Investigation of mountain-wave induced turbulence (MWT) events over the Colorado region on 31 December 2011

On 31 December 2011, 18 severe turbulence events were reported over the Colorado region on the lee-ward side of the Rocky Mountains. Most of them (17) occurred during the 9 hours between 1330 and 2230 UTC and above 30,000 ft (about z = 10 km). The detailed structures of mountain waves and turbulence are investigated for this case using the Advanced Research Weather Research Forecast (ARW-WRF) model that has three nested domains with the finest horizontal grid spacing of 1.5 km in domain 3. At 1800 UTC 31 Dec. 2011, a northwesterly jet stream exceeding 70 m/s approaches the Rocky Mountains along with an upper-level trough at 300 hPa. Westerly flow is dominant at 700 hPa (mountain top height) in domain 1 of the WRF model results, which is similar to those observed in the RUC analysis data. Non-zero SGS TKE appears on the lee-side of the mountain due to the wave breaking, which is consistent with the locations of observed turbulence encounters. The wave breaking is related to complicated wave structures that are generated in dominant westerly or northwesterly flow. Their amplitudes at z = 11 km on the lee side of the mountains are the largest with vertical velocities of +2.3 and –3.1 m/s. At z = 15 km, the wave amplitudes are significantly reduced, which implies that the waves are filtered out or break down in the layer between z = 11 and 15 km. The maximum zonal wind is near z = 8 km and the wind decreases rapidly with height. Therefore, the regions of zero wind appear near z = 12 km, resulting from wave saturation, and eventually, a wave-induced critical level (WICL) of the stationary mountain waves exists near z = 12 km.