Variations Associated with Cores and Gaps of a Pacific Narrow Cold Frontal Rainband

Abstract

A NOAA P-3 instrumented aircraft observed an intense, fast-moving narrow cold frontal rainband (NCFR) as it approached the California coast on 19 February 2001 during the Pacific Coastal Jets Experiment. Airborne Doppler radar data obtained while the frontal system was well offshore indicated that a narrow ribbon of very high radar reflectivity convective cores characterized the rainband at low levels with echo tops to ?4?5 km, and pseudo-dual-Doppler analyses showed the low-level convergence of the prefrontal air. The NCFR consisted of gaps of weaker reflectivity and cores of stronger reflectivity along its length, perhaps as a result of hydrodynamic instability along its advancing leading edge. In contrast to some earlier studies of cold frontal rainbands, density-current theory described well the motion of the overall front. The character of the updraft structure along the NCFR varied systematically along the length of the precipitation cores and in the gap regions. The vertical shear of the cross-frontal low-level ambient flow exerted a strong influence on the updraft character, consistent with theoretical arguments developed for squall lines describing the balance of vorticity at the leading edge. In short segments at the northern ends of the cores, the vertical wind shear was strongest with the updrafts and rain shafts more intense, narrower, and more erect or even downshear tilted. At the southern ends of the cores and just north of the gaps, the wind shear weakened with less intense updrafts that tilted upshear and contained a broader band of rainfall. Simulations using the nonhydrostatic nested grid version of the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5) are used to investigate the core and gap regions, focusing on the relationship between the character of the modeled updrafts and the balance between the cold-air-induced vorticity and the prefrontal ambient shear vorticity. The cold air behind the NCFR, which forces new convection along its leading edge, is probably maintained by large-scale advection of cold air plus evaporative cooling processes within the heavy rain region of the NCFR. Observations confirm the model results; that is, that the updraft character depends on the balance of vorticity at the leading edge. Downshear-tilted updrafts imply that convection at the northern ends of cores may weaken with time relative to the frontal segments at the southern ends, because inflow air would be affected by passage through the heavy rain region before ascent. A mechanism for line modification is thus implied.