## 4 Radar Observations of the Downslope Wind Storm and Turbulence
### Flight-Level Measurements  
Measurements at flight level (\(\sim\)4.27 km MSL, or \(\sim\)0.6 km above Medicine Bow Peak) for the same four UWKA passes are shown in Figure 9. As mentioned in Section 4a, all four passes do not cover the same distance; however, the position of the MBM crest is centered at 0 km (vertical dotted line) in all panels in Figure 9. (The underlying terrain profiles are shown in Figure 9e.) All four passes reveal a drastic change in temperature, vertical velocity, wind, and turbulence from the upwind side to the immediate lee of the crest. Potential temperature increased by as much as 7 K within 8-10 km behind the crest (Figure 9a), due to \(\sim\)2 m\(\cdot\)s\({}^{-1}\) subsidence (Figure 9b). This sinking flow also accelerated by 6-10 m\(\cdot\)s\({}^{-1}\) (Figure 9c) but remained mostly laminar (Figure 9d), until it suddenly became turbulent about 8 km behind the crest. The region of lee turbulence contained downdrafts and updrafts ranging from \(-\)7 m\(\cdot\)s\({}^{-1}\) to +12 m\(\cdot\)s\({}^{-1}\) (Figure 9b), and locations where the zonal wind becomes slightly negative (easterly wind) (Figure 9c). Atmospheric turbulence at the flight level is evaluated using the EDR, measured by a MacCready Turbulence Probe [(47)]. The EDR measured by the MacCready Probe is proportional to the standard deviation of the vertical acceleration an aircraft experiences [(48)]. The MacCready EDR measurement has been adopted by the International Civil Aviation Organization as a measure of turbulence intensity in pilot reports. Measured EDR values are used to categorize the turbulence intensity at flight level. For medium-sized aircraft such as the one used in this study, EDR thresholds of 0.014, 0.050, 0.125, 0.220, 0.350, and 0.500 m\({}^{2/3}\) s\({}^{-1}\) refer to the turbulence categories'smooth to light', 'light', 'light to moderate','moderate','moderate to severe', and'severe' [(23)]. The latter three regions are highlighted in Figure 9d. Figure 9: Flight-level atmospheric variables for the same four along-wind flight tracks shown in Figure 8. The vertical dotted line in all panels (x = 0) corresponds with the crest of the MBM. (**a**) Potential temperature; (**b**) vertical air velocity (the dashed line represents 0 m s\({}^{-1}\)); (**c**) zonal wind; (**d**) eddy dissipation rate (EDR) measured by the MacCready turbulence meter, with colored layers indicating turbulence severity based on the classification of [PERSON] et al. [(23)]; and (**e**) underlying terrain profile (the dashed line in (**e**) represents the flight level). The UWKA experienced severe turbulence during the second and third passes and moderate to severe turbulence on the two other passes (Figure 9d). This indicates that moderate to severe turbulence was present for at least five hours in the lee of the MBM. The location and magnitude observed EDR values at flight level in all four transects (0.2 < EDR < 0.4 m\({}^{2/3}\) s\({}^{-1}\) about 10-30 km downwind of the crest) correspond well with those in the WRF simulation at the same level, but at an earlier time (Figure 7), although at 1 km resolution WRF cannot capture the observed fine-scale variability (Figure 9d). WRF simulated EDR values at times matching the flight transects are only one-third to one-half the observed values (not shown). It is challenging for a single numerical model run (driven by three-hour resolution reanalysis data in the outer domain) to realize the conjunction of conditions leading to wave breaking and downslope wind storm at precisely the right time [49; 50]. The intense turbulence is consistent with remarkably high levels of TKE at flight level (Figure 10). Within the available spectral range, the power spectra of vertical velocity in the lee follow the \(-5/3\) inertial subrange slope both near the ground and at flight level (Figure 10), thus within the \(\sim\)30 km of lee-side stretch, small-scale vertical velocity variations (at a scale of less than a few km) are due to turbulence.