Comprehensive Radiative Transfer Model Research Articles

Infrared Scattering of Cloud in an Isothermal Atmosphere

Abstract In the absence of scattering, thermal contrast in the atmosphere is the key to infrared remote sensing. Without the thermal contrast, the amount of absorption will be identical to the amount of emission, making the atmospheric vertical structure undetectable using remote sensing techniques. Here we show that, even in such an isothermal atmosphere, the scattering of clouds can cause a distinguishable change in upwelling radiance at the top of the atmosphere. A two-stream analytical solution, as well as a budget analysis based on Monte Carlo simulations, are used to offer a physical explanation of such influence on an idealized isothermal atmosphere by cloud scattering: it increases the chance of photons being absorbed by the atmosphere before they can reach the boundaries (both top and bottom), which leads to a reduction of TOA upwelling radiance. Actual sounding profiles and cloud properties inferred from satellite observations within 6-h time frames are fed into a more realistic and comprehensive radiative transfer model to show such cloud scattering effect, under nearly isothermal circumstances in the lower troposphere, can lead to ∼1–1.5-K decrease in brightness temperature for the nadir-view MODIS 8.5-μm channel. The study suggests that cloud scattering can provide signals useful for remote sensing applications even for such an isothermal environment.

Inferring Nighttime Ionospheric Parameters With the Far Ultraviolet Imager Onboard the Ionospheric Connection Explorer.

The Ionospheric Connection Explorer (ICON) Far Ultraviolet (FUV) imager, ICON FUV, will measure altitude profiles of OI 135.6 nm emissions to infer nighttime ionospheric parameters. Accurate estimation of the ionospheric state requires the development of a comprehensive radiative transfer model from first principles to quantify the effects of physical processes on the production and transport of the 135.6 nm photons in the ionosphere including the mutual neutralization contribution as well as the effect of resonant scattering by atomic oxygen and pure absorption by oxygen molecules. This forward model is then used in conjunction with a constrained optimization algorithm to invert the anticipated ICON FUV line-of-sight integrated measurements. In this paper, we describe the connection between ICON FUV measurements and the nighttime ionosphere, along with the approach to inverting the measured emission profiles to derive the associated O+ profiles from 150-450 km in the nighttime ionosphere that directly reflect the electron density in the F-region of the ionosphere.

A Lookup-Table-Based Approach to Estimating Surface Solar Irradiance from Geostationary and Polar-Orbiting Satellite Data

Incoming surface solar irradiance (SSI) is essential for calculating Earth’s surface radiation budget and is a key parameter for terrestrial ecological modeling and climate change research. Remote sensing images from geostationary and polar-orbiting satellites provide an opportunity for SSI estimation through directly retrieving atmospheric and land-surface parameters. This paper presents a new scheme for estimating SSI from the visible and infrared channels of geostationary meteorological and polar-orbiting satellite data. Aerosol optical thickness and cloud microphysical parameters were retrieved from Geostationary Operational Environmental Satellite (GOES) system images by interpolating lookup tables of clear and cloudy skies, respectively. SSI was estimated using pre-calculated offline lookup tables with different atmospheric input data of clear and cloudy skies. The lookup tables were created via the comprehensive radiative transfer model, Santa Barbara Discrete Ordinate Radiative Transfer (SBDART), to balance computational efficiency and accuracy. The atmospheric attenuation effects considered in our approach were water vapor absorption and aerosol extinction for clear skies, while cloud parameters were the only atmospheric input for cloudy-sky SSI estimation. The approach was validated using one-year pyranometer measurements from seven stations in the SURFRAD (SURFace RADiation budget network). The results of the comparison for 2012 showed that the estimated SSI agreed with ground measurements with correlation coefficients of 0.94, 0.69, and 0.89 with a bias of 26.4 W/m2, −5.9 W/m2, and 14.9 W/m2 for clear-sky, cloudy-sky, and all-sky conditions, respectively. The overall root mean square error (RMSE) of instantaneous SSI was 80.0 W/m2 (16.8%), 127.6 W/m2 (55.1%), and 99.5 W/m2 (25.5%) for clear-sky, cloudy-sky (overcast sky and partly cloudy sky), and all-sky (clear-sky and cloudy-sky) conditions, respectively. A comparison with other state-of-the-art studies suggests that our proposed method can successfully estimate SSI with a maximum improvement of an RMSE of 24 W/m2. The clear-sky SSI retrieval was sensitive to aerosol optical thickness, which was largely dependent on the diurnal surface reflectance accuracy. Uncertainty in the pre-defined horizontal visibility for ‘clearest sky’ will eventually lead to considerable SSI retrieval error. Compared to cloud effective radius, the retrieval error of cloud optical thickness was a primary factor that determined the SSI estimation accuracy for cloudy skies. Our proposed method can be used to estimate SSI for clear and one-layer cloud sky, but is not suitable for multi-layer clouds overlap conditions as a lower-level cloud cannot be detected by the optical sensor when a higher-level cloud has a higher optical thickness.

