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Ming-Dah Chou

Abstract

Transmission functions associated with water vapor molecular line and e-type absorption in the IR spectral regions are presented in the form of simple analytical functions and small tables, from which atmospheric IR fluxes and cooling rates can be easily computed. For typical clear atmospheres ranging from the tropics to the subarctic region, the difference with respect to line-by-line calculations is <0.15°C day−1 in the cooling rate and ≲1% in fluxes.

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Ming-Dah Chou

Abstract

A simple scheme is developed to infer cloud amount, optical thickness, and height from satellite-measured radiances for use in surface radiation calculations. The essence of the cloud scheme is to specify a cloud reflectivity threshold for identifying pixels that are likely to be totally filled with clouds. Area-averaged values for the three cloud parameters are derived from the reflectivities of these cloudy pixels and the satellite-measured radiances in the visible and IR window channels. By applying the cloud scheme to the ISCCP (International Satellite Cloud Climatology Project) B3 radiance data and using a radiation routine, the surface radiative fluxes are computed for the tropical and subtropical western Pacific regions (30°S-30°N, 100°E–180°). It is found that the three cloud parameters are quite sensitive to the cloud reflectivity threshold, but the surface radiation is rather insensitive to the set of cloud parameters inferred by a scheme that is directly dependent upon the satellite radiance measurements. Over a broad area in the tropical and subtropical western Pacific regions, the difference in the net surface radiative fluxes is <2 W m−2 for the cloud reflectivity thresholds of 0.3 and 0.4.

This study further strengthens the view of other investigators that the net solar flux at the surface can be linearly related to the net solar flux at the top of the atmosphere. For a given amount of solar radiation absorbed by the earth–atmosphere system, the partition of the absorption between the surface and the atmosphere is affected by the solar zenith angle. As a result, the solar zenith angle has a significant effect on the relationship between the fluxes at the surface and at the top of the atmosphere.

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Ming-Dah Chou

Abstract

Monthly surface radiative fluxes in the tropical Pacific between January 1970 and February 1978 have been calculated using a radiative transfer package which includes detailed treatments of the molecular and droplet absorptions and of the surface and cloud reflections. The input data to the radiation package are surface measurements from the National Climatic Center, climatological temperature and humidity profiles from the National Center for Atmospheric Research, and cloud cover from the University of Hawaii. Results show that the distribution of surface radiation follows closely that of cloudiness and, to a lesser extent, that of humidity. Surprisingly, the distribution of IR radiation can hardly be correlated to the surface temperature. Based upon the expected range of uncertainties in the input data, the rms error in the calculated net surface radiation is estimated to be ∼15 W m−2 with the largest contributions from the uncertainties in cloud cover and humidity. This number is comparable to the interannual variation of the monthly net surface radiation, indicating a need to improve the quality of input data. The sensitivity of surface radiation to input data has also been studied. In order to resolve the interannual variations of the monthly net surface radiation, some accuracy requirements for satellite retrievals of atmospheric and cloud parameters are suggested.

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Ming-Dah Chou

Abstract

The method originally developed by Chou and Arking (1981) for computing the absorption of solar radiation by water vapor has been extended to the computations of transmittance and radiance in infrared water vapor sounding channels. It utilizes the wing-scaling approximation and the k-distribution approach. The effects of the instrument response function, zenith angle and the variation of the Planck radiance with wavenumber can be easily and accurately included in computing transmittance and radiance. The method can be effectively applied to any tropospheric water vapor sounding channels in the infrared. Compared to line-by-line calculations, which can be considered as the most accurate but too time consuming to be operational, the rms error in transmittance of the method at any pressure level is less than 0.009 for the three operational HIRS/2 water vapor sounding channels. The rms error in brightness temperature is less than 0.2°C which is about a factor of 2–5 smaller than the instrument noise.

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Ming-Dah Chou

Abstract

The total absorption of solar radiation by water vapor in clear atmosphere is parameterized as a simple function of the scaled water vapor amount. For applications to cloudy and hazy atmospheres, the flux-weighted k-distribution functions are computed for individual absorption bands and for the total near-infrared region. The parameterization is based upon monochromatic calculations and follows essentially the scaling approximation of Chou and Arking, but the effect of temperature variation with height is taken into account in order to enhance the accuracy. Furthermore, the spectral range is extended to cover the two weak bands centered at 0.72 and 0.82 μm. Comparisons with monochromatic calculations show that the atmospheric heating rate and the surface radiation can be accurately computed from the parameterization. Comparisons are also made with other parameterizations. It is found that the absorption of solar radiation can be computed reasonably well using the Goody band model and the Curtis-Godson approximation.

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Ming-Dah Chou

Abstract

A solar radiation routine has been developed for use in climate studies. It includes the absorption and scattering due to ozone, water vapor, oxygen, carbon dioxide, clouds, and aerosols. Rayleigh scattering is also included. The UV and visible region (λ < 0.69 μm) is grouped into four bands. An effective coefficient for ozone absorption and an effective cross section for Rayleigh scattering are computed for each band. In the near-infrared region (λ > 0.69 μm), the broadband parameterization is used to compute the absorption by water vapor in a clear atmosphere, and the k-distribution method is applied to compute fluxes in a scattering atmosphere. The reflectivity and transmissivity of a scattering layer are computed analytically using the delta-four-stream discrete-ordinate approximation. The two-stream adding method is then applied to compute fluxes for a composite of clear and scattering layers. Compared to the results of high spectral resolution and detailed multiple-scattering calculations, fluxes and heating rate are accurately computed to within a few percent.

