Frontal contraction processes leading to the formation of an intense narrow rainband

Abstract

Processes which played an important role in the sudden development of an intense narrow cold frontal rainband (NCFR) along the leading edge of a wintertime anafront are investigated. Quantitative analysis of the Regional Analysis and Forecast System (RAFS) initialization and Nested Grid Model (NGM) forecast fields, satellite-derived water vapor and ozone measurements, and conventional meteorological data are employed in this study. The NCFR developed upon merger of an arctic front with a nearly stationary cold front over the Appalachian mountains. Rapid intensification of an ageostrophic momentum surge associated with the formation of a sub-synoptic played a dominant role in forcing explosive frontogenesis during frontal merger. A deep tropopause fold and related mid-tropospheric subsidence feature within the entrance region of an upper-level jet crossed the mountains at the time of NCFR formation. Synthesis of the surface, NGM, and satellite data suggests that this subsidence resulted in the development of the sub-synoptic high pressure system, whose leading edge was marked by a 5mb pressure jump. Consequential amplifieation and scale contraction of the cross-front pressure gradient indicates that the scale of the merged frontal system contracted just prior to NCFR development. As precipitation developed, the pressure jump took on a microscale character and increased to 8mb amplitude. The subsequent intensity of this frontal rainband is explained by the interaction between vertical shear associated with a strong, pre-frontal, low-level jet and the gravity current-like structure at the leading edge of the merged cold front.