## 2 Observational and Modeling Design  
### Instruments  
This study uses data collected on 11 January 2013 during the AgI Seeding Cloud Impact Investigation (ASCII-13) field campaign [24]. This campaign was not intended to study mountain waves or downslope wind storms. If it was, many more transects across the wind storm would have been flown, over longer distances and a different flight levels. In any event, the data presented here illustrate an impressive, persistent downslope wind storm event encountered by chance. The University of Wyoming King Air (UWKA) conducted two flights over the MBM (Figure 1b) on 11 January 2013, and three rawinsondes were launched from Saratoga (Figure 1b), upstream of the MBM. No radiosondes were launched from downstream locations, but aircraft soundings were collected some 30 km downstream of the MBM, between 4.2 km MSL and the airport in Laramie (2.2 km MSL). Figure 1: Terrain maps showing (**a**) the three WRF model domains (with coastline and state boundaries within the USA, and (**b**) the inner domain showing the model cross section and surface sites. The four UWKA flight tracks were along the model cross-section, but were shorter, ranging from 30 to 60 km in length. The UWKA carried the WCR, a 3-mm (95 GHz) Doppler radar with fixed antennas oriented in various directions. Here we use three antennas: one pointing up (zenith beam), one down (nadir beam), and 30\({}^{\circ}\) forward of nadir (down-forward beam). The beam widths of these antennas ranges from 0.5\({}^{\circ}\) to 0.7\({}^{\circ}\) and the pulse width during this flight was 37.5 m, although data were sampled at 15 m range resolution. WCR profiles were sampled at about 20 Hz, which corresponds to an along-track resolution of 4-5 m at typical UWKA airspeeds of 80-100 m\(\cdot\)s\({}^{-1}\). The minimum detectable (radar equivalent) reflectivity for the most (least) sensitive WCR beam was about -33 (-28) dBZ at a range of 1 km, and the maximum unambiguous radial velocity was \(\pm\)15.8 m\(\cdot\)s\({}^{-1}\) on this flight. Reflectivity values generally were quite low in the downslope wind storm examined here. They were highest on the windward side, but even there they did not exceed 10 dBZ as the storm was rather shallow and produced only light snowfall. This snow sublimated quickly on the lee side, but WCR echoes persisted remarkably long on the lee side, due to a mixture of sublimating ice particles and blowing snow particles. The latter result from strong winds scouring the surface and lofting the saltating ice particles into the turbulent boundary layer [25]. In the absence of deviations from aircraft reference pitch and zero roll, the radial velocity obtained from the up and down beams represents the vertical velocity of the reflectivity-weighted hydrometeors. However, aircraft wobble during flight, especially in turbulent areas, causes these radial velocities to become contaminated by both the aircraft motion and the horizontal winds. The effects of aircraft motion are automatically removed during post-processing, but horizontal wind contamination must be corrected for separately by assuming some horizontal wind profile. For lack of sounding data, [PERSON] et al. [21] used instantaneous flight-level winds (assumed to be constant down to the ground) for this wind profile. Here the Saratoga radiosonde nearest in time to the flight pass is used for this wind profile. As will be shown later, the wind profile changes dramatically across the MBM crest. This change is not accounted for, and results in higher uncertainty in vertical velocity estimation, especially where the aircraft attitude variations (pitch and roll) are large. The resulting hydrometeor vertical velocity is the sum of the air vertical velocity and the (negative) hydrometeor fallspeed. The latter is not estimated, but WCR vertical velocity will be displayed using a color scheme centered at -1 m\(\cdot\)s\({}^{-1}\), such that positive (negative) values can be interpreted as air updrafts (downdrafts).