Numerical Simulations of Sheltering in Valleys: The Formation of Nighttime Cold-Air Pools
Abstract
Numerical simulations of flow over two-dimensional valleys are conducted in order to study the occurrence of pools of cold air that form at the bottom of valleys during stable nighttime conditions. The results show that during strong surface radiative cooling and light-wind events, the near-surface potential temperatures that occur at the bottom of valleys can be several kelvin below the environmental mean. This is true for quite shallow valleys with depths and widths of 50 m and 1 km, respectively, and is a result of in situ sheltering at the valley bottom. For windier conditions or less rapid cooling, the cold-pool temperature contrasts are reduced. For shallow valleys the magnitude of the difference between the potential temperature at the bottom of the valley and the mean value increases with increasing valley depth. However there is a critical valley depth, beyond which the valley flow becomes decoupled from that aloft and there are no further increases in the potential temperature difference. This critical valley depth depends on the wind speed and radiative cooling rate and the results indicate it is a function of a non-dimensional valley depth (or inverse Froude number), which is itself a property of the undisturbed profiles of wind and stability.
Keywords
Froude number Gravity waves Flow separation Turbulent heat fluxPreview
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References
- Allen T and Brown AR (2006). Modelling of turbulent form drag in convective conditions. Boundary-Layer Meteorol 118: 421–429 CrossRefGoogle Scholar
- Anquetin S, Guilbaud C and Chollet J-P (1998). The formation and destruction of inversion layers within a deep valley. J Appl Meteorol 37: 1547–1560 CrossRefGoogle Scholar
- Atkinson BW (1995). Orographic and stability effects on valley-side drainage flows. Boundary-Layer Meteorol 75: 403–428 CrossRefGoogle Scholar
- Barr S and Orgill MM (1989). Influence of external meteorology on nocturnal valley drainage winds. J Appl Meteorol 28: 497–517 CrossRefGoogle Scholar
- Brown AR and Wood N (2003). Properties and parametrization of the stable boundary layer over moderate topography. J Atmos Sci 60: 2797–2808 CrossRefGoogle Scholar
- Brown AR, Athanassiadou M and Wood N (2003). Topographically induced waves within the stable boundary layer. Quart J Roy Meteorol Soc 129: 3357–3370 CrossRefGoogle Scholar
- Chimonas G and Nappo CJ (1989). Wave drag in the planetary boundary layer over complex terrain. Boundary-Layer Meteorol 102: 217–232 CrossRefGoogle Scholar
- Clements CB, Whiteman CD and Horel JD (2003). Cold-air-pool structure and evolution in a mountain basin: Peter sinks. Utah J Appl Meteorol 42: 752–768 CrossRefGoogle Scholar
- Deardorff JW (1978). Efficient prediction of ground-surface temperature and moisture with inclusion of a layer of vegetation. J Geophys Res 83: 1889–1903 CrossRefGoogle Scholar
- Gal-Chen T and Somerville R (1975). On the use of a coordinate transformation for the solution of the Navier–Stokes equations. J Comp Phys 17: 209–228 CrossRefGoogle Scholar
- Gudiksen PH, King CW, Ruffieux D, Neff WD and Leone JM (1992). Measurements and modeling of the effects of ambient meteorology on nocturnal drainage flows. J Appl Meteorol 31: 1023–1032 CrossRefGoogle Scholar
- Gustavsson T (1995). A study of air and road surface temperature variations during clear windy nights. Int J Climatol 15: 919–932 CrossRefGoogle Scholar
- Gustavsson T, Karlsson M, Bogren J and Lindqvist S (1998). Development of temperature patterns during clear nights. J Appl Meteorol 37: 559–571 CrossRefGoogle Scholar
- Holden JJ, Derbyshire SH and Belcher SE (2000). Tethered balloon observations of the nocturnal stable boundary layer in a valley. Boundary-Layer Meteorol 97: 1–24 CrossRefGoogle Scholar
- LeMone MA, Ikeda K, Grossman RL and Rottach MW (2003). Horizontal variability of 2-m temperature at night during CASES-97. J Atmos Sci 60: 2431–2449 CrossRefGoogle Scholar
- Leonard BP, MacVean MK, Lock AP (1993) Positivity-preserving numerical schemes for multidimensional advection. NASA Technical Memo. 106055, ICOMP-93-05Google Scholar
- Mahrt L, Vickers D, Nakamura R, Soler MR, Sun J, Burns S and Lenschow DH (2001). Shallow drainage flows. Boundary-Layer Meteorol 101: 243–260 CrossRefGoogle Scholar
- Mason PJ (1987). Diurnal variations in flow over a succession of ridges and valleys. Quart J Roy Meteorol Soc 113: 1117–1140 CrossRefGoogle Scholar
- Neff WD and King CW (1989). The accumulation and pooling of drainage flows in a large basin. J Appl Meteorol 28: 518–529 CrossRefGoogle Scholar
- Piacsek SA and Williams GP (1970). Conservation properties of convection difference schemes. J Comput Phys 6: 392–405 CrossRefGoogle Scholar
- Stephens GL (1984). The parameterization of radiation for numerical weather prediction and climate models. Mon Wea Rev 112: 826–867 CrossRefGoogle Scholar
- Thompson BW (1986). Small-scale katabatics and cold hollows. Weather 41(5): 146–153 Google Scholar
- Tian W and Parker DJ (2003). Observations and numerical simulation of atmospheric cellular convection over mesoscale topography. Mon Wea Rev 131: 222–235 CrossRefGoogle Scholar
- Wood N (1995). The onset of separation in neutral, turbulent flow over hills. Boundary-Layer Meteorol 76: 137–164 CrossRefGoogle Scholar
- Wood N and Mason PJ (1993). The pressure force induced by neutral, turbulent flow over hills. Quart J Roy Meteorol Soc 119: 1233–1267 CrossRefGoogle Scholar
- Zängl G (2005a). Formation of extreme cold-air pools in elevated sinkholes: an idealized numerical process study. Mon Wea Rev 133: 925–941 CrossRefGoogle Scholar
- Zängl G (2005b). Dynamical aspects of wintertime cold-air pools in an alpine valley system. Mon Wea Rev 133: 2721–2740 CrossRefGoogle Scholar