A global climatological perspective on the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events
Authors: de Vries Andries Jan 
Published: March 2021

Abstract. Extreme precipitation events (EPEs) frequently cause
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 40-year ERA-Interim reanalysis data (1979–2018)
and apply object-based identification methods for (i) daily EPEs, (ii)
stratospheric potential vorticity (PV) streamers as indicators of Rossby
wave breaking, and (iii) structures of high vertically integrated horizontal
water vapour transport (IVT). First, the importance of these two
synoptic-scale processes is demonstrated by case studies of previously
documented flood events that inflicted catastrophic impacts in different
parts of the world. Next, a climatological quantification shows that Rossby
wave breaking is associated with >90 % of EPEs over central
North America and the Mediterranean, whereas intense moisture transport is
linked to >95 % of EPEs over many coastal zones, consistent
with findings of atmospheric river-related studies. Combined Rossby wave
breaking and intense moisture transport contributes up to 70 % of EPEs in
several subtropical and extratropical regions, including (semi)arid desert
regions where tropical–extratropical interactions are of key importance for
(heavy) rainfall. Odds ratios of EPEs linked to the two synoptic-scale
processes suggest that intense moisture transport has a stronger association with the occurrence of EPEs than Rossby wave breaking. Furthermore, the
relationship between the PV and IVT characteristics and the precipitation
volumes shows that the depth of the wave breaking and moisture transport
intensity are intimately connected with the extreme precipitation severity.
Finally, composites reveal that subtropical and extratropical EPEs, linked
to Rossby wave breaking, go along with the formation of upper-level troughs
and cyclogenetic processes near the surface downstream, reduced static
stability beneath the upper-level forcing (only over water), and dynamical
lifting ahead (over water and land). This study concludes with a concept
that reconciles well-established meteorological principles with the
importance of Rossby wave breaking and intense moisture transport for the
formation of EPEs. Another conclusion with major implications is that
different combinations of Rossby wave breaking and intense moisture
transport can reflect a large range of EPE-related weather systems across
climate zones and can thus form the basis for a new classification of EPE
regimes. The findings of this study may contribute to an improved
understanding of the atmospheric processes that lead to EPEs and may find
application in climatic studies on extreme precipitation changes in a
warming climate.