Are increases in extreme accumulated precipitation over tropical oceans with SST warming due to the slowing of deep convective systems?

In the tropics, heavy daily accumulated precipitation has been shown to be associated with the most organized convective systems. Over the oceans, a simple scaling of the extreme precipitation explains the changes of the high percentiles with SST at a rate close to that predicted from thermodynamics and Clausius Clapeyron. This study aims at clarifying the sensitivity of the deep convective systems relevant to extreme precipitation with respect to SST and to reconcile the object-oriented and the grid box perspectives. For that purpose, two satellite-based precipitation products (IMERG, and GIRAFE) are used together with SST from OSTIA and the deep convective cloud properties from CACATOES. The CACATOES database, a Level-3 product derived from TOOCAN, is available for the 9-year period (2012–2020) over the entire tropical belt (30°S–30°N) on a 1° daily grid.

Our results show that the increase in extreme precipitation fraction is associated with longer lived, larger and slower propagating DCS as the SST warms from 300K to 302.5K. As a consequence, the residence time of the mean system over extreme precipitation grid boxes increases with SST. While it may qualitatively explain the increased accumulated precipitation extreme, akin to the slowdown trend of landing hurricanes, it remains to be shown whether this hypothesis is up to quantitative analysis.

Preliminary investigations of the scaling of the precipitation fraction with SST nevertheless reveals a product-dependence that needs to be explored further using a larger ensemble of satellite precipitation products. The physical robustness of the hypothesis is also further analysed by looking at the thermodynamical and dynamical environment, using ERA-5, of the Deep Convective System relevant to the extreme precipitation grid boxes. A moistening of the environment, consistent with the precipitation increase is found. The changes in the dynamical environment will be further discussed at the conference.