Formation and Maintenance Mechanisms of the Stable Layer over the Po Valley and Genoa Cyclone Movement during MAP IOPs
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2005-10-18
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Abstract
This thesis is composed of two papers concerning events that occurred during the Mesoscale Alpine Programme (MAP). The first paper investigates the formation and maintenance mechanisms of a stable layer over the Po Valley during MAP IOP-8. The second paper examines the dynamics of the movement associated with the MAP IOP-1 and IOP-8 Genoa cyclones as they approached the Apennines.
In the first paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate the stable layer over the Po Valley during MAP IOP-8 in order to investigate the mechanisms that led to the formation and maintenance of the stable layer. Blocking and deflection of cool, easterly flow by the western flank of the Alps was found to play a significant role in the formation of the cool, stable layer by helping to build up the cool air over the Po Valley. When the flow shifted to southerly, the western flank of the Alps and the northern flank of the Alps acted in concert to help retain the cool air over the Po Valley. As a result, warm, less stable southerly flow originating from over the Mediterranean Sea was advected atop the cool, stable layer in the Po Valley. This differential advection, along with blocking by the western and northern flanks of the Alps, helped to maintain the stable layer and extend its longevity over the Po Valley. An additional test showed that one commonly assumed mechanism that acts to maintain stable layers in cases of cold air damming, evaporative cooling, did little to alter the development or life cycle of the stable layer in this case study.
In the second paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate the MAP IOP-1 and IOP-8 Genoa cyclones and their tracks. Both cyclones formed near the Gulf of Genoa, propagated eastward, and approached the Apennines over the Italian Peninsula. The IOP-8 surface cyclone slowed down along the upstream (west) side of the mountain and accelerated over the mountain range, but was slightly deflected towards the south and became discontinuous as it crossed the Apennines. For IOP-1, the surface cyclone was deflected towards the south as it approached the upstream side of the Apennines, but remained on the western side of the mountain range until reaching the southern end of Italy. In this study, the tracks of those Genoa cyclones were examined to investigate what led to their differences. The theory of Lin et al. (2005 JAS) on track deflection is hypothesized to explain the differences in the movement of the IOP-8 and IOP-1 cyclones. Based on ECMWF reanalysis data and MM5 modeling results, it was found, however, that the basic-flow and vortex Froude numbers for both cyclones were nearly identical. Thus, the cyclone movement cannot be explained simply by the orographic blocking effects. Instead, it appears that the movement of the IOP-1 surface cyclone was mainly controlled by synoptic forcing.
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MAP, IOP-1, IOP-8, stable layer, Alps, Genoa cyclone, Apennines
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
