Emission Of Longwave Radiation Research Articles

Attribution of Projected Changes in Atmospheric Moisture Transport in the Arctic: A Self-Organizing Map Perspective

Abstract Meridonal moisture transport into the Arctic derived from one simulation of the National Center for Atmospheric Research Community Climate System Model (CCSM3), spanning the periods of 1960–99, 2010–30, and 2070–89, is analyzed. The twenty-first-century simulation incorporates the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A2 scenario for CO2 and sulfate emissions. Modeled and observed [from the 40-yr ECMWF Re-Analysis (ERA-40)] sea level pressure (SLP) fields are classified using a neural network technique called self-organizing maps to distill a set of characteristic atmospheric circulation patterns over the region north of 60°N. Model performance is validated for the twentieth century by comparing the frequencies of occurrence of particular circulation regimes in the model to those from the ERA-40. The model successfully captures dominant SLP patterns, but differs from observations in the frequency with which certain patterns occur. The model’s twentieth-century vertical mean moisture transport profile across 70°N compares well in terms of structure but exceeds the observations by about 12% overall. By relating moisture transport to a particular circulation regime, future changes in moisture transport across 70°N are assessed and attributed to changes in frequency with which the atmosphere resides in particular SLP patterns and/or to other factors, such as changes in the meridional moisture gradient. By the late twenty-first century, the transport is projected to increase by about 21% in this model realization, with the largest contribution (32%) to the total change occurring in summer. Only about one-quarter of the annual increase is due to changes in pattern occupancy, suggesting that the majority is related to mainly thermodynamic factors. A larger poleward moisture transport likely constitutes a positive feedback on the system through related increases in latent heat release and the emission of longwave radiation to the surface.

A global climate model of Titan's atmosphere and surface

We present the formulation of a global climate model (GCM) of Titan's atmosphere and surface and report initial results. The model is a fully three-dimensional, modified version of NCAR's terrestrial global climate model, CAM3. It includes forcing by Saturn's gravitational tides; a treatment of the planetary boundary layer and surface interactions; scattering and absorption of short-wave radiation; absorption and emission of long-wave radiation; thermal conduction in the soil, and a package for computing atmospheric chemistry. The physical properties and distribution of minor species and aerosols are constrained by Cassini observations. The simulations do not include the effects of a methane hydrological cycle, feedback between radiative forcing and transport of radiatively active minor species and aerosol, or aerosol microphysics. We report a set of baseline results obtained after more than ∼30 Titan-years of model integration. The model calculates a tropospheric circulation that is in-family with the results from other published GCMs, with a regime of weak retrograde winds dominating the summer hemisphere and a band of weak prograde winds appearing in the winter hemisphere. In the stratosphere, the model produces a polar jet in the winter hemisphere, but its peak velocity of 12 m s −1 is more than an order of magnitude weaker than the winter polar jet inferred from observations. We find that the globally integrated atmospheric angular momentum (AAM) undergoes a semiannual oscillation similar to that calculated by Tokano and Neubauer [Tokano, T., Neubauer, F.M., 2005. Wind-induced seasonal angular momentum exchange at Titan's surface and its influence on Titan's length-of-day. Geophys. Res. Lett. 32, L24203–L24206], but delayed in seasonal phase by 30° of L s. The associated transfer of angular momentum to the surface implies that Titan's rate of rotation should be accelerating during the epoch of Cassini Mission observations, consistent with results obtained by the Cassini radar investigation [Stiles, B.W., Kirk, R.L., Lorenz, R.D., Hensley, S., Lee, E., Ostro, S.J., Allison, M.D., Callahan, P.S., Gim, Y., Iess, L., Persi del Marmo, P., Hamilton, G., Johnson, W.T.K., West, R.D., 2008. The Cassini RADAR Team: Determining Titan's spin state from Cassini RADAR images. Astron. J. 135, 1669–1680]. The amount of AAM transfer to the surface predicted by the model, when combined with knowledge of the state of rotation, points to a low value for Titan's effective moment of inertia. As has been suggested [Lorenz, R.D., Stiles, B.W., Kirk, R.L., Allison, M.D., Persi del Marmo, P., Iess, L., Lunine, J.I., Ostro, S.J., Hensley, S., 2008. Titan's rotation reveals an internal ocean and changing zonal winds. Science 319, 1649–1651], this may indicate significant decoupling between Titan's outer crust and its mantle, possibly due to the presence of an intervening subsurface ocean. We note, however, that existing Titan GCMs, as presently formulated, do not yield uniform results concerning AAM transfer to the surface and ignore some physical processes that could potentially alter the predicted magnitude and seasonal phasing of the transfer.

