Cloudless Sky Research Articles

Influence of inhomogeneous surface albedo on UV irradiance: Effect of a stratus cloud

The influence of inhomogeneous surface albedo on UV irradiance is investigated assuming the sky to be overcast by a horizontally homogeneous stratus cloud. From calculations with a three‐dimensional (3‐D) radiative transfer model the snow‐induced relative UV irradiance enhancement as well as the 1‐D albedo is derived for two different snow bidirectional reflectance distribution functions (Lambertian and anisotropic) and snow coverages varying between 0 and 100%. The 1‐D albedo is a quantity defined to consider effects of inhomogeneous surface albedo even in 1‐D radiative transfer codes. It turned out that anisotropy of snow reflectance is of minor importance for quantification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud‐free atmosphere. Furthermore, the comparison of the new results with corresponding data obtained for cloudless sky indicates that a stratus cloud, on the one hand, enhances the surface albedo effect by about a factor 2–3, depending on wavelength. On the other hand, the area significantly influencing UV irradiance via its surface albedo is distinctly smaller. Looking, e.g., at wavelength 330 nm and clear sky, the maximum albedo effect of the surface surrounding a quadratic area of 6400 km2 on UV irradiance at area center is 3%. In the case of a stratus cloud the same effect is found for an area of only 1500 km2.

Influence of the environment reflectance on the ultraviolet zenith radiance for cloudless sky

A three-dimensional Monte Carlo code is used to compute the ultraviolet zenith sky radiance; the code is validated by comparison with a successive-orders-of-scattering code. The amplifications of global irradiance, diffuse irradiance, and zenith radiance that are due to multiple reflectances between a snow-covered ground surface and the atmosphere are compared. For an inhomogeneous Lambertian surface, the contribution of the site environment is analyzed; it depends slightly on the atmospheric turbidity and on the surface reflectance distribution. However, in most cases one can expect approximately 12-15% of the reflected photon contribution to come from within 1 km about the observation site, 25-30% come from areas from 1 to 5 km from the site, 43-47% from 5 to 30 km, and still 10-15% reflected at larger distances. An average contribution function is proposed and used to compute an effective reflectance, which permits retrieval of the sky radiance within 2-4% with a one-dimensional model.

The Ultraviolet Environment of Mars: Biological Implications Past, Present, and Future

A radiative transfer model is used to quantitatively investigate aspects of the martian ultraviolet radiation environment, past and present. Biological action spectra for DNA inactivation and chloroplast (photosystem) inhibition are used to estimate biologically effective irradiances for the martian surface under cloudless skies. Over time Mars has probably experienced an increasingly inhospitable photobiological environment, with present instantaneous DNA weighted irradiances 3.5-fold higher than they may have been on early Mars. This is in contrast to the surface of Earth, which experienced an ozone amelioration of the photobiological environment during the Proterozoic and now has DNA weighted irradiances almost three orders of magnitude lower than early Earth. Although the present-day martian UV flux is similar to that of early Earth and thus may not be a critical limitation to life in the evolutionary context, it is a constraint to an unadapted biota and will rapidly kill spacecraft-borne microbes not covered by a martian dust layer. Microbial strategies for protection against UV radiation are considered in the light of martian photobiological calculations, past and present. Data are also presented for the effects of hypothetical planetary atmospheric manipulations on the martian UV radiation environment with estimates of the biological consequences of such manipulations.

