NUMERICAL SIMULATION AND ISENTROPIC ANALYSIS OF FRONTAL CYCLONES OVER THE WESTERN ATLANTIC OCEAN

Frontal cyclogenesis is a widespread weather phenomenon.In spite of the marked improvements in the study of this kind of phenomenon,many problems ramain.Our understanding and prediction of this type of mesoscale vortex is still hampered by(1)the lack of high-resolution data and(2)the absence of theoretical models.This kind of understanding and forecast is still among the most serious challenges to at mospheric scientists.Here by using PSU/N CAR MM5 mesoscale model,a 60-h simulation is performed to reproduce a frontal cyclogenesis over the Western At lantic Ocean during March 13-15,1992.Beginning with Slantwise Vorticity Development(SVD),the genesis,development and propagation are studied by using high-resolution model output in the context of slantwise isentropic surface.And inspiring results are received.The model reproduces well the genesis,track and intensity of the cyclones,their associated thermal structure as well as their surface circulation.The major cyclone(M)deepens 45hPa in the all 60h and 12hPa in 6h from 36 to 42(model time)and 27hPa in 24h from 36 to 60(model time).Profile and isentropic analysis tell us that the cyclogenesis is in very close relation with slantwise isentropic surface;The cyclones always superpose with the core of neutral convection stability with nearly vertical isentropic surface,which coincides with what SVD says.Beginning with Slantwise Vorticity Development(SVD),the development and propagation of the oceanic frontal cyclone are studied by using high-resolution model output in the context of slantwise isentropic surface.The results show that the frontal cyclone deepens rapidly by interaction with large-scale environment after occurring over ocean with weak hydrostatic stability;SVD theory can well translate the development and propagation,which is closely related with slant wise isentropic surface.The downstream slantwise upsliding movement and declination of isentropic surface make vorticity develop(USVD)under favorable conditions(CDCD is SVD index),and result in the moving and development of cyclone.