Bionics in textiles: flexible and translucent thermal insulations for solar thermal applications

Solar thermal collectors used at present consist of rigid and heavy materials, which are the reasons for their immobility. Based on the solar function of polar bear fur and skin, new collector systems are in development, which are flexible and mobile. The developed transparent heat insulation material consists of a spacer textile based on translucent polymer fibres coated with transparent silicone rubber. For incident light of the visible spectrum the system is translucent, but impermeable for ultraviolet radiation. Owing to its structure it shows a reduced heat loss by convection. Heat loss by the emission of long-wave radiation can be prevented by a suitable low-emission coating. Suitable treatment of the silicone surface protects it against soiling. In combination with further insulation materials and flow systems, complete flexible solar collector systems are in development.

The role of snow interception in winter‐time radiation processes of a coniferous sub‐alpine forest

AbstractThe radiation balance in forests is modified by the presence of a (temporally snow‐covered) canopy much more than it is in open areas. The primary effects of the forest canopy are absorption of incoming shortwave radiation, as well as absorption and emission of longwave radiation. In this study, data from a mobile net radiation sensor moving along a 10‐m bar in a coniferous sub‐alpine stand of central Switzerland were analysed to assess the role of intercepted snow in comparison with other governing factors for winter‐time radiation processes in a coniferous sub‐alpine forest. The four winters investigated (2003–2007) covered a broad range of weather and snow conditions including several periods with intercepted snow lasting up to 19 days.For cloudless days, the data show that canopy albedo is elevated by the presence of intercepted snow. For transmissivity, the impact of intercepted snow is less distinct. Our analysis showed that the fraction of diffuse radiation and solar elevation are the main factors affecting shortwave transmissivity. Only after compensating for these effects, could intercepted snow be associated with increased transmissivities. Part of this analysis was carried out with the help of a physically based radiative transfer model. The model results suggest that multiple reflections are relevant to understanding the complex interactions between the factors affecting transmissivity. Finally, snow interception load can be derived reasonably well (R2 = 0·65) from radiation measurements in this sub‐alpine forest. Copyright © 2008 John Wiley & Sons, Ltd.

Surface radiation balance in the ablation zone of the west Greenland ice sheet

We present 4 years of radiation observations and derived cloud properties from the ablation zone of the west Greenland ice sheet (67°N). Data were collected using three automatic weather stations located at 6, 38, and 88 km from the ice sheet margin at elevations of 490, 1020, and 1520 m asl. This part of Greenland is characterized by a ∼150 km wide tundra, a ∼100 km wide ablation zone and an average equilibrium line altitude of ∼1500 m asl. At the lowest station, snow is redistributed by the wind in crevasses and gullies, leading to very little measured winter accumulation. As a result, glacier ice (albedo ≈ 0.55) is at the surface throughout the melting season. At 1020 m asl, the winter snow cover typically disappears in mid to end June. At 1520 m asl, superimposed ice briefly reaches the surface at the end of the ablation season. The combined effect of decreasing surface albedo and increasing cloud optical thickness as summer progresses causes absorbed shortwave radiation to peak in June at low elevations but progressively later at higher elevations. Incoming longwave radiation peaks in August, in response to increased cloud optical thickness and heating of the atmosphere over the snow and ice‐free surroundings. The ice sheet margin experiences continuous melting in June, July, and August. This limits the emission of longwave radiation, causing net longwave radiation to peak in August, further enhancing melt. The sum of these effects is that summer net radiation increases sharply toward the ice sheet margin. To resolve this correctly requires high‐resolution climate models, in the order of 10 km or better.

