Squall lines are bands of thunderstorms of hundreds of kilometers, also called quasi-linear mesoscale convective systems. One key ingredient in the organization of squall lines is the presence of cold pools below precipitating clouds. These are areas of cold air with negative buoyancy anomaly, driven by the partial evaporation of rain and concomitant latent cooling, and observed to span 10-200 km in diameter (Romps & Jeevanjee, 2016;Zuidema et al., 2017). Cold pools spread radially at the surface as gravity currents, and can thus favor upward motion and the development of new deep convective cells at their edge as described in Tompkins (2001a) and impact aggregation (Muller & Bony, 2015).
The isotopic composition of water vapor (HDO or 𝐴𝐴 𝐴𝐴 18 2 𝑂𝑂 ) evolves along the water cycle as phase changes are associated with isotopic fractionation. The isotopic composition of precipitation recorded in paleoclimate archives has significantly contributed to the reconstruction of past hydrological changes across the tropics (Cruz et al., 2009;Wang et al., 2001). Indeed, over tropical oceans, the precipitation is usually more depleted in heavy isotopes as precipitation rate increases, an observation called the amount effect (Dansgaard, 1964). In concert with the precipitation, the water vapor over tropical oceans is also more depleted as precipitation rate increases according to satellite and in situ observations (Kurita, 2013;Worden et al., 2007). Over tropical land, both the precipitation
Squall lines are known to be the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts. Also, as the magnitude of the shear increases beyond a critical shear, squall lines tend to orient themselves. The existing literature suggests that this orientation reduces incoming wind shear to the squall line, and maintains equilibrium between wind shear and cold pool spreading. Although this theory is widely accepted, very few quantitative studies have been conducted on supercritical regime especially. Here, we test this hypothesis with tropical squall lines obtained by imposing a vertical wind shear in cloud resolving simulations in radiative convective equilibrium. In the sub-critical regime, squall lines are perpendicular to the shear. In the super-critical regime, their orientation maintain the equilibrium, supporting existing theories. We also find that as shear increases, cold pools become more intense. However, this intensification has little impact on squall line orientation.Plain Language Summary A squall line is a line of thunderstorms associated with heavy precipitation and only a well-informed meteorologist would notice that these convective bands are sometimes oriented with respect to the wind direction. Yet this is the case, and in this study we seek to understand what sets tropical squall lines orientation, and why. Using numerical simulations, we show quantitatively that the orientation of the squall lines restores the equilibrium between the wind and the cold pool spreading, providing for the first time a quantitative validation of existing theories.
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