Radar Observations and Numerical Modeling of a Precipitating Line during MAP IOP 5

Abstract

During the special observing period of the Mesoscale Alpine Programme, a narrow north?south-oriented frontal precipitation band was observed by two airborne and one operational Doppler radar on 4 October 1999, over the Friuli target area in northeast Italy. Convective precipitation was mostly concentrated to the south of the eastern Alps in regions of smooth orography and warm southerly flow ahead of the cold front, while stratiform rain formed above the mountainous chain where a marked cold northerly flow, wrapping around the eastern Alps, progressed southwestward. Doppler data collected in both convective and stratiform regions of the precipitation line, as well as high-resolution numerical simulations performed with the nonhydrostatic Meso-NH model, are used to investigate the associated flow structure in connection with the underlying topography. The mutually consistent observed and simulated precipitation and wind fields clearly identify low-level flow convergence relative to the cold front and cold air from the north of the Alps as important factors in setting up the upward motions, in addition to orographic forcing. This cold air delimits a warm frontal surface that is overrun by the warm southerly air. Its southward progression has a blocking effect and contributes to modulating the airflow over the mountains. It is observed that the maximum updraft cores occur upstream of the steepest slopes, as a consequence of the convergence between the warm southerly and cold northeasterly flows. Moreover, observed wind acceleration from south to north is related to a horizontal pressure pertubation gradient force, and dynamic pressure perturbation is closely linked to the mountain shape. Additional numerical experiments with smoother orography are performed to identify the relative roles of broader and finer-scale topography. It is found that the smoother the terrain is, the faster the cold and warm fronts progress southeastward, highlighting the role of at least 1-km orography scale in retarding the frontal system.