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

Abstract

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, 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), 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 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.

Unravelling the mechanism of summer monsoon rainfall modes over the west coast of India using model simulations

“…The latest IMERG product places the maximum rainfall correctly over the Western Ghats slopes (e.g., Prakash and Srinivasan (2021); Phadtare et al . (2022)), but it still underestimates the intensity of heavy rainfall episodes ($$$ > $$$25 mm$$$ \cdotp $$$hr$$$ {}^{-1} $$$) compared with the rain‐gauges (Murali Krishna et al ., 2017). Rojas et al .…”

“…Note that, ideally, $$$ F $$$ should be calculated away from the orography using the upstream undisturbed values of $$$ U $$$ and $$$ N $$$. The blocking distance of the Western Ghats extends offshore by 150–300 km (Phadtare et al ., 2022). Mangalore is well within the blocking region of the Western Ghats.…”

“…Diurnal cycle Phadtare et al (2022) showed that the rainfall during the offshore regime is controlled by a strong diurnal cycle, whereas that during the onshore regime has a weak diurnal cycle. Therefore, here we analyse whether the model can simulate the observed diurnal cycle of rainfall during the offshore mode.…”

“…A recent study by Phadtare et al . (2022) showed that, when the incident low‐level flow over the west coast of India is classified according to $$$ F $$$, the classification leads to the offshore–onshore rainfall pattern – the low $$$ F $$$ values are associated with the offshore mode and high $$$ F $$$ values with the onshore mode. Further, they show that the offshore mode is characterized by strong land–sea breeze variations and greater control of the local diurnal cycle over the west‐coast rainfall.…”

“…Several idealized simulation experiments (Chu and Lin, 2000;Jiang, 2003;Chen and Lin, 2005a;Chen and Lin, 2005b;Reeves and Lin, 2007;Miglietta and Rotunno, 2009) have shown that precipitating systems remain upstream of the orography in the blocked case and precipitation occurs over the orographic slopes (heavy) and the lee region (light to moderate) in the unblocked case. A recent study by Phadtare et al (2022) showed that, when the incident low-level flow over the west coast of India is classified according to F, the classification leads to the offshore-onshore rainfall pattern -the low F values are associated with the offshore mode and high F values with the onshore mode. Further, they show that the offshore mode is characterized by strong land-sea breeze variations and greater control of the local diurnal cycle over the west-coast rainfall.…”