MEDEX:Outline of the project, links to MAP

Agusti Jansa, MEDEX coordinator, Instituto Nacional de Meteorología, Centro Meteorológico Territorial en Illes Balears, E-07015 Palma de Mallorca, Spain, jansa@inm.es

Introduction
MEDEX, the MEDiterranean EXperiment on cyclones that produce high impact weather in the Mediterranean, is a Research and Development Project (RDP) framed in the World Weather Research Programme (WWRP) of the World Meteorological Organisation (WMO), after the official endorsement of its first phase by decision of the Science Steering Committee (SSC) for WWRP, in October 2000. At present, WWRP consists of five RDP (MAP, In-Flight Icing, International Tropical Cyclone Landfall Project, THORPEX and MEDEX) and a few Demonstration Projects. All the RDP in WWRP are oriented to the better understanding and the improvement of the forecasting of high social impact weather phenomena. There are some convergences between some of these projects. In particular, MEDEX has convergences with THORPEX and MAP. In this paper the main aspects of MEDEX are outlined and some links between MEDEX and MAP are shown.
MEDEX: outline of the project
In spite of the usually pleasant weather, the Mediterranean area is quite frequently affected by sudden events of extreme adverse weather, often producing high social impacts. The reduction of the dramatic consequences of these extreme weather events is the ultimate motivation of the MEDEX proposal. Improving forecasts of such events is a necessary, though not sufficient, condition for the above achievement. Although not all the extreme weather events in the Mediterranean are related to cyclones and most of the cyclones do not produce extreme weather, it is plausible to assume that Mediterranean cyclones influence most of the high impact phenomena, at least in an indirect way. Preliminary statistical results (Fig. 1, Jansa et al., 2001) show that around 90% of the heavy rain events (in a variety of areas of the Western Mediterranean) have a cyclone in its vicinity. Most of them in such a location that some kind of influence of the cyclone on the heavy rain generation and location can easily be inferred.



Figure 1. Most frequent location (elliptic area) of a cyclone in case of heavy rain in some western Mediterranean regions (isolated dot). The locations are (from left to right): SE France (W and E of the Rhone), N Italy, Catalonia, and Valencia (Jansa et al., 2001)

According to the cited study, most of the cyclones that accompany heavy rain are not strong and deep baroclinic systems, but shallow orographic or thermal depressions. Therefore, in order to focus the project on a tractable near term scientific objective, the main general objective of MEDEX is stated to be the improvement of knowledge and forecasting of cyclones that produce high impact weather in the Mediterranean area. Cyclone is understood in the most general sense of the word, that is, including shallow depressions. Directed towards the general objective, several specific objectives have been identified for the phase 1 of the project (MEDEX Science Plan Phase 1, available at www.inm.es/MEDEX):



To face the first of these objectives, daily objective analyses and daily GTS and non-GTS rainfall and wind data will be used. The second and third objectives will be based on a set of selected events, covering the period 1995-2003. All available data for the selected periods will be used and data of coincident campaigns included. Neither a field phase, nor the deployment of extraordinary means of observation are foreseen for the MEDEX first phase. 27 events have already been preselected for the period 1995- 2000, ranging from Portugal to Israel. A specific MEDEX Data Base is being organised to collect the data to be used. MEDEX phase 1 is foreseen to finish by the end of 2004. The feasibility of a second phase (perhaps including field measurements) will be studied before that.

MEDEX links to MAP
The primary scientific objectives of the Mesoscale Alpine Programme (MAP) have been published in the MAP Design Proposal (1996) as:


The scientific objectives 1a and 1b together are referred to as wet MAP, objectives 2a and 2b were termed dry MAP. There is a connection between MEDEX and dry MAP, because some Foehn related phenomena and wave drag effects, the formation of low level PV banners included, can be a seed for Mediterranean cyclones, some of them producing high impact weather. But the following thoughts will be mainly focused on the link between MEDEX and wet MAP.



