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  • Author or Editor: J. A. Coakley Jr. x
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J. A. Coakley Jr. and G. W. Grams

Abstract

A simple radiative energy balance model has been developed to assess the impact of stratospheric aerosols on the global climate through their effect on the equilibrium global mean surface temperature. With the assumptions that the radiation in the atmosphere can be treated as diffuse radiation and that the effect of the gases in the stratosphere can be approximated by equivalent gray absorbers and scatterers, an analytic expression which depends only on the optical properties of the aerosol and the planetary albedo is derived for the fractional change in the upward flux of terrestrial infrared radiation at the base of the stratospheric aerosol layer. The fractional change in the upward flux of infrared radiation is then directly related to changes in the global mean surface temperature by using existing results of climate model and radiative convective model calculations. Mie theory is used to compute the scattering and absorbing properties of the aerosol for a range of visible and infrared indices of refraction. Sample calculations are presented that show the fractional change in the upward flux of infrared radiation at the base of the layer as a function of particle size for a specified mass concentration of stratospheric aerosols. The results indicate that both small particles (radii ≲0.05 μm) and large particles (radii ≳ 1.0 μm) generally have a greater influence on terrestrial infrared radiation than on incident solar radiation; therefore, these particles contribute to warming at the surface. Particles of intermediate sizes affect the incident solar radiation more strongly than they affect the terrestrial radiation and thereby contribute to cooling at the surface. The results also demonstrate the feasibility of estimating the largest possible surface temperature response to a given increase in the mass concentration of stratospheric aerosols. Calculations were also performed to enable comparison of the results from the present model with those obtained by approximating the effect of an increase in stratospheric aerosols by means of an equivalent reduction in the solar constant. It is shown that the effects of the aerosols on terrestrial radiation must be negligible, and the aerosols must be nonabsorbing at solar wavelengths in order for the results of the present model to agree with those obtained by assuming a reduction in the solar constant.

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J. A. Coakley Jr. and D. G. Baldwin

Abstract

We present an objective analysis scheme for deriving cloud properties from satellite imagery data for oceanic regions. The scheme is based on the spatial coherence method. As this method is applicable only to simple layered systems, we introduce a default estimate of the cloud cover when the systems become complex as in fronts and tropical disturbances. The steps of the scheme are as follows: 1) identify cloud-free regions and cloud layers within (250 km)2 frames; 2) for each (60 km)2 subframe evaluate the statistics of the radiance field needed to retrieve cloud cover; 3) cumulate the subframe statistics in a given geographical region for several days and construct from the cumulated cloud-free radiances a climatology for that region and time period; 4) derive for each (60 km)2 subframe instantaneous estimates of the cloud-free radiances and cloud properties at the time of satellite passing; 5) composite these (60 km)2 subframe results to form the desired space and time averages. We apply the analysis scheme to derive the cloud cover from NOAA-7 AVHRR GAC data for the orbits of three days and nights over the Pacific basin (0–50°N, 135°W–170°E). We find: 1) the statistics of the radiance field used to obtain the cloud cover represent a 15-fold reduction over the input data volume; 2) clouds will satisfy the conditions for spatial coherence retrievals typically for 30–50% of the (250 km)2 frames and for 50% of the (60 km)2 subframes; 3) the majority of (250 km)2 frames contain more than one identifiable layer of clouds; 4) less than 3% of the (60 km)2 subframes exhibit three identifiable layers suggesting that methods for treating one and two-layered systems on the mesoscale should prove adequate for the majority of maritime cloud cases; 5) the typical uncertainty of an instantaneous cloud cover estimate for a (250 km)2 frame is ΔAc ∼ 0.14. Owing to cancellation of random errors, we expect the uncertainty in the corresponding monthly mean cloud cover to be considerably smaller. In preparing satellite data for analysis, one first reads and converts the bit stream into calibrated radiances. Once the data are in the form of calibrated radiances, the additional computer time required to analyze cloud properties is approximately equal to the computer time needed to read and convert the satellite bit stream.

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