A Global Perspective on the Importance of Rossby Wave Breaking and Intense Moisture Transport for Extreme Precipitation Events

Abstract:

Extreme precipitation events (EPEs) cause frequently flooding with dramatic socioeconomic impacts in many parts of the world. Previous studies considered two synoptic-scale processes, Rossby wave breaking and intense moisture transport, typically in isolation, and their linkage to such EPEs in several regions. This study presents for the first time a global and systematic climatological analysis of these two synoptic-scale processes, in tandem and in isolation, for the occurrence of EPEs. To this end, we use ERA-Interim reanalysis data (1979-2018) and apply object-based identification methods for (i) daily EPEs, (ii) stratospheric potential vorticity (PV) streamers, and (iii) structures of high vertically integrated horizontal water vapor transport (IVT). First, the importance of these two processes is demonstrated by previously documented flood events that inflicted catastrophic impacts. Next, a climatological quantification shows that Rossby wave breaking is highly associated with EPEs in higher-latitude regions, in particular near high topography and over the Mediterranean (>85%), whereas intense moisture transport is strongly linked to EPEs over coastal zones (>95%), consistent with atmospheric-river related studies. Combined Rossby wave breaking and intense moisture transport is very relevant for EPEs in many subtropical regions (>65%), where tropical-extratropical interactions are of key importance for (heavy) rainfall. Furthermore, the relationship between the PV and IVT characteristics and the precipitation volumes show that the strength of the upper-level forcing and moisture transport intensity are intimately connected with the extreme precipitation severity. Finally, composites reveal that EPEs, linked to both synoptic-scale processes, go along with the formation of upper-level troughs and cyclogenetic processes near the surface downstream, reduced static stability beneath the upper-level forcing, and dynamical lifting ahead. This study concludes with a concept that reconciles well-established meteorological principles with the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events. The findings of this study may find application in climatic studies on the changes of precipitation extremes in a warming climate.