Effects of water vapour and rock substrates on the microclimates of painted rock art surfaces and their impact on the preservation of the images

Microclimate data from typical sandstone and limestone rock art sites in the west Kimberley region of Western Australia were recorded in both the wet and the dry seasons. The research described in this paper shows that the climatic behaviour in rock shelters is more amenable to direct interpretation when the relative humidity at any given temperature is converted to absolute humidity or water vapour pressure. It has been shown that many of the sites act as ‘closed systems’ with little input or output of moisture until water bearing fronts or drying winds change the local microenvironment. The rate of cooling of points within the shelters is largely determined by exposure to the sky, which facilitates cooling by emission of long wave radiation and by the heat capacity of the moisture in the air. It was found that the heating and cooling rates for limestone sites in the Napier Ranges are more sensitive to changes in moisture than the sandstone sites in the Mitchell Plateau. The survival of painted images on the rock surfaces is directly linked to the way in which the substrates respond to changes in the moisture content of the sites.

A review of vegetation—atmosphere interactions and their influences on mesoscale phenomena

Vegetation strongly influences exchanges of energy and moisture between land and atmosphere through (1) the vegetation's response to incoming radiation and its emission of longwave radiation (2) the vegetation's physical presence, and (3) the plant's transpiration. These processes affect the diurnal temperature range, processes in the atmospheric boundary layer, cloud cover, rainfall, differential heating, and atmospheric circulations. This paper overviews how vegetation interacts with surface energy and moisture budgets and reviews both observational and modelling studies that examine how vegetation affects weather and climate on the mesoscale (ie, phenomena 10s to 100s of kilometres in horizontal size).

Energy-balance model validation on the top of Kilimanjaro, Tanzania, using eddy covariance data

AbstractEddy covariance data collected over a horizontal surface on the largest ice body on Kilimanjaro, Tanzania, over 26–29 July 2005 were used to assess the uncertainty of calculating sublimation with a surface energy balance (SEB) model. Data required for input to the SEB model were obtained from an existing automatic weather station. Surface temperatures that were solved iteratively by the SEB model were used to compute emitted longwave radiation, turbulent heat fluxes using the aerodynamic bulk method and the subsurface heat flux. Roughness lengths for momentum and temperature, which were found to be the most important input parameters controlling the magnitude of modelled (bulk method) turbulent heat fluxes, were obtained using eddy covariance data. The roughness length for momentum was estimated to be 1.7×10–3 m, while the length for temperature was one order of magnitude smaller. Modelled sensible and latent heat fluxes (bulk method) compared well to eddy covariance data, with root-mean-square differences between 3.1 and 4.8 Wm–2 for both turbulent heat fluxes. Modelled sublimation accounted for about 90% of observed ablation, confirming that mass loss by melting is much less important than sublimation on the horizontal surfaces of the remaining plateau glaciers on Kilimanjaro.

Long-term response of daily epilimnetic temperature extrema to climate forcing

Twenty years (1983–2002) of hourly summer temperature data from the epilimnion of Müggelsee, a shallow lake in northern Germany, showed a long-term increase, with the rate of increase of the daily minima (nighttime temperatures) exceeding that of the daily maxima (daytime temperatures). This does not simply reflect the long-term behaviour of air temperature, which did not exhibit a significant degree of day–night asymmetry. A sensitivity analysis based on a heat-balance model revealed that the daily extrema of the lake surface equilibrium temperature responded differently not only to shifts in air temperature, but also to shifts in wind speed, relative humidity, and cloud cover, suggesting that long-term changes in all four variables contribute to day–night asymmetry in the epilimnetic temperature. A comparison of nighttime and daytime estimates of the heat flux components into the lake indicates that the emission of long-wave radiation from the atmosphere is likely to be the main process responsible for day–night asymmetry in the epilimnetic temperature. Although this process is partially dependent on air temperature, it is also dependent on relative humidity and cloud cover. The influence of long-term changes in these additional driving variables on epilimnetic temperatures cannot therefore be neglected.

