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Multiple Precipitation Mechanisms over Mountains Observed by Airborne Doppler Radar during MAP IOP5

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  • 1 Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan
  • | 2 NOAA/National Severe Storms Laboratory, Boulder, Colorado
  • | 3 Laboratoire d’Aerologie (CNRS-UPS), Toulouse, France
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Abstract

This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to document the detailed airflow and precipitation structure over the mountainous regions near the border of northeastern Italy and Slovenia as a cold frontal system moved eastward and encountered the eastern Alps on 4 October 1999, during MAP IOP5. In contrast to previously documented MAP cases, the environmental conditions associated with this case are characterized by a deep layer of strong convective instability in the lower troposphere and by a cold, northeasterly continental flow coming down from the mountains (the so-called bora wind) along the southeastern Alps. Over the study region, there are two primary mountain barriers: the Julian Alps, oriented roughly west–east with a peak mountain height of ∼2500 m and a significant variation in terrain height along its length, and the other, the Dinaric Alps, a relatively lower mountain range oriented northwest–southeast, immediately adjacent to the south of the Julian Alps. How these two mountain barriers and the northeasterly continental flow influence the precipitation, and the nature of orographic precipitation developing in a potentially unstable environment, are explicitly addressed. Comprehensive analyses of airborne Doppler radar measurements reveal significant variations of precipitation in terms of its location, intensity, and type over the mountainous region. Particularly, five distinct forcing types of precipitation are evident for this case: slope convergence triggering, upslope triggering, weak slope convergence triggering, gap exit convergence triggering, and slope convergence forced stratiform. These precipitation characteristics are found to be closely related to the environmental thermodynamics, orographic features, and the complicated interactions between the southerly/southwesterly flow, northerly continental flow, and the orography. Details of these important observational aspects are elaborated upon in this paper.

Corresponding author address: Dr. Cheng-Ku Yu, Department of Atmospheric Sciences, Chinese Culture University, 55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 111, Taiwan. Email: yuku@faculty.pccu.edu.tw

Abstract

This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to document the detailed airflow and precipitation structure over the mountainous regions near the border of northeastern Italy and Slovenia as a cold frontal system moved eastward and encountered the eastern Alps on 4 October 1999, during MAP IOP5. In contrast to previously documented MAP cases, the environmental conditions associated with this case are characterized by a deep layer of strong convective instability in the lower troposphere and by a cold, northeasterly continental flow coming down from the mountains (the so-called bora wind) along the southeastern Alps. Over the study region, there are two primary mountain barriers: the Julian Alps, oriented roughly west–east with a peak mountain height of ∼2500 m and a significant variation in terrain height along its length, and the other, the Dinaric Alps, a relatively lower mountain range oriented northwest–southeast, immediately adjacent to the south of the Julian Alps. How these two mountain barriers and the northeasterly continental flow influence the precipitation, and the nature of orographic precipitation developing in a potentially unstable environment, are explicitly addressed. Comprehensive analyses of airborne Doppler radar measurements reveal significant variations of precipitation in terms of its location, intensity, and type over the mountainous region. Particularly, five distinct forcing types of precipitation are evident for this case: slope convergence triggering, upslope triggering, weak slope convergence triggering, gap exit convergence triggering, and slope convergence forced stratiform. These precipitation characteristics are found to be closely related to the environmental thermodynamics, orographic features, and the complicated interactions between the southerly/southwesterly flow, northerly continental flow, and the orography. Details of these important observational aspects are elaborated upon in this paper.

Corresponding author address: Dr. Cheng-Ku Yu, Department of Atmospheric Sciences, Chinese Culture University, 55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 111, Taiwan. Email: yuku@faculty.pccu.edu.tw

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