Characteristics of radiative heat transfer in the atmospheric surface layer from the results of direct measurements

Results of direct measurements of the long-wavelength (LW) radiative heat influx (RHI) in the atmospheric surface layer (ASL) are presented. These measurements were performed in August 2003 at the IAP RAS base in Tsimlyansk under the conditions of unstable and stable stratification during a weak wind and a cloudless sky and under nonsteady conditions during cumulus cloudiness in the daytime. The underlying surface was dry steppe with spars grass. The in situ RHI measurements were performed with an original optoacoustic receiver having a quasi-spherical angle of view at heights from 0.15 to 4 m. It is shown that the radiative heating in the ASL was many times the actual heating, especially during near-noon hours. In the daytime, the radiative heating attained its maximum at the heights of measurements 0.15–1 m and decreased with height. The radiative heating at these heights in the near-noon hours was on average about 20 K/h, attaining 60 K/h under a cloudless sky and a weak wind. Under inversion stratification, the radiative cooling usually exceeded the actual cooling, amounting on average from 0 to ?8 K/h and changing with height only slightly. Periods with close (in phase) fluctuations of the radiative and actual cooling, sometimes changing to heating, were observed during the night. Regression equations, showing a high correlation between the RHI values at the heights of measurements 0.5 and 1 m and the soil-air temperature differences at the height of measurements, are obtained for different heights. The diurnal mean RHI profiles are characterized by a heating on the order of several K/h in the lower part of the layer of measurements, which decreases with height and changes to cooling at heights of up to 4 m. A change in the effective radiation with height in the layer of measurements, which was obtained through the summation of RHI values at several heights, was significant, attaining on average ?25 W/m² in the near-noon hours and +10 W/m² in the evening hours. The nonradiative (turbulent) heat influx, obtained as the difference between the rates of actual and radiative temperature variations measured in situ, decreased the radiative heating in the daytime many times. The main sources of error in direct RHI measurements are estimated.