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QUANTIFICATION OF DIVERGENCE IN ALADIN

QUANTIFICATION OF DIVERGENCE IN ALADIN. Vanja Blažica, Benedikt Strajnar, Nedjeljka Žagar. The a im of this study. T o quantify the divergent part of the kinetic energy in the mesoscale; To observe horizontal and vertical dependency of the divergent energy distribution. The methodology.

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QUANTIFICATION OF DIVERGENCE IN ALADIN

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  1. QUANTIFICATION OF DIVERGENCE IN ALADIN Vanja Blažica, Benedikt Strajnar, Nedjeljka Žagar

  2. The aim of this study • To quantify the divergent part of the kinetic energy in the mesoscale; • To observe horizontal and vertical dependency of the divergent energy distribution. Themethodology • Quantification throughtheuse of model spectrum • ALADIN/SI 4.4 km, 6-hourforecasts • IC/BC by ECMWF 4D-Var assimilationsystem • Extension zone included in thespectra • One monthaverage (July 2007), withtworunsperday

  3. Background • Vorticity and divergence  Rossby and IG waves • The KE spectrum can be split into divergent and vortical part Left: Average model spectrum over levels between 9 and 13 km. Right : The difference between KE from U+V and fromVOR+DIV in percents.

  4. Results: Averageenergyspectra

  5. PBL and orography spectrum

  6. Results: Thedivergentenergycontribution The percentage of thedivergent energy in thetotal kineticenergy in a selectedlayer.

  7. Results: Thedivergentenergycontribution The percentage of the divergent energy in a selected layer in the total kineticenergy over all layers.

  8. Results: Thedivergentenergycontribution Distribution of percentage of divergent energy in the total energy with respect to height and wavenumber. Relative - for each level separately. Contour interval is 0.05.

  9. Results: Thedivergentenergycontribution • Vertical distribution of the average percentage of divergent energy in the total energy.

  10. Conclusions • The role of the divergent energy increases in the mesoscale, particularly at shortest scales and near the surface. • The vertical dependency is more complex. • The slope of both variables becomes shallower towards the surface. • At scales above/below 100 km most of the divergent energy comes from the free troposphere/PBL. • Below 50 km, divergentenergypresents more than 50 % oftotalenergy in alllayers. • What is thereasonforthebump at cca 50 km? • Is thesimilar slope of PBL energyspectrumand orography spectrum a coincidence? • Whythemaximum in thestratosphere?

  11. Additionalslide: Sensitivity to diffusionscheme • Fromprevious to currentsettings: • the spectral diffusion was reduced (the order from 4 to 2 and the enhancing coefficients by a factor of five) • the SLHD enhancing coefficients were increased (vorticity by two and divergence by ten). Distribution of average percentage of divergence in the total energy with respect to height andwavenumber. Relative - for each level separately. Contour interval is 0.05. Left: previousdiffusionschemesettings. Right: currentdiffusionschemesettings.

  12. U and V componentsofthewindvector at 6th model level (~100 hPa)

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