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Ronald M. Welch and Bruce A. Wielicki

Abstract

Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) digital data are used to remotely sense fog properties. These include fog cell size distribution, cell aspect ratio (the ratio of the length of the major and minor axes of the cells), and cell orientation angle. The analysis is carried out for four fog scenes, three high-inversion radiation fogs in central California, and one advection fog in eastern South Dakota.

Results for these initial fog studies indicate that 1) fogs are stratocumulus in nature, being composed of individual cellular structures; 2) the reflectance properties vary strongly across the cells, suggesting considerable variation in liquid water content; 3) fogs often are patchy, often revealing surface features between fog cells; 4) the ratio of wavelength (λ) between cells and the height of the boundary layer (h) is λ/h ≈ 2–3, in agreement with values obtained for Benard cells and longitudinal rolls observed in cloud systems; 5) the typical horizontal aspect ratio of fog cells is about a factor of 2; and 6) observed quasi-periodic oscillations of measured fog variables may be caused by advection of the cellular structures across the observational site.

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Ronald M. Welch and Bruce A. Wielicki

Abstract

Reflected fluxes are calculated for broken cloudiness (i.e., nonplane parallel) as a function of cloud cover, cloud optical depth, solar zenith angle and surface albedo. These calculations extend previous results for broken cloud reflected fluxes over a black surface.

The present study demonstrates that not only radiances but also radiative fluxes over high albedo surfaces may be decreased by the presence of broken cloudiness. Conventional wisdom states that cloud radiances(brightnesses) are always greater than the background. While most cloud retrieval schemes are built around this assumption, it is incorrect for clouds over high albedo surfaces such as found in polar regions. However, the most startling and counterintuitive conclusion of this study is that nonabsorbing finite clouds over a highly reflecting surface will decrease the system albedo. As a result, surface absorption is increased, the result of multiple scattering between surface and cloud layer, controlled by cloud morphology and cloud optical thickness.

A simple parameterization of the effects of cloud contamination upon retrieved albedo is given in terms of solar zenith angle, cloud optical depth, surface albedo, cloud cover, and plane-parallel could albedo. In this way, the effects of broken cloudiness are modeled in terms of easily computed plane-parallel values.

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Bruce A. Wielicki and Ronald M. Welch

Abstract

Landsat Multispectral Scanner (MSS) digital data are used to remotely sense cumulus cloud properties such as cloud fraction and cloud reflectance, along with the distribution of cloud number and cloud fraction as a function of cloud size. The analysis is carried out for four cumulus fields covering regions approximately 150 km square. Results for these initial cloud fields indicate that: (i) the common intuitive model of clouds as nearly uniform reflecting surfaces is a poor representation of cumulus clouds, (ii) the cumulus clouds were often multicelled, even for clouds as small as 1 km in diameter, (iii) cloud fractional coverage derived using a simple reflectance threshold is sensitive to the chosen threshold even for 57-meter resolution Landsat data, (iv) the sensitivity of cloud fraction to changes in satellite sensor resolution is less sensitive than suggested theoretically, and (v) the Landsat derived cloud size distributions show encouraging similarities among the cloud fields examined.

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Ronald M. Welch, Stephen K. Cox, and John M. Davis

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No Abstract available.

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Xiang Li, Sundar A. Christopher, Joyce Chou, and Ronald M. Welch

Abstract

Using a new angular distribution model (ADM) for smoke aerosols, the instantaneous top-of-atmosphere (TOA) shortwave aerosol radiative forcing (SWARF) is calculated for selected days over biomass-burning regions in South America. The visible and infrared scanner data are used to detect smoke aerosols and the Clouds and the Earth’s Radiant Energy System (CERES) scanner data from the Tropical Rainfall Measuring Mission are used to obtain the broadband radiances. First, the ADM for smoke aerosols is calculated over land surfaces using a discrete-ordinate radiative transfer model. The instantaneous TOA shortwave (SW) fluxes are estimated using the new smoke ADM and are compared with the SW fluxes from the CERES product. The rms error between the CERES SW fluxes and fluxes using the smoke ADM is 13 W m−2. The TOA SWARFs per unit optical thickness for the six surface types range from −29 to −57 W m−2, showing that smoke aerosols have a distinct cooling effect. The new smoke ADM developed as part of this study could be used to estimate radiative impact of biomass-burning aerosols.

