Operational implementation of the Fritsch-Chappell convective scheme in the 24-km Canadian regional model

The objective and subjective evaluations that led to the implementation of the Fritsch and Chappell (FC) convective scheme in the new 24-km Canadian o perational regional model are described in this study. Objective precipitat ion 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 increas ing the horizontal resolution and of including the FC scheme (instead of th e Kuo scheme used in the previous version of the operational model). The co mparison is particularly in favor of the FC configuration for the summertim e parallel preimplementation run, with improved biases and threat scores, w hile it is nearly neutral for the 12 benchmark cases comprised mostly of la rge-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 meso scale convective systems. A wintertime case study, on the other hand, revea ls that precipitation patterns with the two model configurations are quite similar, even though the FC scheme is essentially inactive for weather syst ems organized on such large scales. In contrast with the Kuo simulation, mo st of the precipitation occurs on the grid scale when using FC. This differ ent 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 de epening of coastal large-scale depressions. The different implicit/explicit partitioning for Kuo and FC is clearly expo sed with precipitation statistics from the 12 benchmark cases. With Kuo, it is found that implicit precipitation is produced over areas as large as (a nd even larger than) that associated with grid-scale precipitation; it is a lso 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 cove rs a small fraction of the model domain (i.e., 1%-2%) and that both precipi tation schemes are dominant in their respective areas, that is, weak precip itation for the explicit scheme and more intense precipitation for the impl icit scheme.