AN ULTRAVIOLET (290 TO 325 NM) IRRADIATION MODEL FOR SOUTHERN CANADIAN CONDITIONS

A numerical model to estimate spectral and broadband ultraviolet irradiance (290 to 325 nm) for Canadian conditions is described and validated with Brewer spectro-radiometer measurements at four stations. The model applies the delta-Eddington algorithm to a 50-level, 100-km, plane-parallel atmosphere with cloud inserted between 2 and 3 km. It requires measured total atmospheric ozone depth and hourly observations of cloud amount. In the absence of ozone soundings, model ozone profiles are scaled by the ratio of measured (from the Brewer instrument) to model total atmospheric ozone depths. In the model calculations, SUSIM ATLAS 3 extraterrestrial irradiance measurements are averaged for each nanometer of wavelength to mimic the triangular filter used by the Brewer instrument. Ozone absorption is calculated from the temperature-dependent coefficients of Pauer and Bass (1985), Rayleigh optical depths after Elterman (1968), and aerosol optical properties from MODTRAN. Surface albedo is a function of snow depth and 0.05 for snow-free ground. Model and measured spectral irradiances for cloudless skies agree well, but model values are smaller than measurements for wavelengths below about 305 nm because of enhancement of the Brewer signal by stray light. Model values of daily cloudless sky irradiance using lidar aerosol optical depth measurements from York University after the Mt. Pinatubo eruption in 1991 agree well with measurements. Cloud optical depths were calculated iteratively for overcast conditions. A fixed optical depth of 45 was used to calculate cloudy sky irradiances at the four stations. These agree well with measurements. Mean bias error (MBE) is less than 5% of the mean measured daily irradiance and root mean square error (RMSE) less than 25%, decreasing to below 12% for 10-day averages. Agreement between mean daily measured and calculated spectral irradiances over a month is also good. [Key words: radiation modeling, ultraviolet irradiance, cloud optical depth.]

Comparison of Cloudless Sky Parameterizations of Solar Irradianceat Various Spanish Midlatitude Locations

We have developed a comparison among several cloudless sky parameterization schemes of the solar irradiance incoming at surface level. The data set was recorded at various Spanish mid-latitude radiometric stations, covering different climatic regimes. It consists of hourly values of the relevant quantities covering periods greater than one year at each radiometric station. Diffuse irradiance data have been corrected for shadowband effect using a model developed by the authors and successfully tested at various radiometric stations. At a first stage, the models were run using all the available local information. After that, we have run the models that present the better performance and, considering that under some circumstances not all the input parameters are available replacing some parameters by their monthly values. As the study reveals, the Gueymard and the Iqbal C model, which require the availability of appropriate information on aerosols, perform best. The influence of precipitable water is of second order thus allowing for the use of estimates based on monthly mean values obtained from climatological records. On the other hand, the study performed on the influence of solar elevation angle on models performance reveals that the worst results correspond to lower solar elevations. This could be a result of limitations in the transmittance parameterization for these elevations.

Empirical modeling of hourly direct irradiance by means of hourly global irradiance

A very important factor in the assessment of solar energy resources is the availability of direct irradiance data of high quality. Nevertheless, this quantity is seldom measured and thus must be estimated from measures of global solar irradiance, a quantity that is registered in most radiometric stations. In this work we analyze the results provided by different models in the estimation of hourly direct irradiance values. We have selected several models proposed by Orgill and Hollands, Erbs et al., Reindl et al., Skarveit and Olseth, Maxwell, and Louche et al. With the exception of the model from Louche et al. that estimates direct irradiance values from direct transmittance values, all of the models estimate direct irradiance from the diffuse fraction. The data set used in this study comprises 25 000 hourly values of global and diffuse irradiance. These values were registered in six Spanish locations with different climatic conditions. The results provided by the model depend on the clearness index, k t, and the solar elevation. The best results are obtained for cloudless skies and higher solar elevation. In those conditions we can estimate the direct irradiance with a root square mean error close to 14% of the average measured value. We have estimated the direct irradiance under cloudless sky conditions using a parametric model proposed by Iqbal. In order to analyze the effect of turbidity on the estimation of direct irradiance we have compared the results obtained by the parametric model when using hourly values of the Angstrom turbidity parameter β with those obtained when using monthly means of hourly values of β.

Meteorological phenomena affecting the presence of solid particles suspended in the air during winter.