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Jan Paegle, Wilford G. Zdunkowski, and Ronald M. Welch

Abstract

The Crank-Nicholson method may not give useful results in detailed prediction of the thermal planetary boundary layer unless tune steps on the order of 10 s are used. In similar problems, lower order time differencing methods give reasonable results with time steps as large as 300 s. The reason for the superior behavior of the lower order schemes relative to straightforward application of the Crank-Nicholson technique is due to a better treatment of short waves which appear to be critically important in nonlinear terms.

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Bryan A. Baum, Vasanth Tovinkere, Jay Titlow, and Ronald M. Welch

Abstract

A fuzzy logic classification (FLC) methodology is proposed to achieve the two goals of this paper: 1) to discriminate between clear sky and clouds in a 32 × 32 pixel array, or sample, of 1.1-km Advanced Very High Resolution Radiometer (AVHRR) data, and 2) if clouds are present, to discriminate between single-layered and multilayered clouds within the sample. To achieve these goals, eight FLC modules are derived that are based broadly on airmass type and surface type (land or water): equatorial over land, marine tropical over land, marine tropical/equatorial over water, continental tropical over land, marine polar over land, marine polar over water, continental polar over land, and continental polar/arctic over water. Derivation of airmass type is performed using gridded analyses provided by the National Centers for Environmental Prediction.

The training and testing data used by the FLC are collected from more than 150 daytime AVHRR local area coverage scenes recorded between 1991 and 1994 over all seasons and over all continents and oceans. A total of 190 textural and spectral features are computed from the AVHRR data. A forward feature selection method is implemented to reduce the number of features used to discriminate between classes in each FLC module. The number of features selected ranges from 13 (marine tropical over land) to 24 (marine tropical/equatorial over water). An estimate of the classifier accuracy is determined using the hold-one-out method in which the classifier is trained with all but one of the data samples; the classifier is applied subsequently to the remaining sample.

The overall accuracies of the eight classification modules are calculated by dividing the number of correctly classified samples by the total number of manually labeled samples of clear-sky and single-layer clouds. Individual module classification accuracies are as follows: equatorial over land (86.2%), marine tropical over land (85.6%), marine tropical/equatorial over water (88.6%), continental tropical over land (87.4%), marine polar over land (86.8%), marine polar over water (84.8%), continental polar over land (91.1%), and continental polar/arctic over water (89.8%). Single-level cloud samples misclassified as multilayered clouds range between 0.5% (continental polar over land) and 3.4% (marine polar over land) for the eight airmass modules.

Classification accuracies for a set of labeled multilayered cloud samples range between 64% and 81% for six of the eight airmass modules (excluded are the continental polar over land and continental polar/arctic over water modules, for which multilayered cloud samples are difficult to find). The results indicate that the FLC has an encouraging ability to distinguish between single-level and multilayered clouds.

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Wilford G. Zdunkowski, Ronald M. Welch, and Jan Paegle

Abstract

A dynamic-numerical model is utilized to study the impact of air pollution on the temperature and wind distributions of the planetary boundary layer. The mathematical model uses a rather complete radiative treatment which comprises the entire solar and infrared spectrum ranging from 0.29 to 100 µm. In the solar spectral range, the absorption by water vapor, nitrogen dioxide and industrial haze is fully accounted for in addition to multiple scattering by air molecules and haze particles. In the spectral region of the strong absorption hands of the infrared emission spectrum, the effect of aerosol is very small and is disregarded. The emissivity method is applied here, allowing full treatment of the overlapping effects of water vapor and carbon dioxide. In the window region, however, the effect of aerosol and water vapor absorption and emission is taken into account in addition to multiple scattering by aerosol particles. The radiative treatment accounts for the influence of relative humidity on the particle distribution function and on the complex index of refraction of the aerosol. The spherical harmonic method is used to handle the scattering problem.

