A study of land-atmosphere interactions during summertime rainfall using a mesoscale model
A coupled mesoscale model (e.g., MM5V3) was used to investigate the mechanisms of land-atmosphere interaction during summertime rainfall in the southern Great Plains. Numerical experiments were conducted for two storms and for clear weather conditions using three distinct physical parameterizations of convective processes. Analysis of the dynamics of moisture and energy states in the boundary layer for all model grid cells revealed the existence of dual limit cycles in the model's phase space of Bowen ratio and relative humidity. The scaling behavior of these limit cycles is consistent with self-similarity of the second kind (i.e., in the sense of intermediate asymptotics). Specifically, the dual limit cycles suggest two distinct regimes of land-atmosphere interactions: (1) a divergent (splitting) trajectory of increasing diurnal variations of Bowen ratio and relative humidity for persistently dry weather conditions and (2) a convergent (merging) trajectory of rapidly decreasing Bowen ratio and increasing relative humidity for wet weather conditions. At the onset of rainfall the convergent cycle exhibits intermittent instability and collapses into a very limited region of high relative humidity and low Bowen ratio, the rainfall attractor. The position of the attractor in phase space is determined by surface controls of land-atmosphere interactions (e.g., soil moisture availability in our applications): That is, land surface heterogeneity determines the dynamic range of land-atmosphere interactions. Regionally, the footprint of these two regimes of land-atmosphere interactions is apparent in the correlation between Bowen ratio and the vertical distribution of relative humidity in the troposphere. In dry weather the correlation varies periodically from positive at night to negative during the day and generally does not penetrate into the upper air. Under wet conditions, however, the correlation is strong and persistently positive throughout the troposphere. These results support the argument that increased surface evaporation during rainfall affects stability conditions in the boundary layer, and thus convective activity in the model, therefore establishing a positive feedback mechanism of land-atmosphere interactions consistent with the rainfall attractor. Copyright 2002 by the American Geophysical Union.
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