A radiative transfer module for calculating photolysis rates and solar heating in climate models: Solar-J v7.5

Abstract. Solar-J is a comprehensive radiative transfer model for the solar spectrum that addresses the needs of both solar heating and photochemistry in Earth system models. Solar-J is a spectral extension of Cloud-J, a standard in many chemical models that calculates photolysis rates in the 0.18–0.8 µm region. The Cloud-J core consists of an eight-stream scattering, plane-parallel radiative transfer solver with corrections for sphericity. Cloud-J uses cloud quadrature to accurately average over correlated cloud layers. It uses the scattering phase function of aerosols and clouds expanded to eighth order and thus avoids isotropic-equivalent approximations prevalent in most solar heating codes. The spectral extension from 0.8 to 12 µm enables calculation of both scattered and absorbed sunlight and thus aerosol direct radiative effects and heating rates throughout the Earth's atmosphere.The Solar-J extension adopts the correlated-k gas absorption bins, primarily water vapor, from the shortwave Rapid Radiative Transfer Model for general circulation model (GCM) applications (RRTMG-SW). Solar-J successfully matches RRTMG-SW's tropospheric heating profile in a clear-sky, aerosol-free, tropical atmosphere. We compare both codes in cloudy atmospheres with a liquid-water stratus cloud and an ice-crystal cirrus cloud. For the stratus cloud, both models use the same physical properties, and we find a systematic low bias of about 3 % in planetary albedo across all solar zenith angles caused by RRTMG-SW's two-stream scattering. Discrepancies with the cirrus cloud using any of RRTMG-SW's three different parameterizations are as large as about 20–40 % depending on the solar zenith angles and occur throughout the atmosphere.Effectively, Solar-J has combined the best components of RRTMG-SW and Cloud-J to build a high-fidelity module for the scattering and absorption of sunlight in the Earth's atmosphere, for which the three major components – wavelength integration, scattering, and averaging over cloud fields – all have comparably small errors. More accurate solutions with Solar-J come with increased computational costs, about 5 times that of RRTMG-SW for a single atmosphere. There are options for reduced costs or computational acceleration that would bring costs down while maintaining improved fidelity and balanced errors.

Improved aerosol radiative properties as a foundation for solar geoengineering risk assessment

AbstractSide effects resulting from the deliberate injection of sulfate aerosols intended to partially offset climate change have motivated the investigation of alternatives, including solid aerosol materials. Sulfate aerosols warm the tropical tropopause layer, increasing the flux of water vapor into the stratosphere, accelerating ozone loss, and increasing radiative forcing. The high refractive index of some solid materials may lead to reduction in these risks. We present a new analysis of the scattering efficiency and absorption of a range of candidate solid aerosols. We utilize a comprehensive radiative transfer model driven by updated, physically consistent estimates of optical properties. We compute the potential increase in stratospheric water vapor and associated longwave radiative forcing. We find that the stratospheric heating calculated in this analysis indicates some materials to be substantially riskier than previous work. We also find that there are Earth‐abundant materials that may reduce some principal known risks relative to sulfate aerosols.

