Estimation of the Cumulative Sensible Heat Flux over a Bare Soil and its Daily Mean Soil Temperature without Wind Speed Data.

Abstract

The exchange speed (CHU) for sensible heat flux is one of the most important parameters for estimating sensible and latent heat fluxes by the bulk transfer method. CHU is also important for estimating soil temperature by using the heat balance model combined with the bulk transfer method. CHU is usually treated as a function of wind speed; however, if wind speed data is not available, it is not easy to estimate an accurate and representative value using measured wind speed from another location due to the effect of the surrounding environment. In this study, we investigated the accuracy of estimating the sensible heat flux over a bare soil surface and its daily mean soil temperature by the bulk transfer method using daily mean meteorological data without wind speed. This means that CHU is treated as a constant value. We also investigated a method of parameterization of CHU on a bare soil without observed flux data. The estimation of the daily mean sensible heat flux without wind speed data has a large root mean square error (RMSE). RMSE without wind speed was 18.3 W m-2, whereas that with wind speed was 10.5 W m-2. However, the accuracy of the estimation of cumulative monthly sensible heat flux without wind speed data was almost equal to that with wind speed data. The two values of RMSE of monthly estimations with and without wind speed data were about 5 W m-2. The estimated cumulative sensible heat flux without using wind speed data, over a period of more than several days, agreed with observed sensible heat flux values. With theoretical analysis and the parameterization of CHU on a bare soil without using observed flux data, the results indicate that cumulative sensible heat flux can be estimated from routine meteorological data without wind speed. The daily mean soil temperature was estimated by the method of Hirota et al. (1995) with and without wind speed data. The accuracy of the estimation of the daily mean soil temperature without wind speed data was almost equal to that with wind speed data. The result indicates that even if wind speed data is not used, the model provides a good estimation of the daily mean soil temperature.