Operational Implementation of the Fritsch–Chappell Convective Scheme in the 24-km Canadian Regional Model

Abstract

The objective and subjective evaluations that led to the implementation of the Fritsch and Chappell (FC) convective scheme in the new 24-km Canadian operational regional model are described in this study. Objective precipitation scores computed for a series of 12 benchmark cases equally distributed throughout all seasons and for a parallel preimplementation run of the new version of the model during summer 1998 show the positive impact of increasing the horizontal resolution and of including the FC scheme (instead of the Kuo scheme used in the previous version of the operational model). The comparison is particularly in favor of the FC configuration for the summertime parallel preimplementation run, with improved biases and threat scores, while it is nearly neutral for the 12 benchmark cases comprised mostly of large-scale weather systems. Examination of a summertime case study confirms the superiority of FC over Kuo for the numerical representation of the structure and evolution of mesoscale convective systems. A wintertime case study, on the other hand, reveals that precipitation patterns with the two model configurations are quite similar, even though the FC scheme is essentially inactive for weather systems organized on such large scales. In contrast with the Kuo simulation, most of the precipitation occurs on the grid scale when using FC. This different partitioning of precipitation into implicit and explicit components is more consistent with the mesoscale-resolving capabilities of the model. It is also observed that the new model physics gives rise to more realistic deepening of coastal large-scale depressions. The different implicit/explicit partitioning for Kuo and FC is clearly exposed with precipitation statistics from the 12 benchmark cases. With Kuo, it is found that implicit precipitation is produced over areas as large as (and even larger than) that associated with grid-scale precipitation; it is also shown that with this configuration most of the precipitation occurs at weak rates and is mainly produced by the implicit scheme. The results with FC are more realistic, in the sense that convective precipitation only covers a small fraction of the model domain (i.e., 1%?2%) and that both precipitation schemes are dominant in their respective areas, that is, weak precipitation for the explicit scheme and more intense precipitation for the implicit scheme.