Effective atmospheric boundary layer temperature from longwave radiation measurements

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The atmospheric boundary layer (ABL) temperature is derived from concurrent measurements of two pyrgeometers, one standard pyrgeometer sensitive to the 3–50 μm wavelength range and one modified pyrgeometer sensitive only in the atmospheric window from 8–14 μm. By combining the measurements from the two instruments we retrieve the effective ABL temperature from the radiation emitted by the atmospheric water vapor contained in the atmospheric boundary layer. Measurements from five sites in Switzerland are analyzed, and salient features of the effective ABL temperature and its seasonal and diurnal variability are discussed. The measurements at Davos, Payerne, Zimmerwald, and Locarno‐Monti show a stable inversion layer during the night and the transition to a convective state during daylight, while at Jungfraujoch no systematic features of the effective ABL temperature are detected because of its locations on a high mountain ridge. The four lower‐altitude sites also show distinct diurnal and seasonal patterns of the ABL temperature with respect to 2 m air temperature. The largest inversion situations occur in winter at Davos, with ABL temperatures on average 5.6 K higher than the 2 m air temperatures. The amplitude of ABL variability is also largest at Davos in winter, with an average diurnal variation of 4.1 K between the early morning and the afternoon. Estimates of clear‐sky longwave downward emissions obtained by using a parametrization with the Brutsaert formula can be noticeably improved by using the effective ABL temperature instead of the 2 m air temperature.