Winter is not traditionally considered to be a risky season for people who suffer from pollen allergies. However, increasing numbers of people are showing symptoms in winter. This prompted our investigation into the levels of solid material in the air, and some of the meteorological phenomena that allow their accumulation. This study showed a possible relationship between the phenomenon of thermal inversion, which occurs when very low temperatures, cloudless skies and atmospheric calms coincide, and an increase in the concentration of solid material in the atmosphere. Frequently, this situation is associated with other predictable phenomena such as fog, dew and frost. This may allow a warning system to be derived for urban pollution episodes. The effect caused by parameters such as wind and rainfall was also analysed. Solid material was differentiated into non-biological material from natural and non-natural sources (e.g. soot, dust, sand, diesel exhaust particles, partially burnt residues) and biological material. The latter mainly comprises pollen grains and fungal spores. Owing to its abundance and importance as a causal agent of winter allergies, Cupressaceae pollen was considered separately.

The effect of clouds and surface albedo on UV irradiances at a high latitude site

At high latitudes the Earth's surface is covered by snow large parts of the year. In spring the snow cover significantly increases UV radiation for cloudless as well as cloudy situations. In Tromsø (69.65° N, 18.95° E), Norway, independent measurements have been made of the effective regional spectral surface albedo, the total ozone column, the effective cloud optical depth, and the surface UV irradiance. These measurements are used together with model simulations to study the effect of snow and clouds on surface erythemal radiation doses. Snow on the surface increases the monthly erythemal doses by more than 20%. Relative to cloudless sky, the clouds reduce the monthly erythemal doses by 20–40%. During snow free conditions monthly doses derived from Earth Probe/Total Ozone Mapping Spectrometer (EP‐TOMS) total ozone and cloud reflectivity agree with the measurements within the experimental uncertainties. In presence of snow, however, the EP‐TOMS derived data are too low by 30–40% due to snow covered surfaces being misinterpreted as clouds.

Variation of net radiation over heterogeneous surfaces: measurements and simulation in a juniper–sagebrush ecosystem

As part of a larger study of carbon dioxide and energy exchange, energy components in an open-canopied juniper–sagebrush ecosystem located in the semi-arid region of Eastern Oregon were measured with the eddy covariance technique. Daytime net radiation averaged 20–30% greater than the sum of sensible, latent and soil heat fluxes. On cloudless days several days after a rain event the imbalance was ∼200–250 W m −2. At such times, differences between the surface radiation temperatures of soil and foliage were large, and we investigated whether such differences may generate systematic errors in the measurement of net radiation. A point measurement of net radiation above an open-canopied forest ecosystem is uncertain, because vegetation structure around the measurement location can be highly variable. Depending on location, various fractions of the upwelling radiation from the soil are intercepted by vegetation and do not reach the radiometer. To determine the magnitude of this uncertainty, we measured tree locations and dimensions, and surface radiation temperature ( T r) and shortwave reflection coefficients ( α) of soils and vegetation in a 100 by 100 m area. Geometrical models generated by ray tracing and rendering software were used to calculate the upwelling radiation that would reach radiometers placed at random locations above the surface. In summer, under cloudless skies the measured radiative surface temperatures of soil and vegetation varied considerably, from a mean of 56°C for sunlit soil to 25°C for shaded soil, and 27–29°C for sunlit and shaded vegetation (trees and shrubs). The mean shortwave reflection coefficient varied little between components (with α v=0.10 for vegetation and α s=0.13 for soil). Spatial variability in upwelling radiation ( R u) arises mainly from component variability at viewing angles from ∼30 to ∼60°, where contributions to R u are large and variation in fractional cover between radiometer locations is large. Our measurements and modeling suggest that a radiometer deployed from a tower in a small clearing will only be affected slightly by the clearing since only about 10% of R u arises from viewing angles less than 15° (directly below the radiometer). The spatial variation in the upwelling radiation reaching a sensor above the canopy increases with increasing differences between the radiation temperatures and reflection coefficients of the various ecosystem components. For the radiative properties found at our site, where the radiative temperature of sunlit soil was ∼30°C larger than the temperature of vegetation and shaded components, the spatial variability in the longwave upwelling radiation ( R lu) was less than 20 W m −2. The spatial variation in the shortwave upwelling radiation ( R su) for the small differences in the reflection coefficient of the ecosystem components was less than 10 W m −2. Consequently, the uncertainty associated with estimating the available energy from a single point measurement of net radiation is not enough to explain the lack of energy closure (200–250 W m −2) in this complex open-canopy ecosystem.

