Contrasting stratification and mesoscale airflow in two heavy precipitation events observed in MAP

Preliminary analysis of observations collected during field phase of MAP (the Mesoscale Alpine Programme) has been conducted with the aim of identifying periods of heavy rainfall characterized by contrasting degrees of upstream stability. In general, weaker static stability (and locally unstable conditions) were favored early during the Autumn 1999 special observing period in conjunction with heavy convective precipitation focused along the Alpine slopes. By contrast, a comparatively high degree of stratification characterized persistent rainfall episodes later in the period, when widespread (and more stratiform) precipitation was sometimes observed to extend well upstream of the barrier. In order to explore this hypothesized relationship, we analyze airborne multiple-Doppler radar observations in the context of nearby rawinsonde measurements to explore the mesoscale structure of flow and precipitation within orographic precipitation systems occurring in these contrasting stability regimes. Observations from two heavy precipitation events (viz. the 20 September "IOP2b" and 21 October "IOP8" missions) have been selected to represent comparatively unstable vs. stable conditions, respectively.

A closely related objective is to develop a more quantitative understanding of the connection between channeled low-level flow within deep Alpine valleys and overlying winds in the free atmosphere. Because this transition occurred at heights that in some cases may have been below the reach of the ground-based fixed radar network, We will relate airborne Doppler radar data over the complex terrain to those from a mobile ground-based Doppler radar (the Doppler on Wheels, DOW), which was principally operated within the Toce and Ticino valleys in the Lago Maggiore region. The availability of airborne Doppler-derived winds extending over and beyond this favored region will allow us to evaluate the generality of airflow structures observed by DOW at other locations along the Alpine barrier.