The Montreal Protocol is working. Concentrations of major ozone-depleting substances in the atmosphere are now decreasing, and the decline in total column amounts seen in the 1980s and 1990s at mid-latitudes has not continued. In polar regions, there is much greater natural variability. Each spring, large ozone holes continue to occur in Antarctica and less severe regions of depleted ozone continue to occur in the Arctic. There is evidence that some of these changes are driven by changes in atmospheric circulation rather than being solely attributable to reductions in ozone-depleting substances, which may indicate a linkage to climate change. Global ozone is still lower than in the 1970s and a return to that state is not expected for several decades. As changes in ozone impinge directly on UV radiation, elevated UV radiation due to reduced ozone is expected to continue over that period. Long-term changes in UV-B due to ozone depletion are difficult to verify through direct measurement, but there is strong evidence that UV-B irradiance increased over the period of ozone depletion. At unpolluted sites in the southern hemisphere, there is some evidence that UV-B irradiance has diminished since the late 1990s. The availability and temporal extent of UV data have improved, and we are now able to evaluate the changes in recent times compared with those estimated since the late 1920s, when ozone measurements first became available. The increases in UV-B irradiance over the latter part of the 20th century have been larger than the natural variability. There is increased evidence that aerosols have a larger effect on surface UV-B radiation than previously thought. At some sites in the Northern Hemisphere, UV-B irradiance may continue to increase because of continuing reductions in aerosol extinctions since the 1990s. Interactions between ozone depletion and climate change are complex and can be mediated through changes in chemistry, radiation, and atmospheric circulation patterns. The changes can be in both directions: ozone changes can affect climate, and climate change can affect ozone. The observational evidence suggests that stratospheric ozone (and therefore UV-B) has responded relatively quickly to changes in ozone-depleting substances, implying that climate interactions have not delayed this process. Model calculations predict that at mid-latitudes a return of ozone to pre-1980 levels is expected by the mid 21st century. However, it may take a decade or two longer in polar regions. Climate change can also affect UV radiation through changes in cloudiness and albedo, without involving ozone and since temperature changes over the 21st century are likely to be about 5 times greater than in the past century. This is likely to have significant effects on future cloud, aerosol and surface reflectivity. Consequently, unless strong mitigation measures are undertaken with respect to climate change, profound effects on the biosphere and on the solar UV radiation received at the Earth's surface can be anticipated. The future remains uncertain. Ozone is expected to increase slowly over the decades ahead, but it is not known whether ozone will return to higher levels, or lower levels, than those present prior to the onset of ozone depletion in the 1970s. There is even greater uncertainty about future UV radiation, since it will be additionally influenced by changes in aerosols and clouds.

/pdf/changes-in-biologically-active-ultraviolet-radiation-346ewjl0ou.pdf

Changes in biologically-active ultraviolet radiation reaching the Earth's surface

http://solar.ucsd.edu/urquhart/urquhart_Intra-hour%20forecasting%20ASES.pdf

Intra-hour forecasting with a total sky imager at the UC San Diego solar energy testbed

A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of skyimaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.

/pdf/retrieving-cloud-characteristics-from-ground-based-daytime-3og2qjaejb.pdf

Retrieving Cloud Characteristics from Ground-Based Daytime Color All-Sky Images

https://escholarship.org/content/qt5dk0b8t5/qt5dk0b8t5.pdf?t=q9piey

History and trends in solar irradiance and PV power forecasting: A preliminary assessment and review using text mining

. The recently increasing development of whole sky imagers enables temporal and spatial high-resolution sky observations. One application already performed in most cases is the estimation of fractional sky cover. A distinction between different cloud types, however, is still in progress. Here, an automatic cloud classification algorithm is presented, based on a set of mainly statistical features describing the color as well as the texture of an image. The k-nearest-neighbour classifier is used due to its high performance in solving complex issues, simplicity of implementation and low computational complexity. Seven different sky conditions are distinguished: high thin clouds (cirrus and cirrostratus), high patched cumuliform clouds (cirrocumulus and altocumulus), stratocumulus clouds, low cumuliform clouds, thick clouds (cumulonimbus and nimbostratus), stratiform clouds and clear sky. Based on the Leave-One-Out Cross-Validation the algorithm achieves an accuracy of about 97%. In addition, a test run of random images is presented, still outperforming previous algorithms by yielding a success rate of about 75%, or up to 88% if only "serious" errors with respect to radiation impact are considered. Reasons for the decrement in accuracy are discussed, and ideas to further improve the classification results, especially in problematic cases, are investigated.

