Betweeen 1145 and 1445 UTC 24 September 1999 the French ARAT and Merlin aircraft flew a a joint mission to investigate the boundary layer conditions over the Po Valley in advance of an approaching upper level trough. The S-Pol and RONSARD radar collected clear-air boundary layer data in coordination with the flights. A low-level sounding by the Merlin showed a well mixed moist boundary layer (Fig. 1). Between 0700 and 1800 25 September 1999, the French ARAT aircraft flew a research mission into the Ligurian Sea to measure conditions upwind of the Lago Maggiore region. Measurements included a sounding over the open ocean near 43.5 deg N, 9 deg E (Fig. 2). This sounding also indicated a low-level moist layer, somewhat deeper than that seen over the Po Valley one day earlier.
At 1200 UTC 25 September 1999 the 500 mb trough lay over the Atlantic coastline of Spain and northern Africa (Fig. 3). The MC2 model predicted precipitation in southwest-northeast bands in the Lago Maggiore region (Fig. 4). It also predicted southwesterly flow of high theta-e air into the region at 850 mb (Fig. 5).
The Electra aircraft flew to the Lago Maggiore region on the basis of the MC2 forecast. By 1600 UTC 25 September 1999 convective precipitation had formed in southwest-northeast bands, much like had been forecast and the Electra aircraft was flying in the region (Fig. 6). The flight continued in this region until a little after 1900 UTC 25 September 1999 (Fig. 7, Fig. 8).
Dual-doppler winds were computed in real-time from the Monte Lema and RONSARD Doppler radars. The dual-Doppler winds for the 2 km level confirmed the predicted southwesterly flow into the Lago Maggiore region (Fig. 9). The southwesterlies increased strongly with height (Fig. 10).
As the low-level southwesterly flow encountered the mountains on the west and north sides of the Lago Maggiore region, convective cells formed. This flow continued throughout the night of 25-26 September, during the day on the 26th, and through the night of the 26-27th. The DOW radar operated in the Ticino valley, at the Lodrino airport, from the evening of the 25th to the evening of the 26th. The DOW scientist Matthias Steiner described on and off convective showers, occasional lightning, thunder, and at one time graupel. By sunrise on the 26th, the DOW was in a field covered with several inches of ponding water. The most spectacular visual experience at the DOW site was the hydrological response to the accumulated rain. The mountain sides have very little soil covering the rocky substrate, and the water was cascading down the mountainsides in huge waterfalls into the Ticino River (Fig. 11, Fig. 12).
The S-Pol radar cross section along a radial pointing up the Ticino valley over the DOW site showed continuous upslope flow (in the radial velocity cross section) throughout the 24 hour period of DOW operation (e.g., Fig. 13). The echoes tended to extend up to about 6-8 km and were nearly always cellular. There was little to no evidence of bright band formation on DOW or S-Pol throughout the event. The precipitation was always showery. There was a strong shear with the upslope winds increasing strongly with height (Fig. 10). The shear appeared to carry upper-level echo toward the crest of the Alps. Polarimetric data from the S-Pol radar indicated that the convective precipitation cells in the upslope flow had small pockets of supercooled rainwater and graupel (Fig. 14).
Very large rain amounts accumulated in the mountains to the west and north of the Lago Maggiore region (Fig. 15). The S-Pol radar estimated accumulations over 300 mm over much of the region of the foothills and lower slopes with estimated maxima reaching 400-500 mm in the region just west of Locarno (the northern maximum in Fig. 13). The Osservatorio Ticinese Locarno (OTL), just above the town near the shore of Lago Maggiore, recorded 214 mm. West of Locarno, 3 km west of the confluence of the Maggia and Melezza Rivers, Giovanni Kappenberger, a forecaster at OTL, measured 440 mm of rain. Just above Intragna, at a location 5 km west of the Maggia and Melezza Rivers, Juerg Joss of OTL recorded 325 mm of rain.
