Extended Abstract (140K)7.4
Multiple Expressions of Upstream Orographic Blocking During MAP
Olivier Bousquet, University of Washington, Seattle, WA; and B. F. Smull
Because upstream blocking can displace orographic forcing away from the windward slopes of steep terrain, understanding its behavior and underlying mechanisms is required in order to better forecast the type, amount and location of orographic precipitation generated by major mountain ranges such as the Alps. A particularly strong case of upstream blocking occurred during IOP 8 of MAP (the Mesoscale Alpine Programme). On this day, a pronounced northerly-component flow was observed to extend from the base of the Alps toward the Mediterranean Sea. This flow developed in response to the blocking of baroclinically-induced southeasterly flow impinging on the barrier at low levels (Smull et al., this volume). A particularly notable jet of northerly flow passing through a narrow gap between the Maritime Alps and Appennines evidently helped trigger convection over the Gulf of Genoa, such that convective instability associated with moist unstable air originating over the Mediterranean Sea was released far upstream of the Alps. This resulted in a significant reduction of precipitation over the Alpine slopes relative to what might have occurred in the absence of significant blocking. Even so, this surprising behavior of the low-level flow (in which the influence of the Alps extended over 100 km upstream of the Alps) is not the only unexpected feature observed during this episode.
During IOP 8, an unexpectedly strong downslope flow was observed within major rifts along the south-facing slopes of the Alpine barrier (e.g., the Toce, Val d'Aosta, Maggia, Adda, among others). These multiple down-valley flows within the Lago Maggiore Target Area (LMTA) merged with a larger-scale blocked easterly-component flow along the broad Po Valley. The presence of multiple, simultaneous down-valley flows is a further indication of the strong degree of blocking during IOP 8, and represents an additional source of cool, stable air serving to reinforce the low-level blocked flow banked up along the barrier on this day. In addition to the study of mechanisms responsible for its formation (addressed in Steiner et al., this volume), another important aspect of the valley-scale flow thus lies in the estimation of its eventual impact on the low-level, barrier-scale, circulation and hence, the formation of precipitation. In order to investigate this problem, we exploit airborne Doppler radar data collected during MAP IOP 8 to derive quantitative budgets within several deep valleys of the LMTA and to estimate the potential impact of this additional mass of cool air jetting into the Po basin on the mesoscale (barrier-scale) circulation.
Session 7, Orographic Precipitation II
Tuesday, 18 June 2002, 10:15 AM-1:29 PM
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