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Summertime observations for 11 yr from the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site are used to investigate mechanisms controlling the transition from shallow to deep convection over land. It is found that a more humid environment immediately above the boundary layer is present before the start of late afternoon heavy precipitation events. The higher moisture content is brought by wind from the south. Greater boundary layer inhomogeneity in moist static energy, temperature, moisture, and horizontal wind before precipitation begins is correlated to larger rain rates at the initial stage of precipitation. In an examination of afternoon rain statistics, higher relative humidity above the boundary layer is correlated to an earlier onset and longer duration of afternoon precipitation events, whereas greater boundary layer inhomogeneity and atmospheric instability in the 2-4-km layer above the surface are positively correlated to the total rain amount and the maximum rain rate. Although other interpretations may be possible, these observations are consistent with theories for the transition from shallow to deep convection that emphasize the role of a moist lower free troposphere and boundary layer inhomogeneity. DOI: 10.1175/2010JAS3366.1

The validity of approximating radiative heating rates in the middle atmosphere by a local linear relaxation to a reference temperature state (i.e., 'Newtonian cooling') is investigated. Using radiative heating rate and temperature output from a chemistry--climate model with realistic spatiotemporal variability and realistic chemical and radiative parameterizations, it is found that a linear regression model can capture more than 80% of the variance in longwave heating rates throughout most of the stratosphere and mesosphere, provided that the damping rate is allowed to vary with height, latitude, and season. The linear model describes departures from the climatological mean, not from radiative equilibrium. Photochemical damping rates in the upper stratosphere are similarly diagnosed. Three important exceptions, however, are found. The approximation of linearity breaks down near the edges of the polar vortices in both hemispheres. This nonlinearity can be well captured by including a quadratic term. The use of a scale-independent damping rate is not well justified in the lower tropical stratosphere because of the presence of a broad spectrum of vertical scales. The local assumption fails entirely during the breakup of the Antarctic vortex, where large fluctuations in temperature near the top of the vortex influence longwave heating rates within the quiescent region below. These results are relevant for mechanistic modeling studies of the middle atmosphere, particularly those investigating the final Antarctic warming. DOI: 10.1175/2009JAS3286.1

The correlated-k-distribution (CKD) method is widely used in the radiative transfer schemes of atmospheric models; it involves dividing the spectrum into a number of bands and then reordering the gaseous absorption coefficients within each one. The fluxes and heating rates for each band may then be computed by discretizing the reordered spectrum into O(10) quadrature points per major gas and performing a pseudo-monochromatic radiation calculation for each point. In this paper it is first argued that for clear-sky longwave calculations, sufficient accuracy for most applications can be achieved without the need for bands: reordering may be performed on the entire longwave spectrum. The resulting full-spectrum correlated-k (FSCK) method requires significantly fewer pseudomonochromatic calculations than standard CKD to achieve a given accuracy. The concept is first demonstrated by comparing with line-by-line calculations for an atmosphere containing only water vapor, in which it is shown that the accuracy of heating rate calculations improves approximately in proportion to the square of the number of quadrature points. For more than around 20 points, the root-mean-square error flattens out at around 0.015 K day 1 due to the imperfect rank correlation of absorption spectra at different pressures in the profile. The spectral overlap of m different gases is treated by considering an m-dimensional hypercube where each axis corresponds to the reordered spectrum of one of the gases. This hypercube is then divided up into a number of volumes, each approximated by a single quadrature point, such that the total number of quadrature points is slightly fewer than the sum of the number that would be required to treat each of the gases separately. The gaseous absorptions for each quadrature point are optimized such that they minimize a cost function expressing the deviation of the heating rates and fluxes calculated by the FSCK method from line-by-line calculations for a number of training profiles. This approach is validated for atmospheres containing water vapor, carbon dioxide, and ozone, in which it is found that in the troposphere and most of the stratosphere, heating rate errors of less than 0.2 K [day.sup.-1] can be achieved using a total of 23 quadrature points, decreasing to less than 0.1 K day i for 32 quadrature points. It would be relatively straightforward to extend the method to include other gases. DOI: 10.1175/2010JAS3202.1

