A case study of nocturnal downslope flows during MATERHORN

Nighttime observations will be presented from the base of an east-facing slope on an isolated desert mountain that borders a large basin. The observations were made as part of the MATERHORN field experiment at Dugway Proving Ground, Utah. The upper part of the instrumented slope had a canyon-like nature and was surrounded by higher terrain on both sides, whereas the lower part was a completely open alluvial fan. Tethered-balloon soundings were made on the lower part of the slope at 15-30-min intervals between the late afternoon of 11 May 2013 and the morning of the following day. Additional measurements came from a line of five 20-30-m high meteorological towers running up the slope, which were instrumented with sonic anemometers and temperature sensors at multiple vertical levels, and from a network of surface weather stations.

The evolution of the slope flows during this night can be divided into three periods: (i) the evening upslope-downslope-flow transition; (ii) the undisturbed downslope-flow period; and (iii) the slope-valley interaction period. During the transition period, near-surface temperatures dropped rapidly and the daytime upslope or up-valley wind changed to a downslope direction. While the flow transition occurred almost simultaneously along the entire slope, the shift in wind direction progressed differently at sites in the upper part of the slope compared to sites in the lower part. During the downslope-flow period, a shallow slope-parallel surface inversion was present, with a homogeneous katabatic flow along the slope. During the final period, the inversion that had formed in the basin to the east of the slope sloshed up the sidewall, producing a rapid temperature decrease and modifying the near-surface flow. As the basin inversion retreated from the sidewall, temperatures increased again and a downslope-flow started to develop anew. Downslope flows and comparatively warm near-surface temperatures alternated with colder temperatures when the basin inversion engulfed the lower part of the sidewall.