A NUMERICAL INVESTIGATION OF A MODERATE COASTAL STORM WITH INTENSE PRECIPITATION

In this study: the development of a moderate coastal storm with intens e precipitation that occurred during 12-14 February 1993 is examined u sing a high-resolution version of the Canadian Regional Finite-element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis ev ents and also the distribution and intensity of heavy mixed precipitat ion (rain and snow) associated with the storm. It is found that the cy clogenesis takes place in response to the low-level inshore advection of high-theta(e) air from the maritime boundary layer and the approach of a mid-level shortwave trough with a warm pool above that is previo usly associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm a nd cold fronts near the cyclone centre. Diagnosis of the control and s ensitivity simulations reveals that the low-level inshore warm advecti on and the propagation of the the stratospheric warm pool contribute m ore significantly to the surface pressure falls during the incipient s tage, whereas the mid-level shortwave trough plays an important role i n the cyclogenesis at Inter stages. Overall, latent heat release accou nts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation o f intense precipitation. The diabatic and kinematic structures near th e min-snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE m odel reproduces well the min-snow boundary structures as previously ob served. Moreover a thermally indirect circulation (perturbation) can b e seen in the vicinity of the rain-snow boundary. It is found, however that melting of snow tends to produce a weak negative or negligible i mpact on the cyclogenesis, as opposed to previous hypotheses.