Precipitation in the South East Alps - MAP scientific conclusions review

During MAP SOP the South East Alps were chosen as secondary target area for the »wet« MAP (P1 - Orographic precipitation). The area of the Julian and Karnic Alps is known as an area of climatological precipitation maximum in the Alps. During MAP SOP in the SE Alps area there was a network of additional automatic rain gauges and the Friuli Venetia Giulia radar in Fossalon was operational. Several research flights with P3 and Electra research aircraft were performed. In MAP SOP one excellent and several good IOPs were recorded in the area.

The main results concerning orographic precipitation in the SE Alps can be summarized:

1. The majority of precipitation is often recorded before the cold front reaches the area as heavy convection is triggered by forced ascent over steep southern slopes of the Alps and as the warm air lifts above the level of free convection.

2. The cold front from the west was the most expressed MAP case IOP 5 - was marked by a squall line. At the same time as the cold front was moving across the Po plain, the cold air was already flowing around the eastern Alps in the lower layers of the atmosphere and over the ridges of the Julian and Dinaric Alps. With two cold fronts (a shallow one from the East and a deep one from the West, the warm air was orographically occluded and squeezed out towards the south.

3. A very dense network of recording rain gauges (about 10 km distance) was a big improvement in comparison to the operational setup but in the areas of the highest precipitation accumulation gradients it should be still denser. Spatial interpolation methods for precipitation were tested on MAP data and for daily values the statistical methods gave good results while for hourly precipitation their results were not better than simple mathematical interpolations. Additional data would be needed especially in the areas of the maxima and of the strongest gradients.

4. Fossalon radar proved to be a very useful tool for monitoring the precipitation in the SE Alps but the radar shadows from the first Alpine ridges lead to heavy underestimation of precipitation in the core region of the Julian Alps.

5. The aircraft missions revealed the wind structure of the cold air from the east moving across the Alpine ridges and its contribution to the forced ascent of the SW warm conveyor belt in front of the cold front from the West. The squall line of the IOP 5 (Oct. 5. 1999) is a good example of orographically modified leading edge of cold flow moving along the south side of the Alps.

6. Several modeling studies revealed that meso-beta processes(~ 100 km) contribute a great deal to the precipitation distribution the South of the Alps and that topographic convergence in this scale is responsible for location of the precipitation maxima. The smaller scale topography, its slopes and altitudes, is further modifying the precipitation distribution. The location of triggering regions for convection is determined with even smaller scale (~1 - 3 km) and use of the radar data can help with nonhydrostatic modeling for lead times up to some hours.