Cloud Transmittance Research Articles

Reflection and transmission of solar light by clouds: asymptotic theory

Abstract. The authors introduce a radiative transfer model CLOUD for reflection, transmission, and absorption characteristics of terrestrial clouds and discuss the accuracy of the approximations used within the model. A Fortran implementation of CLOUD is available for download. This model is fast, accurate, and capable of calculating multiple radiative characteristics of cloudy media including the spherical and plane albedo, reflection and transmission functions, absorptance as well as global and diffuse transmittance. The approximations are based on the asymptotic solutions of the radiative transfer equations valid at cloud optical thicknesses larger than 5. While the analytic part of the solutions is treated in the code in an approximate way, the correspondent reflection function (RF) of a semi-infinite water cloud R∞ is calculated using numerical solutions of the radiative transfer equation in the assumption of Deirmendjian's cloud C1 model. In the case of ice clouds, the fractal ice crystal model is used. The resulting values of R∞ with respect to the viewing geometry are stored in a look-up table (LUT). The results obtained are of importance for quick estimations of main radiative characteristics of clouds and also for the solution of inverse problems.

Ultraviolet climatology over Argentina

Satellite‐derived climatologies of UV index (UVI) and erythemal daily dose were determined for Argentina at a geographical resolution of 0.5° latitude by 0.5° longitude. Total Ozone Mapping Spectrometer climatology of key input parameters was used to calculate the clear‐sky ground‐level solar UV irradiance. NASA Surface meteorology and Solar Energy cloud cover data were used to determine the UV attenuation by clouds. Two cases were tested: (1) monthly averages of total cloud cover and (2) monthly averages for three ranges of cloud cover percentage (0–10%, 10–70%, and 70–100%). Case 2, with smaller biases, was selected. Measured erythemal irradiance at seven stations of the Argentine Ultraviolet Monitoring Network was used to validate the satellite‐derived climatologies, as well as to estimate aerosol parameters, surface albedo corrections, and UV cloud transmittance for the calculations. Annual average biases of the monthly mean satellite‐derived UVI and erythemal daily dose with respect to the ground measurements are mostly within ±10%. The strong longitudinal gradient of UV levels toward the Andes Mountains is emphasized. Very high UVI and erythemal daily dose values are registered in the northwestern tropical high‐elevation Andean plateau, with extreme monthly means above 18 and 10 kJ/m2 respectively, in December–January. Even northern low‐elevation regions show averages over 12 and 7 kJ/m2, respectively. On average, clouds attenuate the clear‐sky erythemal irradiance by less than 20% for most of the continental region during all months. UV levels are considerably higher than those for equivalent regions in the Northern Hemisphere.

한반도를 포함한 동아시아 영역에서 오존전량과 유해자외선의 특성과 예측

The average ratio of the daily UV-B to total solar (75) irradiance at Busan (35.23N, 129.07E) in Korea is found as 0.11%. There is also a high exponential relationship between hourly UV-B and total solar irradiance: UV-B=exp (a(75-b))(R=0.93). The daily variation of total ozone is compared with the UV-B irradiance at Pohang (36.03N, 129.40E) in Korea using the Total Ozone Mapping Spectrometer (TOMS) data during the period of May to July in 2005. The total ozone (TO) has been maintained to a decreasing trend since 1979, which leading to a negative correlation with the ground-level UV-B irradiance doting the given period of cloudless day: UV-B=239.23-0.056 TO (R=0.52). The statistical predictions of daily total ozone are analyzed by using the data of the Brewer spectrophotometer and TOMS in East Asia including the Korean peninsula. The long-term monthly averages of total ozone using the multiplicative seasonal AutoRegressive Integrated Moving Average (ARIMA) model are used to predict the hourly mean UV-B irradiance by interpolating the daily mean total ozone far the predicting period. We also can predict the next day's total ozone by using regression models based on the present day's total ozone by TOMS and the next day's predicted maximum air temperature by the Meteorological Mesoscale Model 5 (MM5). These predicted and observed total ozone amounts are used to input data of the parameterization model (PM) of hourly UV-B irradiance. The PM of UV-B irradiance is based on the main parameters such as cloudiness, solar zenith angle, total ozone, opacity of aerosols, altitude, and surface albedo. The input data for the model requires daily total ozone, hourly amount and type of cloud, visibility and air pressure. To simplify cloud effects in the model, the constant cloud transmittance are used. For example, the correlation coefficient of the PM using these cloud transmissivities is shown high in more than 0.91 for cloudy days in Busan, and the relative mean bias error (RMBE) and the relative root mean square error (RRMSE) are less than 21% and 27%, respectively. In this study, the daily variations of calculated and predicted UV-B irradiance are presented in high correlation coefficients of more than 0.86 at each monitoring site of the Korean peninsula as well as East Asia. The RMBE is within 10% of the mean measured hourly irradiance, and the RRMSE is within 15% for hourly irradiance, respectively. Although errors are present in cloud amounts and total ozone, the results are still acceptable.

