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James A. Coakley Jr.

Abstract

An efficient method for computing the radiative–convective equilibrium temperature profile of a vertical–column atmospheric model is described. The equations which specify the equilibrium conditions are linearized to form a matrix equation in which the net radiative heating at levels within regions of radiative equilibrium and the net radiative flux at the tops of regions undergoing convective adjustment are linearly proportional to the differences between initial guess temperatures and equilibrium temperatures at all levels. Because the original equations am nonlinear in the temperature profile, the matrix elements of the linearized equation also depend on the temperature profile. As a result, the equilibrium temperature profile is obtained by sequentially solving the matrix equation. Sample calculations indicate that successive solutions of the matrix equation converge rapidly to the equilibrium temperature profile, and the equilibrium conditions are typically satisfied after four inversions.

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James A. Coakley Jr.

Abstract

Order-of-magnitude estimates are made for the feedbacks between the global mean surface temperature and the mean tropospheric lapse rate, the mean surface relative humidity and the mean atmospheric relative humidity profile. It is found that with the possible exception of the surface temperature-surface relative humidity feedback the magnitudes of these feedback are not sufficient to significantly alter the sensitivity of the global mean surface temperature to changes in the solar constant as calculated using a vertical-column energy balance model of the earth's atmosphere.

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James A. Coakley Jr.

Abstract

The results of simple zonal energy balance climate models are rather sensitive to the parameterizations used to calculate the fluxes of solar radiation absorbed, thermal radiation emitted and energy transported by the atmosphere and oceans. For this reason results are examined for North's (1975a) constant coefficient diffusion model using climatologically consistent radiation parameterizations. With these radiation parameterizations, the calculated climate is less sensitive to changes in the incident solar radiation than was previously found using other parameterizations. In addition, how the model's results are influenced by the biofeedback mechanism recently proposed by Cess (1978) is studied. This feedback accounts for changes in the surface albedo caused by changes in the vegetation that might accompany climate change. Based on the model results, this feedback could be an important link between the climate and the earth's orbit around the sun.

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Bruce A. Wielicki and James A. Coakley Jr.

Abstract

An error analysis is presented for cloud-top pressure and cloud-amount retrieval using infrared sounder data. Rms and bias errors are determined for instrument noise (typical of the HIRS-2 instrument on TIROS-N) and for uncertainties in the temperature profiles and water vapor profiles used to estimate clear-sky radiances. Errors are determined for a range of test cloud amounts (0.1–1.0) and cloud-top pressures (920–100 mb). Rms errors vary by an order of magnitude depending on the cloud height and cloud amount within the satellite's field of view. Large bias errors are found for low-altitude clouds. These bias errors are shown to result from physical constraints placed on retrieved cloud properties, i.e., cloud amounts between 0.0 and 1.0 and cloud-top pressures between the ground and tropopause levels. Middle-level and high-level clouds (above 3–4 km) are retrieved with low bias and rms errors. For instrument noise the 4.3 μm channels provide the smallest errors. For temperature profile and water vapor profile uncertainties the 15 μm channels generally give smaller errors. Errors due to rms temperature profile uncertainties of 2.0°C are found to be larger than errors due to instrument noise, independent of the sounding channels used.

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James A. Coakley Jr. and Bruce A. Wielicki

Abstract

Adopting procedures often applied to model the earth's climate, we use zonal fields from the control run of a general circulation climate model (GCM) (Wetherald and Manabe, 1975) to construct parameterizations of the energy budget components for use in. a simple Budyko-Sellers energy balance climate model (EBM) (North, 1975). Comparing the results of the GCM and the EBM for changes in solar constant, we find that with these parameterizations changes in the surface temperature calculated with the EBM are substantially larger than those calculated with the GCM. Furthermore, when the meridional energy transport in the EBM is held constant so that it simulates the transport in the GCM, the results of the two models diverge hopelessly. On the other hand, with the parameterizations of the EBM modified to simulate the behavior of the GCM fields, the results of the two models agree fairly well. This exercise illustrates the weakness of current methods used to extract parameterizations for EBMs from observations of the present climate.

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Fu-Lung Chang and James A. Coakley Jr.

Abstract

Studies using International Satellite Cloud Climatology Project (ISCCP) data have reported decreases in cloud optical depth with increasing temperature, thereby suggesting a positive feedback in cloud optical depth as climate warms. The negative cloud optical depth and temperature relationships are questioned because ISCCP employs threshold assumptions to identify cloudy pixels that have included partly cloudy pixels. This study applies the spatial coherence technique to one month of Advanced Very High Resolution Radiometer (AVHRR) data over the Pacific Ocean to differentiate overcast pixels from the partly cloudy pixels and to reexamine the cloud optical depth–temperature relationships. For low-level marine stratus clouds studied here, retrievals from partly cloudy pixels showed 30%–50% smaller optical depths, 1°–4°C higher cloud temperatures, and slightly larger droplet effective radii, when they were compared to retrievals from the overcast pixels. Despite these biases, retrievals for the overcast and partly cloudy pixels show similar negative cloud optical depth–temperature relationships and their magnitudes agree with the ISCCP results for the midlatitude and subtropical regions. There were slightly negative droplet effective radius–temperature relationships, and considerable positive cloud liquid water content–temperature relationships indicated by aircraft measurements. However, cloud thickness decreases appear to be the main reason why cloud optical depth decreases with increasing temperature. Overall, cloud thickness thinning may explain why similar negative cloud optical depth–temperature relationships are found in both overcast and partly cloudy pixels. In addition, comparing the cloud-top temperature to the air temperature at 740 hPa indicates that cloud-top height generally rises with warming. This suggests that the cloud thinning is mainly due to the ascending of cloud base. The results presented in this study are confined to the midlatitude and subtropical Pacific and may not be applicable to the Tropics or other regions.

