Land-breeze Circulations: Are they Always Significant?


Land Surface heterogeneity caused by deforestation may induce significant land-breeze circulations and associated transport of heat and moisture.  Theoretical analyses (Wang et al., 1996 and 1998) have shown that the development of these mesoscale circulations  in a dry atmosphere is sensitive to the synoptic forcing such as atmospheric stability and large scale wind.

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Figure1: Land cover patterns over the simulations domain in Rond˘nia,
Amazonia.  Dark areas represent deforested surface.

It has been argued that for a moist atmosphere, the mesoscale circulations over the surface with variable land cover could trigger moist convection and enhance rainfall.  A recent numerical study suggests that the effect of the land-breeze circulations on the distribution of local rainfall and cloudiness over Amazonia is detectable only during a certain period of year (between the rainy and wet season or break-period) when the synoptic forcing is rather weak.  Even though the mesoscale circulation develops to a significant level over the deforested areas, its impact is limited to triggering low level clouds that make little contribution to local rainfall (Wang et al., 1999, submittled to Journal of Hydrometeorology).

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Figure 2:  Spatial distribution of afternoon (2:00pm Nov 4 1990 local time)
cloudiness (vertically integrated liquid cloud water in kg m^{-2}) for (a) control,
land cover is uniform forest, and (b) deforestation run over variable land cover. 
Break periodsimulation under the condition of reduced synoptic wind.


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Figure 3:  Space-time distribution of mesoscale heat flux (W m^{-2}).  (a) control run
corresponding to Figure 2(a) , and (b) deforestation run corresponding to Figure 2(b).

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