Extended Abstract (364K)9.1
Influence of Synoptic and Mesoscale Environments on Heavy Orographic Rainfall Associated with MAP IOP-2B and IOP-8
Yuh-Lang Lin, North Carolina State University, Raleigh, NC; and J. A. Thurman and S. Chiao
The influence of the synoptic and mesoscale environments on heavy orographic rainfall associated with the MAP IOP-2B and IOP-8 is investigated by using the PSU/NCAR MM5 model. During IOP-2B, a deep trough approached the Alps from west and induced low-level jets, which then triggered mesoscale convective systems and orographic rainfall. A quasi-pressure ridge to the east helped to slow down the convective system. An upper-level diffluent region advanced to the location over the Alps at about the time of heavy rainfall. The synoptic situation for IOP-8 was similar to that for IOP-2B, except the low-level flow was more from the southeast. The rainfall in IOP-8 was lighter and extended farther upstream of the southern Alpine slopes. For IOP-2B, the mesoscale surface winds near the Lago Maggiore region came from 3 different origins: (i) the easterly low-level jet from the Adriatic Sea, (ii) the southerly LLJ from the Tyrrhenian Sea, and (iii) the southwesterly LLJ behind the cold front. Three types of rainfall were identified, (1) orographic rain, (2) rainfall produced by flow convergence, and (3) frontal rain. The orographic rainfall in the concave (Lago Maggiore) region was mainly produced by the orographic uplifing of the conditionally and convectively (potentially) unstable easterly jet. During IOP-2B, two layers of convectively unstable layer, one near the surface and one elevated, moved into the Lago Maggiore region. The pool of elevated high qe air from Africa reached Italy at about 9/20/00UTC, which was originally from western Mediterranean Sea. Strong convection was triggered by the release of convective instability in the lower-levels, which then developed vertically and linked with the higher-level clouds to form a strong convective system. The higher-level clouds appear to be triggered by upper-level forcing. The moisture effect on the leeside flow is to produce a foehn wind. The observed and simulated CAPE was relatively low over northern Italy; however, CAPE was much larger over the Mediterranean Sea. Combination of other common ingredients should be able to induced strong convective systems and heavy orographic rainfall. In addition, the layer-lifting of the low-level flow over the southern Alpine slopes tends to release the convective instability, which downplayed the importance of conditional instability. The mesoscale flow structures of IOP-8 were similar to those of IOP-2B, except that: (a) there was a cold dome formed over the southern slopes of the Alps; (b) the upstream airstream was less conditionally unstable; (c) the rainfall region in IOP-8 extended farther upstream (south) of the Lago Maggiore region. These factors added together and resulted a lighter orographic rainfall. Based on upstream soundings, IOP-8 has a much smaller moist Froude number and resulted more rainfall upstream. Although the PV streamer may serve as a predictor for the heavy orographic rainfall over the Alps, the actual mechanism in triggering the convection was due to the orographic uplifting of the convectively unstable layer of airstream from the Mediterranean Sea and the enhancement by the upper-level divergence. It was found that IOP-2B trough was related to the remnants of Hurricane Floyd. It is hypothesized that the upper-level outflow jet from Floyd may have strengthened the trough of IOP-2B.
Session 9, Orographic Precipitation IV
Wednesday, 19 June 2002, 8:00 AM-9:15 AM
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