The high accuracy of flux and heating rate calculations is achieved with a reasonable amount of computing time. With the UV and visible region grouped into four bands, this solar radiation routine is useful not only for climate studies but also for studies on the photolysis in the upper atmosphere and the photosynthesis in the biosphere.

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Ming-Dah Chou

Abstract

The response of radiation budgets to changes in water vapor and clouds in an El Niño episode is investigated using the analyzed sea surface temperature and satellite-derived clouds and the earth radiation budgets for the tropical Pacific (30°N–30°S, 100°E–100°W). Analyses are performed for April 1985 and April 1987. The former is a non-El Niño year and the latter is an El Niño year. Compared to April 1985, when the SST over the central and eastern equatorial Pacific is approximately 2°C lower, the high-level cloudiness in April 1987 increases in the central and eastern equatorial Pacific. Corresponding to the increase in cloudiness, the outgoing longwave radiation and the net downward solar radiation at the top of the atmosphere decrease. The patterns of thew changes are reversed in the western tropical Pacific and the Northern Hemispheric (NH) subsidence region centered at approximately 20°N, indicating an eastward shift of the convection center from the maritime continents to the central equatorial Pacific and a strengthened NH Hadley circulation.

The earth-atmosphere system in the region receives less radiative energy by 4 W m−2 in the warmer month of April 1987 than in the month of April 1985, which is primarily caused by a reduced atmospheric clear sky greenhouse effect in the NH tropical Pacific in April 1987. Clouds have strong effect on both the IR and solar radiation, but the net cited on the radiation budget at the top of the atmosphere changes only slightly between April 1985 and April 1987. The results are consistent with Lindzen's hypothesis that reduced upper-tropospheric water vapor in the vicinity of the enhanced convection region produces cooling that counteracts warming in the Tropics.

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Ming-Dah Chou

Abstract

Simple and accurate parameterizations have been developed for computing the absorption of solar radiation due to O2 and CO2. The parameterizations are based on the findings that temperature has a minimal effect on the absorption and that the one-parameter scaling can be applied to take into account the effect of pressure variation along a path. Furthermore, overlapping of the absorption due to CO2 and water vapor is treated accurately in the parameterizations. Simulations with a zonally averaged multilayer energy balance model show that the absorption of solar radiation due to O2 and CO2 has a small, albeit nonnegligible, effect on climate. The global surface solar radiation is reduced by 2.2 W m−2, and the warming of the surface temperature due to a doubled CO2 concentration is reduced by 10% in the Northern Hemisphere. Because the parameterizations can be easily implemented without perturbing other parts of a radiation routine, it is suggested that the absorption of solar radiation by O2 and CO2 be included in climate studies using numerical models.

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Chidong Zhang and Ming-Dah Chou

Abstract

In the troposphere of the equatorial western Pacific, the water vapor variability dominates the temperature variability in changing the clear-sky infrared (IR) cooling rate. The large water vapor variability, especially its bimodal distribution at certain levels of the upper troposphere, leads to distinct structures of the clear-sky IR radiative cooling rate. The IR cooling rate, its maximum normally in the upper troposphere (∼300 hPa) and minimum in the lower troposphere (∼650 hPa), tends to become vertically uniform when the upper troposphere is abnormally dry. A local, maximum IR cooling rate may occur in the boundary layer when the lower troposphere becomes extraordinarily dry. The changes in IR cooling due to the water vapor variability affect the rate of generation of convective available potential energy (CAPE) and the conditional instability for deep convection. Little or no mean rainfall over an area of roughly 3 × 105 km2 is observed when either the rate of generation of CAPE suffers from a reduction (magnitude of 50 J kg−1 day−1) or IR cooling decreases with height. The observed variability of water vapor results from both local vertical processes and the large-scale horizontal circulation. Horizontal advection accounts for a large fraction of the drying that is responsible for the changes in the IR cooling profile and in the atmospheric instability for deep convection. These results suggest that interactions among water vapor, radiation, and deep convection must be assessed by fully taking the large-scale circulation into consideration. This study is based on an analysis of upper-air soundings collected during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment Intensive Observing Period and calculations of a radiative transfer model.

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Ming-Dah Chou and Albert Arking

Abstract

An efficient method has been developed to compute the absorption of solar radiation by water vapor. The method is based on the molecular line parameters compiled by McClatchey et al. (1973) and makes use of the far-wing scaling approximation and k-distribution approach previously applied by Chou and Arking (1980) to the computation of the infrared cooling rates. The entire near-IR spectrum between 0.83 and 4 μm is treated as one region with the effect of the variation of the incoming solar flux with wavenumber incorporated into precomputed functions. For clear atmospheres, the solar fluxes are computed from a table in which the scaled water vapor amount is the independent variable. For cloudy atmospheres, the k-distribution method is used. Using the line-by-line method as a standard, the maximum error introduced by this method is ∼4% of the peak heating rate. The present method has the additional advantage over previous methods in that it can be applied to any portion of the spectral region containing the water vapor bands.

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