Clear-sky Irradiance Research Articles

Field measurement of clear-sky solar irradiance in Badain Jaran Desert of Northwestern China

The Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) sponsored and conducted an intensive field campaign on dust aerosols in Badain Jaran Desert of Northwestern China from April 20 to June 20, 2010. A set of state-of-the-art broadband radiometers and sun/sky photometers were deployed along with launched radiosonde. In this paper, we compared the simulated solar irradiances by using the SBDART radiative transfer model with those from the ground-based measurements for 69 selected cases of 7 days. It was shown that the averaged aerosol optical depth at 500nm (AOD500) is 0.18±0.09 with AOD500 less than 0.5 for all cases. The single-scattering albedo and asymmetry factor at 675nm are 0.928±0.035, 0.712±0.023, respectively. The AODs retrieved from the CIMEL sun photometer at various wavelengths agree well with those from the PREDE sky radiometer, and the columnar water vapor contents from CIMEL also agree well with radiosonde observations. In the radiative closure experiment, we used a collocated thermopile pyrgeometer with a shadow and ventilator to correct the thermal dome offset of diffuse irradiance measurement. The mean differences between model and measurements are −9.1Wm−2 (−2.6%) for the direct irradiance, +3.1Wm−2 (+2.8%) for diffuse irradiance, and −6.0Wm−2 (−1.3%) for global irradiance, which indicates an excellent radiative closure. Aerosol shortwave direct radiative forcing (ARF) and radiative heating rate are also investigated. The daily mean ARF ranges from −4.8 to +0.4Wm−2 at the top of the atmosphere, −5.2 to −15.6Wm−2 at the surface, and 5.2 to 10.8Wm−2 in the atmosphere. The corresponding radiative heating rates for the whole atmosphere due to dust aerosols are 0.07, 0.11, 0.14, 0.11, 0.10, 0.08, and 0.07K/day for the 7 selected cloudless days. These solar radiative forcing can be considered as the representative impact of background dust aerosol in Northwestern China.

Temporal variability in direct and global irradiance at various time scales as affected by aerosols

The aerosol optical depth (AOD) is known to be a critical input for radiation modeling purposes, and partially determines the accuracy of modeled direct normal irradiance (DNI) and global horizontal irradiance (GHI). This contribution examines to what extent time variations in AOD also determine the observed variability in DNI, particularly at the daily and longer time scales. Two measures of variability are introduced: the Aerosol Variability Index (AVI) characterizes the magnitude of the variability in AOD over specific periods, from daily to yearly, whereas the Aerosol Sensitivity Index (ASI) relates the magnitude of relative variations in irradiance to absolute variations in AOD. AOD measurements at 180 Aeronet sites over the world are used to obtain clear-sky irradiances with the REST2 radiative model, as well as determinations of ASI and AVI. Large geographic variations exist in AVI, whose largest values are found over western Sahara. The variations of ASI follow a different pattern because it decreases when AOD increases. The variability in GHI is typically 2–4 times lower than that in DNI. On a long-term basis, the normal aerosol-induced variability in DNI is less than ±5% at most sites, but some areas might experience a much larger variability, comparable to that created by large volcanic eruptions. The latest such events predate most current modeled DNI or GHI datasets, making resource assessments potentially too optimistic for bankability if based on such limited data series alone.

Cloud effective transmittance at two sites of the Atacama Desert, Chile

[1] Broadband overcast cloud effective transmittance was determined at Arica (18.47°S, 70.31°W, 20 m above sea level (asl)) and Poconchile (18.45°S, 70.07°W, 560 m asl), Atacama Desert, northern Chile, from 10 min averaged pyranometer measurements of total solar irradiance (ToSI) and ultraviolet solar irradiance (UVSI) during the period 2002–2005. The predominant cloud type is marine stratocumulus, characteristic of the southeastern Pacific tropical environment. The region's very regular climate conditions, characterized by overcast mornings and cloudless afternoons, allow the application of an empirical method to determine the expected clear-sky irradiance during cloudy mornings. The cloud effective transmittance (CET) is determined as the ratio of the measured cloudy-sky irradiance over the expected clear-sky irradiance. CETTo = 0.26 (0.31) for ToSI and CETUV = 0.37 (0.43) for UVSI characterize overcast cloudiness at Arica (Poconchile). One-dimensional radiative transfer model calculations in both ToSI and UVSI ranges are also used. The measured and modeled relationships between CETTo and CETUV closely agree. New insights are given to explain the sparsely populated data around CET = 0.8 observed also by other similar studies.