Cloud optical thickness and effective particle radius derived from transmitted solar radiation measurements: Comparison with cloud radar observations

A method is presented for determining the optical thickness and effective particle radius of stratiform clouds containing liquid water drops in the absence of drizzle from transmitted solar radiation measurements. The procedure compares measurements of the cloud transmittance from the ground at water‐absorbing and nonabsorbing wavelengths with lookup tables of the transmittance precomputed for plane‐parallel, vertically homogeneous clouds. The optical thickness derived from the cloud transmittance may be used to retrieve vertical profiles of cloud microphysics in combination with the radar reflectivity factor. To do this, we also present an algorithm for solving the radar equation with a constraint of the optical thickness at the visible wavelength. Observations of clouds were made in August and September 2003 at Koganei, Tokyo, Japan, using a PREDE i‐skyradiometer and a 95‐GHz cloud radar Super Polarimetric Ice Crystal Detection and Explication Radar (SPIDER). The optical thickness and effective radius of water clouds were derived from the i‐skyradiometer. Then, the vertical profile of the effective radius was retrieved from SPIDER, using the optical thickness determined from the i‐skyradiometer. We found that the effective radii derived by using these two instruments were in good agreement.

Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces. Part II: Cloud Optical Depth and Shortwave Radiative Forcing from Pyranometer Measurements in the Southern Ocean

Abstract Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of “raw cloud transmittance” (trc), the ratio of downward irradiance under cloud to downward irradiance measured under clear sky, vary from 0.1 to 1.0. Over sea ice, few values of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-cloud-droplet transition. This sparsely populated region of transmittances is referred to as the Köhler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60° is typically −250 W m−2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. “Effective” optical depths τ (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of τ are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47°S, increasing to 24 in the region of maximum cloud cover at 58°S, and decreasing to 11 at 67°S near the coast of Antarctica.

Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation: 3. Hollow columns and plates

The ability of an assembly of spheres to represent scattering and absorption by a nonspherical ice crystal of the same volume‐to‐area (V/A) ratio was previously evaluated for convex shapes (circular cylinders and hexagonal prisms). Here we extend the comparison to indented and hollow prisms, which are common in ice clouds. In the equivalent‐sphere representation, the crystal mass and surface area are both conserved. Internal surfaces as well as external surfaces contribute to the total surface area; in the model representation both become external surfaces of spheres. The optical depth τ of the model cloud is thus greater than that of the real cloud by the ratio A/4P, where A is the total area of the nonspherical particle and P is the orientation‐averaged projected area. This ratio, which we call “fluffiness,” is unity for convex shapes but may exceed 2 for clusters of hollow bullets. In effect, the scattering at interior surfaces of a hollow crystal becomes classified as multiple scattering in the model of ice spheres. Therefore, rather than directly comparing the asymmetry factor (g) and single‐scattering albedo (ωo) of the hollow crystal to those of the equal‐V/A sphere, it is more appropriate to compare the product τ(1 − g)ωo, because this quantity largely determines the bulk radiative properties of the cloud. Errors in albedo, absorptance, and transmittance of ice clouds, caused by the equal‐V/A representation, are presented for a range of aspect ratios, indentation depths, and ice‐water paths at visible and near‐infrared wavelengths.

