摘要
This study uses eddy kinetic energy analysis and a targeting method to investigate how an extratropical transition（ET）event induced downstream development（the modification of the midlatitude flow downstream of the ET system） in the midlatitude jet environment. The downstream development showed distinct characteristics of ＂coupling development＂ and being ＂boundary-trapped＂. Eddies（potential disturbances） first developed at the upper levels, and these triggered lower-level eddy development, with all eddies decaying away from the tropopause and the surface. Thereafter, a lower-level eddy caught up with the upper-level eddy ahead of it, and they coupled to form a cyclone extending through the whole troposphere. Vertical ageostrophic geopotential flux may be a crucial dynamic factor throughout the eddy＇s lower-level growth, boundary-trapping,and coupling development.Together with barotropic conversion, the ageostrophic geopotential fluxes that were transported from Hurricane Fabian（2003） to the midlatitudes by the outflow led to downstream ridge development in the upper-level jet. The strong downstream advection of eddy kinetic energy in the exit region of the jet streak triggered downstream trough development. The well-known ridge–trough couplet thus formed. The vertical ageostrophic fluxes that were transported downward from the developed upper-level systems converged near the surface and resulted in lower-level eddy growth. Baroclinic conversion was negligible near the boundaries, while it was the main source of eddy kinetic energy at mid-levels. In the upper-level jet, potential energy was converted to the mean kinetic energy of the jet, which in turn was converted to eddy kinetic energy through barotropic conversion.
This study uses eddy kinetic energy analysis and a targeting method to investigate how an extratropical transition（ET）event induced downstream development（the modification of the midlatitude flow downstream of the ET system） in the midlatitude jet environment. The downstream development showed distinct characteristics of ＂coupling development＂ and being ＂boundary-trapped＂. Eddies（potential disturbances） first developed at the upper levels, and these triggered lower-level eddy development, with all eddies decaying away from the tropopause and the surface. Thereafter, a lower-level eddy caught up with the upper-level eddy ahead of it, and they coupled to form a cyclone extending through the whole troposphere. Vertical ageostrophic geopotential flux may be a crucial dynamic factor throughout the eddy＇s lower-level growth, boundary-trapping,and coupling development.Together with barotropic conversion, the ageostrophic geopotential fluxes that were transported from Hurricane Fabian（2003） to the midlatitudes by the outflow led to downstream ridge development in the upper-level jet. The strong downstream advection of eddy kinetic energy in the exit region of the jet streak triggered downstream trough development. The well-known ridge–trough couplet thus formed. The vertical ageostrophic fluxes that were transported downward from the developed upper-level systems converged near the surface and resulted in lower-level eddy growth. Baroclinic conversion was negligible near the boundaries, while it was the main source of eddy kinetic energy at mid-levels. In the upper-level jet, potential energy was converted to the mean kinetic energy of the jet, which in turn was converted to eddy kinetic energy through barotropic conversion.
基金
jointly supported by the International Cooperating Program of Science and Technology (Grant No. 2010DFA24650)
the National Natural Science Foundation of China (Grant No. 41175061)
