## 3 Atmospheric Conditions
### Upstream Vertical Profile
The first three panels (**a–c**) show temperature (red lines) and dewpoint (blue lines) on a skew T diagram, with wind barbs on the side (full barb = 5 m\(\cdot\)s\({}^{-1}\) and flag = 25 m\(\cdot\)s\({}^{-1}\)), at three different times. Panels (**d–f**) show the vertical profiles of potential temperature, zonal wind speed, and wind direction, respectively. The height of Medicine Bow Peak is marked by a black arrow in the last three panels. generate a vertically propagating gravity wave response. The MBM is definitively wide enough in this case. The first condition applies to the MBM. Other than this first condition, the six other conditions may not all be necessary. One could also question whether Saratoga, at \(\sim\)one mountain width to the west of the MBM, is a representative upstream location. Further, it is not clear whether stronger conditions (e.g., stronger low-level wind, higher low-level stability) imply a stronger downslope windstorm. Thus, these conditions are evaluated qualitatively only. In general these six conditions applied (although condition (b) only marginally) early in the UWKA observation period, i.e. for the second sounding, at 20:00 UTC. At this time (g) applied as well, with Laramie (located in the downwind valley, see Figure 1) reporting strong westerly winds and relatively high potential temperature, 287 K (Figure 4), 3 K higher than the potential temperature in the upwind well-mixed layer at 20:00 UTC (Figure 3d). This implies that potentially warmer air was mixed down into the lee-side airmass, i.e., the downslope wind storm reached the downwind valley. But Laramie experienced cooling after 20 UTC, as colder air moved in from the north (Figure 4). During most of the remainder of the UWKA flights the downslope windstorm may not have reached Laramie some 30 km downwind of the MBM, but it may have penetrated into the valley further west. Towards the end of the UWKA observation period, at 03:00 UTC (next day), conditions (d), (e), and (f) no longer applied [maybe (f) marginally did apply as the zonal wind did just reach 0 m\(\cdot\)s\({}^{-1}\) aloft], and condition (g) could not be verified. Most significant is condition (d), the lack of a stable layer near mountaintop level, below a less stable layer (Figure 3d). So conditions were less suitable for a downslope windstorm at 03:00 UTC. Yet at this time (around 03 UTC) the downslope wind storm was most intense, as will be shown below. It is possible that Saratoga was not a suitable upstream location, as it may have been affected by flow modified by the Sierra Madres, to the southwest of Saratoga (Figure 1b). This range also is rather asymmetric (condition a) and may produce downslope wind storms [43], although there is no evidence this happened on 11-12 January 2013. It is possible also that the downslope windstorm, triggered by suitable upstream conditions earlier on, somehow became self-sustaining by a self-induced critical level. The WRF-simulated upstream conditions, in the inner domain (d03), are compared against radiosonde data in Figure 3. The WRF model \"knows\" about the Sierra Madres and should be able to capture their effect on low-level flow and stability above Saratoga. These low-level conditions (below  
Figure 4: Comparison of surface observations (**black**) against WRF model (**red**) for (**a**) potential temperature and (**b**) wind. The model is compared against observations from the Laramie Regional Airport weather station during a 10-hour period on 11–12 January 2013.  
mountaintop level) are summarized in Table 2. The bulk Brunt-Vaisala frequency \(N\) in Table 2 is calculated from the surface to mountaintop level, as a weighted mean of the dry value from the surface to the lifting condensation level (LCL) and the moist value above the LCL, the latter using the equation in [44]. Its average value (\(N\sim\)0.0073 s\({}^{-1}\)) from three soundings compares very well with the time-space matched WRF value (\(N\sim\)0.0074 s\({}^{-1}\)) (Table 2).