F2. Evaporation from Lake Kasumigaura: Bulk Coefficients and Spatial Distribution of Latent Heat Flux
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Michiaki, Sugita; Zhongwang, Wei; Aiko, Miyano; Hiroya, Ikura
Study of lake evaporation includes various aspects of Dr Brutsaert’s interests such as the exchange processes on water surface, the development of internal boundary layer, estimation of evaporation rate, among others. In the present study, issues on air-water interaction were first revisited. Secondly, horizontal distribution of latent heat fluxes over the lake surface was estimated to study its variability. For these purposes, 10 Hz water vapor, temperature, and wind velocity data have been obtained at the Koshin Observatory located near the center of Lake Kasumigaura, the second largest lake in Japan (220 km2 with mean depth of 4 m) since June of 2007. The corresponding fluxes were determined by applying the eddy correlation method to estimate the bulk coefficients. Agreement and disagreement with previous studies on the bulk transfer coefficients were identified. Also, the influence of lake current, wave, gustiness was investigated. Based on the derived functional form for the bulk coefficient, horizontal distribution of latent heat flux was estimated by first deriving a 100-m grid map of air temperature, wind speed and humidity over the lake area from interpolation of the observed data at meteorological stations in and around Lake Kasumigaura. The bulk method was then applied to each grid to derive latent heat flux every six hours over a year, by assuming the same surface temperature as that at the Koshin Observatory. This was acceptable as the satellite infrared images indicated the presence of quasi-uniform surface water temperature, presumably to reflect well-mixed water body of the shallow lake. The results indicate larger fluxes at the lake center with smaller values near lake shores, mainly to reflect wind speed differences. In comparison, humidity and air temperature is of lessor importance to cause latent heat variability. The mean evaporation over the entire lake surface was found to be 976 (±56) mm/y, while that at the Koshin observatory was 878 mm/y.
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