Radiative transfer modeling of the OI 135.6 nm emission in the nighttime ionosphere

AbstractRemote sensing of the nighttime OI 135.6 nm emissions has been a widely used method for measuring theFregion ionospheric plasma densities. In this work, we first develop a comprehensive radiative transfer model from first principles to investigate the effects of different physical processes on the production and transport of the 135.6 nm photons in the ionosphere and then propose a new approach for estimating electron densities from the nightglow. The forward modeling investigation indicates that under certain conditions mutual neutralization can contribute up to ∼38% of the total production of the nighttime 135.6 nm emissions. Moreover, depending on the ionospheric conditions, resonant scattering by atomic oxygen and pure absorption by oxygen molecules can reduce the limb brightness observed by satellite‐borne instruments by up to ∼40% while enhancing the brightness viewing in the nadir direction by typically ∼25%. Further analysis shows that without properly addressing these effects in the inversion process, the peak electron density in theF region (NmF2) obtained using limb observations can be overestimated by up to ∼24%. For accurate estimation of the ionospheric electron density, we develop a new type of inverse model that accounts for the effects of mutual neutralization, resonant scattering, and pure absorption. This inversion method requires the knowledge of O and O2densities in order to solve the radiative transfer equations. Application of the inverse model to the nighttime ionosphere in the noiseless cases demonstrates that the electron density can be accurately quantified with only ∼1% error in NmF2 and hmF2.

Evaluation of modeled microwave land surface emissivities with satellite‐based estimates

AbstractAn accurate estimate of the microwave surface emissivity is necessary for the retrieval of atmospheric quantities from microwave imagers or sounders. The objective of this study is to evaluate the microwave land surface emissivity modeling of the Community Radiative Transfer Model (CRTM), providing quantitative statistic information for further model improvements. First, the model‐simulated emissivity is compared to emissivity estimates derived from satellite observations (TELSEM, Tool to Estimate Land Surface Emissivities at Microwaves). The model simulations agree reasonably well with TELSEM over snow‐free vegetated areas, especially at vertical polarization up to 40 GHz. For snow‐free surfaces, the mean difference between CRTM and TELSEM emissivities at vertical polarization is lower than 0.01 below 40 GHz and increases to 0.02 at 89 GHz. At horizontal polarization, it increases with frequency, from 0.01 at 10.6 GHz to 0.04 at 89 GHz. Over deserts and snow, larger differences are observed, which can be due to the lack of quality inputs to the model in these complex environments. A further evaluation is provided by comparing brightness temperature (Tbs) simulations with AMSR‐E observations, where CRTM emissivity and TELSEM emissivity are coupled into a comprehensive radiative transfer model to simulate the brightness temperatures, respectively. The comparison shows smaller RMS errors with the satellite‐derived estimates than with the model, despite some significant bias at midday with the satellite‐derived emissivities at high frequencies. This study confirms and extends to the global scale previous evaluations of land surface microwave emissivity model. It emphasizes the needs for better physical modeling in arid regions and over snow‐covered surfaces.

Estimating global solar radiation using a hybrid parametric model from MODIS data over the Tibetan Plateau

The Tibetan Plateau plays an important role in global atmospheric circulation with its complex terrain. The downward surface shortwave radiation (DSSR) can be obtained from remote sensing data because of its sparse observations and rugged surface. In this paper, a satellite-based scheme is presented to retrieve all-sky downward surface shortwave radiation, which links a look-up table algorithm and satellite images. The look-up table for clear sky and cloudy sky was created separately using a comprehensive 1D physically based radiative transfer model SBDART to achieve a higher computational accuracy and efficiency compared to the comprehensive radiative transfer model. The estimated DSSR was validated using one year pyranometer measurements from 8 stations in the Tibetan Plateau and compared with GEWEX–SRB data with 1° spatial resolution. The result shows that the largest root mean square error was 60W/m2 (32%) at Guoluo station, and the least root mean square error was 13W/m2 (13%) at Golmud station. The bias was larger in summer and smaller in winter, which may be caused by uncertainties in the assumption from the 1D radiative transfer model and MODIS cloud properties product for broken and inhomogeneous clouds. The algorithm we proposed can be applied globally without local calibration because it is independent of climate and the surface elevation.

