A Climatology of Tropopause Folds by Global Analyses

Summary A global ECMWF data set comprising ten years of daily routine T63 analyses is employed for the derivation of regional and temporal tropopause fold statistics. Due to the small scale of folds relative to grid resolution it was found to be convenient to identify them by joint occurrences of potential vorticity maxima and upper tropospheric Q-vector divergences as indicators. The applicability of the proposed method is shown by diagnostic studies of episodic mesoscale model simulations, with 40–50 km resolution as a basis for reference and comparison with the global data set. Tropopause folds are a frequent and rather uniformly occurring phenomenon of the extratropics pole-ward of 30° latitude. On the global scale, and based on daily counting, there are about 11 000 tropopause fold days per year. In order to account for the different folding depth and intensity a four step classification is proposed to define a tropopause fold activity by weighted sums, in terms of four intensity classes. Tropopause fold activity in the Northern Hemisphere is nearly twice as high and less homogeneously distributed as in the Southern Hemisphere, where only 62% of the tropopause fold frequency is estimated. The frequency of tropopause folds is significantly smaller in the summer than in the other seasons. Received September 10, 1996 Revised June 24, 1997