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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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Petr Chýlek and J. A. Coakley Jr.

Abstract

For a given value of the solar constant we have found that the Budyko-type climate model gives two positions for the edge of the polar ice sheet. The results of the Budyko model suggest that with a decrease in the solar constant of about 1.6%, the ice cap reaches a latitude of about 50°. At this point the model breaks down and the solution for the position of the edge of the polar ice sheet becomes complex. We have based our conclusions on analytic investigations. Numerical calculations were used only to support our analytic results.

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G. Guo and J. A. Coakley Jr.

Abstract

Clouds and the Earth’s Radiant Energy System (CERES) uses a suite of instruments on the Terra and Aqua satellites combined with analyzed weather data and information on surface conditions to estimate surface radiative fluxes. CERES estimates for the Terra satellite were compared with measurements of the surface radiative fluxes collected with the research vessels (RVs) Wecoma and Thomas G. Thompson radiometers for cruises off the Oregon coast undertaken during 2000–03. To assess the shipboard measurements, the radiometer observations were analyzed to identify cloud-free conditions characterized by ∼1–2 h of relatively stable radiative fluxes. Fluxes for the cloud-free conditions were compared with those calculated using profiles of temperature and humidity from analyzed meteorological fields for the times and locations of the measurements and broadband radiative transfer models. For summertime conditions along the Oregon coast, and assuming a marine aerosol having 0.55-μm optical depth of 0.05, modeled and observed values of the shortwave flux agreed to within 1%–2%. Similar comparisons for the downward cloud-free longwave flux were within 1%–3%. This agreement also held for the CERES surface radiative flux estimates with CERES cloud-free fields of view for ocean scenes within 50 km of the ship being compared with 30-min averages of the shipboard measurements centered on the times of the Terra overpass. Using the CERES observations to identify cloud-free conditions for the Wecoma revealed that in some cases the shipboard measurements of the shortwave flux varied erratically. Criteria were adopted to avoid such behavior, yielding periods in which the surface radiative fluxes were reasonably stable for a range of cloud-free and cloudy conditions. With the criteria applied, the absolute magnitude of the mean differences between the shipboard measurements and the CERES estimates for the downward shortwave flux were within 2%, with RMS differences less than 6% within each month of CERES–shipboard matchups. The absolute magnitude of the mean differences for the downward longwave flux was less than 2%, with RMS differences less than 5%.

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Norman G. Loeb and J. A. Coakley Jr.

Abstract

The validity of plane-parallel (1D) radiative transfer theory for cloudy atmospheres is examined by directly comparing calculated and observed visible reflectances for one month of Global Area Coverage Advanced Very High Resolution Radiometer satellite observations of marine stratus cloud layers off the coasts of California, Peru, and Angola. Marine stratus are an excellent testbed, as they arguably are the closest to plane-parallel found in nature. Optical depths in a 1D radiative transfer model are adjusted so that 1D model reflectances match those observed at nadir on a pixel-by-pixel basis. The 1D cloud optical depth distributions are then used in the plane-parallel model to generate reflectance distributions for different sun–earth–satellite viewing geometries. These reflectance distributions are directly compared with the observations. Separate analyses are performed for overcast and broken cloud layers as identified by the spatial coherence method.

When 1D reflectances are directly compared with observations at different view angles, relative differences are generally small (≲10%) in the backscattering direction for solar zenith angles ≲60° and show no systematic view angle dependence. In contrast, 1D reflectances increase much more rapidly with view angle than the observed reflectances in the forward-scattering direction. Relative differences in the forward-scattering direction are ≈2–3 times larger than in the backscattering direction. At solar zenith angles ≳60°, the 1D model underestimates observed reflectances at nadir by 20%–30% and overestimates reflectances at the most oblique view angles in the forward scattering direction by 15%–20%. Consequently, when inferred on a pixel-by-pixel basis, nadir-derived cloud optical depths show a systematic increase with solar zenith angle, both for overcast and broken cloud layers, and cloud optical depths decrease with view angle in the forward scattering direction. Interestingly, in the case of broken marine stratocumulus, the common practice of assuming that pixels are overcast when they are not mitigates this bias to some extent, thereby confounding its detection. But even for broken clouds, the bias remains.

Because of the nonlinear dependence of cloud albedo on cloud optical depth, errors in cloud optical depth lead to large errors in cloud albedo—and therefore energy budget calculations—regardless of whether cloud layers are overcast or broken. These findings suggest that as a minimum requirement, direct application of the plane-parallel model approximation should be restricted to moderate–high sun elevations and to view angles in the backscattering direction. Based on Monte Carlo simulations, the likely reason for the discrepancies between observed radiances and radiances calculated on the basis of 1D theory is because real clouds have inhomogeneous (i.e., bumpy) tops.