Effects of the Vertical Profiles of Cloud Droplets and Ice Particles on the Visible and Near-Infrared Radiative Properties of Mixed-Phase Stratocumulus Clouds

Such cloud radiative propenies as reflectance, transmittance, and absorptance of mixed-phase clouds have been simulated for the stratocumulus cloud observed on 30 January 1999 within the Japanese Cloud and Climate Study (JACCS) program, in which simultaneous observations of the cloud microphysical and radiative properties were conducted. The stratocumulus cloud was in mixed-phased condition, and highly heterogeneous vertically and horizontally with different mixing ratios of liquid water droplets and ice particles. A vertically-homogeneous, plane-parallel mixed-phase cloud model could reproduce the observed visible (VIS; wavelength < 0.7 μm) band radiative properties, but it could not reproduce the near-infrared (NIR; > 0.7 μm) band radiative properties. A multi-layered, plane-parallel mixed-phase cloud model could consistently reproduce the observed VIS-band and NIR-band radiative properties within the measurement accuracy. It is found that the venical profiles of water droplets and ice particles are an important factor to determine the radiative properties of mixed-phase clouds. The simulated results suggested that the NIR-band reflectance and absorptance could vary by more than 0.1 due to different vertical distributions of the cloud microphysical properties even the model clouds had constant liquid-water-path and ice-water-path. The visible and near-infrared solar reflection can be also affected by the vertical profiles of cloud microphysical properties; it is suggested that the conventional passive remote sensing under the assumption of vertical-homogeneity, may bring large errors in estimation of the microphysical propenies of mixed-phase clouds.

Version 2 data of the National Science Foundation's Ultraviolet Radiation Monitoring Network: South Pole

Spectral ultraviolet (UV) and visible irradiance has been measured at the South Pole between 1991 and 2003 by a SUV‐100 spectroradiometer, which is part of the U.S. National Science Foundation's UV Monitoring Network. Here we present a new data edition, labeled “Version 2.” The new version was corrected for wavelength shift errors and deviations of the spectroradiometer from the ideal cosine response. A comprehensive uncertainty budget of the new data set was established. Below 400 nm the expanded standard uncertainty (coverage factor 2) varies between 4.6 and 7.2%, depending on wavelength and sky condition. The uncertainty of biologically relevant UV irradiances is approximately 6%. Compared to the previously published data set, Version 2 UV data are higher by 5–14%, depending on wavelength, solar zenith angle (SZA), and year of observation. By comparing Version 2 data with results of a radiative transfer model, the good consistency and homogeneity of the new data set were confirmed. The data set is used to establish a UV climatology for the South Pole, focusing on the effects of aerosols, clouds, and total column ozone. Clouds are predominantly optically thin; 71% of all clouds have an optical depth between 0 and 1. The average attenuation of UV irradiance at 345 nm by clouds is less than 5% and no attenuations greater than 23% were observed. Attenuation by homogeneous clouds is generally larger in the visible than in the UV. The wavelength dependence of cloud attenuation is quantitatively explained with the wavelength‐dependent radiance distribution on top of clouds and the incidence‐angle dependence of cloud transmittance. Largest radiation levels occur in late November and early December when low stratospheric ozone amounts coincide with relatively small SZAs. Owing to the large effect of the “ozone hole,” short‐ and long‐term variability of UV during the austral spring is very high. When the ozone hole disappears, DNA‐damaging irradiance can decrease by more than a factor of two within 2 days. Typical summer UV index values range between 2 and 3.5 and vary by ±30% (±1σ) between different years. Linear regression analyses did not indicate statistically significant UV trends owing to the large year‐to‐year variability and the fact that the network was established only after the first occurrence of the ozone hole. Current measurements therefore document variability on an elevated level.

