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The formation of fair-weather cumuli (FWC) has been analyzed in this study based on both a simple mixed layer model and a subset of the data collected from the Atmospheric Radiation Measurement (ARM) Program at the southern Great Plains (SGP) site. By analyzing conditions for the formation of FWC, the authors illustrate how different processes--such as the surface heat fluxes, the surface thermodynamic conditions, the entrainment processes at the boundary layer top, the vertical thermodynamic structure above the boundary layer, and large-scale subsidence--control the formation of clouds. The results of the analysis show that it is the highly nonlinear interaction among these factors that gives rise to the formation of FWC. For this reason, the occurrence of FWC may not simply follow changes in the surface conditions. The analysis indicates that the entrainment of moisture and surface processes play important roles in the formation of FWC, and the net effects of these processes can be evaluated' by a parameter (1 - [[beta].sub.2])/B, where [[beta].sub.2] is the ratio between the entrainment moisture flux and the surface moisture flux, and B is the extended Bowen ratio defined as the ratio of the surface buoyancy flux to the surface latent heat flux. The stratification above the inversion is another key parameter that influences cloud formation. The weaker the stability, the greater the potential for cloud formation. In most situations the net effect of subsidence is to reduce the relative, humidity at the top of the mixed layer and thus is unfavorable for cloud formation, but the intensity of this reduction may vary depending on conditions of the boundary layer. In some specific conditions such as a moist boundary layer over an area with relatively small surface Bowen ratio, the net effect of subsidence on the relative humidity budget at the top of the mixed layer can be weak even though subsidence reduces the mixed layer depth substantially. In this study, some issues related to cloud onset and fractional cloudiness are also discussed based on the ARM SGP observational data.

An improved parameterization is presented for estimating effective atmospheric emissivity for use in calculating downwelling longwave radiation based on temperature, humidity, pressure, and solar radiation observations. The first improvement is the incorporation of an annual sinusoidal variation in effective clear-sky atmospheric emissivity, based on typical climatological variations in near-surface vapor pressure. The second is the continuous estimation of fractional cloudiness by taking the ratio of observed solar radiation to a modeled clear-sky solar radiation. Previous methods employed observer-estimated fractional cloudiness. Data from the Atmospheric Radiation Measurement (ARM) program were used to develop these improvements. The estimation of cloudiness was then used to modify the effective clear-sky atmospheric emissivity in order to calculate 30-min averages of downwelling longwave radiation. Monthly mean bias errors (mbe's) of -9 to +4 W [m.sup.-2] and root-mean-square errors (rmse's) of 11-22 W [m.sup.-2] were calculated based on ARM data over a 1-yr period. These mbe's were smaller overall than any of the six previous methods tested, while the rmse's were similar to the best previous methods. The improved parameterization was then tested on FIFE data from the summer of 1987. Although the monthly mbe's were larger, the rmse's were smaller. It is also shown that data from upper-air soundings can be used to calculate the effective atmospheric emissivity rather than specifying the aforementioned sinusoidal variation. Using ARM upper-air soundings, this method resulted in larger mbe's, -7 to + 11 W [m.sup.-2] especially during the summer months, and similar rmse's. The success of the method suggests that it has application at any observing site within reasonable proximity of an upper-air sounding, while removing the empiricism used to specify the annual sinusoidal variation in emissivity.

The equation of radiative energy balance in a homogeneous plane-parallel cloud accounts for the photon transport in the vertical direction only. From this equation, the absorption can be evaluated as a difference between the net radiative fluxes measured on the cloud top and bottom. Real clouds exhibit extreme horizontal variability of their optical and, hence, radiative properties. So the radiative energy balance equation for inhomogeneous clouds includes an extra term to describe the net energy transport in the horizontal directions (radiative horizontal transport). This study investigates the horizontal transport in nonhomogeneous stratocumulus clouds and its influence on the accuracy of absorption estimates. Using a realistic fractal model of marine stratocumulus clouds and Monte Carlo method, it is shown that the magnitude of horizontal transport 1) is comparable with those of albedo, transmittance, and absorptance and 2) depends strongly on the fractal dimension of cloud optical depth. Because of the horizontal transport, there is no unique dependence between optical and radiative properties of given pixel. The neglect of the horizontal transport is a major source of uncertainty in absorption estimates. Two possible ways of improving the inhomogeneous cloud absorption estimates from field measurements are discussed.

