Cold-Frontal Potential Vorticity Maxima, the Low-Level Jet, and Moisture Transport in Extratropical Cyclones

An elongated cold-frontal maximum in the lower-tropospheric potential vorticity (PV) field accompanies some midlatitude cyclones. These PV maxima are often of diabatic origin, and are hypothesized to contribute substantially to the strength of the low-level jet (LLJ) and moisture transport in the cyclone warm sector. Diagnosis of a representative cyclone event from the central United States during February 1997 is presented with the goals of (i) elucidating the mechanisms of development and propagation of the cold-frontal PV band, and (ii) clarifying the relation between this PV maximum and the LLJ. A confluent upper trough and modest surface cyclone followed a track from the south-central United States northeastward into southern Ontario between 26 and 28 February 1997, accompanied by flooding and widespread straight-line wind damage. A LLJ, with maximum wind speeds in excess of 35 m s−1, was positioned at the western extremity of the cyclone warm sector, immediately east of an elongated PV maximum in the lower troposphere. Results of an Ertel PV budget confirm the importance of latent heat release to the development and eastward propagation of the PV band. Cancellation was observed between the vertical PV advection, which yielded negative (positive) tendencies beneath (above) the cold-frontal PV maximum, and the nonadvective PV tendency, which was positive (negative) beneath (above) the level of maximum heating. The nonadvective PV flux is directed opposite the absolute vorticity vector; therefore vertical wind shear (associated with westward-tilting absolute vorticity vectors) led to eastward nonadvective propagation of the PV maximum. Quasigeostrophic PV inversion indicates that the cold-frontal PV maximum contributed between 15% and 40% to the strength of the LLJ within the cyclone warm sector. The results of this study suggest that a complex interdependence can exist between cold-frontal rainbands, lower-tropospheric PV maxima, the LLJ, and warm-sector moisture transport. The implications of this linkage for numerical weather forecasting are discussed.