Surface energy budget over inland water

Qianyu Zhang

To better understand water-atmosphere interactions requires direct measurements of the surface energy budget and trace gas exchange over inland waters. In this study, an eddy covariance system was used to measure turbulent fluxes of sensible (H) and latent heat (LE) in 2008 and 2009 over the Ross Barnett Reservoir (hereafter “the Reservoir” which was ice-free) in Mississippi, U.S.A., to study physical processes that control daily, intra-seasonal, seasonal, and interannual variations in the surface energy budget. Our results indicate that H and LE were distinctively out-of-phase with net radiation (Rn) on different timescales. Fueled by the previously stored heat in the water, the turbulent transfer of H and LE from the water to the atmospheric surface layer (ASL) was still substantial on nights with a negative Rn and in winters when Rn was very small. The annual means of net radiation, H, and LE for the two years were 110.6, 15.7, and 83.7 W m–2, respectively. About 81% of the Rn absorbed by the water was transferred to the atmosphere through LE and the remainder was transferred through H. On a monthly basis, the upward, positive temperature and humidity gradients, the unstable ASL, and the sufficient mechanical mixing were observed, leading to persistent positive H and LE. Intraseasonal and seasonal variations in turbulent exchanges of H and LE were strongly affected by alternative incursions of large-scale air masses brought in by different synoptic weather systems (e.g., cyclones or anticyclones) throughout the two years. Southerly winds with tropical maritime air masses (warm and humid) from the Gulf of Mexico and northerly winds with continental air masses (cool and dry) produced two distinctive atmospheric forcings for the water-air interactions. Interannual variability in the surface energy budget was modulated by variations in mean climate conditions as well as by differences in these synoptic weather events. It suggests that possible shifts in northerly and southerly winds associated with changes in synoptic weather events would have significant impacts on seasonal, annual, and interannual variations in the water surface energy budget, evaporation rates, and hydrologic processes in this region.

Surface energy budget over inland water

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