Stably Stratified Flow in a Shallow Valley
- 394 Downloads
Stratified nocturnal flow above and within a small valley of approximately 12-m depth and a few hundred metres width is examined as a case study, based on a network of 20 sonic anemometers and a central 20-m tower with eight levels of sonic anemometers. Several regimes of stratified flow over gentle topography are conceptually defined for organizing the data analysis and comparing with the existing literature. In our case study, a marginal cold pool forms within the shallow valley in the early evening but yields to larger ambient wind speeds after a few hours, corresponding to stratified terrain-following flow where the flow outside the valley descends to the valley floor. The terrain-following flow lasts about 10 h and then undergoes transition to an intermittent marginal cold pool towards the end of the night when the larger-scale flow collapses. During this 10-h period, the stratified terrain-following flow is characterized by a three-layer structure, consisting of a thin surface boundary layer of a few metres depth on the valley floor, a deeper boundary layer corresponding to the larger-scale flow, and an intermediate transition layer with significant wind-directional shear and possible advection of lee turbulence that is generated even for the gentle topography of our study. The flow in the valley is often modulated by oscillations with a typical period of 10 min. Cold events with smaller turbulent intensity and duration of tens of minutes move through the observational domain throughout the terrain-following period. One of these events is examined in detail.
KeywordsCold-air drainage Cold pool Nocturnal boundary layer Stratified turbulence Terrain
Two reviewers and Peter Sheridan provided extensive important suggestions that I gratefully acknowledge. This project received support from Grant AGS-1115011 from the National Science Foundation. Christoph Thomas provided the Sodar measurements. The Earth Observing Laboratory of the National Center for Atmospheric Research provided all other measurements.
- Fedorovich E, Shapiro A (2009) Structure of numerically simulated katabatic and anabatic flows along steep slopes. Ann Geophys 57:981–1010Google Scholar
- Katurji M, Zhong S, Kiefer M, Zawar-Reza P (2013) Numerical simulations of turbulent flow within and in the wake of a small basin J Geophys Res 118. doi: 10.1002/jgrd.50519
- Lundquist J, Pepping N, Rochford C (2008) Automated algorithm for mapping regions of cold-air pooling in complex terrain. J Geophys Res 113. doi: 10.1029/2008JD009879
- Mahrt L, Richardson S, Seaman N, Stauffer D (2010) Nonstationary drainage flows and motions in the cold pool. Tellus 62:698–705Google Scholar
- Mori M, Kobayashi T (1996) Dynamic interaction between observed nocturnal drainage winds and a cold air lake. J Meteorol Soc Jpn 74:247–258Google Scholar
- Savage LC, Zhong S, Yao W, Brown WJO, Horst TW, Whiteman CD (2008) An observational and numerical study of a regional-scale downslope flow in northern Arizona. J Geophys Res 113. doi: 10.1029/2007JD009623
- Stoll R, Porté-Agel F (2009) Surface heterogeneity effects on regional-scale fluxes in stable boundary layers: surface temperature transitions. J Atmos Sci 15:1392–1404Google Scholar
- Vosper S, Hughes JK, Lock AP, Sheridan PF, Ross AN, Jemmett-Smith B, Brown A (2013) Cold-pool formation in narrow valleys. QJR Meteorol Soc 127:429–448Google Scholar