Climatology of clear‐air turbulence in upper troposphere and lower stratosphere in the Northern Hemisphere using ERA5 reanalysis data

Spatial and temporal distributions of Clear-Air Turbulence (CAT) in the Northern Hemisphere were investigated using 41 years (1979-2019) of the European Centre for Medium-range Weather Forecast Reanalysis version 5 (ERA5) data. We used two groups of CAT diagnostics to determine occurrence frequencies: (a) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, -divergence, and divergence tendency], and (b) theoretical instability indicators [Richardson number (Ri), potential vorticity (PV), and Brunt-V?is?l? frequency]. The empirical indices showed high frequencies of moderate-or-greater (MOG)-level CAT potential over the East Asian, Eastern Pacific, and Northwestern Atlantic regions in winter. Over East Asia, the entrance region of strong upper-level jets showed the highest frequencies in TI1, TI2, and TI3 due mainly to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of jets had relatively high frequencies of these indices and also Ri. PV frequency was high on the southern side of jet primarily due to negative relative vorticity. Long-term increasing trends of MOG-level CAT potential also appeared in those three regions mainly due to warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and increased VWS due to the strong meridional temperature gradients in the mid-troposphere induced by warming in the tropics and cooling in eastern Eurasia. These trends over East Asia, if continued, are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean. Plain Language Summary Strong bumpiness of cruising aircraft can lead to physical injuries for crews and passengers, which is mostly occurred due to atmospheric turbulence by enhanced wind shear near upper-level jet stream. Spatial characteristics of turbulence and their long-term trends at cruising altitude in Northern Hemisphere were examined using the up-to-date global reanalysis data. The result showed that East Asia, Eastern Pacific, and Northwestern Atlantic regions have a higher frequency of turbulence than other areas. In particular, East Asia was found to be the highest frequency and the largest increasing trend of strong turbulence, which is collocated with a heavy air-traffic region between East Asia and North America across the North Pacific Ocean. This implies that the potential of encountering turbulence over East Asia could be increased further, if the jet stream will be strengthened continually due to climate change in this region.