Influence of Local Land-Surface Processes on the Indian Monsoon: A Numerical Study

Abstract

Twelve July integrations were made with the GLAS (Goddard Laboratory for Atmospheres) GCM (General Circulation Model) to investigate the influence of changes in the land-surface fluxes over the Indian subcontinent on the monsoon circulation and rainfall. The runs consist of an ensemble of three integrations for each of four separate cases: i) a control, ii) an experiment with increased land-surface albodo, iii) an experiment with increased land-surface albedo and reduced land-surface roughness, and iv) an experiment with increased land-surface albedo, reduced surface roughness and no evapotranspiration. All the prescribed land-surface anomalies were limited to the Indian region. An intercomparison of the ensemble means of monthly fields produced by the experiments with those of the control showed that the Indian Monsoon was significantly weakened by both the increase of surface albedo and by the reduction in surface roughness. Higher surface albedo reduced the monsoon rainfall in conformity with Charney's hypothesis; the low surface roughness made the horizontal transport of moisture in the PBL (planetary boundary layer) more westerly, which reduced the cross-isobaric moisture convergence and hence the rainfall over northwestern India while correspondingly increasing it over China. The curl of surface stress divided by the Coriolis parameter (k· ? ? τs)/f represents the boundary layer convergence. There is a remarkable correspondence between changes of this field and rainfall for all the experiments. Since the magnitude of prescribed changes in surface albedo and surface roughness could plausibly be produced by deforestation, the results suggest that major changes in the tall natural vegetation over the Indian subcontinent would have a significant influence on its July rainfall. The last experiment delineated the role of evapotranspiration over India. It was found that the rainfall was essentially unaltered by the absence of evapotranspiration, because the increased moisture convergence produced by the enhanced sensible heating of the PBL largely compensated for the reduction in evapotranspiration.