Large Temperature Fluctuations due to Cold-Air Pool Displacement along the Lee Slope of a Desert Mountain

Abstract

Large temperature fluctuations (LTFs), defined as a drop of the near-surface temperature of at least 3°C in less than 30 min followed by a recovery of at least half of the initial drop, were frequently observed during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program. Temperature time series at over 100 surface stations were examined in an automated fashion to identify and characterize LTFs. LTFs occur almost exclusively at night and at locations elevated 50–100 m above the basin floors, such as the east slope of the isolated Granite Mountain (GM). Temperature drops associated with LTFs were as large as 13°C and were typically greatest at heights of 4–10 m AGL. Observations and numerical simulations suggest that LTFs are the result of complex flow interactions of stably stratified flow with a mountain barrier and a leeside cold-air pool (CAP). An orographic wake forms over GM when stably stratified southwesterly nocturnal flow impinges on GM and is blocked at low levels. Warm crest-level air descends in the lee of the barrier, and the generation of baroclinic vorticity leads to periodic development of a vertically oriented vortex. Changes in the strength or location of the wake and vortex cause a displacement of the horizontal temperature gradient along the slope associated with the CAP edge, resulting in LTFs. This mechanism explains the low frequency of LTFs on the west slope of GM as well as the preference for LTFs to occur at higher elevations later at night, as the CAP depth increases.

The Importance of Soil‐Type Contrast in Modulating August Precipitation Distribution Near the Edwards Plateau and Balcones Escarpment in Texas

“…As summarized previously (e.g., Fernando et al, 2015;, the dynamic effects of mountainous terrain can cause different atmospheric processes, including barrier winds Lee & Xue, 2013;McCauley & Sturman, 1999;Parish, 1982;Schwerdtfeger, 1979;Xu, 1990), flow around and over mountains (Chen & Feng, 2001;Hu & Liu, 2005;Jeglum et al, 2017;Malkus, 1955;Pierrehumbert & Wyman, 1985;Smith, 1982;Takane et al, 2017;Yang & Chen, 2008), wave motions (Armi & Mayr, 2011;Brown et al, 2003;Grubisic et al, 2008;Jackson et al, 2013;Reinecke & Durran, 2009;Serafin et al, 2017;Smith, 2004), wakes (Epifanio & Rotunno, 2005), gap winds (Hitzl et al, 2014;Pan & Smith, 1999;Wang et al, 2016;Whiteman & Doran, 1993), and flow separation (Sheridan et al, 2007;Vosper et al, 2006), to name a few examples. Dynamic effects generated by barriers to atmospheric flow have been shown to play important roles in development of precipitation (Chen et al, 2013;Dettinger et al, 2004;Lee & Xue, 2013;Medina et al, 2005;Pedgley, 1970;Rotunno & Ferretti, 2001).…”

Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

“…) or the formation of wakes and vertically oriented lee vortices which alter the manner of the ambient flow interaction with the CAP (Jeglum et al . ) may further be influential. However, the present study does not consider variations in seasonal, land surface, latitudinal, valley morphology, etc.…”

“…Three-dimensional (3D) effects relating to horizontal aspect (long alpine valleys versus basin-like SLV (e.g. Lareau and Horel, 2015a;Sabatier et al, 2018) or the formation of wakes and vertically oriented lee vortices which alter the manner of the ambient flow interaction with the CAP (Jeglum et al 2017) may further be influential. However, the present study does not consider variations in seasonal, land surface, latitudinal, valley morphology, etc.…”

Distinguishing Time Scales of Katabatic Flow in Complex Terrain

“…Not so textbook are the subtle vertical and horizontal banded features in the temperature trace. The vertical bands after the shade front passes are large temperature fluctuations (LTF's −3 • C or greater temperature drops in a 30-min period) or oscillations that [42] identified and associated with cold-air pool displacements. The horizontal bands are not image production artefacts, but rather are signatures of small-scale drainage channels.…”