Froude-number-based rainfall regimes over the Western Ghats mountains of India

The variations in the character of monsoonal rainfall over the Western Ghats region on the west coast of India are studied using radiosondes, satellite observations and reanalysis products. Summer monsoon rainfall over this region occurs in alternate offshore and onshore phases. It is shown that these phases are primarily controlled by the strength of the low-level westerly jet. Thus, a classification based on the Froude number, F = U/NH, of the onshore flow is proposed, where, H is the mountain height, U is the mean wind speed and N is the mean Brunt-Väisäla frequency over depth H. At low F (< 0.5), onshore winds are weak and the diurnal thermal fluctuation over the orography is strong; the land-sea and mountain-valley circulations are enhanced leading to a stronger diurnal control over the rainfall. A nocturnal offshore propagation of rainfall from the west coast is seen during this phase. Rainfall over the rainshadow region to the east of the Western Ghats also increases due to a weaker lee effect while it decreases over the Western Ghats due to a greater blocking effect. At high F (> 1), the orographic blocking of the low-level winds is weak. Thus, rainfall is enhanced over the Western Ghats and reduced over the rainshadow region due to a stronger lee effect. In this phase, the diurnal thermal fluctuation over the orography is weak. The bulk Richardson number is less than 1 suggesting a dominance of vertical wind shear over the buoyancy forces. The level of free convection and convective inhibition over the west coast are also very low. Hence, at high F, rainfall over the west coast mainly results from the mechanical uplifting of the westerly winds by the Western Ghats with no preference to a particular time of the day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models.