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Analysis of banner cloud dynamics using a newly developed LES model

Analysis of banner cloud dynamics using a newly developed LES model. Daniel Reinert and Volkmar Wirth. Institute for Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany. 1 st International Workshop of EULAG Users Bad Tölz , 06 October 2008. TexPoint fonts used in EMF.

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Analysis of banner cloud dynamics using a newly developed LES model

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  1. Analysis of banner cloud dynamicsusing a newly developed LES model Daniel Reinert and Volkmar Wirth Institute for Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany 1st International Workshop of EULAG Users Bad Tölz, 06 October 2008 TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAA

  2. Outline • The phenomenon of banner clouds • Some examples • Existing theories of banner cloud formation • The applied LES model • Some model specifics (complex orography, turbulent inflow) • Model setup • Numerical simulation of banner clouds • Dominant mechanism of formation • Relative importance of dynamics versus thermodynamics • Summaryandconclusions

  3. The phenomenonofbannerclouds Some examples

  4. Examples Banner clouds occur when sufficiently moist air flows across steep (often pyramidal shaped) mountain peaks or quasi 2D ridges. 27.08.2005 Duration: 19:07 – 20:20 UTC Wind Wind 4 m/s Matterhorn (Swiss Alps) Zugspitze (Bavarian Alps) • cloudisconfinedtotheimmediatelee • windwardsideremainscloud-free Characteristics:

  5. Postulated mechanism of formation (I) Mixing of two air masses with distinct properties (temperature, humidity) Banner cloud = Mixing fog ? (Humphreys, 1964) Adiabatic expansion in a region of accelerated flow at the mountain’s tip, based on the Bernoulli-effect (Beer, 1974) Bernoulli Simple scaling analysis  Pressure reduction due to Bernoulli can not be more than a few hPa  local cooling can not be more than a few tenths of a degree  Itisunlikelythatthepressuredecreaseitselfcausesleesidecondensation Same coolingresults form dry adiabaticliftingofonlyΔz ≈ 20m !

  6. Postulated mechanism of formation (II) Favoured mechanism: Banner clouds as visible result of forced upwelling in the upward branch of a lee vortex (Glickman, 2000) • Objectives: • Verification of postulated mechanism 3 using LES • Clarify necessity of inhomogeneous conditions (temperature, humidity) • Relative importance of thermodynamics for reinforcement and maintenance Mechanism 3 Schween et al (2007)

  7. The applied LES model

  8. The applied LES model • Developedduringbannercloudproject (Reinert et al, 2007); based on a former mesoscale (RANS) model Moist physics: Two-moment warm microphysical bulk scheme (Chaumerliac, 1987) U0 Turbulent inflow: Modified perturbation recycling method Arbitrarily steep topography Method of viscous topography (Mason and Sykes, 1978) • SGS-model • Lilly-Smagorinsky • TKE-closure • (Deardorff, 1980) Topography Time dependent, anelastic N-S-equations on 3D cartesian grid (Arakawa-C) qv T

  9. Turbulent inflow (auxiliary simulation)

  10. Turbulent inflow  Model consistent turbulence for u, v, w, qv, θ

  11. Qualitative exampleofgeneratedturbulence • Coherent structures visualized by Q-criterion (Jeong and Hussain, 1995) • Isosurface coloured with vertical velocity • Isosurface of relative humidity (rh=65%) • Isosurface coloured with vertical velocity

  12. Methodofviscoustopography(Mason and Sykes, 1978) • Treatment ofairandtopographyastwofluidswithvastly different viscosities • Modificationofviscousstresseswithingridcellsintersectedbyorography • Applicationof a modified, interpolatedviscosity / exchangecoefficient • Accounts forexactpositionoforography; refinementofstepwiseapprox. Model discretization: Goal: chooseνint such thatfluxcalculatedbythe model equalsfluxassuming u=0 ms-1atpoint R Assumption: constant fluxes within interpolation layer. Example for

  13. Model setup • Numerical simulation of flow around idealized pyramidal-shaped obstacle • Simulationswereconducted on wind tunnel scaleandatmosphericscale Here: Simulations on atmospheric scale will be shown. H = 1000 m L = 930 m α= 65° x = 25m y = 25m Re= 5.6 × 108 U0= 9 ms-1 • - Turbulent inflow with logarithmic velocity profile • 260(x) × 126(y) × 64(z) grid cells

  14. Thermodynamic situation Idealised profiles motivated by measurements at Mount Zugspitze Virt. pot. temp. and spec. humidity Stability analysis • Lifting cond. level below pyramid tip for large parts of boundary layer depth

  15. Numerical simulation of banner clouds Mechanism of banner cloud formation

  16. Wind vectorsof time meanflow inflow Stagnation point But ! • Significantupwelling in thelee • Highlyasymmetricflowfieldregardingwindward versus leewardside • Upwellingregionhas larger verticalextent in thelee Does mechanism also work for horizontally homogeneous conditions ? Resultssupportpostulatedmechanism 3 Lagrangian information about vertical displacement on lws and wws necessary