Estimation of photosynthetically active radiation under cloudy conditions

Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth’s surface. This cloud radiative forcing is different for the different solar spectral bands. In this work, we analysed the influence of cloud radiative forcing over the photosynthetically active radiation. Knowledge of the photosynthetically active radiation is necessary in different applications, but due to the absence of widespread measurements of this radiometric flux, it must be estimated from available variables. Cloudless sky parametric models compute the global photosynthetically active radiation at surface level by addition of its direct beam and diffuse components. To compute this flux under all sky conditions one must consider the influence of clouds. This could be done by defining a cloud transmittance function. We have developed such a cloud transmittance function considering three different types of clouds. The efficacy of the cloud radiative forcing scheme has been tested in combination with a cloudless sky parametric model using independent data sets. For this purpose, data recorded at two radiometric stations are used. The combination of an appropriate cloudless sky parametric model with the cloud transmittance scheme provides estimates of photosynthetically active radiation with mean bias deviation about 4% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum shows the spectral dependency of the cloud radiative effect.

Parametric models to estimate photosynthetically active radiation in Spain

Different applications dealing with plant physiology, biomass production and natural illumination in greenhouses require knowledge of the photosynthetically active radiation. In absence of measurements of this flux, one must rely on parametric approaches. In this way, the radiant energy is computed using available atmospheric parameters. In the present work, we have developed a comparison among several cloudless sky parameterization schemes. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almerı́a, a seashore location, while the second one is located at Granada, an inland location. The performance of the models has been tested in relation to their predictive capability of direct, diffuse and global components of the photosynthetically active radiation. After our study, it appears that the information concerning the aerosol radiative effects is fundamental to obtain a good estimation, especially for the direct and diffuse components. In order to improve the fitting of the model to the experimental data, several modifications have been suggested. The modified version of the models provide estimates of direct and global components of the photosynthetically active radiation with mean bias deviation below 2%, and root mean square deviation close to experimental error, with slightly worse results for the diffuse component.

