ABSTRACT

In this paper, the sensitivity of precipitation associated with ordinary baroclinic continental winter storms to small variations in the synoptic-scale circulation is explored using ensembles of observed regional analogs. When seeking time-coherent analogs for periods of 1–2 days in length on the basis of anomaly correlation scores at both the surface and 500 hPa, the observed precipitation structures are strongly dependent on circulation details, reflecting subtle differences at both the synoptic scale and mesoscale. It has also been found that when seeking time-coherent regional analogs on the basis of the observed precipitation distributions, such analogs cannot be identified. However, coherent precipitation structures that are identified for shorter periods (order 1 day) are found to occur in association with highly dissimilar circulations. Thus, the sensible weather that results from a particular circulation is crucially dependent on the details of the flow. The authors believe that these findings provide some explanation for the relatively slow advance of precipitation forecast skill in the face of continuously improving forecasts of the large-scale circulation.

The study of the evolution of the base case and its closest circulation analog is split into two successive 24-h periods. During the first 24 h, the substantial differences in the observed precipitation are linked to the complex interplay between precipitation events prior to the period of interest (which led to the generation of upwind sources of moisture) and synoptic-scale dynamics (primarily differences in the structure and intensity of the subtropical jet, which led to slight deviations in the low-level flow between the two cases and subsequent differences in moisture advection from the Gulf of Mexico and moisture convergence). During the second 24 h, differences in the observed precipitation are tied to differences in the convective response despite similarly potentially unstable environments, reflecting differences in synoptic and mesoscale triggering mechanisms. To more fully resolve the relative importance of these mechanisms, experiments with a mesoscale numerical model will be reported in a subsequent paper.