SURFACE FLUX PARAMETERIZATION IN THE TIBETAN PLATEAU KUN YANG ⋆ Core Research for Evolutional Science and Technology (CREST), the University of Tokyo, Tokyo, Japan TOSHIO KOIKE and DAWEN YANG The University of Tokyo, Tokyo, Japan (Received in final form 29 January 2002) Abstract. This study investigates some basic aspects related to surface-flux parameterization in the Tibetan Plateau, based on the measurement at three sites. These sites are essentially flat and covered by very sparse and short grasses in the monsoon season. The main contributions include: (1) an optimization technique is proposed to estimate aerodynamic roughness length based on wind and temperature profiles. The approach is not sensitive to random measurement errors if the number of data samples is large enough. The optimized values reasonably vary with surface characteristics. (2) At the three sites, kB -1 (the logarithm of the ratio of aerodynamic roughness length to thermal roughness length) experiences seasonal and diurnal variations in addition to a dependence on surface types. The mean values for the individual sites vary over a range of 2.7 to 6.4 with large standard deviations. (3) A formula for estimating the value of kB -1 is proposed to account for the effect of seasonal variation of aerodynamic roughness length and diurnal variation of surface temperature. With the formula, the flux parameterization with surface temperature estimates sensible heat flux better than profile parameterization for all the sites. Keywords: GAME-Tibet, Roughness length, Surface-flux parameterization, Surface temperature, Tibetan Plateau. 1. Introduction As the largest and highest plateau in the world, the Tibetan Plateau affects the atmo- spheric circulation through land surface processes in addition to direct topographic influences. On the plateau, the temperature and energy partition experience not only a strong diurnal variation due to intensive solar radiation but also dramatic seasonal variations due to frequent rainfall during the Asian summer monsoon period (Ye and Gao, 1979; Zhou et al., 2000). Because the heat and water vapour from the surface are directly transported vertically to warm and moisten the middle troposphere over the Plateau, the land-atmosphere interactions not only affect the development of the local boundary layer but also change the horizontal gradient of temperature and moisture at a continent scale. Therefore, the energy and water ⋆ Corresponding author and address: Dr. Kun Yang, River Lab., Dept. of Civil Engineering, Uni- versity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: yangk@hydra.t.u-tokyo.ac.jp Boundary-Layer Meteorology 116: 245–262, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.