## 3 Atmospheric Conditions
### Upstream Vertical Profile  
Three soundings were released from Saratoga (Figure 1), upstream of the MBM, within three- to five-hour intervals. The elevations of Saratoga and MBM peak are about 2060 and 3661 m MSL, respectively, and the horizontal distance between these two locations is 42 km. A lower mountain range, the Sierra Madres southwest of Saratoga, should also be noted. The second sounding was released just before the first UWKA flight, and the last sounding was released near the end of the second UWKA flight. These soundings reveal the change in atmospheric vertical structure during an eight-hour period. The reasons we discuss the first two soundings (before the UWKA flight) are (a) that it takes some time (roughly one hour) for conditions over Saratoga to be advected to the lee side of the MBM, (b) that earlier conditions may have been essential to the formation of a breaking wave, which then may have become somewhat self-sustaining through a self-induced critical level, and (c) the model produced a high-amplitude non-linear wave at an earlier time, before the UWKA observation period. The first sounding, released at 17:00 UTC (10 am local time), reveals a well-mixed, windy boundary layer (below 2.4 km MSL) capped by a weak inversion at \(\sim\)2.8 km MSL (Figure 3a,d). The wind steadily backed from near the surface up to 6 km MSL (Figure 3e,f). Rapid backing occurred across a stable layer between 5.5-6.1 km MSL with southeasterly wind above this stable layer. Such reversal of the cross-barrier flow, from low-level westerly wind to (south)easterly wind starting about 2 km above the MBM crest, may be important, as it may support downslope wind storm formation [(42; 16)]. The wind reversal level is referred to as a critical level, as the phase speed of orographic internal gravity waves is zero. The stability of this critical level increases dramatically in the next three hours (compare the first to the second sounding in Figure 3d). Significant cooling occurred below this elevated inversion in those three hours. The well-mixed boundary layer was deeper (\(\sim\)1.2 km deep), probably on account of  
Figure 2: Synoptic condition from the 12 km NAM data at 0000 UTC on 12 January 2013. Wind barbs (full barb = 10 knots) are shown at each level, including at the surface (10 m) in (**d**). The white box in each panel shows the location of the Medicine Bow Mountains. the cold-air advection, but otherwise the fundamental vertical structure remains unchanged from three hours earlier. The third sounding seven hours later (Figure 3c) is, however, fundamentally different from the first two soundings: the elevated inversion layer (~5.5-6.1 km MSL) and the southeastly flow above it are absent, and cloudiness probably occurred up to the (very low) tropopause (Figure 3c). The temperature was also lower below the (vanished) elevated inversion layer, but only slightly, compared to seven hours earlier. The low-level wasted would remained equally strong, \(\sim\)15 m\(\cdot\)s\({}^{-1}\). [PERSON] and [PERSON] (2010) list seven conditions conducive to downslope wind storms: (a) a high, asymmetric mountain with a steeper lee slope; (b) strong cross-barrier winds (>15 m\(\cdot\)s\({}^{-1}\)) at and just above mountaintop level; (c) the wind at this level should be close to normal to the barrier (within 30\({}^{\circ}\)); (d) a stable layer near or just above mountaintop level, and a less stable layer above; (e) cold-air advection and large-scale subsidence to reinforce this stability structure; (f) reverse wind shear and a critical level aloft; and (g) the lack of a cold pool in the lee valley, which may inhibit the downslope storm from penetrating to the surface. In addition, the mountain half-width cannot be so small that the advection time scale is shorter than a buoyancy period; otherwise the mountain cannot  
Figure 3: Vertical profile of the upstream atmosphere, at Saratoga, according to radiosonde data (solid lines) and the inner-domain WRF model (dashed lines).