A model to calculate solar radiation fluxes on the Martian surface

We present a new comprehensive radiative transfer model to study the solar irradiance that reaches the surface of Mars in the spectral range covered by MetSIS, a sensor aboard the Mars MetNet mission that will measure solar irradiance in several bands from the ultraviolet (UV) to the near infrared (NIR). The model includes up-to-date wavelength-dependent radiative properties of dust, water ice clouds, and gas molecules. It enables the characterization of the radiative environment in different spectral regions under different scenarios. Comparisons between the model results and MetSIS observations will allow for the characterization of the temporal variability of atmospheric optical depth and dust size distribution, enhancing the scientific return of the mission. The radiative environment at the Martian surface has important implications for the habitability of Mars as well as a strong impact on its atmospheric dynamics and climate.

Measurement and Simulation of Topographic Effects on Passive Microwave Remote Sensing Over Mountain Areas: A Case Study From the Tibetan Plateau

Knowledge about the surface soil water content is essential because it controls the surface water dynamics and land-atmosphere interaction. In high mountain areas in particular, soil surface water content controls infiltration and flood events. Although satellite-derived surface soil moisture data from passive microwave sensors are readily available for most regions globally, mountainous areas are often excluded from these data (or at least flagged as biased) due to the strong topographic influence on the retrieved signal. Even though a substantial volume of literature is available dealing with topographic effects on spaceborne brightness temperature, no systematic analysis has been reported. Therefore, we present a comprehensive analysis of topographic effects on brightness temperature at C-band using a two-step approach. First, a well-controlled field experiment is carried out using a mobile truck-mounted C-band radiometer to analyze the impact of geometric and adjacent effects on the radiometer signal. Additionally, a comprehensive radiative transfer model is developed accounting for both effects and tested on the ground-based data. Second, recorded Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) data over the Tibetan Plateau were used to analyze the error due to the impact of topography using the developed model. The results of the field experiment clearly show that the geometric effect of a single hill has a much larger impact on brightness temperature compared to the adjacent effect of multiple hills, whereby, due to the geometric effect, the bias is up to +20 K for horizontal and -13 K for vertical polarization. For the adjacent effect, the bias is less than 3 K for both polarizations. Additionally, the developed radio transfer model was able to reproduce both effects with high accuracy. For the AMSR-E data, the model shows that the brightness temperature recorded is biased in the same way as the ground-based measurements and that uncertainties induced by the wide existence of atypical mountain regions in the Tibetan Plateau will have a great impact on the retrieving error (maximum 30%). The largest impact on the retrieval error, on the other hand, is calculated for the soil moisture with a maximum relative error of 44%. The negligible impact can be attributed to false parameterization of the soil texture, soil surface temperature, and sky temperature. Finally, the overall absolute error in the estimated water content is quantified on average with 4%, whereby single pixels indicate a maximum absolute error of up to 16%. In conclusion, we show that recorded spaceborne brightness temperatures are highly biased by topographic effects in mountainous regions using a comprehensive radiative transfer model. Additionally, we suggest using this model to invert the effective surface emissivity of mountain areas for standard processing of higher level data products such as surface soil water content.

A simple model for cloud radiative forcing

Abstract. We present a simple model for the longwave and shortwave cloud radiative forcing based on the evaluation of extensive radiative transfer calculations, covering a global range of conditions. The simplicity of the model equations fosters the understanding on how clouds affect the Earth's energy balance. In comparison with results from a comprehensive radiative transfer model, the accuracy of our parameterization is typically better than 20%. We demonstrate the usefulness of our model using the example of tropical cirrus clouds. We conclude that possible applications for the model include the convenient estimate of cloud radiative forcing for a wide range of conditions, the evaluation of the sensitivity to changes in environmental conditions, and as a tool in education. An online version of the model is available at http://www.iac.ethz.ch/url/crf.

