Climatology of upper‐tropospheric turbulence: Capabilities and limitations of aircraft reports and ERA5 reanalysis diagnostics

Abstract A climatology of upper‐tropospheric turbulence is constructed from eight years of automated Aircraft Communications Addressing and Reporting System (ACARS) turbulence reports from commercial aircraft. Seasonal frequency maps for consistently reported turbulence intensities show contiguous planetary‐scale occurrence frequency maxima over the North Atlantic and North Pacific winter storm tracks, and along tropical flight routes over both oceans. The analyzed turbulence intensities are encountered less often over North America, except for a widespread increase in the east during spring. Low Richardson numbers calculated from European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis Version 5 (ERA5) reanalysis fields and interpolated to report locations are diagnosed most frequently in regions where turbulence is observed most frequently. The frequency maps of the 99th percentile of the Richardson number largely mirror the large‐scale patterns in the turbulence frequency maps. In comparison, the turbulence index (TI1) diagnoses increased potential for turbulence most frequently at high latitudes north of where turbulence is observed most frequently. For the analyzed 99th percentile thresholds of the diagnostics, the probability of detection (POD) is around 20%, despite the overprediction at average turbulence frequencies of 0.15%. Although the POD increases with higher diagnostic percentiles, this comes at the cost of an exponentially decreasing precision. Representative precision is achieved, if at all, only for a small fraction of the observed turbulence. Near high clouds, as indicated by low satellite infrared brightness temperatures, turbulence frequencies increase by a factor of more than five. However, a non‐negligible POD is again only achieved at the expense of significant overprediction by the diagnostics. So, whereas is observed most frequently in regions where small Richardson numbers are diagnosed most frequently in ERA5, the rare occurrence of the analyzed turbulence intensity remains hidden in the diagnostic frequency maps.