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Summer 2000

Europe delivers prime weather for MAP

by Bob Henson

The Doppler on Wheels radar got its first European tour of duty scanning the Alps for heavy rains during MAP. (Photo by B ob Henson.)

The atmosphere proved as reliable as the equipment for the Mesoscale Alpine Program's field phase last fall. Centered on the Alps, this was one of Europe's largest meteorological field programs to date. The study period extended from 7 September to 15 November, hitting the peak seasons for two sets of phenomena: mountain waves and foehn windstorms near the Alps (Dry-MAP) and heavy rains and flooding across northern Italy (Wet-MAP). In their first gathering since the fall field phase, MAP participants met in Bohinjska Bistrica, Slovenia, in May to look at initial results and plan further analysis.

"The weather events were very favorable, fulfilling minimum conditions for all eight scientific objectives," reports Joach Kuettner, who heads the U.S. project office for MAP and holds the UCAR Distinguished Chair for Atmospheric Sciences and International Research (see accompanying article). "In fact, during four or five episodes, all these objectives were simultaneously satisfied, creating a real priority problem for the mission planning team."

There were 17 intensive observing periods (IOPs) during MAP, each running one to three days. The IOPs coincided with a bumper crop of the kinds of weather MAP had sought out, according to MAP's Austrian coordinator, Reinhold Steinacker (University of Vienna). For example, there were 12 days with gravity waves, compared to a ten-year autumn average of 5.6. The number of days with heavy rain along the southern Alps was 160% above the mean.

The hunt for breaking waves

One of MAP's goals, and a particular interest of Kuettner's, was to verify predictions of breaking mountain waves from high-resolution models. Although mountain waves have been studied often, it's been difficult to pinpoint the location of the turbulent areas presumably associated with gravity waves breaking. Many glider pilots soaring in mountain waves (including Kuettner) have encountered this sudden and quite dangerous type of severe turbulence. The MAP experience indicates that this phenomenon may be triggered on such small scales that even the highest-resolution mesoscale models can't pin down its location in advance. For example, the Canadian MC2 model targeted an area of possible wave breaking on 20 September, but the MAP flights that day found only large-amplitude waves (speeds of 6 to 8 meters per second) with no tendency to break.

Kuettner still hopes that further analysis may reveal some breaking waves captured during MAP. "It was the one science objective that didn't work out as expected. However, even this negative result is of scientific value, especially for the modelers who need to improve their resolution and their turbulence parameterization schemes. There's no reason to cry about it. Nature always surprises you."

Among MAP's successes were the first verification of potential vorticity (PV) banners predicted by mesoscale models. PV represents a blend of horizontal circulation and vertical stability. Since PV is conserved as it flows, PV banners that form near mountains can maintain their integrity far downstream. A PV feature "can move off and help enhance or even form a cyclone a thousand kilometers away," notes Kuettner. On 20 October, the data hinted at several PV banners extending to the north of the Alps. Their existence was indicated by mesoscale models run before and after the fact and confirmed by a joint mission including the NOAA P-3, NSF/NCAR Electra, and France's Merlin IV and Fokker 27. In another key result, a ground-based NOAA scanning Doppler lidar, together with data from the P-3, provided unprecedented detail on "gap flow" through the Alps' Brenner Pass.

Wet-MAP investigators focused on heavy upslope rains along the southern Alps. Operational radars were on hand from Switzerland (Monte Lema) and Italy, and research radars from NCAR (S-Pol), France (RONSARD), Switzerland, Germany, and the University of Washington (OPRA). The blend of convective, orographic, and frontal forces at work is making for an analysis challenge. Brad Smull (University of Washington) noted the added value of Wet-MAP flights that used the airborne Doppler radars aboard the NOAA P-3 and NSF/NCAR Electra: "We were able to obtain a comprehensive view of precipitation and airflow patterns over this complex landscape, even in regions blocked from the view of more distant radars."

In one of several other components of Wet-MAP, Matthias Steiner (Princeton University) worked closely with the Doppler on Wheels portable radar, a collaboration between NCAR and the University of Oklahoma. One of the most intriguing observations was a persistent, and sometimes quite strong, flow down and out of the Italian Alps valleys beneath the general upslope flow, particularly in the Toce River valley during the eighth IOP (20-21 October 1999). Both the NOAA P-3 and the DOW captured this downslope flow, seemingly at odds with the heavy upslope rains then occurring.

As dry air over the Po Valley was pushed northwestward toward the Alps, its stability apparently prevented it from crossing over the barrier, said Steiner. "The air seemed to be seeking ways to escape by diving under the upsloping air and pushing back out along the valley bottom. This down-valley flow was likely enhanced, if not initiated, by the evaporation of precipitation, cooling the air within the valley and causing subsidence." S- Pol and airborne radar observations hint that such down-valley flow, never before documented in such detail, might also have taken place elsewhere in the region. As they combine ground-based and airborne radar data with other sensors, Steiner, Smull, Robert Houze (University of Washington), David Jorgensen (NOAA), and colleagues hope to determine connections between the valley-based and larger-scale air flows and to see how these flows might have interacted to cause blocking and push the heaviest rains away from the immediate Alpine slopes.

MAP's participants included about 250 people from 14 nations, including 120 Americans. Along with UCAR and NOAA, MAP involved the U.S. Naval Research Laboratory, 8 U.S. universities, 12 weather services, and 25 research institutes in Europe and Canada. The dry and wet phases of MAP's field phase were split between command centers in Innsbruck, Austria; Milan, Italy; and Bad Ragaz, Switzerland. Operations directors James Moore and José Meitín (Milan) and Richard Dirks (Innsbruck), all from the UCAR Joint Office for Science Support, orchestrated the flights in a region of heavy airline traffic. According to Kuettner, "The difficulties usually connected with multiple operations centers were overcome by excellent planning, close communications, and the good will of all participants."

In fact, there was a good deal of informal collaboration between researchers involved in the wet and dry phases, according to Ronald Smith (Yale University), scientific director for the second half of the MAP field phase. "Scientists in subdisciplines which have had a tendency to move apart in recent years found themselves with a new appreciation of how the whole atmosphere and how our whole discipline can work together. I recall intense discussions linking meteorology and flood hydrology, dynamic and thermally driven valley winds, synoptic and mesoscale circulations among those who, at their home institutions, do not have colleagues in these other fields."

A complete report on the IOPs is being prepared under the direction of Philippe Bougeault (Meteo-France), scientific director for the first half of MAP's field phase. That report will appear later this year in the Bulletin of the American Meteorological Society. Data from last fall's field phase is now available on line from the MAP Data Centre in Zurich, Switzerland. A preliminary IOP report is available from Becky Meitin, UCAR Joint Office for Science Support, rtm@ucar.edu.


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