Airborne Doppler Observations of a Convective System over the Eastern Alps during MAP IOP5

In contrast to relevant dynamics of terrain influence in the presence of steady, horizontally uniform onshore flow, which have been explored to some degree by previous studies, interactions of unsteady, diabatic flow with topography remain largely unexplored. Under suitably environmental conditions such interaction can result in the development of local intense rainfall and high surface winds in the vicinity of the mountains. Unfortunately, the lack of detailed observations nearby topography has largely limited our knowledge of the severe weather conditions and underlying dynamical processes accompanying the orographic modification of these events, which in turn lead to a poor forecasting skill for their occurrence. Special measurements, including in-situ and Doppler radar data, collected from instrumented aircraft during MAP (Mesoscale Alpine Program) provide highly detailed views of mesoscale structure of airflow and precipitation not only farther upstream but also directly over the inland sloped terrain. The main objective of this study is to use airborne Doppler radar observations to document the mesoscale structure and evolution of a mountainous convective system that formed near the border of northern Italy and western Slovenia on 4 October 1999 during MAP IOP5.

Convection associated with this event was initiated at ~0500 UTC over the southern slopes of the Alps while a synoptic cold front (upper-level trough) approached the eastern edge (middle part) of the Alps. Ground-based radar observations at the Fossalon site indicate that precipitation within the system developed into a highly organized nature within the next 2 hours, featured by a narrow but intense band extending from the inland sloped terrain to a location well far upstream of topography over the Gulf of Venice. During the period, both NOAA P-3 and NCAR Electra aircraft made comprehensive Doppler observations over the northern portion of the convective system as it advanced eastward and interacted increasingly with the mountains. Pseudo-dual-Doppler synthesis results from the initial Doppler leg indicate that at low levels the leading edge of the convective rainband was characterized by an obvious convergence between prevailing southwesterlies upstream of the mountains and southeasterlies/easterlies found along the mountain slopes. It appears that the low-level flow feeding the convective system was generally downslope in character. Moreover, a distinct larger-scale confluent zone, which resulted from the interaction of southwesterlies with northerly flow presumably originated from the north side of the Alps, was also evident near the level of the highest mountain crest (~2 km). Mesoscale ascent associated with this observed feature as well as advection of precipitation particles from the leading convection by the southwesterly upper-level flow would probably contribute to the occurrence of pronounced stratiform precipitation over the mountain slopes and higher terrain.