Evaluation of the SHAW Model in Simulating the Components of Net All-Wave Radiation

Radiation exchange at the surface plays a critical role in the surface energy balance, plant microclimate, and plant growth. The ability to simulate the surface energy balance and the microclimate within the plant canopy is contingent upon accurate simulation of the surface radiation exchange. A validation exercise was conducted of the Simultaneous Heat and Water (SHAW) model for simulating the surface radiation exchange (including downward long-wave and upward short- and long-wave radiation) over a maize canopy surface using data collected at Yucheng in the North China Plain. The model simulated upward short-wave and net all-wave radiation well with model efficiencies (ME) equaling 0.97 and 0.98, respectively. Downward and upward long-wave radiation were overestimated by 12.1 and 8.3 W m-2 with ME equaling 0.68 and 0.89, respectively. Two modifications to the model were implemented and tested to improve the simulated long-wave radiation exchange. In one modification, alternative schemes were tested to simulate cloudy sky long-wave radiation, and the best algorithm was employed in the model. With this modification, both downward and upward long-wave radiation were simulated better, with ME rising to 0.88 and 0.91, respectively. A second modification was implemented to use leaf temperature rather than canopy air temperature to compute emitted long-wave radiation. Although more theoretically correct, this modification did not improve simulations compared to the original model because upward long-wave radiation was already overpredicted and midday leaf temperatures at this site were typically higher than canopy air temperatures. Thus, the modification resulted in even higher overprediction of upward midday long-wave radiation. However, this modification removed some of the bias in nighttime emitted long-wave radiation. While the SHAW model simulates the radiation balance and transfer processes within the canopy reasonably well, results point to areas for model improvement.

Euler–Lagrange equation of the most simple 1-D climate model based on the maximum entropy production hypothesis

A simple one-dimensional (1-D) climate model in the meridional direction is considered based on the maximum entropy production hypothesis. This model calculates latitudinal distribution of the emitted long-wave radiation and meridional heat transport for a given latitudinal distribution of the absorbed solar radiation. By treating the problem analytically, an Euler–Lagrange equation and a numerical method solving it are obtained. Calculations based on the maximum entropy production hypothesis qualitatively explain an enhancement of the poleward heat transport observed in a last glacial maximum simulation with a coupled general-circulation model. Copyright © 2005 Royal Meteorological Society

The climate during the Maunder Minimum: a simulation with the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM)

A model simulation of the climate during Maunder Minimum (MM) (1645–1715) was performed using the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM). A multi-year equilibrium integration with prescribed solar insolation, atmospheric composition and sea surface temperatures (SSTs) for MM conditions was compared with a present-day (PD) simulation. We find that during MM the stratosphere was significantly warmer (up to 3 K) than during PD, and dynamically more disturbed in winter. The warming is due to the dominant effect of the lower atmospheric CO 2 concentration during MM, which leads to a reduced emission of long-wave radiation, and compensates the cooling due to the reduced solar irradiance. The troposphere was about 1–1.5 K cooler in the annual mean during MM. The global mean surface air temperature decreased by 0.86 K. Northern hemisphere winters were on average characterized by cooler and drier weather over the northern parts of the continents, with an increase in precipitation in the southern parts. These climate anomalies are shown to be related to a shift in the North Atlantic Oscillation (NAO) towards a predominantly low phase during MM. The simulated climate anomalies are in very good agreement with reconstructions from proxy-data. Changes in the dynamical coupling between the troposphere and stratosphere were found in the MM simulation, indicating the importance of the stratosphere for climate change.

