Origin of the water vapor responsible for the European extreme rainfalls of August 2002: 2. A new methodology to evaluate evaporative moisture sources, applied to the August 11-13 central European rainfall episode

Abstract

[1] An innovative technique has been developed to assess moisture sources contributing to precipitation. It represents an advance with respect to previously developed methodologies because it allows to discriminate more effectively between terrestrial versus oceanic sources, and also to detect with greater precision the relative importance of remote versus local sources together with the sequence of evaporation associated with a rainfall event. It is based upon the use of a mesoscale model to simulate a selected precipitation episode and a Lagrangian trajectory model to evaluate three-dimensional back-trajectories in order to track the vapor parcels transporting the target precipitation to their surface evaporative sources. An extreme sequence of rainfall events occurred over central Europe between August 11th and 13th is chosen to put the methodology into test. A domain centered in the region and spanning 6372 km in the W-E direction and 5940 km in the N-S direction is chosen to assess moisture sources. Results show the evolving role of various sources throughout the duration of the event, indicating that they shift from the southern to northern and eastern source regions, while a simultaneous decrease of marine sources with respect to terrestrial sources occurs from the onset of the event to its ending phase. The capability of discriminating moisture sources with precision is an important requirement to better understand the mechanisms of extreme rainfalls.

Oceanic sources of continental precipitation and the correlation with sea surface temperature

“…[4] Many of these atmospheric moisture tracking studies focused on a particular basin, country, or terrestrial region and identified the moisture sources or sinks for that region taking account of interannual or seasonal variability [e.g., Druyan and Koster, 1989;Numaguti, 1999;Yoshimura et al, 2004;Bosilovich and Chern, 2006;Nieto et al, 2006;Sodemann et al, 2008;Dominguez et al, 2009;Drumond et al, 2010;Gangoiti et al, 2011aGangoiti et al, , 2011bBagley et al, 2012;Keys et al, 2012;Tuinenburg et al, 2012]. More global characterizations of moisture sources showed the import and export of water vapor between nations [Dirmeyer et al, 2009a], or the oceanic versus terrestrial contributions to continental precipitation [e.g., van der Ent et al, 2010;Goessling and Reick, 2011] [5] Our own recent work [van der Ent et al, 2010;van der Ent and Savenije, 2011] showed that globally about 40% of the precipitation on land originates from continental rather than oceanic evaporation.…”

Simultaneous Episodes of Heavy Rainfall in Morocco and Southern Alps: 1. Mesoscale Simulations and Episode Climatology (1979–2016)

“…Both heavy rainfall episodes, in the Alps and Morocco, are analyzed in this manuscript as well as the recent climatology of this type of simultaneous episodes, their time of recurrence since 1979, and the possible occurrence of hot spots in the eastern N‐Atlantic during their initiation. The MesoWat_Source modeling system (Gangoiti, Gómez‐Domenech, et al, ; Gangoiti et al, ) is used for the detailed mesoscale simulation of the simultaneous episode and the subsequent estimation of moisture sources. The main/largest domain of the mesoscale simulation has been enlarged with respect to previous studies (Gangoiti, Sáez de Cámara, et al, ) in order to include more remote sources: to the south, down to the equator, and also to the west, to include the N‐Atlantic Ocean up to the eastern American coast.…”

“…The MesoWat_Source modeling system (Gangoiti, Gómez‐Domenech, et al, ; Gangoiti et al, ) was designed to evaluate the evaporation sources of a prescribed target precipitation. It is based on a series of software modules (Figure ), which share meteorological and positional information of water vapor parcels moving in back‐trajectories initiated at the precipitation levels above the target rainfall.…”

“…The results will be discussed in section . The largest domain (grid #1) is greater than the one used for the ACEF episode (Gangoiti, Gómez‐Domenech, et al, ; Gangoiti, Sáez de Cámara, et al, ). Now, the west‐to‐east domain width doubles that of the August episode and, it is also enlarged to the south, crossing the equatorial line.…”

Water vapour accumulation mechanisms in the Western Mediterranean Basin and the development of European extreme rainfalls

“…The results of this innovative approach, inspired in the pioneering work by Dirmeyer and Brubaker (1999), show that the most important fraction of the attributed evaporation that caused the extreme rainfalls of August 11-13 took place between 6 to 8 days before the precipitation event and that the distribution of evaporative sources changed from day to day along the rainfall episode (Gangoiti et al, 2011). The evaporation sequence corroborates the role and the importance of the WMB accumulation mechanism in the first part of the torrential rainfall episode (from August 11 until August 12 at 12 UTC) as well as the significant contribution of evaporation from northern and eastern land areas in its second part (from August 12 until the early morning of August 13).…”