UV impacts avoided by the Montreal Protocol

Temporal and geographical variabilities in the future "world expected" UV environment are compared with the "world avoided", which would have occurred without the Montreal Protocol on Substances That Deplete the Ozone Layer and its subsequent amendments and adjustments. Based on calculations of clear-sky UV irradiances, the effects of the Montreal Protocol have been hugely beneficial to avoid the health risks, such as skin cancer, which are associated with high UV, while there is only a small increase in health risks, such as vitamin D deficiency, that are associated with low UV. However, interactions with climate change may lead to changes in cloud and albedo, and possibly behavioural changes that could also be important.

Evaluation of radiation scheme performance within chemistry climate models

[1] This paper evaluates global mean radiatively important properties of chemistry climate models (CCMs). We evaluate stratospheric temperatures and their 1980–2000 trends, January clear sky irradiances, heating rates, and greenhouse gas radiative forcings from an offline comparison of CCM radiation codes with line‐by‐line models, and CCMs’ representation of the solar cycle. CCM global mean temperatures and their change can give an indication of errors in radiative transfer codes and/or atmospheric composition. Biases in the global temperature climatology are generally small, although five out of 18 CCMs show biases in their climatology that likely indicate problems with their radiative transfer codes. Temperature trends also generally agree well with observations, although one model shows significant discrepancies that appear to be due to radiation errors. Heating rates and estimated temperature changes from CO2, ozone, and water vapor changes are generally well modeled. Other gases (N2O, CH4, and CFCs) have only played a minor role in stratospheric temperature change, but their heating rates have large fractional errors in many models. Models that do not account for variations in the spectrum of solar irradiance cannot properly simulate solar‐induced variations in stratospheric temperature. The combined long‐lived greenhouse gas global annual mean instantaneous net radiative forcing at the tropopause is within 30% of line‐by‐line models for all CCM radiation codes tested. Problems remain in simulating radiative forcing for stratospheric water vapor and ozone changes with errors between 3% and 200% compared to line by line models. The paper makes recommendations for CCM radiation code developers and future intercomparisons.

Evaluation of numerical weather prediction for intra-day solar forecasting in the continental United States

Numerical weather prediction (NWP) is generally the most accurate tool for forecasting solar irradiation several hours in advance. This study validates the North American Model (NAM), Global Forecast System (GFS), and European Centre for Medium-Range Weather Forecasts (ECMWF) global horizontal irradiance (GHI) forecasts for the continental United States (CONUS) using SURFRAD ground measurement data. Persistence and clear sky forecasts are also evaluated. For measured clear conditions all NWP models are biased by less than 50 W m −2. For measured cloudy conditions these biases can exceed 200 W m −2 near solar noon. In general, the NWP models (especially GFS and NAM) are biased towards forecasting clear conditions resulting in large, positive biases. Mean bias errors (MBE) are obtained for each NWP model as a function of solar zenith angle and forecast clear sky index, kt ∗, to derive a bias correction function through model output statistics (MOS). For forecast clear sky conditions, the NAM and GFS are found to be positively biased by up to 150 W m −2, while ECMWF MBE is small. The GFS and NAM forecasts were found to exceed clear sky irradiances by up to 40%, indicating an inaccurate clear sky model. For forecast cloudy conditions ( kt ∗ < 0.4) the NAM and GFS models have a negative bias of up to −150 W m −2. ECMWF forecasts are most biased for moderate cloudy conditions (0.4 < kt ∗ < 0.9) with an average over-prediction of 100 W m −2. MOS-corrected NWP forecasts based on solar zenith angle and kt ∗ provide an important baseline accuracy to evaluate other forecasting techniques. MOS minimizes MBE for all NWP models. Root mean square errors for hourly-averaged daytime irradiances are also reduced by 50 W m −2, especially for intermediate clear sky indices. The MOS-corrected GFS provides the best solar forecasts for the CONUS with an RMSE of about 85 W m −2, followed by ECMWF and NAM. ECMWF is the most accurate forecast in cloudy conditions, while GFS has the best clear sky accuracy.