The international experiment called the European Aqua Thermodynamic Experiment (EAQUATE) was held in September 2004 in Italy and the United Kingdom to validate Aqua satellite Atmospheric Infrared Sounder (AIRS) radiance measurements and derived products with certain ground-based and airborne systems use ful for validating hyperspectral satellite sounding observations. A range of flights over land and marine surfaces were conducted to coincide with overpasses of the AIRS instrument on the Earth Observing System Aqua platform. Direct radiance evaluation of AIRS using National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed-Interferometer (NAST-I) and the Scanning High-Resolution Infrared Sounder has shown excellent agreement. Comparisons of level-2 retrievals of temperature and water vapor from AIRS and NAST-I validated against high-quality lidar and dropsonde data show that the 1-K/1-km and 10%/1-km requirements for temperature and water vapor (respectively) are generally being met. The EAQUATE campaign has proven the need for synergistic measurements from a range of observing systems for satellite calibration/validation and has paved the way for future calibration/validation activities in support of the Infrared Atmospheric Sounding Interferometer on the European Meteorological Operational platform and Cross-Track Infrared Sounder on the U.S. NPOESS Prepatory Project platform.

Wavenumber spectra of the atmospheric potential energy of Mars at mesoscales (wavelengths of 64-957 km) were obtained as a function of latitude, season, and Martian year using infrared radiance data obtained by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) spacecraft. Spectral slopes tend to be flatter at smaller scales, and the slopes are usually flatter than -1 near small-scale ends. Near large-scale ends, the spectra sometimes show prominent steepening with slopes from -2 to -3. The power peaks in the high latitudes in winter and equinoxes, suggesting that eddies are generated preferentially in baroclinic zones. The seasonal variation at each latitude band, on the other hand, tends to be obscured by large interannual variability. An enhancement in the power was observed around the storm tracks in the Southern Hemisphere. Spectra of the terrestrial stratosphere were also obtained with a similar method from data taken by the Aura satellite and compared to the results for Mars.

With the rapid growth in air travel, there is concern over the radiative impact of contrails and aircraft-induced cirrus on climate. Previous radiation calculations on contrails have almost all used the independent column approximation, which neglects the transport of photons through the sides of the contrail, but in this study the 3D effects are quantified using the Spherical Harmonic Discrete Ordinate Method (SHDOM). The authors have investigated the dependence of shortwave and longwave radiative forcing on contrail aspect ratio, optical depth, solar zenith angle, solar azimuth angle relative to contrail orientation, particle size, particle habit, surface albedo, and surface temperature. It is found that inclusion of 3D transport results in an increase in the positive longwave radiative forcing of the contrail and either an increase or a decrease in the magnitude of the negative shortwave radiative forcing depending on the orientation of the contrail with respect to the sun. Although these two effects are individually quite modest (of order 10%), the fact that the total shortwave and longwave forcings largely cancel during the day means that the relative change in the net radiative forcing due to the 3D effect is substantial; in some cases this results in a doubling of the net forcing of the contrail, in other cases changing its sign. On a more general note, the relatively simple geometry of contrail cirrus provides an ideal test case for explaining the various mechanisms by which 3D photon transport can change the radiative effect of clouds, which can be rather difficult to visualize for more complex cloud scenarios.

A simplified moist general circulation model is used to study changes in the meridional transport of moist static energy by the atmosphere as the water vapor content is increased. The key assumptions of the model are gray radiation, with water vapor and other constituents having no effect on radiative transfer, and mixed layer aquaplanet boundary conditions, implying that the atmospheric meridional energy transport balances the net radiation at the top of the atmosphere. These simplifications allow the authors to isolate the effect of moisture on energy transports by baroclinic eddies in a relatively simple setting. The authors investigate the partition of moist static energy transport in the model into dry static energy and latent energy transports as water vapor concentrations are increased, by varying a constant in the Clausius--Clapeyron relation. The increase in the poleward moisture flux is rather precisely compensated by a reduction in the dry static energy flux. These results are interpreted with diffusive energy balance models (EBMs). The simplest of these is an analytic model that has the property of exact invariance of total energy flux as the moisture content is changed, but the assumptions underlying this model are not accurately satisfied by the GCM. A more complex EBM that includes expressions for the diffusivity, length scale, velocity scale, and latitude of maximum baroclinic eddy activity provides a better fit to the GCM's behavior.

