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Turbulence Diagnostics at the Aviation Weather Center

Turbulence Diagnostics at the Aviation Weather Center. Steverino Silberberg & Jesse Sparks Aviation Weather Center Turbulence Seminar February 2012. Turbulence Diagnostics at the AWC: How?. Calculated from model output and soundings Models:

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Turbulence Diagnostics at the Aviation Weather Center

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  1. Turbulence Diagnosticsat theAviation Weather Center Steverino Silberberg & Jesse Sparks Aviation Weather Center Turbulence Seminar February 2012

  2. Turbulence Diagnostics at the AWC: How? • Calculated from model output and soundings • Models: • difficulty forecasting small scales (sub-grid scale) • damp out small scale motions  stop computational instability • saturate with mesoscale instabilities by F24 • signal to noise issue • Goal  use larger scale signal from model turb diagnostics to point to potential turb regions • Nearly all use VWS

  3. Turbulence Diagnostics: Which? • Absolute Vorticity • Divergence Tendency • Ellrod & Ellrod_new • Ellrod-Knox • GTG • Lighthill-Ford • Richardson Number • Turbulent Kinetic Energy

  4. Absolute Vorticity • Abs Vort < 0 may result in inertial instability • Anticyclonic curvature&/or shear can develop such that Abs Vort < 0 • When Abs Vort < 0 • horizontal acceleration of air parcels • spontaneous breakdown of balance may result in gravity wave formation

  5. Divergence Tendency • Gravity wave generation due to unbalanced flow and geostrophic adjustment • Total divergence tendency causes gravity waves in a stable environment • At AWC: div tend computed from primary terms in the non-linear balance eqn (div tendency eqn)  not the true change in divergence • Div Tend  reduces Ri  turbulence

  6. Ellrod: TI1 • VWS (layer) X Total Deformation (top level) • likelihood of turbulent waves and eddies (if stability is sufficiently low) • Only accounts for mechanical generation of turbulent waves and eddies from shear • Does not account for static stability which could dampen out the turbulent energy • Very skillful in determining the existence of turbulence • Limited skill in determining the intensity of turbulence

  7. EllrodvsEllrod_new • Ellrod • contoured every 4x10-7 s-2 • Ellrod_new • contoured every 8x10-7 s-2 • needed after GFS upgrades increased the strength of upper tropospheric windspeed 10-20 kt resulting in stronger vertical wind shear and deformation

  8. Ellrod-Knox • (VWS x Def)(Div Tend)(constant) • Eqn varies by model and model’s output grid size • Will be available with the RAP upgrade to RUC • Will be available for GFS and NAM • Appearance • Maxima similar to Ellrod, smaller maxima over larger areas due to div tend • Performance: • Higher PODyes for MOG turb than Ellrod • Captures more pireps at the expense of over forecasting

  9. Lighthill-Ford • Spontaneous imbalance of atmospheric flow  produces gravity waves  gravity waves lower Ri  turbulence • In testing in Testbed & Breakfast • Main terms: • Advection of Relative Vorticity • Divergence x relative vorticity • Curl of the gradient of divergence • Jacobian of east-west & north-south velocity

  10. Richardson Number • Static Stability / VWS • Measure dynamic (Kelvin-Helmholtz) instability caused by wind shear • Values < 1 means that VWS is stronger than stability and can result in turbulence • Values > 1 means that it is too stable for turbulence

  11. Richardson Number • Weaknesses • Models do not resolve vertical wind shear and stability at the scale necessary to detect mesoscale areas prime for turbulence • Values < 1 found in regions of both smooth air and turbulent air based on pireps • Ri by itself is not a good parameter for turbulence forecasting; must be used with other parameters

  12. Tubulent Kinetic Energy: “(TKE)” • KE = Avg(KE) + Deviation from Avg(KE) • Deviation from Avg(KE) = TKE • True TKE • TKE Storage + TKE advection = • Buoyancy Production (heat flux) – • Wind Shear Production + • Turbulent Transport of TKE by eddies – • Pressure Correlation (TKE redistributed by Pressure Perturbations) – • Dissipation of TKE (the conversion of TKE into heat) • At AWC, we only use wind shear production term • VWS X VWS • Therefore, only measures the mechanical generation of turbulent waves and eddies from shear

  13. Wind Shear TKE Productionin NSHARP_AWC • 0.54 X Layer Thickness X VWS Squared • VWS = magnitude ΔV/ΔZ (where V is a vector)

  14. Turbulence Diagnostics • Absolute Vorticity • Divergence Tendency • Ellrod & Ellrod_new • Ellrod-Knox • GTG • Lighthill-Ford • Richardson Number • Turbulent Kinetic Energy

  15. Turbulence Diagnostics at the AWC • Calculated from model output and soundings • Models: • difficulty forecasting small scales (sub-grid scale) • damp out small scale motions  stop computational instability • saturate with mesoscale instabilities by F24 • signal to noise issue • Goal  use larger scale signal from model turb diagnostics to point to potential turb regions • Nearly all use VWS

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