The effect of change in surface roughness on the precipitation regime in the coastal area of NE-Brazil
1997
Roth, R.
The NE of Brazil is one of the earth's problematic semiarid areas. However, in the coastal area along the coast from Natal to Salvador precipitation is high enough to allow for a fully developed tropical vegetation. It is shown that this enhancement of annual precipitation is due to the dynamic convergence within the planetary boundary layer and that this effect can quantitatively be evaluated by simple model considerations. To do this the mass flux perpendicular to the coastline is calculated within the lower troposphere for an undisturbed PBL (Planetary Boundary Layer) over land and over sea, by using the resistance law of the PBL and by integrating the equation of monion between the bottom and some height above the PBL. By using the equation of continuity the mean vertical motion due to the coastal convergence can be calculated. The relation of the calculated total amount of lifting near the coast on the rate of precipitation is calculated. Compared with the increase of precipitation due to orography it gives the same increase in precipitation per meter of lifting in the coastal area quantitatively and since this is strongly related to the roughness due to the vegetation along the coast it can be shown that deforestation — for instance to grow sugar-cane — in this area the annual amount of precipitation will decrease considerably. Once the coastal area will have been transformed to a desertlike structure, the coastal maximum of precipitation will diminish at all.
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