A squall-like narrow cold-frontal rainband diagnosed by combined thermodynamic and cloud microphysical retrieval

Abstract

Doppler radar, airborne, and sounding data are used to describe the structure of an exceptionally vigorous narrow cold-frontal rainband (NCFR). A combined thermodynamic and cloud microphysical retrieval technique is used to derive the pressure perturbations and buoyancy in balance with the flow field in the vicinity of the NCFR. The NCFR was triggered by a well-defined cold front that penetrated beneath a deep layer of conditionally unstable prefrontal air. The rainband, which persisted for ∼ 6 h, was located over the Gulf Stream, just off the mid-Atlantic Coast of the U.S.A. It consisted of a series of short-lived cells with updraft maxima between 5 and 15 m s −1 and tops around 12 km. The updraft associated with the NCFR was driven primarily by buoyancy in the middle troposphere, which explains its convective nature. Trailing behind the NCFR was a broad region of stratiform precipitation. Pressure perturbations in the NCFR and the trailing stratiform region were produced mainly by a combination of frontal convergence and buoyancy opposition. The pressure perturbations surrounding the cells within the rainband were due primarily to strong updrafts in the presence of wind shear parallel to the rainband. The same dynamic and cloud microphysical processes that operate in stratiform regions behind some squall lines were present in this NCFR. However, in contrast to a squall line with a trailing stratiform region, the low-level cold pool that penetrated beneath the NCFR was maintained primarily by strong cold advection, rather than by evaporational cooling in a shallow rear-to-front flow. The preponderance of advective over diabatic cooling can be used as a criterion to diagnostically differentiate between NCFRs and squall lines.