Utilization of Direct and Diffuse Sunlight in a Dye-Sensitized Solar Cell — Silicon Photovoltaic Hybrid Concentrator System

The concept of a tandem hybrid concentrator solar module was demonstrated from a dye-sensitized TiO2 solar cell (DSSCs) and a silicon p−n junction solar cell. The test system employed DSSC and Si cells with indoor AM1.5G efficiencies of 9.1 and 18.1%, respectively. Two different optical filters were used to selectively reflect and concentrate near-infrared light from the DSSC onto the Si cell. On the basis of outdoor testing in a 2× concentrator−reflector arrangement, the tandem system generated 93 and 96% of the output power of directly illuminated Si cells under altostratus/cirrostratus and clear sky irradiances, respectively, despite a DSSC-to-Si active area ratio of only 0.92. Similar performance is expected at higher (5−10×) concentration ratios. The hybrid arrangement of visible- and IR-absorbing solar cells addresses the problem of lower performance of conventional concentrators under diffuse irradiance conditions. These proof-of-concept results suggest that system level efficiencies approaching 20...

Solar irradiance at the earth's surface: long-term behavior observed at the South Pole

Abstract. This research examines a 17-year database of UV-A (320–400 nm) and visible (400–600 nm) solar irradiance obtained by a scanning spectroradiometer located at the South Pole. The goal is to define the variability in solar irradiance reaching the polar surface, with emphasis on the influence of cloudiness and on identifying systematic trends and possible links to the solar cycle. To eliminate changes associated with the varying solar elevation, the analysis focuses on data averaged over 30–35 day periods centered on each year's austral summer solstice. The long-term average effect of South Polar clouds is a small attenuation, with the mean measured irradiances being about 5–6% less than the clear-sky values, although at any specific time clouds may reduce or enhance the signal that reaches the sensor. The instantaneous fractional attenuation or enhancement is wavelength dependent, where the percent deviation from the clear-sky irradiance at 400–600 nm is typically 2.5 times that at 320–340 nm. When averaged over the period near each year's summer solstice, significant correlations appear between irradiances at all wavelengths and the solar cycle as measured by the 10.7 cm solar radio flux. An approximate 1.8 ± 1.0% decrease in ground-level irradiance occurs from solar maximum to solar minimum for the wavelength band 320–400 nm. The corresponding decrease for 400–600 nm is 2.4 ± 1.9%. The best-estimate declines appear too large to originate in the sun. If the correlations have a geophysical origin, they suggest a small variation in atmospheric attenuation with the solar cycle over the period of observation, with the greatest attenuation occurring at solar minimum.