Radiative transfer model for aerosols in infrared wavelengths for passive remote sensing applications

A comprehensive analytical radiative transfer model for isothermal aerosols and vapors for passive infrared remote sensing applications (ground-based and airborne sensors) has been developed. The theoretical model illustrates the qualitative difference between an aerosol cloud and a chemical vapor cloud. The model is based on two and two/four stream approximations and includes thermal emission-absorption by the aerosols; scattering of diffused sky radiances incident from all sides on the aerosols (downwelling, upwelling, left, and right); and scattering of aerosol thermal emission. The model uses moderate resolution transmittance ambient atmospheric radiances as boundary conditions and provides analytical expressions for the information on the aerosol cloud that is contained in remote sensing measurements by using thermal contrasts between the aerosols and diffused sky radiances. Simulated measurements of a ground-based sensor viewing Bacillus subtilis var. niger bioaerosols and kaolin aerosols are given and discussed to illustrate the differences between a vapor-only model (i.e., only emission-absorption effects) and a complete model that adds aerosol scattering effects.

Calibration of Special Sensor Microwave Imager/Sounder (SSMIS) upper air brightness temperature measurements using a comprehensive radiative transfer model

For purposes of completing the initial calibration and validation work for the Special Sensor Microwave Imager/Sounder (SSMIS) serial number 2 instrument, we have implemented a comprehensive radiative transfer model (RTM) in order to verify on‐orbit upper air brightness temperature measurements for SSMIS channels 19–24. Rigorous computational treatment of the effects of the Earth's magnetic field on absorption by oxygen in the stratosphere and mesosphere provides us a solid baseline for comparison of theoretically based forward RTM calculations with the satellite observations. With the inclusion of flexibility in our model to account for departures from perfect instrument behavior, initial results provide us a reasonable measure of confidence that the agreement of our model with the observations should make the SSMIS‐derived data products sufficiently reliable.

Impact of the Atmospheric Thickness on the Atmospheric Downwelling Longwave Radiation and Snowmelt under Clear-Sky Conditions in the Arctic and Subarctic

Studies show that the energy available to melt snow at high latitudes is almost exclusively provided by radiation. Solar energy determines the period of possible snowmelt, while downwelling atmospheric longwave radiation modifies the timing and triggers the onset of snowmelt. Atmospheric thickness, defined as the vertical distance between the 500- and 1000-mb pressure surfaces, is directly related to the mean temperature and water vapor path of an atmospheric layer and thus has a direct influence on the downwelling longwave radiation and snowmelt. A comprehensive radiative transfer model was applied to calculate the downwelling longwave radiation to the snow surface over the period of snowmelt from 1980 through 1991 using radiosonde data obtained at Barrow and McGrath, Alaska, under clear-sky conditions. The results indicate that the atmospheric thickness has a positive impact on downwelling longwave radiation, which ranges from about 130 W m−2 for an atmospheric thickness of 4850 m to about 280 W m−2 for an atmospheric thickness of 5450 m. This study demonstrates that atmospheric water vapor path has a greater impact on atmospheric downwelling longwave radiation to the snow surface than the mean atmospheric temperature. This study also indicates that when the near-surface air temperature is used to infer downwelling longwave radiation, significant errors can occur. Thus, compared with the results obtained from the atmospheric radiative transfer model, the empirical formula due to Parkinson and Washington underestimates the downwelling longwave radiation when the near-surface air temperature is relatively high and overestimates it when the near-surface air temperature is relatively low. Investigations of the relationship between the atmospheric thickness and the snowmelt onset were conducted. Results indicate that for the period from 1980 through 1991, an atmospheric thickness of 5250 m at Barrow and 5200 m at McGrath in Alaska was sufficient to trigger the onset of snowmelt. The difference in the threshold values of the atmospheric thickness may be due to differences in the atmospheric structure and different contributions of other energy sources such as sensible and latent heat to melt snow. This study also demonstrates that snow cover disappears earlier during warm and wet (higher atmospheric temperature and precipitable water path, and greater atmospheric thickness) springs and later during cold and dry (lower atmospheric temperature and precipitable water path, smaller atmospheric thickness) springs. Atmospheric precipitable water path has a greater impact on snowmelt than the mean atmospheric temperature. Generally, higher atmospheric temperature is correlated with higher atmospheric water vapor path and since atmospheric temperature is closely coupled to the atmospheric water vapor path in the Arctic and Subarctic and since it can be obtained through routine numerical weather prediction models, the atmospheric thickness may be used as a reliable indicator of regional-scale snowmelt in the Arctic and subarctic.

