Wednesday, 6 August 2003: 11:59 AM
Airborne Doppler observations of a cold frontal system encountering the eastern Alps during MAP IOP5
This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to examine the detailed airflow and precipitation structure associated with a south-north oriented cold frontal system as it moved eastward and encountered the eastern Alps near the border of northeastern Italy and Slovenia on 4 October 1999. Topography over the study region is oriented approximately northwest-southeast (with a peak mountain height of ~2500 m), and exhibits, however, significant variation in terrain height along the mountain ranges. Airborne Doppler observations indicated complicated characteristics of airflow and precipitation within the system, which were highly related to the orographic features and their interaction with synoptic flow. There were three principal consequences: (1) A narrow south-north oriented precipitation band was coincident with the surface frontal convergence zone. This precipitation band became disorganized and dissipated rapidly as the cold front encountered the relatively low terrain, where cold northerly flow originated to the north of the Alps could flow over the mountains. (2) In the prefrontal region, relatively short-lived deep convective cells were located upwind of the highest terrain. These cells were oriented roughly parallel to the low-level prefrontal southwesterlies and occurred in regions of most steepest windward slopes. The position of these cells was shown to roughly coincide with that of calculated maximum vertical motions forced by the mountain slopes, suggesting the significance of upslope lifting on triggering the convection. (3) Stratiform precipitation persisted over the windward slopes during the aircraft observation, and it was in association with the flow confluence between the southwesterly flow and northerly flow originated to the north of the Alps. Particularly, considerable horizontal variation of precipitation was also evident, with enhanced (reduced) precipitation rate over lower (higher) terrain. This observed characteristic appeared closely related to different responses of airflow-terrain interaction.