Real midlatitude meteorological cases are simulated over western Europe with the cloud mesoseale model Meso-NH, and the outputs are used to calculate brightness temperatures at microwave frequencies with the Atmospheric Transmission at Microwave (ATM) radiative transfer model. Satellite-observed brightness temperatures (TBs) from the Advanced Microwave Scanning Unit B (AMSU-B) and the Special Sensor Microwave Imager (SSM/I) are compared to the simulated ones. In this paper, one specific situation is examined in detail. The infrared responses have also been calculated and compared to the Meteosat coincident observations. Overall agreement is obtained between the simulated and the observed brightness temperatures in the microwave and in the infrared. The large-scale dynamical structure of the cloud system is well captured by Meso-NH. However, in regions with large quantities of frozen hydrometeors, the comparison shows that the simulated microwave TBs are higher than the measured ones in the window channels at high frequencies, indicating that the calculation does not predict enough scattering. The factors responsible for the scattering (frozen particle distribution, calculation of particle dielectric properties, and nonsphericity of the particles) are analyzed. To assess the quality of the cloud and precipitation simulations by Meso-NH, the microphysical fields predicted by the German Lokal-Modell are also considered. Results show that in these midlatitude situations, the treatment of the snow category has a high impact on the simulated brightness temperatures. The snow scattering parameters are tuned to match the discrete dipole approximation calculations and to obtain a good agreement between simulations and observations even in the areas with significant frozen particles. Analysis of the other meteorological simulations confirms these results. Comparing simulations and observations in the microwave provides a powerful evaluation of resolved clouds in mesoscale models, especially the precipitating ice phase.

In this paper, the emission of internal gravity waves from a local westerly shear layer is studied. Thermal and/or vorticity forcing of the shear layer with a wide range of frequencies and scales can lead to strong emission of gravity waves in the region exterior to the shear layer. The shear flow not only passively filters and refracts the emitted wave spectrum, but also actively participates in the gravity wave emission in conjunction with the distributed forcing. This interaction leads to enhanced radiated momentum fluxes but more importantly to enhanced gravity wave energy fluxes. This enhanced emission power can be traced to the nonnormal growth of the perturbations in the shear region, that is, to the transfer of the kinetic energy of the mean shear flow to the emitted gravity waves. The emitted wave energy flux increases with shear and can become as large as 30 times greater than the corresponding flux emitted in the absence of a localized shear region. Waves that have horizontal wavelengths larger than the depth of the shear layer radiate easterly momentum away, whereas the shorter waves are trapped in the shear region and deposit their momentum at their critical levels. The observed spectrum, as well as the physical mechanisms influencing the spectrum such as wave interference and Doppler shifting effects, is discussed. While for large Richardson numbers there is equipartition of momentum among a wide range of frequencies, most of the energy is found to be carried by waves having vertical wavelengths in a narrow band around the value of twice the depth of the region. It is shown that the waves that are emitted from the shear region have vertical wavelengths of the size of the shear region.

Three-dimensional cloud-resolving model simulations of a mesoscale region around Kwajalein Island during the Kwajalein Experiment (KWAJEX) are performed. Using observed winds along with surface and large-scale thermodynamic forcings, the model tracks the observed mean thermodynamic soundings without thermodynamic nudging during 52-day simulations spanning the whole experiment time period, 24 July-14 September I999. Detailed comparisons of the results with cloud and precipitation observations, including radar reflectivities from the Kwajalein ground validation radar and International Satellite Cloud Climatology Project (ISCCP) cloud amounts and radiative fluxes, reveal the biases and sensitivities of the model's simulated clouds. The amount and optical depth of high cloud are underpredicted by the model during less rainy periods, leading to excessive outgoing longwave radiation (OLR) and insufficient albedo. The simulated radar reflectivities tend to be excessive, especially in the upper troposphere, suggesting that simulated high clouds are precipitating large hydrometeors too efficiently. Occasionally, large-scale advective forcing errors also seem to contribute to upper-level cloud and relative humidity biases. An extensive suite of sensitivity studies to different microphysical and radiative parameterizations is performed, with surprisingly little impact on the results in most cases.

This paper describes the Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) forward model for a clear-sky atmosphere emitting unpolarized radiation. This model is used for the majority of the EOS MLS radiance calculations. The orbital and viewing geometry of MLS on Aura is such that a two-dimensional (vertical and line-of-sight, horizontal) forward model provides the most accurate basis for the inversion performed in retrievals. The model consists of atmospheric radiative transfer convolved with the instrumental field of view and the spectral response. An additional model relating the pointing heights supplied by the Aura satellite operations and MLS scan pointing encoders to the limb tangent pressure and atmospheric temperature is described. The analytic computation of Jacobians needed for retrievals is given. Index Terms--Aura, Earth Observing System (EOS), Microwave Limb Sounder (MLS), radiative transfer, spectroscopy.

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.

