Extreme Precipitation in Tropical Squall Lines

Squall lines are substantially influenced by the interaction of low‐level shear with cold pools associated with convective downdrafts. Beyond an optimal shear amplitude, squall lines tend to orient themselves at an angle with respect to the low‐level shear. While the mechanisms behind squall line orientation seem to be increasingly well understood, uncertainties remain on the implications of this orientation. Roca and Fiolleau (2020, https://doi.org/10.1038/s43247-020-00015-4 ) show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. This article investigates the influence of the interaction between low‐level shear and squall line outflow on squall line generated precipitation extrema in the tropics. Using a cloud resolving model, simulated squall lines in radiative convective equilibrium amid a shear‐dominated regime (super optimal), a balanced regime (optimal), and an outflow dominated regime (suboptimal). Our results show that precipitation extremes in squall lines are 40% more intense in the case of optimal shear and remain 30% superior in the superoptimal regime relative to a disorganized case. With a theoretical scaling of precipitation extremes (C. Muller & Takayabu, 2020, https://doi.org/10.1088/1748-9326/ab7130 ), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its change in vertical mass flux (dynamic component). The reduction of dilution by entrainment explains half of this change, consistent with Mulholland et al. (2021, https://doi.org/10.1175/jas-d-20-0299.1 ). The other half is explained by increased cloud‐base velocity intensity in optimal and superoptimal squall lines. Squall lines are bands of clouds and thunderstorms spanning hundreds of kilometers, also called quasi‐linear mesoscale convective systems. These systems are associated with extreme weather conditions, including extreme rainfall rates. To better understand and therefore predict this high impact phenomenon, this study investigates the physical processes leading to enhanced precipitation rates when clouds are organized into squall lines, using idealized high‐resolution simulations. Interestingly, the dynamics of squall lines, notably their wind structures, are found to play a key role in setting the intensity of extreme rainfall rates. Precipitation extremes are enhanced by about 30%–40% in optimal and superoptima