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Roger Davies

Abstract

The diffusional growth or evaporation of cloud droplets due to net emission or absorption of radiation is studied. Time dependent solutions for droplet temperatures and supersaturation are obtained, taking into account the partitioning of the net radiation budget between the droplets and the ambient air. Radiative perturbations are shown to cause extremely high rates of change in droplet temperatures. Due to efficient exchange of thermal energy within the cloud, the time constant for reducing these rates to the ambient rate is typically less than a few milliseconds and is approximately proportional to the square of the droplet radius. As the droplets evaporate or grow due to radiative effects, the saturation ratio of the ambient air adjusts due to changes in the water vapor density and the temperature of the air. The time constant for adjustment is found to be a few seconds, and the steady state saturation ratio decreases linearly with increased net radiation absorbed by the cloud. Droplet growth caused by longwave emission occurs under slightly supersaturated conditions.

The net radiation budgets of individual droplets and the supersaturation appear to affect the evolution of the droplet size distribution, but are not needed to assess radiatively induced changes in the cloud temperature and liquid water content, which depend only on the total radiation budget of the cloud.

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Lazaros Oreopoulos and Roger Davies

Abstract

Using the same satellite observations as in Part I of this paper, the authors explore ways to remove the cloud albedo bias (or plane parallel albedo bias), the difference between the plane parallel homogeneous albedo and the average albedo of independent pixels, in regions similar in size to climate model grid boxes.

Scaling regional mean optical depths with the reduction factor of R. F. Cahalan et al. provides albedos close to the independent pixel values. Computed albedos approach the independent pixel values within 0.01 for ∼40% of the regions tested and give standard deviations ∼0.02–0.04. Fitting lognormal distributions to the observed optical depth distributions gives albedos within 0.01 of the independent pixel values more than 70% of the time, with standard deviations ∼0.02–0.06. Gamma distributions are less successful than lognormal distributions, giving acceptable results (average bias ∼0.01–0.02, standard deviation ∼0.05–0.08) only when their parameters are estimated from the maximum likelihood estimates method. The poor performance of the gamma distribution when the method of moments is used for parameter estimation (as H. W. Barker et al. did) is attributed to the presence of high optical depth values in our retrieved fields.

To apply any of the above corrections in GCMs, quantities that are not presently provided by these models are required. The reduction factor and “gamma IP” method require the mean logarithm of optical depth, whereas the lognormal method also requires the variance. The authors suggest a parameterization of these quantities in terms of mean optical depth and cloud fraction, variables available in most GCMs. The albedos resulting from the parameterized versions of the correction methods are still much closer to the independent pixel values than the albedos of the plane parallel homogeneous assumption. Although the “lognormal IP” gives the best overall performance, it requires knowledge of two logarithmic moments and numerical integration. It may therefore prove more appealing for observational than modeling applications.

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Lazaros Oreopoulos and Roger Davies

Abstract

Due to cloud heterogeneity and the nonlinear dependence of albedo on cloud water content, the average albedo of a cloudy scene found by calculating the albedo of independent pixels within the scene tends to be different from the albedo calculated using the average cloud water in the scene. This difference, termed the plane parallel albedo bias (PPH bias), which has previously been estimated from limited case studies, is evaluated here for the first time using an extensive set of Advanced Very High Resolution Radiometer data over oceanic scenes. This dataset yields visible PPH biases that range from 0.02 to 0.30, depending in part on the size of the scene, the viewing–illumination directions, and the assumptions made retrieving cloud optical depths.

The PPH biases increase when atmospheric effects are accounted for but are relatively insensitive to assumptions about cloud microphysics. Due to the limitations of a one-dimensional retrieval, they tend to increase with solar zenith angle and to be larger in the backscattering than the forward scattering direction. Placed in the context of those general circulation models that do not provide subgrid-scale information on cloud amount, these biases are even larger. PPH biases in the broadband-reflected shortwave flux from general circulation models are estimated to exceed 30 W m−2, typically requiring the introduction of a compensatory bias in the model’s treatment of cloud water content.