Attenuation of biologically effective UV doses under overcast skies: a case study from the eastern Atlantic sector of the Southern Ocean

This study deals attenuation of ultraviolet solar radiation measured during the SWEDARP 1997/1998 expedition in the eastern Atlantic sector of the Southern Ocean. Experimental data were measured on board a ship and theoretical computation of radiative transfer has been applied. Focus has been placed on biologically effective UV-B doses using three commonly applied biological weighting functions based on: inhibition of carbon dioxide fixation (Science 258 (1992) 646); Generalized plant effects (Stratospheric Ozone Reduction, Solar Ultraviolet Radiation and Plant Life 1986, Springer, Berlin) and DNA lesions (Proc. Nat. Acad. Sci. USA 71 (1974) 3363). The ratio of measured dose to that under clear sky condition, calculated by a model, was defined as transmittance which was studied in relation to other information. Further, the interrelationship between the biologically effective UV doses and various broadband irradiance in the UV-A, UV-B and PAR was established. It shows that the UV-B doses based on the three weighting functions are closely linked to each other and their cloud transmittances are nearly the same. The biologically effective UV-B doses can be estimated with reasonable accuracy from the broadband irradiance in the UV-A, UV-B and PAR regions, with UV-B giving the best results. Univariate analysis between the transmittance and zenith angle, total cloud cover and cloud base height was performed. It is found that attenuation is almost independent of zenith angle. Transmission is reduced by 7.7% if the cloud cover is increased by one octa. The average transmittance of the UV-B doses is 0.40, indicating that clouds have played an important role in reducing the UV radiation.

Attenuation of spectral actinic flux and photolysis frequencies at the surface through homogenous cloud fields

The effect of clouds on derived photolysis rates has been investigated using actinic flux spectra measured from a 2π sr diode‐array spectral radiometer deployed in a series of differing field campaigns. Homogenously cloudy and clear days were selected, and ratios of cloudy‐to‐clear actinic flux spectra were used to derive a cloud transmittance slope (CTS), which is calculated as the gradient of the cloudy/clear ratio plot at 340 &lt; λ &lt; 420 nm. CTS is a measure of the wavelength dependence of actinic flux attenuation by clouds and shows an approximately linear dependence with solar zenith angle. When the solar zenith angle is high, the CTS is largest because of the enhanced path length of radiation through the scattering medium. The dependence of measured CTS under a range of atmospheric conditions is explored; the slopes of the CT being rationalized in terms of the cloud optical depth and single scattering albedo of the cloud. The appearance of a short‐wave enhancement in CT in a multiple scattering environment is noted in agreement with theory. The photolysis frequency coefficients j(O1D) and j(NO2), calculated from the measured actinic fluxes, are compared to observe the effect of wavelength‐dependent attenuation of actinic flux on photochemical reactions. After the subtraction of the effect of changing overhead ozone column, the effect of cloud was quite variable in that j(NO2) was found to be reduced to a greater extent than j(O1D) by some clouds or vice versa depending on the individual case. The results clearly demonstrate that photolysis frequencies in the UVB and UVA do not vary linearly under different atmospheric conditions in a cloudy field.

The accuracy of downward short‐ and long‐wave radiation at the earth's surface calculated using simple models

AbstractEstimates of the downward global solar and long‐wave radiations are commonly made using simple models. We have tested the estimates produced by a number of these simple models against the values predicted by the radiative transfer model used in a climate model in order to determine their suitability for global applications. For clear sky, two simple models were comparable, but under cloudy conditions a combination of a clear‐sky model based on the Angstrom‐Prescott equation (which deals with the downwelling solar radiation) with a cloud transmissivity utilising total cloud fraction proved best. The lowest root mean square errors were 27 W m−2 for clear‐sky global solar radiation and 90 W m−2 for cloudy conditions. For downward long‐wave radiation in clear‐sky conditions, the model of Garratt (1992) performed best with a root mean square error of 24 W m−2. However, in cloudy conditions the model of Idso &amp; Jackson (1969) performed best with a root mean square error of 22 W m−2, and, as it performs nearly as well as that of Garratt (1992) in clear‐sky conditions, it is probably the best choice. Copyright © 2004 Royal Meteorological Society.

