Longwave radiation at the earth’s surface in Estonia; pp. 480–487Full article in PDF format | doi: 10.3176/proc.2015.4.03
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 Ll↓ exceeds the direct solar radiation Eg↓ about three times, and this ratio has an essential seasonal run. In the total upwelling radiation Ll, the infrared part is still greater, up to 92%. Comparing the measured and calculated (according to the Stefan–Boltzmann law) hourly totals of Ll for snow (emissivity ε = 0.85) we found a good linear relationship (R2 = 0.96). However, the measured totals systematically exceeded the calculated values (on average by 18%). Dependence of the downwelling infrared radiation Ll↑ on the near-surface water vapour pressure e is approximated by a power function (R2 = 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, R2 = 0.91 and for cloudless sky b = 0.25 and R2 = 0.93).
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