By using air-vegetation-soil layer coupled model equations as weak constraints, a variational method is developed to compute sensible and latent heat fluxes from conventional observations obtained at meteorological surface stations. This method also retrieves the top soil layer content (daytime only) and the surface skin temperature as by-products. The method is applied to Oklahoma Mesonet data collected in the summer of 1995. Fluxes computed for selected Mesonet stations are verified against those obtained by the surface energy and radiation balance system at Atmospheric Radiation Measurement (ARM) Cloud and Radiation Testbed (CART) sites closest to the selected Mesonet stations. The retrieved values of soil water content are also compared with the direct measurements at the closest ARM CART stations. With data provided by the dense Mesonet, the method is shown to be useful in deriving the mesoscale distributions and temporal variabilities of surface fluxes, soil water content, and skin temperature. The method is unique in that it provides an additional means to derive flux fields directly from conventional surface observations.

The POLDER instrument on board ADEOS provides the first opportunity to apply the two-channel ratio technique in the near infrared to retrieve atmospheric water vapor content at the global scale. Over land, POLDER estimates compare well with radiosonde measurements, with a 3.1-kg [m.sup.-2] rms error. There is a clear negative bias of a few kg [m.sup.-2] however. This bias can be explained by the spectral variation of the target reflectance between the two bands used for the estimate. Over ocean, POLDER estimates compare very well with either radiosonde measurements (6.9 kg [m.sup.-2] rms error) or SSM/I estimates (4.2 kg [m.sup.-2] rms error). A mapped POLDER product shows a good overall similarity with ECMWF analyzed field, with smaller synoptic structures in the POLDER field. An example of monthly product is presented.

The radiometric sensitivity of the POLarization and Directionality of the Earth's Reflectances (POLDER) instrument in the ocean color spectral bands (443, 490, and 565 nm) was checked vicariously by comparing top-of-atmosphere normalized radiances measured by the instrument with those computed for the same geometries using a radiative transfer model. In situ measurements of aerosol optical thickness and marine reflectance at the time of the satellite overpass were used as input to the model. The accuracy of the vicarious calibration coefficients was estimated to better than 3%. A large decrease in the POLDER instrument response was found in the blue, confirming the results previously obtained using alternative techniques. Index Terms - In-situ measurements, POLDER, radiative transfer, radiometric calibration.

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 paper presents an improved version of a previously published model for the diffuse fraction of hourly global irradiance. In addition to hourly solar elevation and clearness index, an hour-to-hour variability index and regional surface albedo are included among the input parameters. Moreover, to prevent excessively high normal incidence beam irradiances at very low solar elevations, the model does not allow a solar elevation dependent maximum beam transmittance to be exceeded. This new model is tuned to 32 years of data from Bergen, Norway. Moreover, a test against independent data from four European stations showed that the model performs better than the models of Erbs et al. (1982), Maxwell (1987) and Perez et al. (1992).

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.

A new Monte Carlo-based three-dimensional (3D) radiative transfer model of high spectral and spatial resolution is presented. It is used to investigate the difference in broadband solar radiation absorption, top-of-the-atmosphere upwelling, and surface downwelling solar radiation in a cloudy atmosphere between 3D and 1D calculations. Spatial variations of these same radiation components (absorption, upwelling, and downwelling), together with pathlength distributions, are analyzed for different wavelengths to describe the main physical mechanisms at work. The model contains all of the important atmospheric and surface radiative constituents. It includes Rayleigh scattering, absorption and scattering by both cloud droplets and aerosols, and absorption by the major atmospheric gases. Inputs include 3D liquid water fields, aerosols, and gas distribution, type, and concentrations. Using satellite imagery of a tropical cloud field as input, model results demonstrate that various plane-parallel (1D) assumptions can underestimate atmospheric absorption when compared to 3D computations. This discrepancy is caused by a complex interaction of gaseous absorption, cloud droplet absorption, and the solar zenith angle. Through a sensitivity analysis, the authors demonstrate that the most important factor is the morphology of the cloud field, followed by the vertical stratification of water vapor.

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.