Long‐term variations in the occurrence and effective solar transmission of clouds as determined from surface‐based total irradiance observations

Time series of cloud solar transmission and cloud occurrence frequency are developed for the past 27 years at four globally remote and climatically diverse surface locations. A new methodology is developed that objectively segregates times of cloud‐free conditions from those times when clouds are detected in high‐time‐resolution total solar irradiance observations that are obtained from pyranometers. The methodology for cloud detection depends on the magnitude and short‐term variability of observed departures from clear‐sky conditions. Expected clear‐sky irradiances are based on interpolated clear‐sky observations. Results of the new cloud detection methodology are compared to four independent cloud detection methods. An effective cloud transmission is determined as the ratio of observed irradiance in the presence of clouds to that expected in the absence of clouds. Selective forward scattering by clouds toward the observation site results in computed effective transmissions frequently being &gt;1.0. It is shown that conditional temporal averaging of effective cloud transmission over periods of three days or more virtually eliminates the unrealistic cloud transmissions exceeding 1.0. Such temporal averaging of the surface measurements is advantageous for comparing against other area‐wide cloud transmission estimates, such as those determined from satellite or by numerical climate models. The cloud occurrence frequency and the effective solar transmission for long‐term observational records are summarized into monthly and annual averages, and their long‐term variability is investigated. Temporal variations in frequency distributions of transmission are used to determine which clouds are responsible for changes in mean cloudiness. A statistically significant upward trend in cloud occurrence frequency, from 76 to 82% between 1976 and 2001, is detected at Barrow, Alaska, where clouds having solar transmission near 0.2 exhibit the largest increase. At the South Pole, decadal timescale oscillations in both cloud characteristics are detected, but no particular cloud category is identified as the source of that oscillation.

Cloud Radiative Forcing of the Arctic Surface: The Influence of Cloud Properties, Surface Albedo, and Solar Zenith Angle

Abstract An annual cycle of cloud and radiation measurements made as part of the Surface Heat Budget of the Arctic (SHEBA) program are utilized to determine which properties of Arctic clouds control the surface radiation balance. Surface cloud radiative forcing (CF), defined as the difference between the all-sky and clear-sky net surface radiative fluxes, was calculated from ground-based measurements of broadband fluxes and results from a clear-sky model. Longwave cloud forcing (CFLW) is shown to be a function of cloud temperature, height, and emissivity (i.e., microphysics). Shortwave cloud forcing (CFSW) is a function of cloud transmittance, surface albedo, and the solar zenith angle. The annual cycle of Arctic CF reveals cloud-induced surface warming through most of the year and a short period of surface cooling in the middle of summer, when cloud shading effects overwhelm cloud greenhouse effects. The sensitivity of CFLW to cloud fraction is about 0.65 W m−2 per percent cloudiness. The sensitivity of ...

A simplified physical model for assessing solar radiation over Brazil using GOES 8 visible imagery