The optical properties of equatorial cirrus were studied during a three-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experiment consisted of vertical lidar (532 nm) and passive infrared filter radiometer (10.84 [[micro]meter]) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, infrared absorption, and visible optical depth and linear depolarization ratio. A standard lidar-radiometer analysis, with some improvements, was used to calculate these quantities. The cirrus was found to vary in altitude from a maximum cloud top of 17.6 km to a minimum cloud base of 6 km with equivalent temperatures of -82 [degrees] C to -7 [degrees] C respectively. The cirrus also varied widely in depth (0.7 to 7.5 km). The mean emittance (for each temperature interval) of the cooler clouds was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures. The mean infrared absorption coefficients were similar to those of midlatitude clouds, except at the extreme temperature ranges, but were higher than those observed in tropical synoptic clouds over Darwin. Infrared optical depths varied from 0.01 to 2.4 and visible optical depths from 0.01 to 8.6. Plots of integrated attenuated backscatter versus infrared emittance, for various ranges of cloud temperature, showed characteristic behavior. Values of the measured quantity k/2[Eta], where k is the visible backscatter to extinction ratio and [Eta] a multiple scattering factor, were found to increase with temperature from 0.14 at -70 [degrees] C to 0.30 at -20 [degrees] C. Values of the quantity 2[Alpha][Eta], where a is the ratio of visible extinction to infrared absorption coefficient, varied from about 1.7 to 3.8, depending somewhat on the cloud temperature. Deduced values of a were as high as 5.3 at the lower temperature ranges, indicating smaller particles. The lidar integrated attenuated depolarization ratio A decreased with temperature, as found previously in midlatitude cirrus. Values of A varied from 0.42 at -70 [degrees] C to 0.18 at -10 [degrees] C. Data obtained from the NOAA/ETL microwave radiometer gave values of water path, varying from 4 to 6 cm precipitable water. A value of the water vapor continuum absorption coefficient at 10.84 [[micro]meter] equal to 9.0 [+ or -] 0.5 [g.sup.-1] [cm.sup.2] [atm.sup.-1] was obtained in agreement with previous observations.

The interaction of clouds and radiation is a particularly difficult issue in the study of climate change. Clouds have a large impact on the earth's radiation budget but the range of spatial and temporal scales and the complexity of the physical processes associated with clouds made these interactions difficult to simulate. The Department of Energy's Atmospheric Radiation Measurement (ARM) program was established to improve the understanding of the interaction of radiation with the atmosphere with a particular emphasis on the effects of clouds. To continue its role of providing data for the study of these interactions, the ARM program deployed an Atmospheric Radiation and Cloud Station (ARCS) in the tropical western Pacific. This site began operation in October 1996. The tropical western Pacific is a very important climatic region. It is characterized by strong solar heating, high water vapor concentrations, and active convection. The ARCS is equipped with a comprehensive suite of instruments for measuring surface radiation fluxes and properties of the atmospheric state and is intended to operate for the next 10 years. The ARCS is an integrated unit that includes a data management system, a site monitor and control system, an external communications system, redundant electrical power systems, and containers that provide shelter for the equipment as well as work space for site operators, technicians, and visiting scientists. The dataset the ARCS produces will be invaluable in studying issues related to clouds and radiation in the Tropics. The site is located in Manus Province, Papua New Guinea, at 2.060 [degrees] S, 147.425 [degrees] E, 300 km north of the island of New Guinea. Two more ARCS are planned for deployment across the tropical Pacific.

The authors present a retrieval technique for the inference of cloud optical depth from data obtained by a ground-based multichannel radiometer for use in climate-related studies. The basic steps of the analysis procedure are considered, including accurate calculations of the atmosphere and cloud properties. An approach is described that uses observed and model-simulated transmittances rather than irradiances and that dogs not depend on an absolute calibration of the instrument. This approach, which specifically deals with the transmittances, also offers substantial computational advantages. The resulting algorithm is applied (but not limited) to the measurements of incoming solar irradiance by a particular ground-based instrument, the so-called Multi-Filter Rotating Shadowband Radiometer (MFRSR). Sample results of inferred cloud optical depth obtained from MFRSR measurements in Fairbanks, Alaska, are presented.

