Features of transparency of the main types of clouds in the UV region

It is known that the variability of the UV flux reaching the underlying surface is affected by such factors as the total ozone content, cloudiness, aerosol, some minor gas components, albedo of the underlying surface and other factors. At the same time, the main contribution to the change in the incident UV flux, especially its long-wave part, where the influence of ozone is minimal, is made by cloudiness.

Earlier, estimates of the variability of the atmospheric transparency in the UV long-wave range (at 380 nm) were obtained for Tomsk for the so-called "fair weather" conditions and meteorological conditions associated with the presence of a large amount of smoke from forest fires in the atmosphere. However, estimates of the influence of clouds of the main forms on the variability of the atmospheric transparency in the above-mentioned spectral range have not been previously carried out. Therefore, the purpose of this study is to obtain these estimates, analyze them and compare them with similar estimates for "fair weather" conditions.

To estimate the spectral transparency of the atmosphere due to clouds of the main forms, we used the data of the NILU-UV-6T multichannel filter radiometer obtained at the IMCES SB RAS geophysical observatory in 2006–2020. The radiometer measures the irradiance at wavelengths of 305, 312, 320, 340, 380, and 400–700 nm. The software supplied with the device allows calculating the relative transparency of the atmosphere at a wavelength of 340 or 380 nm due to clouds and aerosol. We use 380 nm.

Information on clouds and meteorological phenomena was used from the Tomsk meteostation located 6 km from the observatory. To assess the influence of a particular cloud form on transparency, a selection of cases was made with the presence of only one of the main cloud forms at the observation time at the meteorological station in the absence of clouds of other forms. An additional condition when selecting cases with cumulonimbus, nimbostratus and stratified clouds was the presence of precipitation (downpours, continuous and drizzling, respectively). The calculation was based on the average minute transparency values ​​for 30-min intervals relative to the meteorological time, i.e. 00 UTC±15 min, 03 UTC±15 min, etc., when clouds of specific forms were observed. The following characteristics were adopted as criteria for “fair weather” conditions during the comparative analysis of transparency variability: cloud count no more than 5 points, no low-level clouds and no vertical development, no thunderstorms, precipitation, fog, haze, snowstorms, dust storms and smoke, average surface wind speed no more than 6 m/s.

As a result of the study, estimates were obtained for the variability and repeatability of the relative transparency of the atmosphere in the UV long-wave range (380 nm) in the presence of clouds of the main forms. It was determined that the average and median transparency values ​​in the presence of clouds are significantly lower than the values ​​characteristic of “fair weather” conditions. The analysis showed that with a decrease in the height of the lower cloud boundary, a decrease in transparency is observed (R=0.74). The distribution of transparency repeatability in Sc, Ac and Cc is bimodal. The dispersion of transparency values ​​for most clouds, except for St and Ns, is higher than for “fair weather” conditions.

The work was carried out within the framework of the state assignment of the IMCES SB RAS.