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FASTEX IOP 18: A very deep tropopause fold. II: Quasi‐geostrophic omega diagnoses
Author(s) -
Donnadille Jérǒme,
Cammas JeanPierre,
Mascart Patrick,
Lambert Dominique
Publication year - 2001
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712757704
Subject(s) - geology , advection , geostrophic wind , wind shear , tropopause , streak , climatology , curvature , frontogenesis , geophysics , mesoscale meteorology , geometry , physics , wind speed , troposphere , mathematics , oceanography , thermodynamics
A set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold (820 hPa) during the Intensive Observing Period 18 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). In this second part, diagnoses of vertical motion using a Q‐vector partitioning in the natural coordinate system that follows the geostrophic wind are analysed. The partitioning allows the evaluation of vertical motions associated with forcing mechanisms such as confluence and diffluence, thermal advection by the horizontal geostrophic shear (shear advection) and curvature of the flow. The synoptic situation involves the formation of an intense upper‐level jet streak when an Arctic trough and a southern ridge move into phase with each other. Results assessed in the course of the tropopause‐fold life cycle show that subsiding vertical motions associated with each of the forcing mechanisms (confluence, shear advection and curvature) overlap on the cyclonic‐shear side of the entrance region of the jet streak. It is shown that an additional effect of shear advection over the confluence, a necessary ingredient for the development of deep tropopause folds in two‐dimensional contexts, takes place in the present case‐study. However, the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper‐level frontal zone (i.e. with a maximum frontogenetic effect) are, in order of importance, the shear advection and the curvature.