/pdf/automatic-cloud-classification-of-whole-sky-images-4dhgm7pigs.pdf

Automatic cloud classification of whole sky images

In Lauder, Central Otago, New Zealand, two all-sky imaging systems have been in operation for more than 1 yr, measuring the total, opaque, and thin cloud fraction, as well as indicating whether the sun is obscured by clouds. The data provide a basis for investigating the impact of clouds on the surface radiation field. The allsky cloud parameters were combined with measurements of global, direct, and diffuse surface solar irradiance over the spectral interval from 0.3 to 3 mm. Here, the results of ongoing analysis of this dataset are described. As a reference for the magnitude of the cloud influence, clear-sky irradiance values are estimated as a simple function of solar zenith angle and the earth‐sun distance. The function is derived from a least squares fit to measurements taken when available cloud images show clear-sky situations. Averaged over a longer time period, such as 1 month, cloud fraction and surface irradiance are clearly negatively correlated. Monthly means in the ratio of the measured surface irradiance to the clear-sky value had a correlation coefficient of about 20.9 with means of cloud fraction for the months from July 2000 to June 2001. In the present work reductions in the surface irradiance and situations in which clouds cause radiation values to exceed the expected clear-sky amount are analyzed. Over 1 yr of observations, 1-min-averaged radiation measurements exceeding the expected clearsky value by more than 10% were observed with a frequency of 5%. In contrast, a reduction of more than 10% below estimated clear-sky values occurred in 66% of the cases, while clear-sky irradiances (measured irradiance within 610% of estimated clear-sky value) were observed 29% of the time. Low cloud fractions frequently lead to moderate enhancement, because the sun is often unobscured and the clouds are brighter than the sky that they hide. As cloud fraction increases the sun is likely to be obscured, causing irradiance values to fall well below clear-sky values. However, in the case of unobscured sun, there is a tendency for strongest enhancements when cloud fractions are highest. Enhancements, especially at high solar zenith angle, are also often observed in association with thin clouds.

Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

This assessment, by the United Nations Environment Programme (UNEP) Environmental Effects Assessment Panel (EEAP), one of three Panels informing the Parties to the Montreal Protocol, provides an update, since our previous extensive assessment (Photochem. Photobiol. Sci., 2019, 18, 595-828), of recent findings of current and projected interactive environmental effects of ultraviolet (UV) radiation, stratospheric ozone, and climate change. These effects include those on human health, air quality, terrestrial and aquatic ecosystems, biogeochemical cycles, and materials used in construction and other services. The present update evaluates further evidence of the consequences of human activity on climate change that are altering the exposure of organisms and ecosystems to UV radiation. This in turn reveals the interactive effects of many climate change factors with UV radiation that have implications for the atmosphere, feedbacks, contaminant fate and transport, organismal responses, and many outdoor materials including plastics, wood, and fabrics. The universal ratification of the Montreal Protocol, signed by 197 countries, has led to the regulation and phase-out of chemicals that deplete the stratospheric ozone layer. Although this treaty has had unprecedented success in protecting the ozone layer, and hence all life on Earth from damaging UV radiation, it is also making a substantial contribution to reducing climate warming because many of the chemicals under this treaty are greenhouse gases.

/pdf/environmental-effects-of-ozone-depletion-uv-radiation-and-1v1beswx0x.pdf

J. B. Liley

Papers

Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

Environmental effects of ozone depletion, UV radiation and interactions with climate change: UNEP Environmental Effects Assessment Panel, update 2017