Radiative Properties of Mid-latitude Frontal Ice-Clouds Observed by the Shortwave and Longwave Radiometer-Sondes

In the Japanese Cloud and Climate Study (JACCS) cirrus experiment, simultaneous measurements of cloud radiative and microphysical properties were conducted by using the combined-sonde (radiometersonde + hydrometeor-video-sonde (HYVIS)) observation system at the Meteorological Research Institute (MRI), located at (36.05°N, 140.13°E) in Tsukuba, Japan, during early summer seasons from 1995 to 1999. We have analyzed the radiative properties of frontal ice-clouds observed by the shortwave and longwave radiometer-sondes (Asano et al. 2004). To interpret the observed radiative flux profiles, we have also performed radiative transfer calculations for horizontally homogeneous atmospheric models, where the single-scattering properties of ice-clouds were computed by anomalous diffraction theory for ice-crystals observed by HYVIS. On an average of the observed frontal ice-clouds, the shortwave re- flectance, transmittance and absorptance were estimated to be 0.41±0.03, 0.51±0.06, and 0.08±0.09, respectively, for the averaged ice-cloud layer with a mean visible optical thickness of 4.6 and a mean geometrical thickness of 5.4 km (mean volume extinction coefficient of 0.85 km-1). The ice-clouds were significantly heated by absorption of solar radiation in daytime. On the other hand, the mean effective emittance was estimated to be about 0.86±0.37, showing that the frontal ice-clouds never acted as blackbody for longwave radiation. The lower parts of ice-cloud layers were heated by absorption of longwave radiation from the surface and the atmosphere below the ice-clouds, while the upper parts were cooled by emission of longwave radiation to space. The shortwave and longwave heating profiles could make the daytime ice-cloud layers thermodynamically unstable.

ESTIMATING POLYNYA CLOUDINESS

Global cloudiness distributions, though an important component in radiative and hydrological budgets, are neither adequately known nor easily retrieved by the spatial and spectral resolutions afforded by current satellite instrumentation. At high latitudes, cold, high albedo surfaces present a particular challenge to cloud retrieval, offering little or no thermal or visible contrast for cloud-ice discrimination. It is in these frequently cloudy and climate-sensitive regions that changing cloud amounts and optical parameters enact the greatest influence, enhancing or suppressing melt through cloud base emission of longwave radiation or scattering of incident shortwave radiation. Polynyas and leads, seasonally ice-free areas characterized by intense air-sea fluxes of heat and moisture, are useful features for exploring the relationships between cloud cover and the underlying surface. Using polar-optimized CASPR (Cloud and Surface Parameter Retrieval) algorithms to process multi-channel AVHRR radiances, cloud amounts, microphysics, and surface forcing are evaluated and validated against in situ measurements collected in several polynyas and leads across the Western Arctic during the years 1992-2000

Tight coupling between soil moisture and the surface radiation budget in semiarid environments: Implications for land‐atmosphere interactions

Observations are used to examine how soil moisture influences the surface radiation budget, ground heat flux, and available energy in semiarid environments. Defining this relationship is critical to understand interactions between the land surface and the atmosphere, in particular assessing if a feedback exists between soil moisture and rainfall anomalies. We use two summers of data collected from semiarid grassland and shrubland ecosystems in central New Mexico. The response of surface radiation budget components and other variables to soil moisture variations are quantified via linear regression. Then, the variations are scaled over the observed range of soil moisture (15% volumetric water content). The soil temperature is lower by >10°C when the surface soil is wet, compared to when the soil is dry. This temperature decrease results in a measured decrease of 85–100 W m−2 in longwave radiation emitted at the surface. The increase in net longwave radiation is equal in magnitude because downward longwave radiation does not vary with soil moisture. The observed changes in net shortwave radiation are relatively minor (<10 W m−2), as the surface albedo decreases by only 1.5% when soil is wet. Net radiation increases by an amount roughly equal to the decrease in emitted longwave radiation (∼85–100 W m−2). Changes in ground heat flux are not detectable, given the noise in the data. Therefore the available energy, Qa, is higher by 80 W m−2 when the soil is wet. This change is 22% of average Qa at the shrubland site and 19% at the grassland site. The observed soil moisture‐induced Qa variations are large compared to other sources of Qa variability, so they should influence boundary layer moist static energy. However, the intervals during which soil moisture is high and therefore Rn and Qa are enhanced are short, on the order of several days. Therefore feedbacks to rainfall may be limited. Compared to other environments, the influence of soil moisture on Rn and Qa is likely greater in semiarid environments because soil moisture‐induced fluctuations in evaporative fraction and surface temperature are relatively large.

