Numerical Modelling of Valley Cold Air Pools

Transcription

1 Numerical Modelling of Valley Cold Air Pools ICAM, 4 th June 2013 Peter Sheridan and Simon Vosper Thanks to: D. Whiteman, J. Horel, E. Crosman, N. Laureau, S. Hoch, Univ. Utah

2 Outline Motivation Recent field campaigns Effects of valley scale/geometry BLASIUS model 2D idealised simulation results Changing valley width only Changing valley width and depth in constant ratio Background stability Summary/future work

3 Motivation Applications Route-based forecasting: road-icing, fog Air quality, agriculture, climate change (glacier melt, microrefugia) Scale dependence: Small valleys (~ 1km x 100m): Typical English countryside, Sinkholes, Arizona Meteor Crater Diurnal cold pools form on calm, clear nights; simple structure Not resolved by NWP forecast (use post-processing) Large valleys (~ 10km x 1000m): U.S. / Alpine basins; Northern England, Scotland, Wales Complex flows involved, more scope for structural complexity and significant variability within valley Difficult to model accurately / interpret model results Daytime persistence

4 COLPEX Field Campaign (small valleys) COLd air Pooling EXperiment Met Office and NCAS (Univs. of Leeds, Salford and Surrey). Clun Valley, Shropshire: Rolling terrain ~100 m valleysummit, ~1 km wide Ground network combined with upper-air and remote sensing measurements High-resolution nested MetUM (Δx=100m, nz=140) Price et al. (2011), BAMS, 92:

5 COLPEX: Cold pool intensity vs. NH/U Idealised study (Vosper and Brown, 2008 BLM 127: ): Stronger cold pools for larger NH/U Smaller F lw results in a stronger cold pool F lw = LW DN / LW UP Critical NH/U where Δθ stops increasing due to sheltering Analysis confirms critical NH/U~1.0 for COLPEX observations (Sheridan et al QJRMS online)

6 Persistent Cold-Air Pool Study (PCAPS) Salt Lake Valley University of Utah and NCAR scientists Wintertime long lasting (multi-day) cold pools with complex spatiotemporal evolution Lareau et al. (2013) BAMS, 94: 51 63

7 Salt Lake cold pools Factors involved in Salt Lake cases: Horizontal inhomogeneity Daytime persistence Warm/cold advection aloft Possible/temporary mix-out by strong winds, passing troughs Lake effects (advective reinforcement/weakening)

8 Idealised modelling

9 BLASIUS model Boussinesq flow-over-orography model Simple force-restore surface model, diurnal radiation Shading and aspect No microphysics or radiative exchange with model atmosphere: instead set value of nocturnal F lw to represent cloud amount 1 st order Ri dependent mixing length closure scheme 2D simulations initialised using profile from 1D simulation Configuration: Idealised cosine slope Initial constant Brunt Vaisala frequency, N and wind, U, with height in 1D simulation 1 January, latitude 51N Periodic 2D; domain ~ 2 x valley width External parameters: N, U, F lw 3 rd diurnal cycle of 2D simulation examined

10 Constant valley depth, N=0.01/s, U=7.5m/s, F lw =0.6, H=-150m

11 Narrow valley 16 Z 17 Z 18 Z 04 Z

12 Insert wide base W=8km

13 Insert wide base W=8km 15 Z 18 Z 00 Z 06 Z

14 W=2km 15 Z 18 Z 00 Z 06 Z

15 Constant valley shape, U=7.5m/s, F lw =0.6, N=0.01/s

16 Terrain comparison Idealised Clun Valley Salt Lake Valley

17 H=-600m W=8km F lw = Z 18 Z 00 Z 06 Z

18 H=-1500m W=20km F lw = Z 18 Z 00 Z 06 Z

19 Constant valley shape, U=7.5m/s, F lw =0.6, N=0.02/s

20 H=-600m W=8km F lw = Z 18 Z 00 Z 06 Z

21 H=-900m W=12km F lw = Z 18 Z 00 Z 06 Z

22 U=7.5m/s, F lw =0.85

23 H=-1500m W=20km N=0.01/s F lw = Z 18 Z 00 Z 06 Z

24 H=-1200m W=16km N=0.02/s F lw = Z 18 Z 00 Z 06 Z

25 Summary of duration

26 Summary For small valleys, diurnal cold pools cold pool intensity depends on NH/U and F lw Larger horizontal scales: temperature and flow structure within valley becomes more complex, horizontally asymmetric Downslope flow maintains warmth of windward part of valley Flow separation/jump over return gravity current head shelters leeward part Similar minimum temperatures in narrow and very wide valleys Deeper valleys: Support increasingly late cold pool break-up, and eventually daytime persistence of a stable layer Deeper valleys more effectively shelter and confine cold pool during the day? Increased background stability: Shorter wavelength; shooting flow into valley inhibited, sheltering further enhanced so that stable layers now persist in shallower valleys Affects temperature of residual layer and so stability of approach flow at

27 Questions?

28 Future work Wind strength, time of year, latitude, shallower slope, width/height ratio (deeper valleys) 3D simulations, sidewall features Quantitative characterisation of cold pool behaviour More thorough understanding of flow processes involved internal and external to valley Relationship to external conditions

29 Supplementary slides

30 Hourly mean surface sensible heat flux (Wm -2 ) Soon after sunset (18 UTC) the surface heat flux becomes relatively small within the valley

31 Hourly mean contributions to q budget at 2 m Across hills: Advection provides warming tendency. Turbulent flux divergence cools. In valley: Turbulent flux divergence generally cools (apart from warming over local cold spots) but is smaller than across hills. dq/dt adv dq/dt mix UTC (K h -1 )

32 Across hills: Advection and turbulent flux divergence terms approximately cancel. Radiative term makes a larger contribution to cooling In valley: Total dq/dt approximated by sum of advective and turbulent flux divergence. Radiative term relatively small dq/dt adv+mix dq/dt total UTC (K h -1 )

33 18-19 UTC dq/dt adv (K h -1 ) Flow sweeps across valley Advection of cold air into interior of valley via flow separation from valley sides UTC Flow in valley decoupled Cold advection associated with downvalley drainage Air cooled adjacent to valley floor, further up the valley, is transported down the valley

34 Idealised 2D simulations Vosper & Brown 2008: air cooled in situ when depth ~ 100 m, width = 1 km sheltering effect generalised in terms of non-dimensional valley depth, NH/U beyond a critical NH/U, cold pool intensity no longer increases

35 Difference between valleys Difference in cold pool intensity, D[Dq] Small NH/U: cold pools are weak/non-existent in both valleys D[Dq] is small Intermediate NH/U: D[Dq] Clun valley bottom is decoupled; shallower Burfield valley is not D[Dq] is large Large NH/U: critical value is reached in Burfield valley; both valley bottoms are decoupled and cool equally D[Dq] is small again Critical NH/U ~ 1

36 W=20km 15 Z 18 Z 00 Z 06 Z

37 Crown copyright Vertical Met Office cross sections: Hourly mean q and winds COLPEX 100m model (Vosper et al QJRMS in press)

38 Flw 0.6 N 0.02 Flw 0.6 N 0.01 Flw 0.85 N 0.01 Flw 0.85 N 0.02