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: 1) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, divergence, and divergence tendency], and 2) theoretical instability indicators Richardson number (Ri), potential vorticity (PV), and Brunt-Vӓisӓlӓ frequency. The empirical indices showed high frequencies of MOG-level CAT potential over the East Asian, Eastern Pacific, and Northwest Atlantic regions in winter. Over East Asia, the entrance region of the strong upper-level jets, showed the highest frequencies in TI1, TI2, and TI3 mainly due to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of the jet had relatively high frequencies of these and also Ri. PV frequency was high on the southern side of the 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 the warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and 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 are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean.

Downslope windstorms are responsible for wildfires, wind gusts, and turbulence in the lee side of the Taebaek Mountains, called Yeongdong region (YD) in Korea. We classified the synoptic conditions of the windstorms in the YD using a Self-Organizing Map (SOM). For the windstorm events from 1979 to 2019, sea level pressure anomalies were used to train the SOM. It was found that the synoptic patterns could be classified into three representative types: 1) the south high and north low pattern in spring, 2) the west high and east low pattern in winter, and 3) the strong low-pressure system passing the northern part of Korea. At the 850 hPa level, prevailing southwesterly (nortwesterly) flow with warm (cold) advection was dominant in Type 1 (2), and Type 3 presented a well-developed baroclinic system of cyclone. Adiabatic warming by downslope windstorm is the strongest in Type 1, which is likely to have a huge impact on the spread of wildfires. Three mesoscale generation mechanisms were examined under different synoptic patterns. Hydraulic jump theory was dominant for the windstorms in Type 2 due to upstream flows with moderate Froude numbers and inversion layers. The partial reflection of mountain waves was found in all types but more frequent in Type 1 than others. Downslope windstorms with wave breaking at critical levels mostly occurred in Type 1. This objective classification of weather patterns responsible for downslope windstorm in the YD is useful for better prediction and future projection of this event with climate change.