A very simple model of atmospheric transmittance uses as its input regularly observed and archived measures of upper air humidity. The five model parameters were determined empirically to optimize the agreement with radiation measured at six sites in the northwestern contiguous U.S. in summer and winter 1990. The model uses the relative humidity at 500 hPa, the relative humidity at either 950 hPa or 800 hPa depending on the altitude of the station, and the precipitable water. Although the model does not use cloud observations explicitly, it is partitioned into a low-humidity stage and a high-humidity stage with markedly different dependences on relative humidity. In the low-humidity stage the transmittance is approximated as the product of factors for dry air and for water vapor. In the high-humidity stage the transmittance is the product of the transmittances of each of the two layers used; the product is a strongly decreasing function of relative humidity, as a representation of the effect of cloud. A split-sample test using measurements for spring and autumn indicated that the model parameters were not badly distorted by conditions unique to the summer and winter seasons. In terms of the average daily global shortwave radiation received at the surface, the rms errors are 31 W [m.sup.-2] in spring, 33 in summer, 30 in autumn, and 18 in winter. The corresponding relative errors are 0.25, 0.17, 0.20, and 0.31. Because of high-frequency temporal variation in the model residuals, the error declines to about half of the daily error for 7-day averages and to about a quarter of the daily error for 30-day averages.

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.

Global and diffuse radiation and surface meteorological measurements at Edmonton, Montreal, Port Hardy, Toronto and Winnipeg for the years 1977-1984 are analyzed to yield estimates of atmospheric precipitable water and turbidity. Three methods of estimating the precipitable water and two methods of estimating the turbidity are used and compared. Laboratory measurements of pyranometer response as a function of zenith angle are used to correct the global radiation measurements. Circumsolar radiation is removed from the direct radiation obtained from the difference of measured global and diffuse radiation. The magnitude of this circumsolar correction is discussed in the light of recent measurements and calculations of the circumsolar ratio. Turbidity time series are presented, showing a clearly defined El Chichon eruption signature in 1983-1984. A comparison with earlier results is included.

Values of the Angstrom turbidity coefficient, [Beta], at 0.50 [[micro]meter] wavelength have been determined in Lagos, a tropical coastal city in Nigeria, for 17 months between 1990 and 1991 using a Volz sun photometer. The results obtained indicate high variability of aerosol loading, though the city, on average, experiences high turbidity for most parts of the year. An annual average of 0.299 with a standard deviation of 0.187 was found. On average, the month of January experienced the highest turbidity, with a mean value of 0.497, while November experienced the lowest aerosol loading on average with a value of 0.225. The aerosols influencing the city are both of maritime and Saharan origin, coupled with locally produced particulates from industries and automobile combustion. Possible effects of the Mount Pinatubo haze may be the cause of the increase of turbidity values in the second half of 1991 when compared with values for the same period in 1990. Addition of [Beta] in the Liu and Jordan type diffuse fraction correlation indicates little improvement in standard error.

A meteorological experiment was carried out in summer 1995 at two representative sites above and below the timberline in the Dischma Valley, near Davos, Switzerland. The study aimed at investigating the characteristics of the surface energy balance and of the evapotranspiration in a high alpine environment during the vegetation period. At both sites, net radiation is the only energy source. It amounts to about 80 W [m.sup.-2] at the lower and 100 W [m.sup.-2] at the upper sites, respectively. Since the albedo and the longwave radiation budget do not differ significantly, net radiation was found to be linearly dependent on global radiation. The latent heat flux associated with evapotranspiration represents the most important energy sink, averaging to -70 W [m.sup.-2] at the lower and -63 W [m.sup.-2] at the upper locations. It is therefore of comparable magnitude, despite a larger energy availability at the upper site. This is due to a significantly larger Bowen ratio at this upper location. On the other hand, the diurnal course of the Bowen ratio is at both sites such that the latent heat flux can be expressed as a linear function of net or global radiation. For a better characterization, the actual evapotranspiration was compared to Penman's parameterization, which represents the potential limit for saturated surface conditions. The comparison shows that, even during wet periods, evapotranspiration is regulated by the moisture conditions in the soil and the physiological behavior of vegetation.

A neural network is used to calculate the longwave net radiation ([L.sub.net]) at the sea surface from measurements of the Special Sensor Microwave/Imager (SSM/I). The neural network applied in this study is able to account largely for the nonlinearity between [L.sub.net] and the satellite-measured brightness temperatures (TB). The algorithm can be applied for instantaneous measurements over oceanic regions with the area extent of satellite passive microwave observations (30-60 km in diameter). Comparing with a linear regression method the neural network reduces the standard error for [L.sub.net] from 17 to 5 W [m.sup.-2] when applied to model results. For clear-sky cases, a good agreement with an error of less than 5 W [m.sup.-2] for [L.sub.net] between calculations from SSM/I observations and pyrgeometer measurements on the German research vessel Poseidon during the International Cirrus Experiment (ICE) 1989 is obtained. For cloudy cases, the comparison is problematic due to the inhomogenities of clouds and the low and different spatial resolutions of the SSM/I data. Global monthly mean values of [L.sub.net] for October 1989 are computed and compared to other sources. Differences are observed among the climatological values from previous studies by H.-J. Isemer and L. Hasse, the climatological values from R. Lindau and L. Hasse, the values of W. L. Darnell et al., and results from this study. Some structures of [L.sub.net] are similar for results from W. L. Darnell et al. and the present authors. The differences between both results are generally less than 15 W [m.sup.-2]. Over the North Atlantic Ocean the authors found a poleward increase for [L.sub.net], which is contrary to the results of H.-J. Isemer and L. Hasse.

