Abstract
A numerical study is conducted using the Naval Research Laboratory (NRL) limited-area model to study the evolution and structure of a rapidly intensifying marine cyclone observed during intensive observing period 4 (IOP 4; 4–5 January 1989) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) over the North Atlantic Ocean.
The single grid version of the NRL model used in the study has 16 layers in the vertical with a horizontal resolution of 1/3° longitude and 1/4° latitude. The primitive equation, hydrostatic model in sigma coordinates includes parameterized physics of cumulus convection, radiation, and the planetary boundary layer. The National Meteorological Center (NMC) Regional Analysis Forecast System (RAFS) analysis is used to provide the initial and boundary conditions.
Starting from the 0000 UTC 4 January RAFS initialization, the control model simulates the ensuing cyclogenesis, deepening the initial disturbance from 998 to 952 mb in 24 h. While the simulated cyclone is about 15 mb weaker than that observed, the simulation reproduced many of the well-documented observed features of the IOP 4 cyclone, such as the remarkable comma-shaped precipitation pattern, bent-back warm front, warm-core seclusion, and secondary cold front. Control model results show that (i) the strongest temperature and water vapor gradients are aligned with the warm front and secondary cold front, not the primary cold front, (ii) the major precipitation and strongest vertical motion are along the warm front and its bent-back extension, (iii) the cyclonic circulation is displaced well to the southwest of the triple point, and (iv) the cellular convection occurs behind the secondary cold front accompanied by extreme surface sensible and latent beat transfer with a total maximum flux exceeding 3000 W m−2 over the Gulf Stream approximately 100 km offshore of the Carolinas. A detailed analysis of model results is performed and is found to be in excellent agreement with available satellite and mesoscale observations.
Sensitivity experiments are also conducted to identify the importance of various dynamical and physical processes contributing to the rapid intensification. Results from sensitivity tests show that (i) the dynamic processes are more responsible for the rapid intensification and unique structure of the marine cyclone than the physical processes, (ii) both the sea surface heat transfer and the release of latent heat in clouds contribute positively to the cyclogenesis, (iii) physical processes combine to intensify the storm in a nonlinear fashion, and (iv) the formation of unique features associated with the IOP 4 storm such as the bent-back extension of the warm front, warm-core seclusion, and westward development of the low pressure center away from the triple point are not sensitive to physical processes.
