A Numerical Investigation of Low-Level Processes in Rapid Cyclogenesis

Abstract

Several physical processes and properties of the initial state that affect marine cyclogenesis are examined using a mesoscale numerical model. The sensitivity of an idealized cyclone to the effects of latent heat release, surface heat and moisture fluxes as well as the initial meridional temperature gradient and static stability is examined by comparing various numerical stimulations of cyclogenesis in a baroclinic channel-flow model. Idealized initial conditions are derived analytically and are characterized by strong low-level baroclinity and a very weak upper-level trough. These initial conditions are used to examine which factors in baroclinic cyclogenesis are most important for rapid development (1 m h?1 for 24 h or more) and how diabatic processes modify the development rate. A strong low-level meridional gradient (40°C/2000 km) and low static stability (a mean lapse rate of 6.0°C km?1) resulted in rapid development of the model cyclone. The model cyclogenesis is more sensitive to small changes in the initial baroclinity than to physical processes during the development, which suggests that sustained rapid development requires substantial baroclinic instability. Inclusion of latent heat release during the development resulted in only a 10% increase in the average deepening rate. This effect of latent heating depended crucially upon the moisture distribution and is more representative of large-scale stable condensation than strong convection. Modification of the model cyclogenesis by various surface heat and moisture flux distributions indicated that the phase and magnitude of these fluxes relative to the low-level atmospheric baroclinity is important. A distribution of surface heating that enhanced the low-level baroclinity resulted in a 15% increase in growth rare, suggesting an important interaction during certain periods of development. Surface heating distributions that reduced the low-level baroclinity by counteracting thermal advection damped the development of the model cyclone as suggested by previous studies.