This paper reports results of observations of radiative fluxes measured in sub-Sahel Africa during a 2-yr period (1992-94). Shortwave radiation in the solar spectrum (0.2-4.0 [[micro]meter]), photosynthetically active radiation (0.4-0.7 [[micro]meter]), and longwave radiation (4.0-50.0 [[micro]meter]) were observed. In this study, the annual variability and the effects of dust on these fluxes (in particular, on the ratio of photosynthetically active radiation to the total shortwave radiation) were characterized. This ratio, known as the conversion factor, is important in modeling net primary productivity and the total C[O.sub.2] budget. In the past, this ratio was assumed to be constant. The authors' observations indicate that the daily average conversion factor has a strong annual cycle with a minimum of 0.41 in the middle of the dry season and a maximum of 0.55 in the first half of the rainy season, which is consistent with theoretical computations. The 2-yr mean is 0.49, only about 2% less than the nominal value of 0.50. The 2-yr mean values of the daily (24 h) averages of shortwave, photosynthetically active, and longwave downward fluxes are 200, 98, and 398 W [m.sup.-2], respectively. The average shortwave transmittance is 0.482. The two years of observations differed in the mean value of the downward shortwave component by about 7 W [m.sup.-2], the second year having lower values, with a similar decrease in photosynthetically active radiation and an increase in the longwave component by about 5 W [m.sup.-2], which would point to an increased cloudiness during the second year. The long-term objective of this study is to have high-quality ground truth in this climatic region for validating satellite-inferred surface radiative fluxes.

Aerosol optical depth measurements by means of ground-based Sun photometry were made in Bern, Switzerland during two and a half years primarily to provide quantitative corrections for atmospheric effects in remotely sensed data in the visible and near-infrared spectral region. An investigation of the spatial variability of tropospheric aerosol was accomplished in the summer of 1994 in the Swiss Central Plain, a region often covered by a thick aerosol layer. Intercomparisons are made with two Sun photometers operated by the Swiss Meteorological Institute in Payerne (Swiss Central Plain) and Dayos (Swiss Alps, 1590 m a.s.I.). By means of an inversion technique, columnar particle size distributions were derived from the aerosol optical depth spectra. Effective radius, columnar surface area, and columnar mass were computed from the inversion results. Most of the spectra measured in Bern exhibit an Angstrom-law dependence. Consequently, the inverted size distributions are very close to power-law distributions. Data collected during a four month calibration campaign in fall 1993 at a high-mountain station in the Swiss Alps (Jungfraujoch, 3580 m) allowed us to study optical properties of stratospheric aerosol. The extinction spectra measured have shown to be still strongly influenced by remaining aerosol of the June 1991 volcanic eruptions of Mount Pinatubo. Inverted particle size distributions can be characterized by a broad monodisperse peak with a mode radius around 0.25 [[micro]meter]. Both aerosol optical depths and effective radii had not yet returned to pre-eruption values. Comparison of retrieved aerosol optical depth, columnar surface area and mass, with the values derived from lidar observations performed in Garmisch-Partenkirchen, Southern-Germany, yielded good agreement. Index Terms - Optical depth, size distribution, Sun photometry.

This paper describes a new radiation monitoring program, the Integrated Surface Irradiance Study (ISIS), that builds upon and takes over from earlier NOAA networks monitoring components of solar radiation [both the visible component (SOLRAD) and the shortwave component that causes sunburn, UV-B] across the continental United States. ISIS is implemented in two levels. Level 1 addresses incoming radiation only, and level 2 addresses the surface radiation balance. Level 2 also constitutes the SURFRAD (Surface Radiation) program of the NOAA Office of Global Programs, specifically intended to provide radiation data to support large-scale hydrologic studies that will be conducted under the Global Energy and Water Cycle Experiment. Eventually, it is planned for level 2 sites to monitor all components of the surface energy balance. Both levels of ISIS will eventually measure both visible and UV radiation components. At present, there are nine sites that are considered to be at ISIS level 1 standard and an additional four level 2 SURFRAD sites. A 10th level 1 site will be in operation soon. Plans call for an increase in the number of sites of both kinds, up to about 15 ISIS sites, of which 6 will be at the SURFRAD level. Data are available via FTP at ftp.atdd.noaa.gov/pub/isis or at http://www.srrb.noaa.gov (level 2).

Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison. The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insolation is larger than the measurements for clear-sky and total-sky conditions. This experiment has been developed to test retrievals of radiative fluxes and the associated forcings by clouds, aerosols, surface properties, and water vapor. Collaboration is sought; the goal is to extend the domain of meteorological conditions for which such retrievals can be done accurately. CAGEX Version 1 covers April 1994. Subsequent versions will (a) at first span the same limited geographical area with data from October 1995, (b) then expand to cover a significant fraction of the GEWEX Continental-Scale International Project region for April 1996 through September 1996, and (c) eventually be used in a more advanced form to validate CERES.

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.

A new radiation scheme, suitable for two-stream radiation transfer models, was developed for cirrus clouds. Analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter. These are functions of the ice particle size distribution parameters, ice particle shapes, and wavelength. The ice particle shapes considered were hexagonal plates and columns, bullet rosettes, and planar polycrystals. These appear to be the principal crystal types found in cirrus clouds. The formulation of radiative properties accounts for the size distribution projected area and the distance radiation travels through ice particles. For absorption, refraction and internal reflection of radiation were parameterized. By assuming an idealized cirrus cloud, the dependence of the single scatter albedo, reflectance, and emissivity on wavelength, ice particle shape, and size distribution was demonstrated. Reflectance and emissivity exhibited a strong dependence on ice particle shape, with planar polycrystals and bullet rosettes often being twice or more reflective than hexagonal columns and plates. The radiation scheme was tested with microphysical and radiation measurements from two cirrus cloud field studies. It was shown for both case studies that, by matching observed and predicted albedo-emissivity curves, the radiation scheme could predict the observed mean ice particle size and ice water path (IWP), provided the dominant ice particle shape was known or inferred. Retrieved IWP values differed from measurement-derived values by [less than or equal to] 15% for the first case study and 18% on average for the second case study. Hence, it may be feasible to retrieve realistic IWP estimates from satellite data for a given ice particle shape. Other radiation schemes have not been able to explain the second case study, which was characterized by relatively high albedos. These high albedos appeared to result from unusually small hexagonal plate crystals having asymmetry parameter values similar to those of cloud droplets. An improved treatment of the asymmetry parameter was not the primary reason for the good agreement between theory and observations. Rather, key factors appeared to be improved treatments of ice particle photon path, projected area and mass, and the omission of certain physical processes included in Mie theory that may not be appropriate for ice particles. The radiative properties were predicted from analytical expressions, making this scheme useful for predicting radiative properties in large-scale models without excessive increases in computation time.

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.

Six years of hourly global and diffuse irradiation measurements on a horizontal surface performed at Athalassa, Cyprus, are used to establish a relationship between the daily diffuse fraction and the daily clearness index. Two types of correlations - yearly and seasonal - have been developed. These correlations, of first and third order in the clearness index are compared to the various correlations established by Collares-Pereira and Rabl (1979), Newland (1989), Erbs et al. (1982), Rao et al. (1984), Page (1961), Liu and Jordan (1960) and Lalas et al. (1987). The comparison has been performed in terms of the widely used statistical indicators (MBE) and (RMSE) errors; an additional statistical indicator, the t-statistic, combining the earlier indicators, is introduced. The results indicate that the proposed yearly correlation matches the earlier correlations quite closely and all correlations examined yield results that are statistically significant. For large [Mathematical Expression Omitted] values, most of the earlier correlations exhibit a slight tendency to systematically overstimate the diffuse fraction. This marginal disagreement between the earlier correlations and the proposed model is probably significantly affected by the clear sky conditions that prevail over Cyprus for most of the time as well as atmospheric humidity content. It is clear that the standard correlations examined in this analysis appear to be location-independent models for diffuse irradiation predictions, at least for the Cyprus case.

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.