The Sensitivity of the Tropical Hydrological Cycle to ENSO

Abstract Satellite observations of temperature, water vapor, precipitation and longwave radiation are used to characterize the variation of the tropical hydrologic and energy budgets associated with the El Nino–Southern Oscillation (ENSO). As the tropical oceans warm during an El Nino event, the precipitation intensity, water vapor mass, and temperature of the tropical atmosphere are observed to increase, reflecting a more vigorous hydrologic cycle. The enhanced latent heat release and resultant atmospheric warming lead to an increase in the emission of longwave radiation. Atmospheric global climate models, forced with observed sea surface temperatures (SSTs), accurately reproduce the observed tropospheric temperature, water vapor, and outgoing longwave radiation changes. However, the predicted variations in tropical-mean precipitation rate and surface longwave radiation are substantially smaller than observed. The comparison suggests that either (i) the sensitivity of the tropical hydrological cycle to E...

Estimating surface air temperatures, from Meteosat land surface temperatures, using an empirical solar zenith angle model

Temperature values derived from Meteosat are an indication of emitted long-wave radiation, and are not a true indication of ambient air temperature. The authors believe that Solar Zenith Angle (SZA) can be used as a proxy for solar energy reaching the ground surface, and its subsequent effects upon the land surface temperature detected by Meteosat. Raw satellite temperatures often overestimate the actual screen temperature during the day, and underestimate at night. By using a statistical model which relates Meteosat and WMO screen temperature deviations, and SZA values, it has been possible to generate a correction algorithm which minimizes these differences. The algorithm generates a new proxy value, being a simulated ambient (screen) air temperature. The algorithms achieve an accuracy of within 3 C for over 70% of the Meteosat temperatures processed. The operational use of this algorithm requires only the raw Meteosat temperature value, and the SZA. Such temperature corrections are useful for a wide range of environmental monitoring applications. An example is in the field of vector-borne disease modelling which requires proxies for temperature across large regions, and where more conventional meteorological stations are inadequate.

The effect of clumping and stomatal response on evaporation from sparsely vegetated shrublands

The development of two-source models of evaporation has improved estimates of evaporation from sparsely vegetated surfaces over those of single-source models. This paper investigates the importance of clumping of vegetation, as defined by the fractional vegetative cover, for evaporation from soil and shrubs. A sensitivity analysis; of a model that includes clumping of vegetation and three sources of water for evaporation, shrubs, soil under shrubs and bare soil outside shrubs, is presented. Resistances to vapour transfer were derived experimentally for a shrubland in southeastern Spain, which was dominated by the leguminous shrub Retama sphaerocarpa. Stomatal conductances of the shrub were found to decline linearly with the in-canopy water vapour saturation deficit (D 0). The slope of this relationship increased linearly with volumetric soil water content. Soil surface resistances were found to increase quadratically with time after wetting of the soil surface. The model that included clumping (the C model) was compared with single- and two-source models. The tested models included feedbacks between plant resistances, in-canopy vapour pressure saturation deficits, and total evaporation from the surface ( λE t ). The single-source and two-source models overestimated evaporation from the plant ( λE p ) by 1% and 8%, respectively, whereas the C model underestimated λE p by 5%. The overestimates of λE p by the conventional two-source model resulted primarily from over-estimation of the radiation being absorbed by the canopy. The underestimation by the C model may have resulted from underestimation of the radiation absorbed by the canopy resulting from high reflected short-wave and emitted long-wave radiation from the soil. The C and two-source models predicted D 0 within 4%, whereas the single-source model underestimated D 0 by 19%, resulting from the omission of energy fluxes from the soil. The good prediction of evaporation from the plant by the single source model resulted from the combination of under-estimating D 0 and overestimating stomatal conductance.

