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

T HE 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. T HE METHODOLOGY Quantification through the use of model spectrum ALADIN/SI 4.4 km, 6-hour forecasts IC/BC by ECMWF 4D-Var assimilation system Extension zone included in the spectra One month average (July 2007), with two runs per day 2

B ACKGROUND 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 from VOR+DIV in percents. 3

R ESULTS : A VERAGE ENERGY SPECTRA 4

PBL AND OROGRAPHY SPECTRUM 5

R ESULTS : T HE DIVERGENT ENERGY CONTRIBUTION The percentage of the divergent energy in the total kinetic energy in a selected layer. 6

R ESULTS : T HE DIVERGENT ENERGY CONTRIBUTION The percentage of the divergent energy in a selected layer in the total kinetic energy over all layers. 7

R ESULTS : T HE DIVERGENT ENERGY CONTRIBUTION Distribution of percentage of divergent energy in the total energy with respect to height and wavenumber. Relative - for each level separately. Contour interval is

R ESULTS : T HE DIVERGENT ENERGY CONTRIBUTION Vertical distribution of the average percentage of divergent energy in the total energy. 9

C ONCLUSIONS 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, divergent energy presents more than 50 % of total energy in all layers. What is the reason for the bump at cca 50 km? Is the similar slope of PBL energy spectrum and orography spectrum a coincidence? Why the maximum in the stratosphere? 10

A DDITIONAL SLIDE : S ENSITIVITY TO DIFFUSION SCHEME From previous to current settings: 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 and wavenumber. Relative - for each level separately. Contour interval is Left: previous diffusion scheme settings. Right: current diffusion scheme settings. 11

U AND V COMPONENTS OF THE WIND VECTOR AT 6 TH MODEL LEVEL (~100 H P A ) 12