A numerical case study of secondary marine cyclogenesis sensitivity to initial error and varying physical processes

Secondary cyclogenesis has been identified as a difficult forecast challeng e. In this paper, the authors examine the dominant physical processes assoc iated with the predictability of a case of explosive secondary marine cyclo genesis and provide a better understanding of the large variability in the recent model-intercomparison simulations of the case. A series of sensitivi ty experiments, involving changes to the model initial conditions and physi cal parameterizations, is performed using the Canadian Mesoscale Compressib le Community Model with a grid size of 50 km. It is found that errors in the model initial conditions tend to decay with time, and more rapidly so in "dry" simulations. The model fails to produce the secondary cyclogenesis in the absence of latent heating. water vapor bu dget calculations from the control experiment show that the surface moistur e flux from 6 to 12 h is the largest contributor of water vapor to the budg et area in the vicinity of the cyclone center and remains an important mois ture supply throughout the integration period. During the first 12 h, these fluxes are crucial in inducing grid-scale diabatic heating and destabilizi ng the lower troposphere, thereby facilitating the subsequent rapid deepeni ng of the storm. A secondary maximum in surface latent heat flux to the nor th and east of the primary maximum acts to force the cyclogenesis event to the south and east of a coastal circulation center. When the surface evapor ation is not allowed, much less precipitation is produced and the secondary cyclone fails to develop. Calculations of the potential temperature on the dynamic tropopause (i.e., 2-PVU surface) in the absence of surface evapora tion indicate a significantly damped thermal wave when compared with the co ntrol integration. This result for a case of secondary cyclogenesis differs from those general ly found for large-scale extratropical cyclogenesis where upper-level baroc linic forcings tend to dominate, and motivates the need for better physical parameterizations, including the condensation and boundary layer processes , in operational models. The authors speculate that the different treatment of condensation and boundary layer processes may have been partly responsi ble for the enhanced variability in the simulation of this case in a recent ly completed international mesoscale model intercomparison experiment.