Figure 2. Climatological frequency of heavy precipitation (over 20 mm/day) over the Alps during the month of October. Colour coding shows percentage of days (after Frei and Schär, 1998).

Geographically, MAP concerns the Alpine Region. The target territory for MEDEX is the whole Mediterranean area. Obviously, the southern slopes of the main Alps (in Switzerland and Italy) and some other areas in South-Eastern- France, Northern-Italy, Slovenia or Croatia are common territory for both projects, MAP and MEDEX. Heavy precipitation (in the common area) is then a common topic of interest for MAP and MEDEX, although the focus of both projects differs. As stated in the document Scientific Objectives (MAP Data Centre, http://www.map.meteoswiss.ch), the largest amounts of precipitation in the Alps occur during autumn on their southern slopes. Climatologically, the rainfall is mostly concentrated over slopes exposed to air trajectories coming from the Western Mediterranean or the Adriatic sea (Fig. 2, Frei and Schär, 1998). Typical mesoscale weather systems and flow fea tures that favour heavy and long lasting precipitation in the south of the Alps are Mediterranean depressions and fronts (e.g. Sénési et al., 1996). The role of the Mediterranean cyclones (or depressions) in organising an inflow of wet and warm Mediterranean air to feed heavy precipitation in Alpine and non-Alpine Mediterranean locations has been identified for several particular events, including some of the most important cases that were selected for the preliminary phase of MAP (see Jansa et al., 1995, and Jansa et al., 2000, for a summary). Statistically, in 84% of 77 events of heavy precipitation in Ticino, Liguria, Lombardia and Piemonte there is a cyclone within a radius of 600 km from the heavy rain site, mostly located as indicated in Fig. 1 (up-right corner). For South-Eastern- France (W and E of the Rhone river, up-left and up-centre in Fig. 1) the numbers are 90% of 195 events and 91% of 120 events. Part of the concomitance of cyclones and heavy rain could be by chance, not causal, since in fact the presence of depressions in the Mediterranean is quite frequent. But their relevance becomes more evident by comparison with the frequency of the presence of a depression within 600km for a random sample of days (with or without rain), which is only 38% from S-Switzerland and N-Italy and 56% from SE-France (Jansa et al., 2001; or preliminary view in Jansa et al., 1996). As a consequence, a complete understanding of some of the most significant wet-MAP related weather events during the field phase of MAP, probably also requires understanding of the mechanisms that lead to a Mediterranean cyclogenesis, prior to and associated with the heavy rain event.

Figure 3. MSL pressure, 20-Sep.-99 at 06 UTC (MAP IOP 2; left) and 07-Nov.-99 at 00 UTC (MAP IOP 15; right).

Three of the 27 pre-selected MEDEX events (1995-2000) are also MAP IOPs: IOP2B (20-22 Sep. 1999), 5 (2-5 Oct. 1999) and 15 (6-9 Nov. 1999). In the first case, a secondary low in the Mediterranean (L in Fig. 3) intensifies the feeding current (arrow in Fig. 3) towards the Alps. 300 mm/24 h and floods were registered in the Lago Maggiore area. IOP 15 is an archetype of deep Mediterranean lee cyclone, which produced very strong winds and/or heavy rain in areas of Spain, France, Italy, Croatia and even Greece. Valuable additional information about these cases is available in the MEDEX Data Base.

Final remark
The scientific objectives for MEDEX and MAP are clearly distinct. For MEDEX the objective is the Mediterranean cyclogenesis. For MAP it is the orographic perturbation of the air flow. But there are derived common topics of interest: in particular, heavy rain in the Mediterranean Alpine area is a main consequence of both, the Mediterranean cyclogenesis and the orographic perturbation of the air flow. MAP is not totally complete without a reference to the Mediterranean cyclogenesis and MEDEX is not totally complete without considering the orographic enhancement of the precipitation. It is mutually beneficial to maintain contact between both projects. In this sense, MEDEX remains open to additional participation. Interested scientists or institutions may contact the MEDEX coordina tor. To know more about MEDEX: http://www.inm.es/MEDEX.

References

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