The effects of clouds on enhancing surface solar irradiance

Spectral solar irradiances were observed using a visible and UV Multi-Filter Rotating Shadowband Radiometer on the rooftop of the Science Building at Yonsei University, Seoul (, , 86 m) during one year period in 2006. 1-min measurements of global(total) and diffuse solar irradiances over the solar zenith angle (SZA) ranges from to were used to examine the effects of clouds and total optical depth (TOD) on enhancing four solar irradiance components (broadband 395-955 nm, UV channel 304.5 nm, visible channel 495.2 nm, and infrared channel 869.2 nm) together with the sky camera images for the assessment of cloud conditions at the time of each measurement. The obtained clear-sky irradiance measurements were used for empirical model of clear-sky irradiance with the cosine of the solar zenith angle (SZA) as an independent variable. These developed models produce continuous estimates of global and diffuse solar irradiances for clear sky. Then, the clear-sky irradiances are used to estimate the effects of clouds and TOD on the enhancement of surface solar irradiance as a difference between the measured and the estimated clear-sky values. It was found that the enhancements occur at TODs less than 1.0 (i.e. transmissivity greater than 37%) when solar disk was not obscured or obscured by optically thin clouds. Although the TOD is less than 1.0, the probability of the occurrence for the enhancements shows 50~65% depending on four different solar radiation components with the low UV irradiance. The cumulus types such as stratoculmus and altoculumus were found to produce localized enhancement of broadband global solar irradiance of up to 36.0% at TOD of 0.43 under overcast skies (cloud cover 90%) when direct solar beam was unobstructed through the broken clouds. However, those same type clouds were found to attenuate up to 80% of the incoming global solar irradiance at TOD of about 7.0. The maximum global UV enhancement was only 3.8% which is much lower than those of other three solar components because of the light scattering efficiency of cloud drops. It was shown that the most of the enhancements occurred under cloud cover from 40 to 90%. The broadband global enhancement greater than 20% occurred for SZAs ranging from 28 to . The broadband diffuse irradiance has been increased up to 467.8% (TOD 0.34) by clouds. In the case of channel 869.0 nm, the maximum diffuse enhancement was 609.5%. Thus, it is required to measure irradiance for various cloud conditions in order to obtain climatological values, to trace the differences among cloud types, and to eventually estimate the influence on solar irradiance by cloud characteristics.

Air temperatures in a power line corridor over 10 years following afforestation with Eucalyptus globulus

Over the 10 years following the planting of Eucalyptus globulus on an area of 320 ha in 1998, air temperature was measured in the middle of a power line corridor and compared with temperatures recorded at meteorological stations located 11.2 and 35.7 km away. At these reference stations, the annual means of both daily minimum and daily maximum temperature remained approximately constant. In the power line corridor, annual mean daily minimum temperature also remained approximately constant, but annual mean daily maximum temperature fell by about 3.6°C; in particular, between years 4 and 7 the cooling rate was on average about 0.7°C per year. The resulting sigmoid time course of annual mean daily temperature range may partly reflect the evolution of evapotranspiration from the plantation, but seems to be mainly attributable to the shading of the power line corridor by the growing trees, being closely correlated with solar irradiance at the center of the corridor as estimated from tree growth data using sun charts and standard formulae for clear-sky solar irradiance.

A broad‐spectrum sunscreen prevents cumulative damage from repeated exposure to sub‐erythemal solar ultraviolet radiation representative of temperate latitudes

We have previously shown the detrimental effects of 19 sub-erythemal exposures to daily ultraviolet radiation (DUVR, which mimics non-extreme exposure conditions), delivered over 4 weeks to volunteers. This source had UVA (320-400 nm) to UVB (290-320 nm) irradiance ratio of 25, instead of that close to 10 that is typically the case with solar-simulated radiation (SSR) that represents summer global sunlight with a clear sky and quasi-zenith solar irradiance. Here, we report on an extension of this previous study, in which we evaluated the photoprotection afforded by a broad-spectrum daily-care product with a low-sun protection factor (SPF 8, UVA-PF 7 and 3* rated UVA protection). We assessed cellular and molecular markers of photodamage that are relevant to skin cancer and photoageing. This study shows that biological effects of repeated exposure to DUVR can be prevented by a broad-spectrum daily-care product and that the level of protection afforded varies with the studied endpoint. Efficient daily UVR protection, as provided by a broad-spectrum daily-care product, is necessary to prevent the 'silent' sub-erythemal cumulative effects of UVR from inadvertent sun exposure.

