STRUCTURE AND EVOLUTION OF A TROPOPAUSE FOLD DURING GALE IOP-1 - AN ETA MODEL STUDY

A fine-mesh regional model simulation of upper-level cyclogenesis is c arried out to examine the structure and evolution of the accompanying tropopause fold and its relationship to the surface and upper-level cy clones. The initial state for the simulation, conducted using the 80-k m, 16-level version of the National Meteorological Center Eta model, u ses the Level III-b gridded dataset for 1200 UTC, 18 January 1986, dur ing the First Intensive Observing Period (IOP-1) of the Genesis of Atl antic Lows Experiment (GALE) project. Results are presented from a 48 hour integration of the model. The emphasis is on the examination of t he synoptic scale evolution and structure of the upper-level cyclone a nd tropopause fold, both of which were successfully simulated in the m odel. The potential vorticity structure associated with a propagating jet-streak displayed distinctive structure, with its tilt reversing as the jet-streak moved around the base of an amplifying upper-level tro ugh. In addition, the model simulates the intrusion of dry, stratosphe ric air containing high potential vorticity anomalies into the lower t roposphere as well as subsidence warming when the folding of the tropo pause occurs. The model also predicts upper-level frontogenesis as a r esult of a thermally indirect secondary circulation in the exit region of the jet-streak. The success of the model simulation is most likely the result of comprehensive physics and the fine grid resolution empl oyed and, more importantly, the excellent distribution of subsynoptic scale initial data during the GALE project.