The quantification of cloud-induced shortwave atmospheric absorption is a painstaking task and often the subject of contention. Several analytical methods previously used for this purpose are examined in detail applying each method to a set of collocated satellite and surface measurements of radiant fluxes taken in April of 1994 in Oklahoma. It is demonstrated that, if care is not taken, conclusions regarding cloud-induced absorption can be as much a function of the chosen analytical methods as they are of the data themselves. It is argued that the best method for determining the cloud radiative forcing ratio is from the slope of a plot of the cloud radiative forcing ratio at the surface versus the cloud radiative forcing at the top of the atmosphere and/or a normalized analog. Application of this method shows that clouds in Oklahoma, on average, induced an absorption of 4% more of the solar insolation than did clear sky. An examination is made of three recent papers that have reported cloud-induced atmospheric absorption in large excess over that which has been generally considered possible. It is shown that, once uncertainties and biases in the analytical methods are considered, the results of all three papers are consistent with conventional formulations of cloud-radiation interactions.

A cloud retrieval algorithm using NOAA/National Environmental Satellite, Data, and Information Service High-Resolution Infrared Radiation Sounder 2 Microwave Sounding Unit measurements from a polar-orbiting satellite, described in McMillin et al., uses multiple channel pairs with a two-pass procedure for enhancing accuracies. The current paper complements McMillin et al. in several ways. First, it describes the characteristics of the channel pairs used in the algorithm while documenting the logic of the channel selection. It shows that the cloud-top heights and cloud fractions are dependent on the sensing channel pairs. The higher the altitude of the weighting function, the smaller the cloud fractions. Second, it adds an atmospheric attenuation correction and displays its effect on cloud-top heights. Without the attenuation correction, the cloud-top distributions are separated into two bands, possibly as a result of the distance between the heights of the weighting functions of the sensing channel pair. The attenuation correction effectively eliminates the gap, both by lowering the upper band and by elevating the lower band. The cloud fractions from this experimental operation are compared with Air Force Real-Time Nephanalysis for 3 months, and they reveal its strength in detecting low-level stratus.

GOES high-resolution visible imagery is used to define the spatial resolution and scan geometry required for an advanced infrared sounder to fly on future polar-orbiting satellites. The definition is based on optimizing the probability of achieving one or more cloud-free infrared sounder fields of view within the footprint of an Advanced Microwave Sounding Unit (AMSU) assumed to have a linear resolution of 64 km. It is found that an instrument with about 8 km linear (10 km circular), or better, resolution that samples nine, or more, spatially independent fields of view within each AMSU footprint is needed to provide a high probability of achieving uncontaminated, by cloud, infrared sounding radiance observations.

Observations cited by Ramanathan et al. and Cess et al. indicate systematic errors in the solar radiation parameterizations of the current atmospheric general circulation models. Cloudy scenes have an observational excess (or calculational deficit) of atmospheric absorption. Pilewskie and Valero have also reported anomalously large absorption. A simple model is presented here to show how fields of broken clouds cause average photon pathlengths to be greater than those predicted by homogeneous radiative transfer calculations of cloud-atmosphere ensemble with similar albedos, especially under and within the cloud layer. This one-sided bias is a contribution to the anomalous absorption. The model is illustrated quantitatively with a numerical stochastic radiative transfer calculation. More than one-half the anomaly is explained for the parameters used in the numerical example.

