Air motions and precipitation growth in Alpine storms

This study documents two strong Mesoscale Alpine Programme (MAP) storms. Each occurred ahead of a strong baroclinic trough. Even though their 500 and 850 hPa flow patterns were strikingly similar upstream of the Alps, their flow at lower levels was different and resulted in different forms of orographic enhancement of the precipitation associated with baroclinic systems by the Alpine terrain. During Intensive Observing Period (IOP) 2b, the low-level flow and thermodynamic conditions over the Alps north-west of the Lago Maggiore region constituted an unstable atmosphere with Froude number Fr>1 (i.e. unblocked or flow-over regime), while in IOP 8 the flow was stable with Fr<1 (i.e. blocked or flow-around regime in the region immediately upstream of the slopes on the western side of Lago Maggiore). For IOP 2b (IOP 8) the wind field and precipitation patterns north-west of the Lago Maggiore were very similar to the high- (low-)Fr autumn climatology. Thus the two cases represent fundamentally distinct regimes of orographic modification of the baroclinic precipitation. During IOP 8 (blocked case), the strong stability and weak wind speed at low levels prevented the airflow from rising over the slopes north-west of the Lago Maggiore region and forced it to turn away from the mountains. The precipitation over the mountains was produced as the strong flow above 900 hPa was forced over the blocked low-level air as well as the terrain. In contrast, during IOP 2b (unblocked case) the flow was strong at all levels with low static stability; therefore the low-level air rose easily over the abruptly rising terrain. Since the lower-level air rose together with the upper-level air, it could transport upward moisture unavailable in the blocked case; this moisture condensed to add significantly to the precipitation production in the unblocked case. In addition to the high Fr, the air was potentially unstable; the slight instability of the airstream impinging upon the upslopes favoured the development of convective cells over the lower slopes which were embedded in the stratiform background lifting, thus further enhancing the formation of cloud and precipitation on the lower windward slopes. Examination of all the cases observed by polarimetric radar in MAP confirmed the microphysical processes seen in IOPs 2b and 8, and suggests fundamental microphysical differences between the unstable unblocked, and stable blocked cases. In both cases precipitation forms by a simple stratiform process in the form of dry snow aloft, becoming wet snow at the melting layer and falling out as rain below. However, in the unstable unblocked cases rainfall is enhanced by the participation of the low-level flow in the orographic lifting; when the low-level air rises easily over the first high peaks of terrain, raindrops grow rapidly by coalescence at low levels, and graupel forms just above the 0°C level. The coalescence-produced drops and melted graupel particles contribute to heavy rain on the lower Alpine slopes. The orographic enhancement in the stable blocked cases is fundamentally limited since these low-altitude precipitation processes cannot add significantly to the background stratiform precipitation. Copyright © 2003 Royal Meteorological Society.