is sought; the goal is to extend the domain of meteorological conditions for which such retrievals can be done accurately. CAGEX Version 1 covers April 1994. Subsequent versions will (a) at first span the same limited geographical area with data from October 1995, (b) then expand to cover a significant fraction of the GEWEX Continental-Scale International Project region for April 1996 through September 1996, and (c) eventually be used in a more advanced form to validate CERES.

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Thomas P. Charlock and William D. Sellers

Abstract

Changes in aerosol concentrations could imply changes in cloud condensation nuclei concentrations, which could in turn affect cloud optical properties. Calculations made with a one-dimensional radiative-convective model show that this can be a significant mechanism for climate change.

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