## 3 Atmospheric Conditions
### Modeled Flow Field across the Mountain  
Vertical cross sections of vertical velocity, potential temperature, and horizontal wind are shown in Figure 5, for a 10-hour period (from 18 UTC Jan 11 to 04 UTC 12 January 2013), the last five of which overlap with the UWKA transects. These zonal cross sections are obtained from the innermost domain along the line in Figure 1b, which transects the top of the MBM and matches the UWKA flight track. The zonal wind for the corresponding model transects is shown in Figure 6. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Sounding Launch**} & \multicolumn{6}{c}{**Average Value from Surface to Mountaintop**} \\ \hline \multirow{3}{*}{**Date**} & **Time** & \multicolumn{2}{c}{**U (m/s)**} & \multicolumn{2}{c}{**Wind Dir (\({}^{\circ}\))**} & \multicolumn{2}{c}{**N (0.01 s\({}^{-1}\))**} & \multicolumn{2}{c}{**\(h\)**} \\ \cline{3-8}  & **(UTC)** & **obs** & **model** & **obs** & **model** & **obs** & **model** & **obs** & **model** \\ \hline
11 January 2013 & 1700 & 16.7 & 10.1 & 240 & 277 & 1.06 & 0.54 & 1.03 & 0.85 \\
11 January 2013 & 2000 & 14.9 & 10.9 & 265 & 270 & 0.57 & 0.67 & 0.57 & 0.96 \\
12 January 2013 & 0300 & 14 & 8.2 & 271 & 301 & 0.55 & 1.02 & 0.59 & 1.96 \\ \hline Average & & 15.2 & 9.7 & 259 & 283 & 0.73 & 0.74 & 0.68 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characterization of the upwind environment, and evaluation of model performance. Shown are the wind speed \(U\), wind direction, the Brunt–Vääsälä frequency \(N\), and the Froude number \(Fr\). All observations (OBS) are derived from a radiosondes released in Saratoga and model output is time–space-matched to these observations. Here \(N\) is the depth-weighted average dry (moist) value of the Brunt–Väsälä frequency below (above) cloud base (defined as the LCL). The non-dimensional mountain height \(h\) is calculated as \(\frac{NH}{U}\) where \(H\) is the height of Medicine Bow Peak (3,661 m MSL) above Saratoga (2070 m MSL). All variables are averages between the surface (10 m) and mountaintop level. Lee gravity waves are present as early as 18 UTC (Figure 5a), and high amplitude trapped lee waves can be seen at 20 UTC (Figure 5b). A deep isentropic layer resulting from a dividing streamline [3] starts to develop over the MBM at 22 UTC (Figure 5c), near 5 km MSL. The upstream stratified air plunges down below this isentropic layer, especially at 22 UTC when this plunging flow transitions to an updraft (hydraulic jump) reaching 8 m s\({}^{-1}\) (at x = 52 km in Figure 5c). This subsiding, near-surface flow reaches a speed of 25 m s\({}^{-1}\) some 15 km downwind of the crest (Figure 6c). This speed is twice as large as the upstream wind speed. Two vortices flank the hydraulic jump at 22 UTC, both with an  
Figure 5: Model vertical cross sections of vertical velocity (color filled), potential temperature (black lines), and wind vectors along the transect line shown in Figure 1, from west to east, at two-hour increments starting in panel (**a**) at 18:00 UTC on 11 January. Two counter-rotating vortices (yellow dashed lines) are shown schematically in panel (**c**). Figure 6: As in Figure 5, but showing zonal wind speed. easterly flow component. These vortices are drawn schematically in Figures 5c and 6c. Trapped lee waves persist for some distance further downwind. The amplitude of the wind storm is suppressed at 00 UTC and 02 UTC (Figure 5d,e), but is larger again, although more shallow, at 04 UTC (Figure 5f). Transects of model turbulent kinetic energy (TKE, m\({}^{2}\) s\({}^{-2}\)) and eddy dissipation rate (EDR, m\({}^{2/3}\) s\({}^{-1}\)) are shown in Figure 7, valid for 20 UTC, a time when the downslope windstorm and hydraulic jump are best developed. We would like to estimate EDR from model output, because EDR is a well-established measure of the subjective feel of turbulence experienced by aircraft passengers. EDR values for specific-size aircraft are linked to turbulence severity categories in pilot reports.