Synoptic and Mesoscale Environments for Orographic Rainfall Associated with MAP IOP-8

Abstract

In this study, we have adopted Penn State/NCAR Mesoscale Model version 5 (MM5) to simulate the synoptic and mesoscale environments conducive to orographic rainfall associated with Mesoscale Alpine Programme (MAP) Intensive Observation Period 8 (IOP-8). The model sensitivity tests on cumulus parameterization schemes, microphysical parameterization schemes, and terrain resolution were also included in this study.

A deep trough system associated with low-level jet approached the Lago Maggiore target area at 0000UTC 20 October 1999. During the same time period, a high pressure system was located to the east of the trough system at the same time. The high-low pressure system then remained quasi-stationary through 1200 UTC 20 October and 1200UTC 21 October. The southerly flow advected conditionally unstable air, i.e. high [subscript e], up to the Po Valley and the southern Alpine slopes. The sounding upstream of the Ligurian Apennines appears to contain high convective available potential energy (CAPE). Meanwhile, an easterly flow penetrated the Po Valley along the foothill of the southern Alps. The easterly flow met the southerly flow near the northern coast of the Adriatic Sea and Ligurian Sea to help enhance the orographically induced low-level convergence. As a result, the low-level convergence near the Ligurian Apennines was stronger. The easterly flow was confined in the Po Valley between Alps and Apennines and kept moving toward the west. Eventually, it flowed out through the gap between Maritime Alps and Ligurian Apennines and formed a mesoscale vortex with the southerly flow around the western Po Valley. The relative cold and stable easterly flow then piled beneath to provide a stable environment. It was proved that the cold air serves as a cold dome to make the southerly flow easily ride on it. Therefore, the upward motion near the southern Alpine slopes was very weak and not able to produce convective rainfall. Only shallow clouds developed and stratiform precipitation was shown from both model results and observations.

On the other hand, the southerly flow produced heavy orographic rainfall over the Ligurian Apennines. Along with the low-level convergence, which was enhanced by the confluence of easterly and southerly flow near the Ligurian Sea and Apennines, the upper-level divergence also played an important role in triggering and maintaining the convective systems near this region. The right entrance of jet streak was co-located with the Ligurian Apennines surrounding area through the model integration. As seen in the model simulation, the coupling of upper-level and lower-level forcing was essential for producing rainfall over Ligurian Apennines and Ligurian Sea during IOP-8.

Based on model sensitivity tests on microphysical parameterization schemes, we found that the Reisner scheme tended to underpredict the precipitation over the southern Alpine slopes. The Goddard LFO scheme produced a reasonable amount of snow particles but overpredicted the amount of graupel, which may help explain the overprediction of rainfall over the southern Alpine slopes. As shown by the cumulus parameterization schemes sensitivity tests, the Grell scheme was not active enough to produce enough rainfall. Most of the rainfall was produced via microphysical parameterization scheme. In comparison, the Kain-Fritsch scheme did produce a fair amount of precipitation near the southern Alps and the ocean. However, it appears the Grell scheme is more suitable for the stable environment near the Lago Maggiore target area because it produced a reasonable amount of rainfall.

For the sensitivity tests on Ligurian Apennines, we found that it has a significant impact on the rainfall distribution associated with MAP IOP-8. Without the Ligurian Apennines, the cold air was spread toward the ocean and the heavy rainfall was shifted toward the southern Alpine slopes.