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A transition from a predominantly offshore to an onshore rainfall phase over the west coast of India was simulated using three one-way nested domains with 12, 4, and 1.33 km horizontal grid spacing in the Weather Research and Forecasting model. The mechanism of offshore-onshore rainfall oscillation and the orographic effects of the Western Ghats are studied. A convective parametrization scheme was employed only in the 12 km domain. A trough extending offshore from the west coast facilitates offshore rainfall. This trough is absent during the onshore phase, and rainfall occurs over the coast mainly via orographic uplift by the Western Ghats. The model overestimates rainfall over the Western Ghats at all resolutions as it consistently underestimates the boundary-layer stratification along the coast. Weaker stratification weakens the blocking effect of the Western Ghats, resulting in anomalous deep convection and rainfall over its windward slopes. The 4 and 1.33 km domains simulate the offshore-to-onshore transition of rainfall but fail to capture a sufficient contrast in rainfall between land and sea compared with observations. The 12 km domain produces light rainfall, anchored along the coast, throughout the simulation period and, hence, gravely underestimates the offshore rainfall. The offshore rainfall persisted in the 4 and 1.33 km domains in a sensitivity experiment in which the Western Ghats were flattened. This suggests that orographic effects do not significantly influence offshore rainfall. In another experiment, the convective parametrization scheme in the 12 km domain was turned off. This experiment simulated the offshore and onshore rainfall phases correctly to some extent but the rainfall intensity was unrealistically high. Thus, a model with a horizontal grid spacing of 0(~ 1 km), in which convection evolves explicitly, is desired for simulating the west-coast rainfall variations. However, improvements in the representation of boundary-layer processes are needed to capture the land-sea contrast. [ABSTRACT FROM AUTHOR]

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 controlled primarily by the strength of the low‐level westerly jet. Thus, a classification based on the Froude number, F=U/NH$$ F=U/ NH $$, of the onshore flow is proposed, where, H$$ H $$ is the mountain height, U$$ U $$ is the mean wind speed, and N$$ N $$ is the mean Brunt–Väisäla frequency over depth H$$ H $$. At low F$$ F $$ (< 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$$ F $$ (> 1), 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$$ F $$, rainfall over the west coast results mainly from mechanical uplifting of the westerly winds by the Western Ghats, with no preference for a particular time of day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models. [ABSTRACT FROM AUTHOR]

The west coast of India, dominated by the Western Ghats mountain range, is among the rainiest places in the Tropics. The interaction between the land–sea contrast of the coast, the monsoonal westerlies, and the oblique mountains is subject to complex intraseasonal variability, which has not previously been explored in depth. This study investigates that variability from the perspective of the land–sea contrast, using empirical orthogonal function analysis to discern regimes of onshore and offshore rainfall over southwest India and the eastern Indian Ocean. Locally, it is found that the rainfall is most sensitive to midtropospheric humidity: when this is anomalously high, deep convection is encouraged; when this is anomalously low, it is suppressed. A moisture‐tracking algorithm is employed to determine the primary sources of the anomalously wet and dry midtropospheric air. There are important secondary contributions from low‐level vorticity and cross‐shore moisture flux. The dominant control on intraseasonal variability in coastal precipitation is found to be the Boreal Summer Intraseasonal Oscillation (BSISO): over 75% of the strongest offshore events occur during phases 3 and 4; and about 40% of the strongest onshore events occur during phases 5 and 6. The location of monsoon low‐pressure systems is also shown to be important in determining the magnitude and location of coastal rainfall. [ABSTRACT FROM AUTHOR]