The dynamical part of the analysis consists of the numerical solution of a coupled system of partial differential equations comprising the equation of horizontal mean motion, the thermodynamic equations of the air and the soil, and the transport equations of moisture and pollution. Various models of the exchange coefficient are used to study the impact of model assumptions on the computed distributions of temperature, pollutant material and wind. It is found that the choice of the exchange model is not critical but has some effect on the model computations. The present calculations show that the maximum impact of air pollution on the evolution of temperature and wind profiles is highly significant, thus verifying the previous conclusions of Zdunkowski and McQuage (1972).

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Qingyuan Han, William B. Rossow, Joyce Chou, and Ronald M. Welch

Abstract

The most common approach used to model the aerosol indirect effect on clouds holds the cloud liquid water path constant. In this case, increasing aerosol concentration increases cloud droplet concentration, decreases cloud droplet size, and increases cloud albedo. The expected decrease in cloud droplet size associated with larger aerosol concentrations has been found to be larger over land than over water and larger in the Northern than in the Southern Hemisphere, but the corresponding cloud albedo increase has not been found. Many previous studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds when aerosols change. This study examines the relationship between geographic and seasonal variations of cloud effective droplet size and cloud albedo, as well as cloud liquid water path, in low-level clouds using International Satellite Cloud Climatology Project data. The results show that cloud albedo increases with decreasing droplet size for most clouds over continental areas and for all optically thicker clouds, but that cloud albedo decreases with decreasing droplet size for optically thinner clouds over most oceans and the tropical rain forest regions. For almost all clouds, the liquid water path increases with increasing cloud droplet size.

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Sundar A. Christopher, Min Wang, Todd A. Berendes, Ronald M. Welch, and Shi-Keng Yang

Abstract

Using satellite imagery, more than five million square kilometers of the forest and cerrado regions over South America are extensively studied to monitor fires and smoke during the 1985 biomass burning season. The results are characterized for four major ecosystems, namely, 1) tropical rain forest, 2) tropical broadleaf seasonal, 3) savanna/grass and seasonal woods (SGW), and 4) mild/warm/hot grass/shrub (MGS). The spatial and temporal distribution of fires are examined from two different methods using the multispectral Advanced Very High Resolution Radiometer Local Area Coverage data. Using collocated measurements from the instantaneous scanner Earth Radiation Budget Experiment data, the direct regional radiative forcing of biomass burning aerosols is computed. The results show that more than 70% of the fires occur in the MGS and SGW ecosystems due to agricultural practices. The smoke generated from biomass burning has negative instantaneous net radiative forcing values for all four major ecosystems within South America. The smoke found directly over the fires has mean net radiative forcing values ranging from −25.6 to −33.9 W m−2. These results confirm that the regional net radiative impact of biomass burning is one of cooling. The spectral and broadband properties for clear-sky and smoke regions are also presented that could be used as input and/or validation for other studies attempting to model the impact of aerosols on the earth–atmosphere system.

These results have important applications for future instruments from the Earth Observing System (EOS) program. Specifically, the combination of the Visible Infrared Scanner and Clouds and the Earth’s Radiant Energy System (CERES) instruments from the Tropical Rainfall Measuring Mission and the combination of Moderate Resolution Imaging Spectrometer and CERES instruments from the EOS morning crossing mission could provide reliable estimates of the direct radiative forcing of aerosols on a global scale, thereby reducing the uncertainties in current global aerosol radiative forcing values.

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