A TEST OF THE RADIATIVE ENERGY BALANCE OF THE SHAW MODEL FOR SNOWCOVER

Snow and ice present interesting challenges to hydrologists. Simulating the radiative balance over snow, which is an important part of surface–atmosphere interactions, is particularly challenging because of the decay in albedo over time and the difficulty in estimating surface temperature and incoming long-wave radiation fluxes. Few models are available that include a comprehensive energy and water balance for cold season conditions. The simultaneous heat and water model (SHAW) is a detailed, physical process model of a vertical, one-dimensional canopy–snow–residue–soil system which integrates the detailed physics of heat and water transfer through a plant canopy, snow, residue and soil into one simultaneous solution. Detailed provisions for metamorphosis of the snowpack are included. The SHAW model was applied to data for one winter/spring season (November to May) on a ploughed field in Minnesota without prior calibration to test the performance of the radiation components. Maximum snow depth during this period was 30 cm. For the nearly 100 days of snowcover, the model accounted for 69% of the variation in net solar radiation, 66% of the variation in incoming long-wave radiation, 87% of the variation in emitted long-wave radiation, 26% of the variation in net long-wave radiation and 55% of the variation in net radiation balance. Mean absolute error in simulated values ranged from 10 W m−2 for emitted long-wave radiation to 27 W m−2 for the entire net radiation balance. Mean bias error ranged from 8 W m−2 for emitted long-wave radiation to −16 W m−2 for the entire net radiation balance. When the entire 170 days of simulation, which included periods without snowcover, were included in the analysis, the variation in observed values increased greatly. As a result, the variation in observed values accounted for by the model increased to 97, 71, 93, 56 and 94%, respectively, while the mean absolute and mean bias errors in simulated values remained nearly the same. Model modifications and parameter adjustments necessary to improve winter-time simulation were investigated. Simulation results suggest that the SHAW model may be a useful tool in simulating the interactive influences of radiative transfer at the surface–atmosphere interface.

Biases in Earth radiation budget observations: 2. Consistent scene identification and anisotropic factors

Simple threshold scene identification methods are developed to reduce the effects of errors in scene identification on the anisotropy of reflected and emitted radiances inferred from Earth Radiation Budget Experiment (ERBE) scanner observations. The ERBE maximum likelihood estimate (MLE) scene identification is assumed to be accurate for nadir fields of view. Various combinations of neighboring ERBE scanner fields of view at nadir are used to determine the population of cloud scene types as a function of field of view size. Longwave and shortwave thresholds are then determined for each of the ERBE solar zenith, satellite view zenith, and relative azimuth angular bins so that the population of cloud scene types at a particular satellite view zenith angle is consistent with the field of view size at the particular satellite view zenith angle. Differences between the anisotropy of reflected sunlight and emitted longwave radiation obtained using the new scene identification method and that obtained using the ERBE MLE method show that the ERBE radiative fluxes have satellite view zenith angle dependent biases. Thresholds are also developed for cloud scene identification with fields of view that are constructed to have a constant size with satellite view zenith angle. The angular dependence of reflected sunlight and emitted longwave radiation for scenes identified with these thresholds show little dependence on field of view size. This lack of dependence is a necessary condition for using scanning radiometer data to obtain radiative fluxes.