Climatology of Clear‐Air Turbulence in Upper Troposphere and Lower Stratosphere in the Northern Hemisphere Using ERA5 Reanalysis Data

Abstract

The jet stream and associated vertical wind shears (VWSs) are strongest in middle latitudes in the upper troposphere (z = 8-12 km), and their existence is largely explained by the meridional temperature gradient (MTG) between the tropics and polar regions via the thermal wind relation (e.g., Holton, 1992;Lee et al., 2019). Jets are closely associated with dynamic weather phenomena such as storm tracks and cyclogenesis. However, they are also responsible for aviation weather hazards including upper-level turbulence encountered by cruising aircraft in the upper troposphere and lower stratosphere (UTLS) (e.g., Kim et al., 2016;Sharman et al., 2012). These turbulence encounters are commonly referred to as Clear-Air Turbulence (CAT) which often occurs in the vicinity of the jet stream without visually detectable convective clouds (e.g., Kim et al., , 2018Lester, 1994;Sharman et al., 2006). The CAT may be generated by a variety of mechanisms, including but not limited to, shear instability, frontogenesis near upper-level jet/frontal systems (Dutton & Panofsky, 1970;Ellrod & Knapp, 1992), and emissions of inertia gravity waves via geostrophic adjustments at the exit region

Evidence for Large Increases in Clear‐Air Turbulence Over the Past Four Decades

“…Interestingly, of all the CAT diagnostics that the studies have in common, the two that do not require the computation of a vertical derivative (namely the deformation and divergence) are very similar between the two studies, lending support to this explanation for the differences. In this paper we have also examined 21 diagnostics, many more than J. H. Lee et al (2023), and we have examined the whole globe rather than just the northern extra-tropics (20-60°N).…”

“…In this paper we have also examined 21 diagnostics, many more than J. H. Lee et al. (2023), and we have examined the whole globe rather than just the northern extra‐tropics (20–60°N).…”

“…To make the results more applicable to global aviation, the diagnostics will be calculated on the 197 hPa pressure level, corresponding approximately to a flight level of 39,000 ft (FL390), rather than the tropopause, and for the entire globe as opposed to just the northern hemisphere considered by JS07 and J. H. Lee et al. (2023).…”

“…These 21 diagnostics have previously been validated using aircraft measurements of CAT and have generally been found to be skillful (e.g., Sharman et al, 2006). To make the results more applicable to global aviation, the diagnostics will be calculated on the 197 hPa pressure level, corresponding approximately to a flight level of 39,000 ft (FL390), rather than the tropopause, and for the entire globe as opposed to just the northern hemisphere considered by JS07 and J. H. Lee et al (2023).…”

“…Marlton et al (2021) and Simmons et al (2020) showed that the commissioning and decommissioning of meteorological satellites can introduce biases. J. H. Lee et al (2023) have recently updated JS07's work (see Section 4 for a discussion).…”