Dry layers are frequently observed in atmospheric soundings from the climatologically humid western Pacific warm pool region. Some 2400 soundings from the TOGA COARE field program were objectively examined for humidity drops (layers in which relative humidity decreases rapidly with height), indicative of the bases of dry layers. These occur throughout the lower and middle troposphere, with frequency peaks near 550 (the 0 [degrees] C level), 800, and 950 mb. A composite constructed from these sounding data indicates the following. 1) Almost all dry layers are too dry (and not warm enough) to be interpreted as conservative vertical displacements. Rather, they apparently consist of filaments or tongues of low moist static energy air advected into the column, often from the subtropics. 2) Dry tongues are anomalously virtually warm near their bases with a slight cool layer below; that is, they sit atop sharp stable layers or inversions. The authors hypothesize that radiation is responsible for the thermal structure of dry tongues. The radiative effects of humidity structures in the troposphere are reviewed and illustrated. A composite-derived radiative heating perturbation, acting for 3.5 days in an idealized model of a dry tongue [approximately] 300 km in width (values consistent with case studies), reproduces fairly well the high vertical wavenumber components of the composite thermal structure. Dynamics acts to spread the effect of the radiative heating perturbation over a wider area and to concentrate the temperature perturbations near the dry tongue base, as observed. The deep layer-mean warmth of the composite dry tongue arises from a slight correlation between dry tongue occurrence in this dataset and a [approximately] 1 [degree] C global-scale intraseasonal variation of tropical tropospheric temperature. A dry tongue affects convective clouds both directly, through its thermal structure, and indirectly, through dry air entrainment. Low-level dry tongues can prevent deep convection outright, while the stable layers associated with dry tongues at higher altitudes may cause convection to detrain mass. Humidity drops, stable layers, and a proxy for layer clouds all have similar altitude distributions.

On many planets there is a continuous heat supply to the surface and a continuous emission of infrared radiation to space by the atmosphere. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. Atmospheric convection is a natural heat engine that might operate in this system. Based on the heat engine framework, a simple theory is presented for atmospheric convection that predicts the buoyancy, the vertical velocity, and the fractional area covered by either dry or moist convection in a state of statistical equilibrium. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. The mechanical work is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels. Increases in the fraction of energy dissipated at warmer temperatures, at the expense of decreases in the fraction of energy dissipated at colder temperatures, lead to increases in the apparent efficiency of the convective heat engine. The volume integral of the work produced by the convective heat engine gives a measure of the statistical equilibrium amount of convective available potential energy (CAPE) that must be present in the planet's atmosphere so that the convective motions can be maintained against viscous dissipation. This integral is a fundamental global number qualifying the state of the planet in statistical equilibrium conditions. For the earth's present climate, the heat engine framework predicts a CAPE value of the order of 1000 J [kg.sup.-1] for the tropical atmosphere. This value is in agreement with observations. It also follows from our results that the total amount of CAPE present in a convecting atmosphere should increase with increases in the global surface temperature (or the atmosphere's opacity to infrared radiation).

The seasonal changes in the turbulent and radiative energy fluxes above and within the vegetation canopy of a red oak forest in Harvard, MA, have been measured. The observations, which began in the summer of 1991, revealed several seasonal variations which were confirmed by satellite imagery. These include the incidence of global solar albedo, photosynthetically active radiation albedo and the seasonal pattern of the canopy's surface energy budget.

The sensible heat flux (H), latent heat flux (LE) and soil heat flux over heterogeneous land surfaces were derived using single- and dual-source models of soil-vegetation-atmosphere surface energy transfer and radiometric surface temperature measurements. The results, which were compared with observations over semiarid plains and tallgrass prairie regions, showed a 20% difference between the dual-source model's estimates of the turbulent fluxes H and LE and the data set.

Microwave brightness temperatures at three frequencies (23.8, 36.5, and 90 GHz) of a carefully smoothed silt-loam were measured over a large range of near-surface soil moisture conditions. The microwave data were collected simultaneously with ground observations of soil moisture, soil temperature, and bulk density. The atmospheric downwelling radiation is computed from the French weather forecast model outputs and cloud coverage surface observations. Assuming that roughness effects are negligible for the studied surface, Fresnel coefficients are used to model soil reflectivities. The relationship between soil moisture and soil reflectivity at high frequencies is then analyzed through existing semi-empirical models of the soil complex dielectric permittivity.

Previous studies on the use of A. Angstrom's model for predicting global solar radiation are inconsistent with its use of coefficients and lack sufficient climatologic evidence to be considered valid. Past research also failed to develop the approaches required to generalize and improve the model. Further studies should be conducted to address issues concerning climatological influences on radiation and sunshine, statistical and stochastic models to support Angstom's approach and wide scale effects of sunshine recorders.

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.