Measuring Vertical Heat Flux with RASS

Summary A wind‐profiling Doppler radar equipped with a radio acoustic sounding system (RASS) may be used to estimate the vertical profile of the vertical flux of heat in the atmosphere. Simultaneous measurements of the time‐varying temperature and vertical air velocity are combined to give the convective heat flux using the eddy‐correlation method. The accuracy of the estimates depends on the fundamental accuracy of the temperature and vertical velocity measurements. Also, in common with all eddy‐correlation methods, uncertainties are introduced by the need to define a suitable averaging time and to remove trends. A problem unique to RASS is the possible presence of ground and intermittent clutter at close ranges, which can cause errors in the vertical air velocity measurements. These considerations are discussed with particular reference to observations using a UHF radar wind profiler situated in an urban environment, where clutter is a serious problem. A Rank‐Order Signal Processing Algorithm (ROSPA) for recognizing and eliminating outliers in the vertical velocity, is introduced. It is explained how ROSPA uses both a minimum filter and a median filter on the velocity data. It is shown, using a comparison with nearly clutter free data from a rural site, that the filtering substantially improves the quality of the noisy urban data. The paper then compares RASS‐measured urban and rural heat flux profiles, along with the heat flux profile measured by an instrumented airplane. It is concluded that the main obstacles to RASS heat flux measurements are the effects of winds and turbulence in the boundary layer, rather than clutter. Received September 24, 1998 Revised January 27, 1999