Evaluating the Impact of Clouds on Total Daily UVB Irradiances in Arid and Semi-arid Regions of Iran

Although the contribution of Ultra-violet (UV) radiation is about 5-7% of the whole solar energy; nevertheless, it plays an important role in regulating the biological and photochemical processes within the Earth-atmospheric system. Ultra-violet radiation is well-known for its significant influence on human health and the environment. High UV doses have negative effects on the skin (erythema (sunburn), skin cancer) and cause eye diseases and immune suppression. However, moderate UV doses have positive effects causing vitamin D production. Apart from the solar elevation, ozone and cloudiness are the main factors affecting UV level and providing significant year-to-year variability of UV radiation. The effect of clouds on UV radiation is as varied as the clouds change. Fully overcast skies lead to reductions in surface UV irradiance. On average, scattered or broken clouds also cause reductions, but short-term or localized UV levels can be larger than for cloud-free skies if direct sunlight is also present. It is noted that long-term cloud type and amount trends are largely unknown due to the relatively short data record of comprehensive cloud observations and the high variability of clouds on interannual and longer time scales. So far, most studies have focused on in-vitro impacts of UV radiation on human health and plant physiology. Unfortunately, not much research has addressed the effect of ozone and clouds distribution on total daily UVB irradiances in central arid deserts of Iran. Meanwhile, these limited investigations have used Tropospheric Ultraviolet-Visible (TUV5) radiation. The present work is aimed to evaluate the influence of clouds and ozone on daily UVB in different sky conditions.
To estimate the total daily UVB irradiances (280-315 nm), 13-year (2005-2017) historical data from 22 meteorological sites (9 provinces) were applied as the input of the TUV5 multilayer radiative transfer model. The Tropospheric Ultraviolet-Visible (TUV) model is used widely by the scientific community for applications including atmospheric photochemistry, solar radiometry, and environmental photobiology. The model calculates spectral radiance, irradiance, and actinic flux over 120-750 nm at an underlying resolution of 0.01 nm, as well as weighted spectral integrals including wavelength bands (visible, UVA, UVB, UVC), photolysis coefficients (112 reactions), and biologically active irradiances (UV index, DNA damage, vitamin D production, etc.).  Atmospheric inputs include vertical profiles of N2, O2, O3, NO2, SO2, clouds, and aerosols. The propagation of radiation through multiple atmospheric layers (concentric spherical shells for direct solar beam, plane-parallel for diffuse radiation) is computed using a fast 2-stream approximation or a multi-stream discrete ordinates scheme. Version 5.3 provides updated spectroscopic data for a number of photolysis reactions (7). The aforesaid dataset includes Total column ozone (TCO), Cloud optical depth (COD), Aerosol optical depth (AOD), and Surface albedo (SALB), which were freely extracted from ://disc.gsfc.nasa.gov satellite-based images.
TUV5 Model estimated total daily UVB radiation for three different sky conditions (Clear-sky, whole sky cover, real sky) and the results compare to each other. The maximum daily UVB for clear-sky and overcast conditions (whole cloud cover) was found in summer and for the south and south-east of the region (Kerman, Fars, and Yazd provinces). The observed decline in daily UVB due to the clouds varied from 33% in summer to 67% in autumn, which highlights the importance of total cloud cover (overcast) in reducing the UVB radiation in the study sites. For the real sky condition (all-sky), the maximum daily UVB irradiances were found in southern parts of the region for most of the seasons. However, as the Indian summer Monsoon result, the maximum UVB has slightly moved toward the northwest of the region. Meanwhile, the inter-comparison of daily UVB maps with total column ozone (TCO), cloud optical depth (COD), aerosol optical depth (AOD), and surface albedo (SALB) maps show that the geographical position of maximum UVB radiation in southern provinces is in good agreement with the total column ozone and cloud optical depth. In this regard, variations of monthly SALB and AOD have less influence on the determination of displacing the maximum UVB.
Results of the present work highlight the high biological risk of solar UVB irradiances during clear-sky days over the study region. For full cloud cover (overcast condition), the maximum and minimum UVB are observed in the south and northeast of the region, respectively. A relative comparison of total daily UVB in clear-sky conditions versus the UVB of overcast conditions highlights the fact that clouds can significantly reduce the biological risk from 33% in summer to 67% in autumn. The UVB reduction by clouds is more pronounced during cold seasons due to the combined interaction of larger solar zenith angle (lower sun angle) with clouds and ozone.