Solar radiation assessment by satellite is constrained by physical limitations of imagery and by the accuracy of instantaneous local atmospheric parameters, suggesting that one should use simplified but physically consistent models for operational work. Such a model is presented for use with GOES 8 imagery applied to atmospheres with low aerosol optical depth. Fundamental satellite‐derived parameters are reflectance and cloud cover. A classification method applied to a set of images shows that reflectance, usually defined as upper‐threshold Rmax in algorithms assessing cloud cover, would amount ∼0.465, corresponding to the transition between a cumuliform and a stratiform cloud field. Ozone absorption is limited to the stratosphere. The model considers two spectral broadband intervals for tropospheric radiative transfer: ultraviolet and visible intervals are essentially nonabsorbing and can be processed as a single interval, while near‐infrared intervals have negligible atmospheric scattering and very low cloud transmittance. Typical values of CO2 and O3 content and of precipitable water are considered. A comparison of daily values of modeled mean irradiance with data of three sites (in rural, urban industrial, and urban coastal environments), September–October 2002, exhibits a bias of +5 W m−2 and a standard deviation of ∼15 W m−2 (0.4 and 1.3 MJ m−2 for daily irradiation). A comparison with monthly means from about 80 automatic weather stations (covering a large area throughout the Brazilian territory) still shows a bias generally within ±10 W m−2 and a low standard deviation (&lt;20 W m−2), but the bias has a trend in September–December 2002, suggesting an annual cycle of local Rmax values. Systematic (mean) errors in partial cloud cover and in nearly clear‐sky situations may be enhanced using regional values for atmospheric and surface parameters, such as precipitable water, Rmax, and ground reflectance. The larger errors are observed in situations of high aerosol load (especially in regions with industrial activity or forest or agricultural fires). The last case is evident when sites in the Amazonian region or São Paulo city are selected. When considering daily values averaged within 2.5° × 2.5° cells, the standard error is lower than 20 W m−2; present results suggest an annual cycle of mean bias ranging from +10 to −10 W m−2, with an amplitude of ∼10 W m−2. These values are close to the proposed requirements of 10 W m−2 for the mean deviation and 25 W m−2 for the standard deviation. It is expected that the introduction of a reference grid containing mean values of parameters within a cell could induce a decrease in the standard deviation of mean errors and the correction of their annual cycle. A model adaptation for assessing the effect of high aerosol loads is needed in order to extend improvements to the whole Brazilian area.

Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces: A Parameterization in Terms of Optical Depth, Solar Zenith Angle, and Surface Albedo

A multilevel spectral radiative transfer model is used to develop simple but accurate parameterizations for cloud transmittance as a function of cloud optical depth, solar zenith angle, and surface albedo, for use over snow, ice, and water surfaces. The same functional form is used for broadband and spectral transmittances, but with different coefficients for each spectral interval. When the parameterization is applied to measurements of “raw” cloud transmittance (the ratio of downward irradiance under cloud to downward irradiance measured under clear sky at the same zenith angle), an “effective” optical depth τ is inferred for the cloud field, which may be inhomogeneous and even patchy. This effective optical depth is only a convenient intermediate quantity, not an end in itself. It can then be used to compute what the transmittance of this same cloud field would be under different conditions of solar illumination and surface albedo, to obtain diurnal and seasonal cycles of cloud radiative forcing. The parameterization faithfully mimics the radiative transfer model, with rms errors of 1%–2%. Lack of knowledge of cloud droplet sizes causes little error in the inference of cloud radiative properties. The parameterization is applied to pyranometer measurements from a ship in the Antarctic sea ice zone; the largest source of error in inference of inherent cloud properties is uncertainty in surface albedo.

Some considerations on the infrared cloud forcing

Long‐wave spectral cloud forcing is computed for realistic clouds of various optical thickness embedded in the tropical and subarctic winter standard atmospheres. The strict connections between the integrated long‐wave cloud forcing (LCF) and cloud's altitude and transmissivity are shown. LCF, especially in presence of ice clouds, grows almost linearly with cloud transmissivity, and largest values are found in presence of high and opaque cirri. We have also considered the effect of incomplete spectral coverage in broadband long‐wave radiometry when estimating LCF. The error has been computed as a function of edge cutoff (two different filters are used) and cloud's transmissivity and top altitude. Uncorrected measurements from sensors with incomplete spectral coverage generally underestimate the real LCF, especially for the highest cirrus clouds in the tropics. A correction is possible since the percentage error is practically constant with cloud's transmissivity.

