A Northern Hemispheric climatology of indices for clear air turbulence in the tropopause region derived from ERA40 reanalysis data

Clear air turbulence (CAT) has relevance for aviation but also for cross‐tropopause transport of chemical constituents. This study presents a 44‐year climatology (from 1958 to 2001) of indicators for CAT for the Northern Hemisphere tropopause on the basis of reanalysis data (ERA40) from the European Centre for Medium‐Range Weather Forecasts. Small Richardson numbers ( Ri ), negative squared Brunt‐Väisälä ( N 2 ) and negative potential vorticity (PV) values are used as indicators for Kelvin‐Helmholtz, hydrostatic, and symmetric instability, respectively. Additionally, an empirical indicator (TI) is applied. All indicators have winter frequency maxima for CAT over the North American east and west coasts. Other local maxima are found over the western part of the North Atlantic and North Pacific, the Himalayas, central Europe, and eastern China. In summer, frequencies are smaller, except for N 2 , and patterns are shifted poleward. Frequencies of TI and PV depend strongly on jet position, whereas Ri has moderate dependence. N 2 frequencies are largest over land and are not markedly influenced by jets. The frequency maxima relative to the jets differ with a northern maximum for TI, a southern maximum for PV, and one for Ri that is essentially along the jet axis. For the 44‐year period pronounced nonlinear trends are identified with an increase of 40–90% over the North Atlantic, United States, and European sector. For the two phases of the North Atlantic Oscillation and the Pacific/North American flow pattern the interannual variability of CAT, indicated by TI and PV, is significant (in contrast to Ri and especially N 2 ).