Remote Sensing of Surface and Cloud Properties in the Arctic from AVHRR Measurements

Abstract Algorithms to retrieve cloud optical depth and effective radius in the Arctic using Advanced Very High Resolution Radiometer (AVHRR) data are developed, using a comprehensive radiative transfer model in which the atmosphere is coupled to the snowpack. For dark surfaces AVHRR channel 1 is used to derive visible cloud optical depth, while for bright surfaces AVHRR channel 2 is used. Independent inference of cloud effective radius from AVHRR channel 3 (3.75 μm) allows for derivation cloud liquid water path (proportional to the product of optical depth and effective radius), which is a fundamental parameter of the climate system. The algorithms are based on the recognition that the reflection function of clouds at a nonabsorbing wavelength (such as AVHRR channel 1) in the solar spectrum is primarily a function of cloud optical thickness, whereas the reflection function at a liquid water absorbing wavelength (such as AVHRR channel 3) is primarily a function of cloud particle size. For water clouds ove...

Bidirectional anisotropic reflectance of snow and sea ice in AVHRR Channel 1 and 2 spectral regions. I. Theoretical analysis

The bidirectional reflectance anisotropy in Advanced Very High Resolution Radiometer (AVHRR) Channel 1 and 2 spectral regions for snow- and ice-covered surfaces was investigated using a comprehensive radiative transfer model for the coupled atmosphere, snow, ice, and ocean system. The model has been developed to include surface roughness effects. The importance of the scattering characteristics of snow and the surface roughness of sea ice to the bidirectional reflectance of these surfaces is presented. The modeled anisotropic factor, however, shows less sensitivity to aerosol loading and snow soot contamination than to solar elevation, snow phase function, and ice surface roughness. Results show high anisotropic reflectance factors (ARFs) and different reflectance features for snow and ice surfaces in both the AVHRR Channel 1 and 2 spectral regions. The model-observation comparison indicates that the model is able to predict the general characteristics of the bidirectional reflection of snow and sea ice. The introduction of surface roughness in the model can well explain the extremely strong forward or high anisotropic reflectance of sea ice.

Optical remote sensing of marine constituents in coastal waters: a feasibility study

Optical remote sensing of ocean color is a well-established technique for inferring ocean properties. However, most retrieval algorithms are based on the assumption that the radiance received by satellite instruments is affected only by the phytoplankton pigment concentration and correlated substances. This assumption works well for open ocean water but becomes questionable for coastal waters. To reduce uncertainties associated with this assumption, we developed a new algorithm for the retrieval of marine constituents in a coastal environment. We assumed that ocean color can be adequately described by a three-component model made up of chlorophyll a, suspended matter, and yellow substance. The simultaneous retrieval of these three marine constituents and of the atmospheric aerosol content was accomplished through an inverse-modeling scheme in which the difference between simulated radiances exiting the atmosphere and radiances measured with a satellite sensor was minimized. Simulated radiances were generated with a comprehensive radiative transfer model that is applicable to the coupled atmosphere-ocean system. The method of simulated annealing was used to minimize the difference between measured and simulated radiances. To evaluate the retrieval algorithm, we used simulated (instead of measured) satellite-received radiances that were generated for specified concentrations of aerosols and marine constituents, and we tested the ability of the algorithm to retrieve assumed concentrations. Our results require experimental validation but show that the retrieval of marine constituents in coastal waters is possible.