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James A. Coakley Jr. and Robert D. Cess

Abstract

We insert the effect of naturally occurring tropospheric aerosols on solar radiation into the NCAR Community Climate Model (CCM). The effect of the aerosol depends on concentration and type (continental, maritime), surface albedo, solar zenith angle and cloud cover. The experiments were performed for perpetual July boundary conditions. Globally averaged, the aerosol reduces the solar radiative flux absorbed at the top of the model atmosphere by 3.0 W m−2, at the surface by 4.4 W m−2 and in the lower troposphere it increases the flux absorbed by 1.4 W m−2. Owing to fixed sea surface temperatures the climate simulated by the CCM is hardly affected by these perturbations. The global surface temperature change is −0.08 K (−0.27 K for continents), zonal surface temperature changes are limited to a few tenths of a degree and regional surface temperature changes rarely surpass −1 K. Between 30°S and 60°N the aerosol suppresses convective activity by reducing solar beating for land surfaces. As a result the upper troposphere, which for these regions and time of year is heated largely through moist convective processes, cools more so than the surface and lower troposphere which are directly affected by the interaction of the aerosol with solar radiation. Small but significant changes in climate are obtained for isolated portions of the globe. The most notable changes occurred for a region of Africa just north of the equator where the aerosol pushed the model towards “desertification.” That is, for this region the radiative forcing due to the aerosol gave rise to changes in convection and wind fields which in turn led to a significant reduction in precipitation. The processes involved in the changes wore like thaw discussed by Charney et al. for the role of surface albedo in desertification.

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James A. Coakley Jr. and Christopher D. Walsh

Abstract

One-kilometer Advanced Very High Resolution Radiometer (AVHRR) observations of the effects of ships on low-level clouds off the west coast of the United States are used to derive limits for the degree to which clouds might be altered by increases in anthropogenic aerosols. As ships pass beneath low-level clouds, particles from their plumes serve as condensation nuclei around which new cloud droplets form. The increased droplet concentrations lead to a decrease in droplet sizes. The change in sizes is manifested as an increase in the reflected sunlight observed at 3.7 μm in satellite imagery data. Images at 3.7 μm are used in a semiautomated procedure for identifying polluted portions of clouds and distinguishing them from the nearby unaffected portions. Radiances at 0.64, 3.7, and 11 μm are used to determine visible optical depths, droplet effective radii, and cloud emission temperatures for both the polluted and unpolluted portions. The analysis of several hundred 30-km segments of ship tracks reveals that changes in visible optical depths are about half the values expected, given the changes observed for the droplet radii and assuming cloud liquid water amount remains constant. Simple radiative transfer calculations indicate that the shortfall in the optical depth change is unlikely to be due solely to the absorption by the polluting particles. It is likely that polluted clouds lose liquid water. The equivalent loss is approximately 15%–20% of the initial cloud liquid water.

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William R. Tahnk and James A. Coakley Jr.

Abstract

A time history of the calibration coefficients for channels 1 and 2 of the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-12 and NOAA-15 spacecraft is presented. The history is based on reflectances observed for the interior zones of the Antarctic and Greenland ice sheets previously obtained with the NOAA-9 AVHRR, which serves as the calibration standard. Reflectances observed in December and January for the Antarctic ice sheet are used to characterize sensor performance. Reflectances observed in May and June for the interior of the Greenland ice sheet are used to detect any substantial midyear shifts in the coefficients. Prelaunch calibration coefficients for the NOAA-12 AVHRR are shown to be in error and the coefficients drift with time. The coefficients are compared with those reported in earlier studies, and the time rate of change of the calibration is found to be smaller than previously reported. The observations for the NOAA-15 AVHRR are consistent with the prelaunch calibration coefficients for channel 1 but indicate a slight shift in the coefficients for channel 2. The calibration coefficients for the NOAA-15 AVHRR appear to be stable. A slight drift in the response of channel 2 in the low-reflectance range is barely detectable.

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James A. Coakley Jr. and Takahisa Kobayashi

Abstract

Radiative transfer calculations for a one-dimensional column model of the atmosphere containing a plane-parallel, homogeneous cloud are used to show that the common procedure of assuming that fields of view for high resolution satellite imagers are either overcast or cloud-free will lead to biases in the infrared albedo and surface insulation for typical (250 km)2 weekly to monthly mean climate scales. The biases arise because cloud fields on the (1–8 km)2 scale typical of satellite imager resolution are often broken rather than overcast and because the anisotropy of the radiance held for overcast regions differs from that for cloud-flee regions. Furthermore, the anisotropy for overcast regions is a nonlinear function of cloud optical depth; consequently, there is no equivalent overcast system that will reproduce the anisotropy of the radiance field reflected by broken clouds. The results of the calculations indicate that the biases are a function of the sun-earth-satellite viewing geometry. For example, for a solar zenith angle of 30° and for typical satellite viewing geometries the biases are estimated to reach 5% in the planetary albedo and −5% in surface insulation. The biases increase with increasing solar zenith angle, but on average the percentage bias is fairly insensitive to cloud optical depth. The constancy of the percentage bias should allow it to be largely removed from climatological datasets.

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