FLUX DETERMINATION OVER A SMOOTH SURFACE UNDER STRONGLY UNSTABLE CONDITIONS MICHIAKI SUGITA 1 , TETSUYA HIYAMA 2, NOBUHIKO ENDO 2 and SHAO-FEN TIAN 1 1Environmental Research Center, University of Tsukuba, Tsukuba, Ibaraki 305, Japan; 2DoctoraI Program in Geoscience, University ofTsukuba, Tsukuba, lbaraki 305, Japan (Received in final form 8 September, 1994) Abstract. Careful micrometeorologicalmeasurements on an empty parking lot allowed determination of the surface fluxes of sensible heat H and of momentum by applying profile equations derived from Monin-Obukhov similarity theory with two sets of the stability correction function for momentum 9 ,~ and sensible heat ~h. These fluxes were compared with reference values of H independently determined by means of an eddy correlation technique. In general, better agreement was found between H values derived from profiles with the stability functions of Brutsaert (1992) and reference H values, than when the Businger-Dyer functions were used to derive H. The disagreement in the latter comparison was especially serious under strongly unstable conditions, with the value of y = -z/L (where z is the height and L is the Obukhov length) larger than 10. A closer look at the procedure for calculating H from the profiles revealed that the large differences between the H values derived with these two different versions of the stability correction functions were caused by the small differences of the ,It h values, and not by the larger differences of the 9 ~ values. This result stems from the strong sensitivity of the resulting H values on the choice of g~h. 1. Introduction A technique to calculate surface fluxes from profiles of the relevant variable, i.e., the profile method, has been applied successfully to numerous data sets obtained under neutral to moderately unstable or moderately stable atmospheric conditions. However, not much is known about the validity of this technique under strongly unstable conditions. Indeed, the stability correction functions required to calculate surface fluxes were mostly derived on the basis of data obtained in the stability range 0 < y < 2 (where y = -z/L, z is the height above the ground and L the Obukhov length). Thus it is not clear if the conventional profile method is still applicable under strongly unstable conditions to determine surface fluxes. Although such conditions seldom occur near the Earth's surface in the normal natural environment, recent successes in the application of the technique to data measured at higher elevations of the order of z ~ 100 m (e.g., Brutsaert and Kustas, 1987; Sugita and Brutsaert, 1990; Brutsaert and Parlange, 1992) in order to determine regional surface fluxes, have made it necessary to scrutinize this issue because y tends to be larger at higher elevation. Kader and Yaglom (1990) recently presented observational data with 9 values in excess of 2, and proposed similarity functions for each of three sublayers within the surface later, namely, a dynamic sublayer for 0.04 > y > 0, a dynamic-convective sublayer for 1 > y > 0.1, and a free convection sublayer for y > 2, approximately. Their proposal for these similarity functions has subsequently been adopted by Boundary-LayerMeteorology 73: 145-158, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.