Water molecules are made up of one oxygen atom and two hydrogen atoms. But some water molecules are naturally heavier than others, containing heavier oxygen or hydrogen atoms than usual. The relative proportion of water molecules of different weights can be measured in the rain, and recorded over hundreds or thousands of years in cave speleothems (stalagtites and stalagmites). It has been observed that in the largest storms, such as cyclones or squall lines, the rain is poorer in heavy molecules. Several studies have exploited this property to reconstruct past variations in the frequency of tropical cyclones or squall lines, based on compositional variations observed in speleothems. We tried to understand why the rain is more depleted in heavy molecules in tropical cyclones and squall lines. To do this, we use for the first time high-resolution simulations (2-4 km horizontal) of the internal dynamics of tropical cyclones and squall lines, and of their water molecule composition. We show that it is mainly rain evaporation and rain-vapor exchanges that deplete water vapor and rain of heavy molecules. This result reinforces our confidence in the reconstructions of past variations in the frequency of tropical cyclones and squall lines from speleothems.
This work was published in J. Adv. Model. Earth Syst.