Observations and numerical simulations of narrow cold-frontal rainbands

Narrow cold frontal rainbands (NCFRs) are narrow bands of heavy precipitation that often accompany surface cold fronts. The strong updraft that produces the NCFR is forced by a forward-moving, sharp boundary separating advancing cold air from warm-sector air. Many studies have documented the organization of NCFRs into a broken line of “precipitation cores”, which are approximately elliptical regions of intense precipitation 5-50 km in length, and which are oriented at a slight angle to the synoptic-scale front. The present study focuses on two cases of NCFRs. The first is an observational analysis of a case off the Washington coast, observed by Doppler radar and wind profilers, on 2 February 2001. This case is unique in that the cold front with which the band was associated, as well as the inflow into the band, were not surface-based, but were detached from the surface, situated above a ~1-km deep layer of rearward moving cool air that passed unimpeded underneath the cold front. This case demonstrates the non-necessity of a true gravity current (with flow stagnation at the surface) for the formation of a narrow cold frontal rainband. The second case is a modeling simulation of the NCFR in the North Atlantic that was well-documented by Wakimoto and Bosart study. This modeling study explores the competing theories for the formation and maintenance of the corrugated structure, including horizontal shear instability, trapped gravity waves, and microphysical processes.