Monthly average clear-sky broadband irradiance database for worldwide solar heat gain and building cooling load calculations

This paper establishes the formulation of a new clear-sky solar radiation model appropriate for algorithms calculating cooling loads in buildings. The aim is to replace the ASHRAE clear-sky model of 1967, whose limitations are well known and are reviewed. The new model is derived in two steps. The first step consists of obtaining a reference irradiance dataset from the REST2 model, which uses a high-performance, validated, two-band clear-sky algorithm. REST2 requires detailed inputs about atmospheric conditions such as aerosols, water vapor, ozone, and ground albedo. The development of global atmospheric datasets used as inputs to REST2 is reviewed. For the most part, these datasets are derived from space observations to guarantee universality and accuracy. In the case of aerosols, point-source terrestrial measurements were also used as ground truthing of the satellite data. The second step of the model consists of fits derived from a REST2-based reference irradiance dataset. These fits enable the derivation of compact, but relatively accurate expressions, for beam and diffuse clear-sky irradiance. The fitted expressions require the tabulation of only two pseudo-optical depths for each month of the year. The resulting model, and its tabulated data, are expected to be incorporated in the 2009 edition of the ASHRAE Handbook of Fundamentals.

An Upper Threshold of Enhanced Global Shortwave Irradiance in the Troposphere Derived from Field Measurements in Tropical Mountains

Abstract Extraordinarily high values of global irradiance (up to 1832 W m−2) incident upon a horizontal surface were observed during a 4-yr meteorological field campaign in the southern Ecuadorian Andes Mountains (4°S). The unexpected magnitude gave rise to a thorough revision of the instrumentation and an assessment of the radiation database. Infrastructure at the sites and software were critically examined, and the sensor and datalogger manufacturers were contacted. The observed enhanced irradiance was quantified with a simple clear-sky model for global radiation. The efforts showed that atmospheric conditions and not artifacts had produced the high values. Cloud radiative effects could be singled out as the exclusive source of the “superirradiance.” Mean (bihourly) maximum enhancement was 119.6% of the potential (clear sky) irradiance; absolute maximum enhancement occasionally reached to over 170%. Thereby, under ideal conditions, the upper threshold of global irradiance is apparently ∼200% of the potential (clear sky) direct radiation [i.e., at the point of observation, downwelling diffuse cloud radiation can (almost) equal the magnitude of its apparent “source”]. The observations were made between altitudes of 1500 and 3400 m MSL in different climates ranging from perhumid to semiarid (i.e., in very cloudy climates and in less cloudy climates). The conditions that were found to explain the magnitude of the extremely enhanced irradiance are not confined to tropical or mountainous environments only, but rather they can occur at any spot in the troposphere where clouds exist. Therefore, the findings appear to be of general validity.

Typical distribution of the solar erythemal UV radiation over Slovakia

Abstract. Maps of solar erythemal ultraviolet (EUV) irradiance daily doses were created for every month with a horizontal resolution of 500 m at the geographical domain 47.15 N–49.86 N×16.94 E–22.81 E covering the territory of Slovakia. The cloud modification factor for the EUV radiation (cmfUV) was modeled utilizing the relation between the cloud modification factor of global and EUV radiation. The maps of the cmfUV factor were created by utilizing measurements of global irradiance performed at nine observatories during the period 1995–2004 and modeling of the cmfUV dependence on altitude. Maps of the EUV irradiance daily dose corresponded to clear-sky conditions and EUV irradiance daily dose affected by average cloudiness were constructed for mean monthly total ozone, its upper and lower monthly limits, for two probability levels of snow cover occurrence as criteria for the snow effect incorporation in the model and for one day representing typical values for every month. The map-set can be regarded as an atlas of solar EUV radiation over Slovakia.

