A new formulation for the Bowen ratio over saturated surfaces

Abstract

Analytical expressions are presented for calculating the Bowen ratio, Bo = Hs/HL, from a quantity Bo* that is derived primarily from the surface temperature Ts and the assumption that the near-surface air is saturated (but not supersaturated) with water vapor. Here Hs is the surface sensible heat flux, and HL is the surface latent heat flux. These analytical expressions obtained from 17 tabulations of Hs, HL, and Ts taken from the literature were examined. These data came from experiments over snow-covered ground and sea ice and over water in the Great Lakes, the marginal seas, and the open ocean at a variety of latitudes. Surface temperatures ranged from -41° to 29°C. Over 90% of the data fell into one of three heat flux cases: Hs > 0 and HL > 0, Hs < 0 and HL < 0, or Hs < 0 and HL > 0. From the tabulated data, a single equation that relates Bo to Bo*(Ts) for each case is developed, unifying the prediction of Bo from Ts for most saturated surfaces on earth. Last, the sensitivity of Bo to uncertainties in the quantities needed to estimate it is investigated. This analysis suggests that the analytical expressions should generally predict Bo to better than ±40%. It is concluded that predicting Bo from surface temperature alone yields good results for large-scale or longer time (e.g., daily) averages but is less accurate when both the spatial and temporal averaging are limited. Consequently, the results will probably find most use in global climate models, in coupled ice-ocean-atmosphere models, and for data assimilation and quality control.