Improving the numerical prediction of a cyclone in the Mediterranean by local potential vorticity modifications

Abstract

This study explores the benefit that can be drawn from incorporating local potential vorticity (PV) modifications into a numerical forecast. The case under study is the severe cyclogenesis that occurred in the Western Mediterranean from 9 to 12 November 2001. This case was shown to be extremely sensitive to the upper-level flow, which governed to a great extent the deepening of the depression and the location and intensity of its associated precipitation. Corrections of PV were guided by METEOSAT-7 water vapour observations and restricted to the upper troposphere, mainly altering the topography of the dynamical tropopause. Using both the French operational global model ARPEGE and the non-hydrostatic mesoscale model Meso-NH, it is shown that carefully designed PV corrections lead to a substantial improvement in the simulation of the storm, both in terms of surface pressure, cloud cover and precipitation forecasts. Furthermore, the impact of the modifications is shown to be a maximum when they are introduced at the time corresponding to the incipient stage of cyclogenesis.

Mediterranean hurricanes: large-scale environment and convective and precipitating areas from satellite microwave observations

“…It indirectly confirms the role of upper level structures for short range forecasting of sub-synoptic vortices over the Mediterranean Sea, as shown e.g. by Argence et al (2009), that demonstrated that rather minor perturbations in the depth of the upper-level trough at the initial stage of cyclogenesis could propagate and intensify during numerical simulations, strongly impacting the forecast of a medicane.…”

Tropical transition of a Mediterranean storm by jet crossing

“…It would be interesting to evaluate the role of the upperlevel jet using a PV inversion technique as in Argence et al (2009), for example. However, modifying the upperlevel trough would change the triggering of convection; hence the development of the medicane over the Ionian Sea before its passage below the upper-level jet.…”

“…Söhne et al, 2008), and values less than 260 K associated with high-and mid-level clouds. The MSG BT at 6.2 µm is sensitive to upper-tropospheric humidity, high BTs being associated with dry intrusion of stratospheric origin in a baroclinic region (Argence et al, 2009). Following Funatsu et al (2007), we used the observations from AMSU-B moisture channels (3-5) to detect the presence of hydrometeors through the scattering of radiation, which lowered the BT compared to its surroundings.…”

“…Here, the threshold of BT less than 250 K was chosen to diagnose deep, high clouds (as in Argence et al, 2009). A first peak in number of deep, high clouds occurred at 1800 UTC on 25 September.…”

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

“…Although slight differences in the upper-level PV distribution can have a large impact on the simulation of heavy precipitation events (e.g. Fehlmann and Quadri, 2000;Romero et al, 2005;Argence et al, 2009), Figure 7 shows that spatial distribution of the precipitation in the Piedmont region is relatively well reproduced by the CHRM. There is a slight eastward shift in the location of the precipitation, which is due to the eastward shift of the PV streamer at 16 November compared to the ECMWF analyses.…”