Air motions and precipitation growth in Alpine storms

Abstract

SUMMARYThis study documents two strong Mesoscale Alpine Programme (MAP) storms. Each occurred ahead of a strong baroclinic trough. Even though their 500 and 850 hPa ow patterns were strikingly similar upstream of the Alps, their ow 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 ow 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 ow-over regime), while in IOP 8 the ow was stable with Fr < 1 (i.e. blocked or ow-around regime in the region immediately upstream of the slopes on the western side of Lago Maggiore). For IOP 2b (IOP 8) the wind eld 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 modi cation of the baroclinic precipitation.During IOP 8 (blocked case), the strong stability and weak wind speed at low levels prevented the air ow 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 ow above 900 hPa was forced over the blocked low-level air as well as the terrain. In contrast, during IOP 2b (unblocked case) the ow was strong at all levels with low static stability; therefore the low-level air rose easily over the abruptly rising terrain. Since the lowerlevel air rose together with the upper-level air, it could transport upward moisture unavailable in the blocked case; this moisture condensed to add signi cantly 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 con rmed 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 ow in the orographic lifting; when the low-level air rises easily over the rst 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 case...

A numerical study of three catastrophic precipitating events over southern France. II: Mesoscale triggering and stationarity factors

“…This behaviour is very similar to what was found in southerly flow near the Alps (e.g. Medina and Houze, 2003).…”

Effect of the Andes Cordillera on Precipitation from a Midlatitude Cold Front

“…Other observations of fronts decelerated by upstream flow blocked by topography have been made in the European Alps (e.g., Kurz 1990), the Appalachian Mountains (O'Handley and Bosart 1996), the coastal mountains of western Canada (Doyle and Bond 2001), the Wasatch mountains of northern Utah (Cox et al 2005), and the Pacific Northwest (Braun et al 1999;Yu and Bond 2002). Precipitation distribution in the Alps and Apennines has also been related to both the degree of flow blocking by topography (e.g., Medina and Houze 2003) and the amount of entrainment of moist low-level air into unblocked flow aloft (e.g., Bousquet and Smull 2006). Prefrontal low-level blocking has been observed to aid rapid weakening of a cold front as it approached the Pacific Cascades (Bond et al 2005), and topographic barrier jets have been observed by synthetic aperture radar in the Gulf of Alaska (Winstead et al 2006).…”

The relation between airflow and orographic precipitation on the southern side of the Alps as revealed by weather radar

“…Similar findings were also obtained by Neiman et al (2002) and James and Houze (2005), who studied orographic precipitation enhancement mechanisms over California's coastal mountains. A conceptual model for the orographic precipitation mechanisms in stable blocked and unstable unblocked flows was proposed by Medina and Houze (2003). This conceptual model highlights the leading role of the upstream mesoscale flow kinematic and thermodynamic characteristics in originating different precipitation regimes over the mountains and nearby areas.…”