Analysis of the Near-IR Spectrum of Saturn: A Comprehensive Radiative Transfer Model of Its Middle and Upper Troposphere

Spectra from 1.7 to 3.3 μm acquired at the NASA Kuiper Airborne Observatory include two of Saturn's near-IR atmospheric transmission windows that are at least partially obscured by telluric H 2O and CO 2absorptions at ground-based telescopes. This entire spectral region was fitted to a model that included gaseous absorption by H 2, CH 4, NH 3, and PH 3and the effects of multiple scattering by haze. The objectives were to determine accurate elemental abundance ratios (e.g., C/H, P/H, etc.) and to characterize the size, distribution, and composition of the haze particles in Saturn's atmosphere. The results for C/H and P/H are 8.5 × 10 −4and 4.3 × 10 −7, respectively. No evidence of gaseous NH 3was found. The upper limit to the NH 3mixing ratio at Saturn's radiative-convective boundary is ≊10 −9. Ammonia is decidedly undersaturated at atmospheric pressures lower than ≊1 bar. The upper limit to gaseous NH 3at 3 μm is extremely low compared to detected amounts derived from observations at visible, mid-IR, and microwave wavelengths. These differences can be reconciled on the basis of different mechanisms for spectral line formation in these disparate spectral regions. A search for solid phase NH 3was also negative. From thermochemical arguments it has been widely assumed that NH 3ice crystals comprise the upper clouds on Saturn, although no incontrovertible spectroscopic proof has ever been presented. Strong bands of solid NH 3at 3 μm therefore offer an important test of this assumption. Saturn's observed spectrum was placed on an absolute reflectivity scale which then could be compared with synthesized spectra of candidate haze particles. The calculations demonstrated that the reflectances of pure, polydisperse NH 3ice crystals with effective radii ranging from 0.1 to 2.25 μm are not compatible with Saturn's 3-μm spectrum. A reasonable fit to Saturn's continuum spectrum can only be achieved by using bright, micron-sized scattering haze particles mixed in with H 2, CH 4, and PH 3in Saturn's middle and upper troposphere.

Measured UV spectra compared with discrete ordinate method simulations

A comprehensive radiative transfer model is used to calculate surface spectral ultraviolet irradiance under cloud‐free conditions. The results are compared with measurements made at Lauder, New Zealand (45°S, 170°E) before and after the eruption of Mount Pinatubo and including a snow‐covered surface. There is reasonable agreement between measured and calculated diffuse, direct, and global irradiances over the range 300 to 450 nm. Discrepancies may be due to calibration errors in the UV measurements, or in the extra terrestrial irradiances input to the model. Ratios of diffuse/direct irradiances are independent of such calibration uncertainties and therefore provide a sensitive test of the model. If appropriate ozonesonde data, surface albedo, and aerosol optical properties are used, the model ratios are in satisfactory agreement with measurements over a wide range of observing conditions. For cases in which the atmospheric optical properties are best known the agreement is better than 8% in the UV‐B range, and for wavelengths 320 to 450 nm the deviation is smaller. The comparison suggests that the ultraviolet radiation exposure can be computed with confidence for clear sky conditions if the appropriate atmospheric molecular density profiles, ozonesonde data, surface albedo, and aerosol optical properties are available. The UV radiation model is used to investigate the impact of changes in solar zenith angle, ozone abundance, surface albedo, and aerosol loading on UV radiation reaching the surface of the Earth. The ratios of diffuse to direct irradiance depend critically on solar zenith angle, surface albedo, and aerosol extinction. Ozone changes have pronounced effects on the global UVB irradiance but have only a minor effect on these ratios.