Observed and simulated kinematic, thermodynamic, and microphysical structure of generating cells in a wide cold-frontal rainband

During the PACJET-2001 field program, the NOAA P-3 aircraft sampled a strong, convective cold front over the eastern Pacific Ocean off the coast of Southern California. The flight track was such to permit both along-front and cross-front cross-sections of in-situ thermodynamic, kinematic, and microphysical data, as well as obtaining Doppler radar data. Reflectivities along the distinct narrow cold-frontal rainband (NCFR) reached over 50 dBz, and the NCFR was organized into typical core and gap regions. A wide cold-frontal rainband with more moderate precipitation extended approximately 100 km to the west behind the NCFR. Mesoscale model simulations of this case used grid-spacings to 1.7 km while maintaining an adequate grid-spacing aspect ratio requiring high vertical resolution. The focus will be on validating the small mesoscale kinematic, thermodynamic, and microphysical structure of the generating cells producing the wide cold-frontal rainband. The simulation realistically produced these generating cells as represented by the vertical velocity field, though substantial differences are seen in the microphysical validations. To our knowledge, this is the first documentation of a successful simulation of rainband generating cells in a case with key validation data. The implications of the grid resolution and the microphysical parameterizations on the success of simulations of cold-frontal rainbands will be discussed. Because the simulation successfully represented the generating cells and wide cold-frontal rainband, the model output is examined for the dynamical mechanism generating these cells. Analyses show that negative moist potential vorticity (MPV) was present at 700 mb near the base of the multiple generating cell updrafts, and that updrafts were oriented in bands along the mid-level shear vector both in the observations and in the model. This suggests that these generating cells were produced either by conditional symmetric instability (CSI) or, if the air is stable to viscous CSI, by large-scale quasi-geostrophic forcing of this low-stability, negative MPV region as documented by Xu (1992). Additional insights into the formation mechanism will be provided.