The use of cloud fraction as a means of incorporating horizontal cloud inhomogeneity in radiative transfer calculations is widespread in the atmospheric science community. This study addresses some issues pertaining to the use of cloud fraction by performing radiative transfer in inhomogeneous two-dimensional media. Two approximation techniques are developed to solve the equation of transfer for the spatial averages of diffuse flux. The first is based on a first-order closure technique that leads to a one-dimensional equation of transfer with a modified source term. The computational speed of this technique is equivalent to the plane-parallel method. The second method of approximation performs a full two-dimensional computation of the direct solar beam that is used as the pseudo-source in an independent pixel diffuse radiative transfer calculation. Both methods require a knowledge of the spatial distribution of the direct solar transmission, rather than cloud fraction, for their specification. Tests are performed comparing the accuracy of these two new methods, as well as the plane-parallel and independent pixel methods, with the complete solution using the spherical harmonic spatial grid method. The media have horizontal variations in extinction that are sinusoidal but truncated according to the cloud fraction. The 2D direct-beam independent pixel method is generally the most accurate approximation, especially for forward-scattering phase functions. The source closure method is substantially more accurate than the independent pixel or cloud-fraction weighted plane-parallel methods.

Implementation of a broadband radiation parameterization in the UCLA cumulus ensemble model (CEM) is discussed in this study, with emphasis on the specific problems associated with adequate calculation of radiative transfer processes in the CEM. The radiation parameterization is based on the Harshvardhan et al. broadband radiative transfer model with cloud optical properties as formulated by Stephens et al. The lowest CEM layer is divided into a thick layer and a very thin layer near the surface for the longwave radiation calculation. Diagnostic tests have been performed to compare the accuracy of this parameterization with results from a more complicated radiative transfer model. Simulations with fully interactive radiation are performed to compare two methods for invoking the radiation module. The time interval for calling the radiation module has to be very small for the conventional method, which keeps the radiative heating rate constant until the radiation module is called again. It can be larger for the 'accumulated' method, which accumulates and averages cloud microphysical properties within the time interval and applies the amount of radiative heating over the time interval at the time when the radiation module is called. Additional sensitivity tests are also performed to justify the omission of the radiative effects of rapidly falling precipitating particles. Other sensitivity tests are related to the adequacy of the domain size and horizontal resolution for studying the cloud-radiation interaction problems discussed in Part II.

The two-dimensional UCLA cumulus ensemble model is used to examine the impact of cloud-radiation interactions on the macroscopic behavior of cumulus ensembles. Two sets of simulations are performed with noninteractive (NI) and fully interactive (FI) radiative transfer, and with prescribed large-scale advective effects. The time-varying horizontally averaged radiative heating rates [Q.sub.R] from the FI simulations are used to prescribe the time-varying, horizontally homogeneous [Q.sub.R] in the NI simulations. The effects of both longwave radiation and diurnally varying solar radiation are examined from these two sets of simulations. The diurnally varying solar radiation can drive a diurnal cycle of deep convection over the tropical oceans by stabilizing the large-scale environment during the daytime relative to the nighttime. The results presented in this study confirm the dominant role of the direct radiation-convection interaction mechanism for the diurnal cycle of oceanic precipitation. Comparison of the results of the FI and NI simulations suggests that the horizontal differential heating mechanism of Gray and Jacobson plays a secondary role in the diurnal cycle of precipitation. The presence of interactive radiation in the FI simulation, however, postpones the maxima and minima of convective activity due to the greater persistence of upper-tropospheric clouds. These clouds can survive against the solar absorption effects during the daytime. The impact of longwave-cloud interactions on the macroscopic behavior of cumulus ensembles is a slightly stronger modulation of cumulus activity by large-scale processes. Upper-tropospheric clouds are somewhat more active and last longer in the presence of interactive radiation. The longwave-cloud interactions are achieved by the so-called continuously destabilizing mechanism, which has its greatest effects on thin cloud layers.

This paper compares accuracy for the daily averaged reflection, transmission, and absorption of solar flux derived from the delta-four-stream approximation and a few selected two-stream approximations. In the chosen variety of two-stream approximations, a two-stream approximation suggested by Zhu produces the most accurate reflection and transmission for conservative scattering with optical thickness [[Tau].sub.1] [is less than] 3. Otherwise, the delta-Eddington approximation is superior to other two-stream approximations. The delta-four-stream approximation produces errors that are smaller than the delta-Eddington approximation by at least a factor of 2.