Comparison of UV irradiance measurements at Summit, Greenland; Barrow, Alaska; and South Pole, Antarctica

Abstract. An SUV-150B spectroradiometer for measuring solar ultraviolet (UV) irradiance was installed at Summit, Greenland, in August 2004. Here we compare the initial data from this new location with similar measurements from Barrow, Alaska, and South Pole. Measurements of irradiance at 345 nm performed at equivalent solar zenith angles (SZAs) are almost identical at Summit and South Pole. The good agreement can be explained with the similar location of the two sites on high-altitude ice caps with high surface albedo. Clouds attenuate irradiance at 345 nm at both sites by less than 6% on average, but can reduce irradiance at Barrow by more than 75%. Clear-sky measurements at Barrow are smaller than at Summit by 14% in spring and 36% in summer, mostly due to differences in surface albedo and altitude. Comparisons with model calculations indicate that aerosols can reduce clear-sky irradiance at Summit by 4–6%; aerosol influence is largest in April. Differences in total ozone at the three sites have a large influence on the UV Index. At South Pole, the UV Index is on average 20–80% larger during the ozone hole period than between January and March. At Summit, total ozone peaks in April and UV Indices in spring are on average 10–25% smaller than in the summer. Maximum UV Indices ever observed at Summit, Barrow, and South Pole are 6.7, 5.0, and 4.0, respectively. The larger value at Summit is due to the site's lower latitude. For comparable SZAs, average UV Indices measured during October and November at South Pole are 1.9–2.4 times larger than measurements during March and April at Summit. Average UV Indices at Summit are over 50% greater than at Barrow because of the larger cloud influence at Barrow.

Towards closure between measured and modelled UV under clear skies at four diverse sites

Abstract. The purpose of this work is determine the extent of closure between measurements and models of UV irradiances at diverse sites using state of the art instruments, models, and the best available data as inputs to the models. These include information about aerosol optical depth (unfortunately not extending down as far into the UVB region as desirable because such information is not generally available), ozone column amounts, as well as vertical profiles of temperature. We concentrate on clear-sky irradiances, and report the results in terms of UV Index (UVI). Clear-sky data from one year of measurements at each of four diverse sites (Lauder – New Zealand, Mauna Loa Observatory – Hawaii, Boulder – Colorado, and Melbourne – Australia) have been analysed in detail, also taking account of different measurements of ozone, including satellite-derived values, as well as ground measured values, both from Dobson instruments and as retrieved from the UV spectra under study. Previous studies have generally focussed on data from a single site, and for shorter periods. As such, it is the most comprehensive study of its kind to date. At Lauder, which is the cleanest low altitude site, we obtained agreement between measurement and model at 5% level, which is consistent with the best agreement found previously. At Mauna Loa Observatory, similar agreement was achieved, but model calculations need to allow for reflections from cloud that are present below the observatory. At this site, there are occasional problems with using satellite-derived ozone. At Boulder, mean agreements were similar but the dispersion around the mean was slightly larger, corresponding to larger uncertainties in the aerosol inputs to the model. However, at Melbourne, which is the only non-NDACC (Network for the Detection of Atmospheric Composition Change) site, there remain unexplained discrepancies. The measured values are significantly lower than the calculated values. We investigate the extent to which this discrepancy can be explained by incomplete knowledge of aerosol extinctions in the UV at this site. We conclude that further information about aerosol optical depth and single scattering albedo in the UVB region is needed to resolve the issues. For more polluted sites (the four considered locations show in general small aerosol load), the uncertainties in the aerosol input parameters would lead to less confidence on the modelling approach. At the three NDACC sites, the closure provided by the study gives confidence in both the measurements and our ability to model them. This study revealed a limitation in the use of PTFE diffusers when temperatures are lower than approximately 20°C. It also documents the range of clear sky UVI values expected at these diverse sites.

Prediction and validation of cloudless shortwave solar spectra incident on horizontal, tilted, or tracking surfaces

The SMARTS spectral model can advantageously be used to predict clear-sky irradiance spectra on surfaces of any tilt and orientation, e.g., for the simulation of spectrally selective technologies like photovoltaic devices or coated fenestration systems. To evaluate the intrinsic accuracy of the model, its current version undergoes here a three-step validation exercise, involving reference radiative transfer codes, and two series of sophisticated spectral and ancillary measurements performed at different locations. Provided that the most important inputs are known with sufficient accuracy, it is concluded that the model performance is very high, with typical differences of 1–2% when compared to reference models, and uncertainties largely within the overall experimental error (≈5%) when compared to spectroradiometric measurements. The effect of imprecise spectral reflectance data of the foreground on the diffuse irradiance incident on shaded vertical surfaces is discussed.

