On Monin–Obukhov Scaling in and Above the Atmospheric Surface Layer: The Complexities of Elevated Scintillometer Measurements

Abstract

In scintillometry Monin–Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature \({(C_{T^2})}\) . In order to prevent saturation a scintillometer can be installed at an elevated level. However, in that case the observation level might be located outside the atmospheric surface layer (ASL) and thus the validity of MOST questioned. Therefore, we examine two concepts to determine the turbulent surface sensible heat flux from the structure parameter at elevated levels with data obtained at 60-m height on the Cabauw tower (the Netherlands). In the first concept (MOSTs) \({C_{T^2}}\) is still scaled with the surface flux, whereas in the second (MOSTl) \({C_{T^2}}\) is scaled with the local sensible heat flux. The \({C_{T^2}}\) obtained from both concepts is compared with direct observations of \({C_{T^2}}\) using a sonic anemometer/thermometer. In the afternoon (when the measurement height is located within the ASL) both concepts give results that are comparable to the directly observed values of \({C_{T^2}}\) . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in \({C_{T^2}}\) that occurs when the measurement height is located in the entrainment zone disqualifies the use of MOST. Second, during the morning transition, local scaling shows the correct pattern (zero flux and a minimum in \({C_{T^2}}\)) but underestimates \({C_{T^2}}\) by a factor of ten. Third, from the best linear fit a we found that the slope of MOSTl gave better results, whereas the offset is closer to zero for MOSTs. Further, the correlation between the direct observations and MOST-scaled results is low and similar for the two concepts. In the end, we conclude that MOST is not applicable for the morning hours when the observation level is above the ASL.