Recent trends in surface sensible heat flux on the Tibetan Plateau

Abstract

Over the last three decades, the Tibetan Plateau has exhibited a significant increase in air temperature and a significant decrease in wind speed. How the surface heat source has changed is an important issue in monsoon research. Based on routine meteorological data, this study investigates the differences between methods for estimating trends in surface sensible heat flux on the Tibetan Plateau for the period 1984–2006. One is a physical method based on micro-meteorological theory and experiments, and takes into account both atmospheric stability and thermal roughness length. The other approach includes conventional empirical methods that assume the heat transfer coefficient is a constant value or a simple function of wind speed. The latter method is used widely in climatologic studies. Results from the physical method show that annual mean sensible heat flux has weakened by 2% per decade, and flux seasonal mean has weakened by ∼2%–4% except in winter. The two commonly used empirical methods showed high uncertainties in heat flux trend estimates, although they produced similar climatologies. Annual mean heat flux has weakened by 7% per decade when a fixed transfer coefficient is used, whereas the trend is negligible when the transfer coefficient is assumed a function of wind speed. Conventional empirical methods may therefore misrepresent the trend in sensible heat flux.