Clear-sky classification procedures and models using a world-wide data-base

Clear-sky data need to be extracted from all-sky measured solar-irradiance dataset, often by using algorithms that rely on other measured meteorological parameters. Current procedures for clear-sky data extraction have been examined and compared with each other to determine their reliability and location dependency. New clear-sky determination algorithms are proposed that are based on a combination of clearness index, diffuse ratio, cloud cover and Linke’s turbidity limits. Various researchers have proposed clear-sky irradiance models that rely on synoptic parameters; four of these models, MRM, PRM, YRM and REST2 have been compared for six world-wide-locations. Based on a previously-developed comprehensive accuracy scoring method, the models MRM, REST2 and YRM were found to be of satisfactory performance in decreasing order. The so-called Page radiation model (PRM) was found to underestimate solar radiation, even though local turbidity data were provided for its operation.

A case study to prepare for the utilization of aerosol forecasts in solar energy industries

Precise aerosol information is indispensable in providing accurate clear sky irradiance forecasts, which is a very important aspect in solar facility management as well as in solar and conventional power load prediction. In order to demonstrate the need of detailed aerosol information, direct irradiance derived from Aerosol Robotic Network (AERONET) ground based measurements of aerosol optical depth (AOD) was compared in a case study over Europe to irradiance calculated using a standard aerosol scenario. The analysis shows an underestimation of measurement-derived direct irradiance by the scenario-derived direct irradiance for locations in Northern Europe and an overestimation for the Mediterranean region. Forecasted AOD of the European Dispersion and Deposition Model (EURAD) system was validated against ground based AERONET clear sky AOD measurements for the same test period of February 15th to 22nd, 2004. For the time period analyzed, the modelled AOD forecasts of the EURAD system slightly underestimate ground based AERONET measurements. To quantify the effects of varying AOD forecast quality in their impact on the application in solar energy industry, measured and forecasted AOD were used to calculate and compare direct, diffuse, and global irradiance. All other influencing variables (mainly clouds and water vapour) are assumed to be modelled and measured correctly for this analysis which is dedicated to the specific error introduced by aerosol forecasting. The underestimated AOD results in a mean overestimation of direct irradiance of +28 W/m 2 (+12%), whereas diffuse irradiance is generally underestimated (−19 W/m 2 or −14%). Mean global irradiance values where direct and diffuse irradiance errors compensate each other are very well represented (on average +9 W/m 2 or +2%).

Modification of Summertime Arctic Cloud Characteristics between a Coastal and Inland Site

Abstract Cloud characteristics at two sites on the North Slope of Alaska separated by ∼100 km have been examined for the warmer months of 2001–03 using data collected from microwave radiometers, ceilometers, rotating shadowband radiometers, and pyranometers. Clouds at the inland site, Atqasuk, were found to have approximately 26% greater optical depths than those at the coastal site, Barrow, and the ratio of measured irradiance to clear-sky irradiance was nearly 20% larger at Barrow under cloudy conditions. It is hypothesized that a significant factor contributing to these differences is the upward fluxes of heat and water vapor over the wet tundra and lakes. Support for this hypothesis is found from the behavior of the liquid water paths for low clouds, which tend to be higher at Atqasuk than at Barrow for onshore winds but not for offshore ones, from differences in sensible heat fluxes, which are small but significant over the tundra but are nearly zero over the ocean adjacent to Barrow, and from the mixing ratios, which are significantly higher at Atqasuk than at Barrow. Results from a simple model further indicate that latent heat fluxes over the tundra and lakes can account for a significant fraction of the differences in the estimated boundary layer water content between Barrow and Atqasuk.