A Case Study of the Nocturnal Boundary Layer Evolution on a Slope at the Foot of a Desert Mountain

Abstract

Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in Utah. A case study of nocturnal boundary layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary layer development over the slope could be divided into three distinct phases during this night: 1) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. 2) The flow transition was followed by a 3–4-h period of almost steady-state boundary layer conditions, with a shallow slope-parallel surface inversion and a pronounced downslope flow with a jet maximum located within the surface-based inversion. The shallow slope boundary layer was very sensitive to ambient flows, resulting in several small disturbances. 3) After approximately 2300 mountain standard time, the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.

The evolution and sensitivity of katabatic flow dynamics to external influences through the evening transition

“…The total depth of the katabatic layer (not shown) deepens through the ET at all sites, typically exceeding the height of the towers within 2 h of local sunset. In their case‐study during the spring campaign, Lehner et al () reported an approximate katabatic depth of 30 m through the ET, observed with a tethered balloon system located between ES2 and ES3.…”

“…Due to frequent valley interaction at all ES towers, which typically occurs later into the night (Lehner et al, ; Grachev et al, ), this study focuses on the timing and structure of the katabatic flow through the ET. Here, we have defined the ET as 1 h before to 3 h after local sunset, where local sunset is defined at each site as the moment where the direct insolation drops to zero as the shadow front eclipses the tower.…”

“…The strength, structure and onset of the katabatic flow are a function of the local surface energy balance, ambient stratification, slope elevation and azimuth angles, surrounding topography, roughness elements, and other circulations covering a range of scales (Neff and King, ; Whiteman, ; Zardi and Whiteman, ). The katabatic onset may occur simultaneously over the entire slope (‘sheet transition’; Fernando et al , ; Villagrasa et al , ), propagate down the slope as a front (‘front transition’; Hunt et al , ; Brazel et al , ; Pardyjak et al , ; Fernando et al , ), or follow the direction of the shadow front, upslope for upward propagating shadows (Nadeau et al ) and downslope for downward propagating shadows (Lehner et al ). Furthermore, the katabatic structure and timing can vary dramatically from one day to the next, with the onset of the katabatic flow sometimes occurring significantly before or after local sunset (e.g.…”

“…External influences include ambient winds (e.g. Banta and Cotton, ; Arritt and Pielke, ; Mahrt and Larsen, ; Savage et al , ), ambient stratification (Neff and King, ), cloud cover (Barr and Orgill, ; Villagrasa et al, ), shadow fronts (Nadeau et al, ; Lehner et al, ), land cover, and soil type and soil moisture. While the influence of ambient winds has been the subject of much previous work, other external variables influencing the katabatic flow have received much less attention.…”

Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study

“…the Alps, Rotach et al, 2004;Nadeau et al, 2013, or Salt Lake Valley, Doran et al, 2002Monti et al, 2002). However, shallow drainage flows (SDFs) or density currents have been less studied (Mahrt et al, 2001;Soler et al, 2002;Udina et al, 2013;Oldroyd et al, 2014;Lehner et al, 2015a), in part because of their smaller scale, that often makes them more difficult to detect. Their proximity to the surface and their ability to change the surface conditions make them important and interesting phenomena worthy of analysis in SBL studies.…”