Sources of errors in measurements of PAR

Due to optical processes in the atmosphere the spectral distribution of global, direct solar and diffuse radiation is different and depends on solar elevation, atmosphere transparency etc. Because of various optical properties of phytoelements, the spectral distribution of penetrated and reflected radiation differs from that of incoming radiation. Owing to radiative transfer processes, photosynthetically active radiation (PAR) and its proportion in integral solar radiation is not constant. The aim of the paper is to study theoretically these changes and to estimate systematic errors caused by deviation of the spectral sensitivity of pyranometers and PAR sensors from the spectral sensitivity of ideal sensors. The spectral distribution of radiation was estimated by combining different literature data and the spectral sensitivity of a typical pyranometer using available data for spectral transparency of glass domes and spectral reflectivity of black paints. Simultaneous use of energetic and quantum treatments of radiation in biogeophysics and subsequent existence of different definitions of PAR is a source of uncertainties and misunderstanding. The basic concepts, units and conversion factors between integral radiation and PAR, as well PAR efficiency for different kinds of solar radiation, and spectral corrections for some radiation sensors will be discussed below. Our calculations show that the conversion factor U PAR is 1–3% higher for penetrated radiation and about 6% higher for reflected radiation compared with U PAR Q=0.219 W s μmol −1 for global radiation under a cloudless sky. Due to the different spectral sensitivity of ideal energetic and ideal quantum sensors for PAR, the radiation measured through them differs 3–13% depending on the kind of radiation. Hence PAR quantum efficiency is 3–13% higher when PAR is defined as measured by the ideal energetic PAR sensor than when it is estimated by the ideal quantum PAR sensor. Their quantum efficiency depends essentially on the kind of radiation: for diffuse radiation it is increased by 13% but for reflected radiation it is decreased to about one fourth compared with global and direct solar radiation. For penetrated global radiation, quantum efficiency decreases rapidly with canopy depth, being 8–9 times lower at the bottom of a dense canopy than above it. For the LI-COR LI-190SA Quantum Sensor, systematic spectral errors do not exceed 1%. For the Kipp and Zonen PAR LITE sensor, errors are between 1 and 8% depending on the kind of radiation. For the LI-COR LI-200SA Pyranometer Sensor, systematic spectral errors for the global radiation reflected or penetrated by the canopy are high (20–40%), and this instrument cannot be recommended for measurements inside the canopy as is warned also by the manufacturers.

Comparison of groud-based and TOMS-EP total ozone data for antarctica and Northern Midlatitude Stations (1996–1999)

A comparison of the total ozone data by the TOMS-EP satellite and ground based Dobson spectrophotometer (Ukrainian Antarctic Station ‘Akademik Vernadsky’, 65.25S, 64.27W), and M124 UV filter radiometer (Kyiv University Observatory ‘Lisniky’, 50.30N, 30.53E) measurements was made. Comparison of the TOMS-EP and Dobson daily total ozone values for Antarctic Peninsula gives a decrease of the mean relative difference from 3.1% in 1996/1997 to 1.4% in 1998/1999. Comparison of the TOMS-EP and M124 data gives a changing relative difference of near zero (from −0.4% to 0.11% in 1997–1999). Days with cloudless sky and with totally overcast sky were selected from the data sets. The mean relative difference of TOMS — Dobson data changed during 1996–1999 for cloudless days from 6% to 11% and for cloud cover from −0.5% to −1.3%. Northern midlatitude observations give a relative difference of TOMS — M124 data for cloudless sky of 0.15% and for totally overcast sky of −3.5%. This comparison shows that differences in cloud cover influence the total ozone values obtained from ground-based and satellite measurements in the regions considered.

Clash of the Clans: An Extract from a Bit of Earth

Street near the docks was the meeting place of the gainfully employed and the desperately unemployed. Hundreds of miners, who had fled from the troubles in Bandong Valley, Larut and elsewhere, had converged on the area. The backlanes and sidelanes were choked with idle men. They squatted beside the open drains, shoved their way along the five-foot paths, overflowed onto the roads and blocked the progress of the pushcarts, rickshaws, buggies, gharries and bullock carts inching their way through the heart of the trading district. All were hoping to earn a few cents, which would buy the day's bowl of rice. But jobs were scarce. Traders and merchants were jittery about their investments in the turbulent Malay States. The town was full of rumours of impending war. To the few policemen patrolling the area, mainly Sikhs and Amboynese employed in the Colonial Police Department, the babble of Chinese tongues was incomprehensible. They walked through the throng, unseeing and unhearing. They did not see the knots of coolies swelling. Neither did they hear talk of murder and slaughter nor rumours of impending war between the Hakka and Cantonese miners. Tai-kor Wong was killed in bed! His head chopped off and flung out of the window ! 'The gods have eyes, lor Let the White Cranes taste our sword of vengeance! Kill those daughter-fuckers, mother-fuckers and sons of pigs! As the morning wore on, the talk grew more virulent and strident. Tales of past battles were recalled and relived. Venom and rage spread among the coolies, smiting the knots of men huddled in the shadows. The sun rose higher in the cloudless sky, and soon not many patches of shade were left. More coolies squeezed into the covered walkways of the shops. Tempers ran short. A word here, a word there, a few words overheard and repeated, an imagined insult, a stare, a push and before anyone knew it, something had ignited the tinderbox. A scuffle broke out just as Baba Wee's gharry swung into view. A young coolie was pushed into its path. The two horses neighed and reared up on their hind legs. The Indian driver was nearly knocked off his seat. Whoa!

