Superenhancement of Precipitation at the Center of Tropical Islands

Abstract

We identified a canonical pattern of the diurnal cycle (DC) of precipitation on tropical islands, with exceptions for small and very large islands, using an 18-year climatology of satellite estimates. Medium-to-large islands receive more precipitation than the surrounding ocean with superenhancement of precipitation in their center respective to their periphery. Differential heating of the island surface generates a sea breeze front (SBF) propagating inland in the afternoon, triggering precipitation. SBF segments from different coasts meet in the island center, further invigorating precipitation. Small islands exhibit neither DC nor precipitation enhancement most likely because ventilation prevents the genesis of the land-sea breeze circulation. On the largest islands, the SBFs decay after sunset before reaching the center; hence, the superenhancement is not significant, also precipitation lasts longer and cloud remnants delay the DC on the next day, which we attribute to the presence of high mountains.Plain Language Summary Satellite estimates of rainfall show that medium-to-large tropical islands receive more rain than the surrounding ocean, and rainfall amounts are even higher at the center of these islands. The increased precipitation is caused by the surface and atmospheric diurnal cycle. Land heats faster than the ocean in the morning, and a sea breeze develops. The sea breeze lifts the warm and moist air over the island and the rising air condenses as it cools with ascent, forming clouds and thus rainfall. The sea breeze penetrates inland during the afternoon and meets the sea breeze from other coasts of the island in its center which further enhances rainfall. Small islands do not exhibit this behavior most likely because the wind transports warm surface air away from the island and brings cool air from the sea, preventing the sea breeze formation and the development of a diurnal cycle. The largest islands are too big for the sea breeze segments to meet in their center before sunset; thus, no superenhancement of rainfall occurs in the center of these islands.

High-resolution mesoscale wind-resource assessment of Fiji using the Weather Research and Forecasting (WRF) model

“…For the leeward and the windward sides of the larger islands, a strong diurnal cycle is observed and modelled, which can be seen in Figures 8 and 9. For the outer islands, which are relatively smaller, a weaker diurnal cycle is observed (Figure 10), which is expected for small tropical islands as ventilation prevents the genesis of the land-sea breeze circulation [40]. This small observed diurnal cycle is not correctly simulated by the WRF model, which only simulates a small early-morning increase in wind speed.…”

Weak cooling of the troposphere by tropical islands in simulations of the radiative‐convective equilibrium

“…Once mature and precipitating, the clouds further enhance rainfall over and next to the islands (Sobel et al ., 2011; Wang and Sobel, 2017; Ruppert and Chen, 2020); and sometimes they organize into Mesoscale Convective Systems (Houze et al ., 1981; Mori et al ., 2004) or excite offshore‐propagating gravity waves (Mapes and Houze, 1993; Love et al ., 2011; Coppin and Bellon, 2019; Ruppert and Zhang, 2019). Factors controlling the spatial distribution and amount of precipitation over islands, in particular interactions between convection and sea breezes, have been studied extensively (e.g., Yang and Slingo, 2001; Qian, 2008; Robinson et al ., 2008; Cronin et al ., 2015; Ulrich and Bellon, 2019). The effects of islands on the mean tropical climate has so far received less attention.…”

Detecting and Tracking Coastal Precipitation in the Tropics: Methods and Insights into Multiscale Variability of Tropical Precipitation

“…Indeed, the diurnal cycle is dominated by islands in the warm and humid Maritime Continent in the biggest category and the 8000-32 000 km 2 category. The dependence of the diurnal cycle on the size of the islands is beyond the scope of this study; it is further investigated in Ulrich and Bellon (2019).…”