Numerical Study on the Roles of Land Surface Evapotranspiration
in Atmospheric Water Circulation in the Meiyu Season over East Asia
Akira Kawabata ($B@nH*(B $BNh(B)
Numerical experiments were carried out using a regional atmospheric model, to clarify the roles of land surface evapotranspiration in atmospheric water circulation especially with respect to supply of water vapor in the Meiyu season. The calculations were performed for June and July 1998 when the Meiyu front was stagnated over southern and northern China, respectively. The numerical experiments consist of two kinds of calculations. One is the Standard experiment calculated with real distribution of evapotranspiration efficiency. Another is the NoLE (No Land Evapotranspiration) experiment calculated with the land surface evapotranspiration cut off.
Decrease in land surface evapotranspiration brought not only decrease in water vapor supply to the atmosphere but also increase in sensible heat flux over the land area. As a result, the southerly moisture flux into the Meiyu front was intensified especially in the lower atmosphere. Over the Meiyu frontal region, precipitation and water vapor amount in the lower atmosphere increased in the NoLE experiment for June, while decreased for July. The significant decreases were found over the other regions for both months.
It is found that the decrease in water vapor supply from the land surface can be compensated by the enhanced horizontal moisture flux and the convergence around the Meiyu frontal region. The comparison of the results for June and July suggests that evapotranspiration from the land surface over the region to the south of the Meiyu front plays an important part in water vapor supply to the Meiyu front. Since southerly moisture flux was small because of the upstream moisture convergence, land surface evapotranspiration is found to be crucial for precipitation over the region to the north of the Meiyu front.
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