A study of zenith luminance on Madrid cloudless skies

The zenith luminance ( L z) has been measured for cloudless skies at the IDMP station in Madrid. Mean values obtained at every 15 min have been fitted vs. solar elevation ( α). A 5th degree polynomial connecting log ( L z) against ( α) gives the best correlation in terms of the correlation coefficient ( r=0.9358). A possible explanation of the dependence of the zenith luminance on solar elevation at low values is given. An exponential model obtained by other authors ( Vázquez and Bernabeu, 1997) for a nearby temporary station in the same city does not account for the qualitative dependence of L z vs. α for α<≈15°, and also overestimates experimental values for high solar elevations. The effect of turbidity on L z is studied categorizing available data in three specified groups of T v, the visual or luminous/illuminance turbidity. Best fits with 5th degree polynomials connecting log ( L z) against ( α) are given for: T v<3, 3< T v<5, and T v>5.

The European Community Cloudless Sky Radiance Model. An Evaluation by Means of the Skyscan'834 Data Set

Having in mind the high complexity of the sky radiance distribution, the Commission of the European Communities, in the frame of the Solar Energy R&D in the European Community Programme, developed among other related fields of research a clear sky radiance model. This model, the so-called EC-Model, was intended for the estimation of the diffuse irradiance falling on an inclined surface by integration of the sky radiance field. It represents a progress development over its predecessor, the Berlin model, and takes into account all the scientific knowledge about this topic, lying heavily on the principles of the Liebelt formulation and the work of Steven and Unsworth on the observed relative radiance distribution. The European Solar Radiation Atlas was in part prepared by the adoption of the EC-Model and could not take into account Valko’s new results about sky radiance distributions, because of time coincidence and that only previous results were ready at the time it was published. In this work, we focus on the EC-Model behaviour, against experimental sky radiance data from Toronto, Canada. The SKYSCAN’834 Data Set is a well-known database, which we have used in order to investigate the EC-Model behaviour both under its original formulation (the one that has been used in the development of the Atlas) and under the modified version suggested by Valko’s results. Even though we are considering data from outside Europe, the results of this test let us establish the model limitations and the modifications that should be done to it.

Performance validation of MURAC, a cloudless sky radiance model proposal

The behaviour of a new model, previously proposed, for the directional distribution of the clear sky diffuse radiance is presented in this work. This new model, called MURAC, includes two parameters for which simple expressions without local dependence were proposed. In this way MURAC claims to be a general applicability model for the angular distribution of cloudless sky radiance. We present the model performance analysis by means of the well-known SKYSCAN 83/84 DATA SET, which involves angular sky radiance distributions recorded at Toronto, Canada. This analysis indicates that MURAC is able to estimate instantaneous sky radiance values with a mean bias error of −3.7% and a root mean square error of 63.6%. These results, in the same order of magnitude as other sky radiance models, seem to confirm our expectation.

Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer

As a part of the Boreal Ecosystem-Atmosphere Study (BOREAS), measurements of the spectral reflectance anisotropy of three boreal forest canopies were studied for cloudless sky conditions at the phenological growth stages which were at or near maximum leaf area index at each site. The three sites were relatively homogeneous mature stands of black spruce, jack pine, and aspen located in the southern boreal zone of central Saskatchewan. Measurements of the spectal bidirectional reflectance factors with a 15° instrument field of view in three spectral bands centered at 662 nm, 826 nm, and 1658 nm were made with the PARABOLA instrument over a range of solar zenith angles typically varying from 35° (near solar noon) to 70°. The measured reflectance factors showed large anisotropy at all three sites and for all three wavelengths, with prominant backscatter peak reflectances, and strong retro solar view angle (hot spot) maximum reflectances in the visible (662 nm) and shortwave infrared (1658 nm) for the jack pine and black spruce sites, with a less pronounced hot spot at the aspen site. Pronounced effects of canopy and understory shadowing in the visible, as a function of solar zenith angle (SZA), were observed for the black spruce and jack pine sites, with resultant large linear increases in computed normalized difference and simple ratio vegetation indices as SZA increased for near-nadir view angles. Hemispheric spectral reflectances or spectral albedos were computed from angular integration of PARABOLA measured bidirectional reflectances. Visible (662 nm) hemispheric reflectances for the jack pine and black spruce canopies showed very little variation with solar zenith angle, while near-infrared hemispheric reflectances increased strongly with increasing SZA. Estimates were made of the total shortwave albedo for the aspen and jack pine sites from irradiance and reflectance weighting of the spectral hemispheric reflectances in the three measured wavelengths. Comparison of estimated to pyranometer measured total albedo showed all estimates to be biased high, but only by about 0.007–0.018, depending on which of two sets of pyranometer measured albedos were utilized for the comparison. The measured bidirectional reflectance factor (BRF) data sets reported in this study coupled with ancillary data of biophysical parameters collected at the same sites by BOREAS researchers provide a unique data set for the development and characterization of canopy bidirectional reflectance modeling and for the interpretation of remotely sensed data for boreal forest canopies.

Enhanced absorption of UV radiation due to multiple scattering in clouds: Experimental evidence and theoretical explanation

Measurements of spectral ultraviolet‐B irradiance under optically thick clouds show strongly enhanced attenuation by molecular and particulate absorbers. The atmospheric photon path is enhanced owing to the presence of a highly scattering medium, leading to an amplification of absorption by tropospheric ozone and aerosol. Calculations with discrete ordinate and Monte Carlo models show that photon paths in realistic water clouds may be enhanced by factors of 10 and more compared to cloudless sky. Model calculations further show that UV spectra measured under thick clouds can be well simulated within 10%, indicating that the involved processes are quantitatively described by current models. These findings are of important consequence for all ground‐based remote sensing applications which take advantage of measuring scattered radiation in order to infer atmospheric trace gas abundances. These algorithms, for example, the calculation of total ozone from global irradiance or zenith sky radiance, are subject to large errors, when neglecting the influence of cloud scattering on the derived data. In the present study, errors of more than 300 Dobson Units (DU) have been found, if such methods were applied without care in the presence of thick clouds.

Effects of snow cover on UV irradiance and surface albedo: A case study

A heavy snowfall, followed by several days of cloudless skies before significant snow melt had occurred, enabled a quantitative study of the effects of snow on downwelling UV spectral irradiances at the National Institute for Water and Atmospheric Research UV measurements site in Lauder, New Zealand. The largest UV enhancements (&gt;70%) were seen during partly cloudy conditions immediately after the snowfall. A radiative transfer model was used to quantify the enhancements due to the snow cover and the spectral albedo of the snow under clear‐sky conditions. The first cloudless day on which the radiative transfer model could be used with confidence occurred 7 days after the snowfall. By this time, the maximum enhancements due to snow at solar zenith angle (SZA) 70° were approximately 22% in the UV‐A region. In the UV‐B region, the enhancements were approximately 28% and tended to increase slightly at larger SZA. The corresponding surface albedo was 0.62±0.08, and comparison with supplementary measurements indicated that the albedo decayed with time. Any spectral or SZA dependencies in the enhancements were below the measurement uncertainties in the UV region. Comparisons with supplementary data indicated that the albedo immediately after the snow was greater than 0.8.