B Constant Rs

30V 15*

Figure 4. July precipitation for experiments over the African region: (a) control run and (b) constant surface albedo over Africa (from CN03).

30V 15*

Figure 4. July precipitation for experiments over the African region: (a) control run and (b) constant surface albedo over Africa (from CN03).

effect associated with cross-continental flow is termed ventilation, and the interaction of the summer monsoon heating with its circulation is termed the interactive Rodwell-Hoskins (IRH) mechanism (CNS). To study mechanisms for the poleward extent of the summer monsoon rain zones, we examined those processes that affect the monsoon rainfall: the local processes, ventilation and the IRH mechanism. The magnitude of land-ocean heating contrast affects the monsoon circulation and then the monsoon rainfall, so the processes that change the land-ocean contrast are also examined.

4.1. Local processes

The summer monsoon rainfall is initiated by seasonal marching of solar heating via Fnet. Positive Fnet is found over summer continents except for Saharan Africa, where the surface albedo is high (Fig. 3). Based on (3), positive Fnet heats the atmosphere and induces convection when the horizontal transport is small. Over regions with small Fnet, such as the Sahara, the associated rainfall should be very low, even with enough soil moisture supply. When reducing the surface albedo over the Sahara in a pair of experiments for examining the effect of Fnet, the summer monsoon rainfall over northern Africa increases, and the monsoon rain zone extends farther northward due to stronger Fnet (Fig. 4). The net surface heat flux Fsnet over land is near zero for an average longer than one day. When the low surface albedo reflects less shortwave radiation to the space, Fnet increases. The variation of Fnet over the summer continents also changes the land-ocean heating contrast that modifies the summer monsoon circulation and its feedback on the monsoon rainfall. For most regions, this feedback is stronger than the direct Fnet effect, so the direct Fnet effect is difficult to separate from other effects, except for regions with high surface albedo. This feedback associated with the monsoon circulation will be discussed in the following section.

Local soil moisture is another effect that affects the summer monsoon rainfall. Considering a steady-state moisture budget derived from (2), evaporation also contributes to the summer monsoon rainfall besides the moisture transport by vertical motion (w term) and horizontal advection (v ■ V term). Over continents, soil moisture is an important factor in the evaporation process, and it can be affected by the surface type and vegetation. A schematic

0 0

Post a comment