Observations and numerical modeling of sub and super-critical flow at White Sands Missile Range

Numerous observational and modeling studies have revealed a wide variety of flows around, through and above terrain obstacles. Most such studies, however, have considered fairly simple terrain such as an isolated summit or a long barrier perpendicular to an impinging flow. Real terrain and real atmospheric conditions are more complex than that used in the above mentioned studies and the resulting phenomena are similarly more complicated and diverse.

Much of the White Sand Missile Range lies to the lee of the Organ and San Andres Mountains in southern New Mexico. The Organ Mountains include a rugged quasi-circular 1500 m high massif with a diameter of about 10 km. It is connected to a narrow and remarkably steep and narrow 1 to 1.5 km SE-NW oriented ridge. The ridge, in turn, connects to the approximately 100 km long south to north oriented barrier of the San Andres Mountains. Several passes and one additional massif complicate the San Andres barrier. Because of the variation in terrain heights, it is possible during a given situation at WSMR to have both sub- and super-critical flows in juxtaposition greatly complicating the resulting flow and presenting significant challenges to numerical models.

Many US Army missions are greatly impacted by the highly variable weather conditions in and around complex terrain such as at WSMR, but the Army's capability to forecast and diagnose such conditions remains limited. To better understand and to evaluate and improve the capability of numerical models to forecast the effects of terrain on weather conditions, the Army Research Laboratory collected surface data from 5 10 m instrumented towers sited in the lee of the Organ Mountains during the first three months of 2004. In addition, data from the White Sands Missile Range (WSMR) Surface Automated Meteorological System (SAMS) and other nearby surface stations such as the Remote Automated Weather Stations (RAWS) and the wind profiling radar at WSMR were collected. The total data set enables meso-beta and -gamma scale depiction of the wind flow in the lee of the Organ mountains; this is augmented in the vertical using horizontal and vertical wind components from the wind profiling radar.

We will show high resolution (~1 km grid spacing) results from the Weather Research and Forecasting and National Taiwan University/ Purdue models for a variety of flow situations such as hydraulic jump, lee waves, juxtaposition of super-critical and sub-critical flows and so on. An example is shown below. This was a day that included partially trapped lee waves extending downwind from the Organ and San Andres mountains across the Tularosa Basin. Just leeward of the Organ Mountains there were strong downslope winds; just a bit farther downwind the strong downslope winds transitioned to reversed flow at the surface associated with a hydraulic jump and the first of the lee waves. The observed train waves to the lee of the mountains are consistent with a decrease in height of the Scorer parameter and are well represented by the model's surface wind winds and are in agreement with the wind profiler observations. The 10 m winds of the model and observations are shown in the figure below. The complete results for this day including comparison of model and observed pressure perturbations and those for other days and much different situations will be shown in detail.