An increase in the planetary albedo of the earth-atmosphere system by only 10% can decrease the equilibrium surface temperature to that of the last ice age. Nevertheless, albedo biases of 10% or greater would be introduced into large regions of current climate models if clouds were given their observed liquid water amounts, because of the treatment of clouds as plane parallel. Past work has addressed the effect of cloud shape on albedo; here the focus is on the within-cloud variability of the vertically integrated liquid water. The main result is an estimate of the 'plane-parallel albedo bias' using the 'independent pixel approximation,' which ignores net horizontal photon transport, from a simple fractal model of marine stratocumulus clouds that ignores the cloud shape. The use of the independent pixel approximation in this context will be justified in a separate Monte Carlo study. The focus on marine stratocumulus clouds is due to their important role in cloud radiative forcing and also that, of the wide variety of earth's cloud types, they are most nearly plane parallel, so that they have the least albedo bias. The fractal model employed here reproduces both the probability distribution and the wavenumber spectrum of the stratocumulus liquid water path, as observed during the First ISCCP Regional Experiment (FIRE). The model distributes the liquid water by a cascade process, related to the upscale cascade of energy transferred from the cloud thickness scale to the mesoscale by approximately 2D motions. For simplicity, the cloud microphysical parameters are assumed homogeneous, as is the geometrical cloud thickness; and the mesoscale-averaged vertical optical thickness is kept fixed at each step of the cascade. A single new fractal parameter, 0 [is less than or equal to] f [is less than or equal to] 1, is introduced and determined empirically by the variance of the logarithm of the vertically integrated liquid water. In the case of conservative scattering, the authors are able to estimate the albedo bias analytically as a function of the fractal parameter f, mean vertical optical thickness [[Tau].sub.v], and sun angle [Theta]. Typical observed values are f = 0.5, [[Tau].sub.v] = 15, and [Theta] = 60 [degrees], which give an absolute bias of 0.09, or a relative bias equal to 15% of the plane-parallel albedo of 0.60. The reduced reflectivity of fractal stratocumulus clouds is approximately given by the plane-parallel reflectivity evaluated at a reduced 'effective optical thickness,' which when f = 0.5 is [[Tau].sub.eff] [approximately equals] 10. Study of the diurnal cycle of stratocumulus liquid water during FIRE leads to a key unexpected result: the plane-parallel albedo bias is largest when the cloud fraction reaches 100%, that is, when any bias associated with the cloud fraction vanishes. This is primarily due to the variability increase with cloud fraction. Thus, the within-cloud fractal structure of stratocumulus has a more significant impact on estimates of its mesoscale-average albedo than does the cloud fraction.

The Atmospheric Radiation Measurement (ARM) Program, supported by the U.S. Department of Energy, is a major new program of atmospheric measurement and modeling. The program is intended to improve the understanding of processes that affect atmospheric radiation and the description of these processes in climate models. An accurate description of atmospheric radiation and its interaction with clouds and cloud processes is necessary to improve the performance of and confidence in models used to study and predict climate change. The ARM Program will employ five (this paper was prepared prior to a decision to limit the number of primary measurement sites to three) highly instrumented primary measurement sites for up to 10 years at land and ocean locations, from the Tropics to the Arctic, and will conduct observations for shorter periods at additional sites and in specialized campaigns. Quantities to be measured at these sites include longwave and shortwave radiation, the spatial and temporal distribution of clouds, water vapor, temperature, and other radiation-influencing quantities. There will be further observations of meteorological variables that influence these quantities, including wind velocity, precipitation rate, surface moisture, temperature, and fluxes of sensible and latent heat. These data will be used for the prospective testing of models of varying complexity, ranging from detailed process models to the highly parameterized description of these processes for use in general circulation models of the earth's atmosphere. This article reviews the scientific background of the ARM Program, describes the design of the program, and presents its status and plans.

