Effects of the New England Coastal Front on the Distribution of Precipitation

Frank D. Marks Jr. Department of Meteorology, Massachusetts Institute of Technology, Cambridge 02139

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Pauline M. Austin Department of Meteorology, Massachusetts Institute of Technology, Cambridge 02139

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Abstract

Precipitation patterns have been analyzed for six wintertime storms in New England where coastal fronts developed and for two without coastal fronts. In all of the storms the predominant precipitation features, as observed by radar, were mesoscale bands which contained convective cells, a pattern typical of extratropical cyclones. Vertical profiles of potential temperature, humidity and wind indicate that most of the condensation occurred in a layer of warm moist air lifted by synoptic-scale ascent ahead of the baroclinic disturbance. Cumulus convection was initiated in a shallow unstable region at the top of the warm moist layer. The coastal front circulations apparently developed independently of the large-scale baroclinically induced circulations and were very shallow, typically 300 m in depth. They had durations of 7–15 h and existed during most of the time when the precipitation bands were passing over eastern Massachusetts. The effect of the coastal fronts was to enhance the precipitation over an area about 80 km wide along a line between Boston, Massachusetts, and Providence, Rhode Island, with the average increase ranging from 13 to 147%. The mechanism for precipitation enhancement appears to be creation of low cloud by the coastal front circulation. The cloud droplets are then accreted by snow-flakes which originated at higher levels.

Abstract

Precipitation patterns have been analyzed for six wintertime storms in New England where coastal fronts developed and for two without coastal fronts. In all of the storms the predominant precipitation features, as observed by radar, were mesoscale bands which contained convective cells, a pattern typical of extratropical cyclones. Vertical profiles of potential temperature, humidity and wind indicate that most of the condensation occurred in a layer of warm moist air lifted by synoptic-scale ascent ahead of the baroclinic disturbance. Cumulus convection was initiated in a shallow unstable region at the top of the warm moist layer. The coastal front circulations apparently developed independently of the large-scale baroclinically induced circulations and were very shallow, typically 300 m in depth. They had durations of 7–15 h and existed during most of the time when the precipitation bands were passing over eastern Massachusetts. The effect of the coastal fronts was to enhance the precipitation over an area about 80 km wide along a line between Boston, Massachusetts, and Providence, Rhode Island, with the average increase ranging from 13 to 147%. The mechanism for precipitation enhancement appears to be creation of low cloud by the coastal front circulation. The cloud droplets are then accreted by snow-flakes which originated at higher levels.

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