A general circulation model study of a tropopause‐folding event at middle latitudes

Abstract

AbstractThe UK Universities' Global Atmospheric Modelling Programme General Circulation Model (UGCM) has been run at high horizontal resolution to investigate a tropopause‐folding event at middle latitudes. Geopotential‐height, potential‐vorticity and specific‐humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut‐off low associated with the tropopause fold. the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper‐level frontal region and the corresponding jet‐streaks as they progressed through the region of the developing trough. A series of vertical cross‐sections at the ends of the jet streaks demonstrated that a fold could be captured by the UGCM using potential vorticity and specific humidity, although more small‐scale features were evident in the specific‐humidity contours than in the corresponding potential‐vorticity fields. the corresponding baroclinic zone was also resolved, although both the slope and thickness were rather larger than sometimes observed. Both the spatial and temporal extent of the fold compared well with observations. In addition the model simulated the ageostrophic winds in the entrance, exit and middle‐streak regions. A direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak, in good agreement with theory and with observational data. A combination of confluence, cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance, whereas the forcing at the jet exit was attributed mainly to diffluence and curvature. Q vectors were employed to investigate the vertical motion in the vicinity of the fold, and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak. Finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model: it was smaller in size than the fold of the cyclonic trough. A Q‐vector analysis confirmed the position of the small fold along the jet and revealed large‐scale ascending motion that may have been responsible for the small size of fold. the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause‐folding event at mid‐latitudes is discussed.