  17. Initializationof passive tracer • Advectionof passive tracerΦ, satisfying informationaboutmeanverticaldisplacementΔz ofairmasses on windward versus leewardside. z z0 z0 x

  18. Initializationof passive tracer • Advectionof passive tracerΦ, satisfying informationaboutmeanverticaldisplacementΔz ofairmasses on windward versus leewardside. Δz: mean vertical displacement z z0 z1 z0 z0 x

  19. Time averagedverticaldisplacementz of passive tracer • Highlyasymmetric • largest positive Δz in theimmediatelee  No necessity for additional leeward moisture sources or distinct air masses • Magnitude ofasymmetryis a measurefortheprobabilityofbannercloudformation Overall: strong structuralsimilaritywith real bannercloud.

  20. Simulation withmoisturephysicsswitched on • Simulation ofrealisticallyshapedbannercloud • Substantiateresults/conclusionsdrawnfromtheformer (dry) runs Objectives: Setup: • no additional (leeward) moisture sources • no distinct air masses • no radiation effects

  21. Simulation withmoisturephysicsswitched on • Simulation ofrealisticallyshapedbannercloud • Substantiateresults/conclusionsdrawnfromtheformer (dry) runs Objectives: inflow

  22. Simulation withmoisturephysicsswitched on • Simulation ofrealisticallyshapedbannercloud • Substantiateresults/conclusionsdrawnfromtheformer (dry) runs Objectives: Setup: • no additional (leeward) moisture sources • no distinct air masses • no radiation effects Mean specific cloud water content in x-z-plane

  23. Impact ofmoisturephysics • Resultsforoneinvestigatedthermodynamicsituation: • Moisturephysics do not significantlyimpacttheupwardbranchoftheleewardvortex. • Moisturephysicsgiveriseto a moderate increaseofleewardturbulence. Onecouldthinkofthefollowingimpacts: • Impact on meanflow: • Potential toreinforce/sustaintheupwardbranchoftheleewardvortex  Help tosustainbannercloudsduringepisodeswithweakdynamicalforcing • Impact on leewardturbulence: • Banner cloudsgiveriseto a destabilizationoftheleewhichmayincreaseleewardturbulence

  24. Summaryandconclusions • leewardmoisturesources • distinctairmasses • radiationeffects The numericalsimulationsrevealed • Banner cloudformationdownwindof pyramidal shapedmountainscanbeexplainedthrough: • Forcedupwelling in theupwardbranchof a leewardvortex • Flow fieldishighlyasymmetricregardingtheLagrangianverticaldisplacement  Banner cloudscan form underhorizontallyhomogeneousconditions  Noneedfor additional featureslike: • Theoriesbased on mixingfogor Bernoulli-Effectare not necessary in order toexplainbannercloudformation. • Moisturephysicsprobablyofsecondaryimportanceforbannerclouddynamics

  25. Thankyouforyourattention

  26. References • Beer, T. (1974): Atmospheric Waves. Adam Hilger Press • Deardorff, J. W. (1980): Stratocumulus-capped mixed layers derived from a three-dimensional model. Boundary Layer Meteorology, 18, 495-527 • N. Chaumerliac et al. (1987): Sulfur scavenging in a mesoscale model with quasi-spectral microphysics: Two-dimensional results for continental and maritime clouds. J. Geophys. Res. 92, 3114-3126. • Glickman, T. S., ed. (2000): Glossary of Meteorology, American Meteorology Society, Allen Press, second. Edn. • Humphreys, J. W. (1964): Physics of the air. Fourth Ed.; Dover Publ. • Jeong, J. and F. Hussain (1995): On the identification of a vortex. J. Fluid Mech. 285, 69-94 • Mason, P. J. and R. I. Sykes (1978): A simple cartesian model of boundary layer flow over topography. J. Comput. Phys. 28, 198-210 • Reinert, D. et al. (2007): A new LES model for simulating air flow and warm clouds above highly complex terrain. Part I: The dry model, Boundary Layer Meteorology, 125, 109-132 • Schween, J. et al. (2007): Definition of ‘banner clouds’ based on time lapse movies. Atmos. Chem. Phys 7, 2047-2055

  27. Meteorologicalconditionsforbannercloudformation • Whether a bannercloudformsor not isdeterminedbyboththethermodynamicalsituation (T(z), qv(z) upstream) andthedynamicalsituation(flowfieldinducedbymountain) • Thermodynamicalsituation (T(z), qv(z)) anddynamicalsituation must match • Followingschematic: • Characterizationofthermodynamicalsituation: Verticalprofileof LCL derivedfrominflowdataset • Characterizationofdynamicalsituation: Verticalprofilesoftracerdisplacement

  28. Meteorologicalconditionsforbannercloudformation No cloud Banner cloud Nobannercloud cloud on wwandlwside

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