Computational results have been obtained for the spherical albedo, global transmission, and global absorption of plane-parallel layers composed of cloud droplets. These computations, obtained using the doubling method for the entire range of single scattering albedos (0 less than or equal to omega(sub 0) less than or equal to 1) and for optical depths between 0.1 and 100, are compared with corresponding results obtained using selected multiple scattering approximations. Both the relative and absolute accuracies of asymptotic theory for thick layers, three diffuse two-stream approximations, and two integrated two-stream approximations are presented as a function of optical thickness and single scattering albedo for a scattering phase function representative of cloud droplets at visible wavelengths. The spherical albedo and global absorption computed using asymptotic theory are found to be accurate to better than 5% for all values of the single scattering albedo, provided the optical thickness exceeds about 2. The diffuse two-stream approximations have relative accuracies that are much worse than 5% for the spherical albedo over most of the parameter space, yet are accurate to within 5% in the global absorption when the absorption is significant. The integrated delta-Eddington scheme appears to be the most suitable model over the entire range of variables, generally producing relative errors of less than 5% in both the spherical albedo and global absorption.

The simulations of tropical convection and thermodynamic states in response to different imposed large-scale forcing are carried out by using a cloud-resolving model and are evaluated with the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment observation. The model is forced either with imposed large-scale vertical velocity and horizontal temperature and moisture advections (model 1) or with imposed total temperature and moisture advections (model 2). The comparison of simulations with observations shows that bias in temperature and moisture simulations by model 1 is smaller than that by model 2. This indicates that the adjustment of the mean thermodynamic stability distribution by vertical advection in model I is responsible for better simulations. Model I is used to examine effects of different parameterized solar radiative and cloud microphysical processes. A revised parameterization scheme for cloud single scattering properties in solar radiation calculations is found to generate more solar heating in the upper troposphere and less heating in the middle and lower troposphere. The change in the vertical heating distribution is suggested to stabilize the environment and to cause less stratiform cloud that further induces stabilization through cloud-IR interaction. The revised scheme also causes a drier middle and lower troposphere by weakening vertical moisture flux convergence. Also tested is the effect of a revised parameterization scheme for cloud microphysical processes that tends to generate more ice clouds. The cloud-induced thermal effect in which less ice cloud leads to less infrared cooling at cloud top and more heating below cloud top is similar to the effect of no cloud-radiation interaction shown in a sensitivity experiment. However, the exclusion of cloud-radiation interaction causes drying by enhancing condensation, and the reduction of ice clouds by the microphysics scheme induces moistening by suppressing condensation.

A study was carried out in order to estimate the effect of melting particles on simulated brightness temperatures at microwave frequencies between 10.7 and 85.5 GHz for precipitation over the ocean. The meteorological model framework is based on the assumption that the strongest radiometric effect is due to the drastically increased permittivity of melting particles driven by the volume fraction of liquid water. Thus, effects caused by particle aggregation and breakup are neglected. Different approaches for calculating the effective permittivity of mixed particles are compared. The resulting extinction coefficients, single scattering albedos, and asymmetry parameters indicate a maximum effect when the particle is composed of a water matrix with air-ice inclusions. In particular the extinction coefficient may vary by more than two orders of magnitude right below the freezing level dependent on frequency and the applied mixing formula. In the melting region also the strongest dependence of the optical properties on the droplet spectrum is observed. Extreme local differences of 100% between the particle optical properties employing either a Marshall-Palmer or gamma-type drop size distribution occur. When radiative transfer calculations are carried out, average deviations of 20-30 K at low frequencies (10.7 and 19.35 GHz) are observed, mainly due to the strong dependence of the extinction coefficient on the implemented melting process. However, this effect is driven by the applied mixing formula rather than the drop size distribution; that is, for particles composed of a water matrix and air-ice inclusions independently of melting stage the emission excess seems to be overexpressed. The systematic effect of including the melting process in radiative transfer calculations for the development of surface rain retrievals was also investigated. Over 550 model atmospheres were used to estimate the relative deviation of surface rain-rate estimates using a set of operational rain retrieval algorithms. Neglecting the melting effect may lead to severe overestimations of surface rain rates by up to 100% in stratiform conditions. However, if the melting layer is either weakly expressed or nonuniformly distributed in space, the relative overestimation is much lower. If the effective permittivity of melting particles is calculated using the weighted mixing approach of Meneghini and Liao, considerably less effect of melting particles on passive microwave emission is observed.