Disentangling the Forcing Mechanisms of a Heavy Precipitation Event along the Alpine South Side Using Potential Vorticity Inversion

Abstract

Extreme precipitation events along the Alpine south side (AS) are often forced by upper-level positive potential vorticity (PV) anomalies over western Europe. These so-called PV streamers go along with a dynamical forcing for upward motion, a reduction of the static stability in the troposphere (hence facilitating convection), and are associated with low-level winds that transport moisture toward the Alps.A case of heavy precipitation is examined using the 40-yr ECMWF Re-Analysis data. Piecewise PV inversion (PPVI) and the limited-area Climate High Resolution Model (CHRM) are used to assess the influences of mesoscale parts of the streamer on the precipitation event. The impacts on the vertical stability are quantified by the convective available potential energy (CAPE) and an index of static stability. Very sensitive areas in terms of the stability are located beneath the southern tip of the streamer; smaller changes in the stability are observed in the Alpine region.The moisture transport toward the Alps is sensitive to the amplitude of the streamer, which influences the amount of water that can be transported along its eastern flank.The impacts of the topography on the flow are assessed by calculating an average inverse Froude number. Whether or not the air parcels are blocked by or lifted over the barrier (going along with suppressed and enhanced precipitation, respectively) depends on the vertical stability and the impinging wind velocity, two parameters that are inherently linked to the PV streamer and its substructure.

Forcings and feedbacks on convection in the 2010 Pakistan flood: Modeling extreme precipitation with interactive large‐scale ascent

“…By separating the large-scale flow into components associated with individual disturbances using techniques such as piecewise PV inversion [Schlemmer et al, 2010] or Helmholtz decomposition, one may further estimate precipitation due to individual systems.…”

Central European tornado environments as viewed from a potential vorticity and Lagrangian perspective

“…For the calculation of CAPE, we rely on the operational post-processing routines for mixedlayer CAPE (layer depth 50 hPa) for the COSMO model (more information can be obtained from http://www.cosmo-model. org), which was adapted to the ECMWF setting by Schlemmer et al (2010). SRH is based upon the analysis of the tilting of horizontal vorticity into the vertical by Davies-Jones et al (1990).…”

“…The examination of the PV structure is instructive in that anomalous, upper-level PV features can be associated with enhanced advection of low-level moisture, forced lifting and atmospheric destabilization (Funatsu and Waugh, 2008;Massacand et al, 1998;Hoinka and Davies, 2007;Schlemmer et al, 2010). With respect to jet stream/streak structure, it has been shown that the ageostrophic circulation associated with jet streaks can also result in forced lifting (Uccelini and Johnson, 1979;Rose et al 2004).…”

Impact of North Atlantic evaporation hot spots on southern Alpine heavy precipitation events

“…Massacand et al, 1998;Fehlmann and Quadri, 2000;Richard et al, 2003;Argence et al, 2008;Ducrocq et al, 2008;Nussier et al, 2008;Schlemmer et al, 2010) considered events in autumn or early winter, when the Mediterranean is still relatively warm. This can cause considerable evaporation of moisture, which may be transported towards the Alpine ridge due to the flow induced by an upper-level PV streamer and contribute to the occurrence of such heavy precipitation events.…”