Dynamics and Boundary Layer Processes in Two Asian Cyclones

Abstract

An explosive and nonexplosive cyclone that developed during March 1986 along the coast of Japan are analyzed to illustrate the possible interaction of moist baroclinic dynamics, boundary layer structure and surface heat and moisture fluxes in their development. The moist symmetric stability is examined in the warm frontal ascent regions to investigate the contribution of surface energy fluxes to the moist baroclinic dynamics. Results indicate that moist symmetric neutrality occurs in the updraft regions of both cyclones and suggest a direct coupling between surface energy fluxes and the moist baroclinic processes. Surface flux distributions for the two cyclones show that large positive surface heat and moisture fluxes to the northeast of the low persisted in the explosive cyclone but were quite transient in the nonexplosive cyclone. Boundary layer equivalent potential temperature (?e) budgets reveal that approximately half of the local surface ?e increase in the warm frontal region throughout the development of the explosive cyclone is due to surface fluxes with the remainder due to horizontal advection. This contrasts sharply with the nonexplosive cyclone where there was a nearly equal initial contribution to local surface ?e increases by the surface fluxes in the warm frontal region that diminished rapidly during the development. Maintenance of the strong surface fluxes in the updraft region of the explosive cyclone is attributed to the interaction of the warm frontal dynamics with a downstream upper-level jet streak. Strong ageostrophic advection of cold air in the boundary layer resulted from the thermally direct vertical circulation of the warm front and jet entrance region. The nonexplosive cyclone lacked this strong thermally direct vertical circulation. Consequently, the large surface energy flux contributions in the explosive cyclone depended significantly on the baroclinic processes of the cyclone. This relationship was not present in the weaker cyclone.