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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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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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Thomas P. Charlock
and
William D. Sellers

Abstract

No abstract available.

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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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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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Thomas P. Charlock
and
Fred G. Rose

Abstract

The spatial and temporal relationships between fluctuations in geopotential height and high-cloud fractional area in low-pass (periods greater than 10 days) and intermediate-pass (10–30 days) time scales are investigated and compared with relationships in the fast-pass (2.5–6 days) time scale. NMC 500-hPa height and Nimbus-7 THIR/TOMS high-cloud data are used for extended 6-month (October-March) winters (1979–85). Summary correlation maps describe the spatial phase relationships between the heights and the clouds over the full domain of the northern extratropics.

As we move from the slower low-pass and intermediate-pass regimes to the fast-pass regime, the temporal variance of the height field decreases, but the temporal variance of the high cloudiness increases. Surprisingly, the 500-hPa height and high-cloud fields are more strongly correlated in the slower time scales. The spatial phase relationships between the height and cloud fields are generally different in the low-pass and fast-pass regimes.

Over portions of the jet exit regions, one-half of the variance in the low-pass cloudiness can be explained by a correlation with the height fluctuations at nearby points. Over these areas, the low-pass height fluctuations are approximately equivalent barotropic, and the heights are correlated with clouds quite strongly downstream (to the east) but only weakly upstream (to the west). This contrasts with the spatial phase relationships found in the more baroclinic fast-pass regime, where fluctuations in height are correlated with clouds by about the same absolute magnitude either downstream or upstream. An analysis of the temporal evolution of intermediate-pass height and cloud fields reveals the cloud signal of two-dimensional Rossby wave dispersion over a small portion of the northern extratropics.

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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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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
Fred G. Rose

Abstract

A time filter that passes waves with periods in the 2.5-6.0 day band is applied to a six-winter record of the Nimbus-7 THIR/TOMS high cloudiness and the NMC 500-mb geopotential height in the northern extratropics. The strongest correlations between fluctuations in geopotential and high cloudiness are found in the baroclinic waveguides, where the fields of both geopotential and high cloudiness exhibit 1arge variabilities. Over many grid points in the waveguides, positive anomalies in high cloud areas are found to be approximately one-third of a wavelength to the east of negative anomalies in 500-mb heights (band-pass troughs), and negative anomalies in high cloud arms are found to be approximately one-sixth of a wavelength to the west. A map of the standardized anomalies in the cloud area associated with height fluctuations above the mean forms a simple negative of the map of the cloud anomalies associated with height fluctuations below the mean. The analysis presented here suggests that the high cloud structures of baroclinic waves are less spatially coherent than the internal geopotential height structures. Over the North Pacific, small-scale (latitudinal wavenumber 13-18) fluctuations in geopotential appear to play a greater role in forcing high cloudiness than do medium-wale (latitudinal wavenumber 7-12) fluctuations in geopotential.

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