Mesoscale Precipitation Areas in a Warm Frontal Wave

Abstract

A detailed case study is made of mesoscale precipitation areas (MPA's) within a warm frontal wave with the following aims: 1) To identify large (LMPA) and small (SMPA) mesoscale precipitation areas within the wave and elucidate their inter-relationships throughout their lifetimes. 2) To clarify the relationships between the behavior outlined in 1) and the synoptic-scale airflow. Synoptic-scale airflows were identified by isentropic relative flow analysis and cross sections of wet-bulb potential temperature described the thermodynamic structure of the front. Each autographic raingage record from 327 stations in England and Wales was divided into approximately 130 13 min periods and precipitation maps for each period were produced giving a sequence of precipitation distributions as the front crossed England and Wales. Three types of LMPA's were found: one linear and parallel to the front, one linear and perpendicular to the front, and one nonlinear and moving through the front. The linear LMPA's comprised 3?4 SMPA's and the different lifetimes and movements of the SMPA's led to the different lifetimes, orientations and movements of the LMPA's. In all cases, the primary cause of the MPA's appeared to be the release of potential instability by the synoptic-scale uplift. MPA development and demise showed marked sensitivity to the synoptic-scale airflow. The nonlinear LMPA developed in a comparatively stable atmosphere, with no ready explanation for this. The ?budding blocks? of mesoscale systems within cyclones appear to be the small, rather than the large, mesosystems. We remain confronted by the problem of why this should be so.