The resolution of the satellite sensor and the averaging/sampling of the satellite substantially influences the calculated PPH bias. The authors find a significant drop in albedo bias (∼0.02–0.05) when averaging/sampling original local area coverage (LAC) data to global area coverage (GAC) resolution or when Landsat data were averaged to LAC resolution. These results, along with stochastic simulations of internal LAC pixel variability indicate that the bias discrepancies among variable resolution satellite data are mostly due to the neglect of subpixel cloud fraction, which makes clouds appear thinner than they actually are.

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Ákos Horváth and Roger Davies

Abstract

Satellite wind measurements represent an invaluable contribution to the description of the flow field over the oceans. Conventional cloud-tracking techniques suffer from the inability to simultaneously determine wind speed and height. Currently, the uncertainty in the independently calculated heights is the major factor limiting the accuracy of cloud motion winds. Near-simultaneous multiangle imagery from the multiangle imaging spectroradiometer (MISR) forms the basis of a unique method able to simultaneously retrieve cloud motion and height. The coupled motion and height parallaxes can be unscrambled from three properly selected multiangle views through a purely geometric, stereoscopic approach. Results based on simulated data indicate that for a mesoscale domain the average along-track and cross-track horizontal wind components may be obtained with an accuracy as good as 3–4 m s−1, and 1–2 m s−1, respectively, and with a corresponding height error of 300–400 m. The technique also possesses a limited capability to distinguish between low and high features moving at different velocities in a multilayer cloud field.

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Lazaros Oreopoulos and Roger Davies

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The relationship between sea surface temperature (SST) and albedo or cloud cover is examined for two tropical regions with high values of cloud radiative forcing and persistent marine stratocumulus (mSc)–one off the west coast of Peru, the other off the west cost of Angola. The data span five years, from December 1984 to November 1989. Albedos are from the Earth Radiation Budget Experiment, cloud covers are from the International Satellite Cloud Climatology Project, and SSTs are from the Climate Analysis Center.

Negative correlation coefficients between albedo and SST are found to be about −0.8 when the seasonal variation of the entire dataset is analyzed. The interannual variation and the spatial variation of individual months also yields correlation coefficients that are negative. The correlation between cloud cover and SST is found to be similar to but weaker than the correlation between albedo and SST, suggesting a decrease in cloud amount and a decrease in cloud albedo with increasing SST for these regions. The corresponding albedo sensitivity averages −0.018 K−1 with local values reaching −0.04 K −1. These findings are valid from 19°C to 25°*C for the Peru mSc and 22°C to 27°C for the Angola mSc. These temperatures approximately bound the domains over which mSc is the prevalent cloud type within each region.

These results imply a potential positive feedback to global warming by marine stratocumulus that ranges from ∼0.14 W m−2 K−1 to ∼1 W m−2 K−1, depending on whether or not our results apply to all marine stratocumulus. While these values are uncertain to at least ±50%, the sensitivity of albedo to see surface temperature in the present climate may serve as a useful diagnostic tool in monitoring the performance of global climate models.

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Tamás Várnai and Roger Davies

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This paper examines the processes through which cloud heterogeneities influence solar reflection. This question is important since present methods give numerical results only for the overall radiative effect of cloud heterogeneities but cannot determine the degree to which various mechanisms are responsible for it. This study establishes a theoretical framework that defines these mechanisms and also provides a procedure to calculate their magnitude. In deriving the framework, the authors introduce a one-dimensional radiative transfer approximation, called the tilted independent pixel approximation (TIPA). TIPA uses the horizontal distribution of slant optical thicknesses along the direct solar beam to describe the radiative influence of cloud heterogeneities when horizontal transport between neighbors is not considered. The effects for horizontal transport are then attributed to two basic mechanisms: trapping and escape of radiation, when it moves to thicker and thinner cloud elements, respectively.

Using the proposed framework, the study examines the shortwave radiative effects of cloud-top height and cloud volume extinction coefficient variations. It is shown and explained that identical variations in cloud optical thickness can cause much stronger heterogeneity effects if they are due to variations in geometrical cloud thickness rather than in volume extinction coefficient. The differences in albedo can exceed 0.05, and the relative differences in reflectance toward the zenith can be greater than 25% for overhead sun and 50% for oblique sun. The paper also explains a previously observed phenomenon: it shows that the trapping of upwelling radiation causes the zenith reflectance of heterogeneous clouds to increase with decreasing solar elevation.