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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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B. Sechrist, J. A. Coakley Jr., and W. R. Tahnk

Abstract

The response of already polluted marine stratocumulus to additional particles was examined by studying the clouds where two ship tracks cross. Nearly 100 such crossings were collected and analyzed using Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) multispectral imagery for the daytime passes off the western coast of the United States during the summer months of 4 years. To reduce biases in the retrieved cloud properties caused by the subpixel spatial structure of the clouds, results are presented only for ship tracks found in regions overcast by extensive layers of marine stratus. When two ship tracks cross, one of the tracks exhibits much larger changes in droplet radii when compared with the surrounding unpolluted clouds and is referred to as the dominant ship track. The clouds at the crossing typically exhibit properties that are closer to those of the dominant than to those of the subordinate ship track. To determine whether the additional particles at the crossing affect the dominant track, local gradients in the retrieved cloud properties near the crossing were determined for both ship tracks. Based on the gradients, the clouds at the junction were found to have significantly smaller droplet radii and significantly larger column droplet number concentrations than were predicted based on their values in both ship tracks on either side of the crossing. Comparing the effects of particle loading at the crossings and elsewhere along the ship tracks revealed that the effects decreased as the column droplet number concentration of the clouds being affected increased.

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Christopher S. Bretherton, E. Klinker, A. K. Betts, and J. A. Coakley Jr.

Abstract

Cloud fraction is a widely used parameter for estimating the effects of boundary-layer cloud on radiative transfer. During the Atlantic Stratocumulus Transition Experiment (ASTEX) during June 1992, ceilometer and satellite-based measurements of boundary-layer cloud fraction were made in the subtropical North Atlantic, a region typified by a 1–2 km deep marine boundary layer with cumulus clouds rising into a broken stratocumulus layer underneath an inversion. Both the diurnal cycle and day-to-day variations in low-cloud fraction are examined. It is shown that ECMWF low cloudiness analyses do not correlate with the observed variations in cloudiness and substantially underestimate the mean low cloudiness.

In these analyses, the parameterization of low cloud fraction is primarily based on the inversion strength. A comparison of ECMWF analyses and ASTEX soundings (most of which were assimilated into the analyses) shows that the thermodynamic structure of the boundary layer and the inversion strength are well represented (with some small but significant systematic biases) in the analyses and preserved (again with some biases) in 5-day forecasts.

However, even when applied to the actual sounding the ECMWF low cloud scheme cannot predict the observed day-to-day variations or the diurnal cycle in low cloud. Other diagnostic schemes based on lower tropospheric stability, cloud-top entrainment instability, boundary-layer depth, and vertical motion do equally poorly. The only successful predictor of low cloud frontier from sounding information is the relative humidity in the upper part of the boundary layer.

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D. A. Randall, J. A. Coakley Jr., C. W. Fairall, R. A. Kropfli, and D. H. Lenschow
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Jonathan P. Taylor, Martin D. Glew, James A. Coakley Jr., William R. Tahnk, Steven Platnick, Peter V. Hobbs, and Ronald J. Ferek

Abstract

The influence of anthropogenic aerosols, in the form of ship exhaust effluent, on the microphysics and radiative properties of marine stratocumulus is studied using data gathered from the U.K. Met. Office C-130 and the University of Washington C-131A aircraft during the Monterey Area Ship Track (MAST) experiment in 1994. During the period of MAST, stratocumulus clouds were studied during 11 flights and a wide range of levels of background pollution was observed. The impact of the aerosol emitted from the ships was found to be very dependent on the background cloud microphysical conditions. In clouds of continental influence, the susceptibility of the cloud to further aerosol emissions was low, with a correspondingly weak microphysics and radiation signature in the ship tracks. In clean clouds, changes in droplet concentration of a factor of 2, and reductions in droplet size of up to 50%, were measured. These changes in the microphysics had significant impacts on the cloud radiative forcing. Furthermore, as a result of the cloud droplet size being reduced, in some cases the drizzle was suppressed in the clean clouds, resulting in an increase in liquid water path in the polluted ship track environment. The impact of this combined change in liquid water path and droplet radius was to increase the cloud radiative forcing by up to a factor of 4.

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J. A. Coakley Jr., P. A. Durkee, K. Nielsen, J. P. Taylor, S. Platnick, B. A. Albrecht, D. Babb, F.-L. Chang, W. R. Tahnk, C. S. Bretherton, and P. V. Hobbs

Abstract

The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon, NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes, visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why the majority of ship tracks observed from satellites off the coast of California are found south of 35°N. A small rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when low-level clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.

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