Estimation of Surface Turbulent Fluxes via Variational Assimilation of Air Temperature and Specific Humidity

The heat fluxes between the land surface and overlying atmosphere strongly influence the characteristics of the planetary boundary layer. These fluxes play a significant role in the land-atmosphere interaction. In this study, screen-level air temperature and specific humidity measurements were assimilated into a coupled land surface-atmospheric boundary layer (ABL) model within a variational data assimilation (VDA) framework. The two main unknown parameters of the VDA are the neutral bulk heat transfer coefficient (C_HN) and evaporative fraction (EF). C_HN scales the sum of turbulent heat fluxes, and EF scales their partitioning. The new model takes advantage of information in the sequences of reference-level air temperature and specific humidity to partition available energy between the turbulent heat fluxes.

        The new approach is tested over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site (Kansas) where verification data on surface heat fluxes, and ABL height, potential temperature and humidity are available. The estimated heat fluxes show a good agreement with the observations. The results indicate that the developed model can effectively characterize the daily variations in the heat fluxes estimates and the assimilation scheme is able to use the implicit information in the sequences of air temperature and humidity to partition available energy among the turbulent heat fluxes. The model also can provide accurate estimates of ABL height, potential temperature, and humidity.