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Larry Di Girolamo and Roger Davies

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The authors have developed a cloud mask technique that may be applied to the efficient selection of “clear enough” scenes for image navigation. While the mask can be applied generally, the motivation for its development comes from its intended use on Multiangle Imaging Spectroradiometer (MISR) imagery. The difficulties in detecting clouds in the presence of land–water boundaries when using prenavigated imagery is overcome by using a simple two-step direct threshold technique. The two steps involve the thresholding of two observables derived for each pixel. The first is a 0.86-μm reflectance. The second is a new observable, D = | NDVI |b R 1 −2, where NDVI = (R 2R 1)(R 2 + R 1)−1, R 2 is the 0.86-μm reflectance, R 1 is the 0.67-μm reflectance, and b is chosen so as to maximize the separation between clear and cloudy pixels. The success of the cloud mask is shown by applying it to degraded AVIRIS data. The authors make comparisons with a more popular NDVI technique to show the advantage of our method.

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Harshvardhan, James A. Weinman, and Roger Davies

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The transport of infrared radiation in a single cuboidal cloud has been modeled using a variable azimuth two-stream (VATS) approximation. Computations have been made at 10 μm for a Deirmendjian (1969) C-1 water cloud of single scattering albedo, ω = 0.638 and asymmetry parameter, g=0.865. Results indicate, that the emittance of the top face of the model cloud is always less than that for a plane parallel cloud of the same optical depth. The hemispheric flux escaping from the cloud top has a gradient from the censor to the edges which are warmer when the cloud is over warmer ground. Cooling rate calculations in the 8–13.6 μm region show that there is cooling out of the sides of the cloud at all levels even when there is heating of the core from the ground below.

The radiances exiting from model cuboidal clouds were computed by path integration over the source function obtained with the two-stream approximation. Results suggest that the brightness temperature measured from finite clouds will overestimate the cloud-top temperature.

Some key results of the model have been compared with Monte Carlo simulations. Overall errors in flux and radiance average a few degrees for most cases.

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Miriam Blaskovic, Roger Davies, and J. B. Snider

Abstract

Surface measurements made at San Nicolas Island during the intensive field observation marine stratocumulus phase of the First International Satellite Cloud Climatology Progam Regional Experiment, July 1987, are analyzed to retrieve the average diurnal variation of marine stratocumulus and related surface variables. Cloud thickness and integrated liquid-water content show a clear decrease during the day from sunrise to sunset, increasing thereafter. The average liquid-water density in the cloud is closely related to the cloud thickness, decreasing as the cloud thickness decreases. The cloud-base height has a diurnal range of 150 ± 30 m, rising from sunrise till midafternoon. The cloud-top height has a similar diurnal range of 130 ± 30 m, but the main descent occurs in the late afternoon. Surface air temperature also increases at sunrise, directly in phase with the cloud-base lifting, and has a diurnal range of 2°C.

The diurnal behavior of the cloud base appears to be consistent with model-predicted uncoupling of the cloud layer and the subcloud layer as the turbulent flux of moisture is inhibited by solar heating near the cloud base. Similarly, variation in surface air temperature is consistent with the inhibition of the turbulent flux of heat between the two layers, shielding the surface from the effect of longwave cooling from the cloud top. The variation in cloud-top height, however, does not appear to he readily explainable by present diurnal models.

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James A. Coakley Jr. and Roger Davies

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We report the observation of a feature that is characteristic of the reflection of solar radiation from absorbing, finite clouds. When absorption takes place, more radiation can be reflected by broken cloud fields than by extensive unbroken cloud fields. We observe this feature in solar radiation at 3.7 μm reflected by low-level, single-layered systems of water clouds over the Pacific Ocean. Interpreting the effect as due to geometrical factors, we note that absorption causes the reflected radiances to be highly anisotropic, so that they are generally greater from the sun-facing cloud sides than from the cloud tops. Diffusive leakage of radiation through the cloud sides is also reduced, and as a result maximum reflectivities occur in situations that maximize the contributions to the reflected radiation from the sides relative to that from the tops. Interpreting the effect as due to changes in liquid water content and cloud droplet sizes, we note that the observations at 0.63, 3.7 and 11 μm are consistent with a cloud model in which the liquid water content and droplet sizes are greater in the cloud centers and smaller at their edges. We present theoretical calculations that qualitatively support both interpretations.

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