Longwave radiation at the earth’s surface in Estonia

Abstract

Solar radiation has been continuously measured in Estonia since 1950, but the recordings of longwave radiation began only about ten years ago. This paper presents the first description of the characteristic features of up-and downwelling longwave radiation in the Baltic Sea region. In the radiation balance the longwave fluxes have an important role. In the annual totals of radiation incident upon the ground surface in Estonia, the longwave atmospheric downwelling radiation L l  exceeds the direct solar radiation E g  about three times, and this ratio has an essential seasonal run. In the total upwelling radiation L l , the infrared part is still greater, up to 92%. Comparing the measured and calculated (according to the Stefan-Boltzmann law) hourly totals of L l  for snow (emissivity ε = 0.85) we found a good linear relationship (R 2 = 0.96). However, the measured totals systematically exceeded the calculated values (on average by 18%). Dependence of the downwelling infrared radiation L l  on the near-surface water vapour pressure e is approximated by a power function (R 2 = 0.73). This is in good accordance with the results of studies carried out at other geographical sites. The influence of clouds on the fitted power function is noteworthy. Separate analysis of the hours with full cloudiness of low clouds and the cloudless hours confirmed the validity of the power function. However, a difference was found in their parameters (for overcast sky the exponent b = 0.20, R 2 = 0.91 and for cloudless sky b = 0.25 and R 2 = 0.93).

Evaluation of Radiation Components in a Global Freshwater Model with Station-Based Observations

“…However, here also, the small number of stations and months available for upward radiation components data (Table 2), as well as the small footprint for upward radiation measurement, limits the assessment. However, currently, the emissivity value is kept constant within WaterGAP which is a problem especially when land surface is covered by snow which influences emissivity values [52][53][54]. It should, therefore, be tested if a snow-cover-dependent emissivity value leads to improvements for WaterGAP calculation of L↑.…”