Multichannel moderate-bandwidth filter instrument for measurement of the ozone-column amount, cloud transmittance, and ultraviolet dose rates

A six-channel moderate-bandwidth filter instrument for measurement of UV and visible radiation has been developed. The characteristic of the instrument are described, including the spectral and the angular responses. Furthermore the calibration procedure is outlined. Combining information from several channels, one may determine the total ozone-column amount, various biological dose rates, a cloud transmission factor, and the effective cloud optical depth. The methods used to determine these parameters are presented, and the measured parameters are compared with similar ones obtained from other instruments. The total ozone as measured by the instrument agrees with measurements from a standard Brewer to -0.05% +/- 2.04% over a two-year period. Two weeks of cloudless Commission Internationale de L'Eclairage dose rates agree with those from a Bentham double monochromator spectroradiometer to 0.99 +/- 0.03.

Version 2 total ozone mapping spectrometer ultraviolet algorithm: problems and enhancements

Satellite instruments provide global maps of surface UV irradiance by combining backscattered radiance measurements with radiative transfer models. The accuracy of the models is limited by uncertainties in input parameters representing the atmosphere and the Earth's surface. To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow/ice, and a preliminary aerosol correction. We compare results of the version 1 TOMS UV algorithm and the proposed version. We evaluate different approaches for improved treatment for average cloud attenuation within a satellite pixel, with and without snow/ ice on the ground. In addition to treating cloud transmission based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate atmospheric UV irradiance transmission throughout the day. A new method is proposed to obtain a more realistic treatment of the effects from snow-covered terrain. The method is based on an empirical relation between UV reflectivity and measured snow depth. The new method reduces the bias between the TOMS UV estimations and ground-based UV measurements for snow periods. We also briefly discuss the complex problem of estimating surface UV radiation in presence of UV-absorbing aerosols. The improved (Version 2) algorithm can be applied to reprocess the existing TOMS UV irradiance and exposure estimates (since November 1978) and to future satellite sensors (e.g., GOME-2, OMI on EOS/Aura, and Triana/EPIC).

Lidar ratio and depolarization ratio for cirrus clouds.

We report on studies of the lidar and the depolarization ratios for cirrus clouds. The optical depth and effective lidar ratio are derived from the transmission of clouds, which is determined by comparing the backscattering signals at the cloud base and cloud top. The lidar signals were fitted to a background atmospheric density profile outside the cloud region to warrant the linear response of the return signals with the scattering media. An average lidar ratio, 29 +/- 12 sr, has been found for all clouds measured in 1999 and 2000. The height and temperature dependences ofthe lidar ratio, the optical depth, and the depolarization ratio were investigated and compared with results of LITE and PROBE. Cirrus clouds detected near the tropopause are usually optically thin and mostly subvisual. Clouds with the largest optical depths were found near 12 km with a temperature of approximately -55 degrees C. The multiple-scattering effect is considered for clouds with high optical depths, and this effect lowers the lidar ratios compared with a single-scattering condition. Lidar ratios are in the 20-40 range for clouds at heights of 12.5-15 km and are smaller than approximately 30 in height above 15 km. Clouds are usually optically thin for temperatures below approximately -65 degrees C, and in this region the optical depth tends to decrease with height. The depolarization ratio is found to increase with a height at 11-15 km and smaller than 0.3 above 16 km. The variation in the depolarization ratio with the lidar ratio was also reported. The lidar and depolarization ratios were discussed in terms of the types of hexagonal ice crystals.

A New Method for Determining Cloud Transmittance and Optical Depth Using the ARM Micropulsed Lidar

Cirrus clouds play an important role in the climate through their optical and microphysical properties. The problem with measuring the optical properties of these clouds can be partially addressed by using lidar systems. The calibration of backscatter lidar systems, in particular, typically relies on the known molecular (Rayleigh) backscatter, which is a function of temperature, pressure, and chemical composition of the air. This paper presents an improved method for determining the cloud transmittance, and thus optical depth, derived from backscatter lidar measurements. A system of equations is developed in terms of a proposed metric that is required to possess a minima, and has a unique solution for the gain, offset, and transmittance. The new method is tested on a synthetic case as well as using data from two different lidar systems that operate at two different wavelengths. The method is applied to lidar data collected by the lidar operating at the central Pacific island of Nauru under the auspices of the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program.