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2.2 Mixing length parameterisation in ALARO -0 (preliminary results) The way of the mixing length formulation appears to be very important in current ALARO–0 model that uses the first order closure pseudo-TKE parameterisation of turbulent fluxes (Geleyn et al., 2006, Redelsperger et al., 2001). Classical first order closure turbulence schemes in ARPEGE/ALADIN were using constant profiles of mixing length based on observational studies and on the knowledge of the typical variation of eddy viscosity in the atmosphere (e.g. O` Brien, 1970). A new empirical formulation was introduced in 2005 by J.- F. Geleyn and J. Cedilnik (Cedilnik, 2005, hereafter GC05) which made the mixing length profile dependent on the height of the planetary boundary layer (PBL). The formula for the mixing length for momentum/enthalpy l yields: m / θ κ ( z + z0 / θ ) ( z + z ) ⎛ 1 l m / θ = , (2) κ 0 / θ + ε m / θ ⎞ 1+ ⎜ ⎟ λm / θ ⎝ β m / θ + ε m / θ ⎠ where κ is the Von Kármán constant, z 0 is the roughness length and function prescribed as: ε m / θ −α m / θ ( z + z ) 0 / θ + hPBL β m / θ ε m / θ is an exponential = e (3) Parameter hPBL is the diagnostic height of the PBL (Ayotte et al., 1996).The profile of l m / θ can be further modified via parameters λ m / θ , α m / θ , β m / θ . Minor modifications of this formulation were tested in 2006 in order to have a faster decrease of the mixing length over h PBL and a dependency of the asymptotic mixing length λ m / θ on the height of the PBL. The pTKE parameterisation using the formulas (2) and (3) for mixing length was tested in single column model on second GABLS experiment (see e.g. Cuxart et al., 2006). It provided satisfactory results for stably stratified PBL (e.g. further improvement of the PBL height diagnostics). However, the GC05 type of mixing length and its modification do not match well situations with large mechanical production of turbulent kinetic energy above PBL that occur in the region of strong jet stream (figure 6, left). The mixing length parameterisation dependent on TKE and integral buoyancy calculation seemed to be better for this purpose (Bougeault and Lacarrère, 1989, hereafter BL89). Nevertheless, direct application of the BL89 type of mixing length in the pTKE turbulence scheme has a consequence of TKE overestimation and further, rather negative impact on certain meteorological parameters (too strong surface wind and wind gusts). Hence, a combination of the empirical GC05 and BL89 formulation was tested to achieve a reasonable compromise. The resulting formula yields: l ( l −l ) = l + k (4) m / θ 0 m / θ m / θ BL89 0m / θ Here l0 m / denotes a first-guess for the mixing length represented by the GC05 θ parameterisation (2) and (3), l BL89 is the BL89 formulation, which gives information about the 254
TKE and the integral buoyancy of the transported parcel. Function k m / θ is used to modulate the rate of the l BL89 influence and it is prescribed by empirical formula. 3-D experiments with formulation (4) showed more production of TKE in areas of strong jets or low static stability against the reference GC05 mixing length (figure 6, right). This improvement was achieved without exaggerating the values of TKE in the PBL or at the tropopause (which could violate the stability of the computation). Conclusion: Preliminary results on case studies achieved with the merged empirical and Bougeault-Lacarrère parameterisation show rather neutral impact on surface meteorological quantities (wind, mean sea level pressure). Differences can be observed rather on parameters, which are very sensitive on turbulent fluxes (e.g. on vertical profiles of diagnosed TKE). However, some properties and non-local aspects of this scheme seem to be promising and their influence will be further evaluated. Figure 6: Vertical cross-section of wind velocity (isotachs, dashed, every 5 m/s) and turbulent kinetic energy (in color, kg . m 2 . s -2 ) in 12 hour forecast valid to 15 December 2005 from the reference experiment with modified GC05 scheme of mixing length (left) from experiment with for the merged GC05 and BL89 parameterisation. Thick solid line denotes the height of the PBL. The cross-section is zonal, nearly following the 51.5° N latitude. Acknowledgment Contribution of other ALARO-0 developers D. Banciu, M. Bellus, R. Brožková, B. Carty, D. Drvar, K. Essaouaini, J.-F. Geleyn, L. Gerard, M. Janoušek, I. Stiperski, A. Trojaková, M. Tudor, F. Váňa, J.-M. Piriou is acknowledged. References: Ayotte, K. W., P. P. Sullivan, A. Andren, S. C. Doney, A. A. M. Holtslag, W. G. Large, J. C. McWilliams, C.- H. Moeng, M. J. Otte, J. J. Tribbia, and J. C. Wygaard, 1996: An evaluation of neutral and convective planetary boundary parameterization relative to large eddy simulations. Boundary-Layer Meteorology, 79, 131-175 Bougeault, P., Lacarrère, P., 1989: Parameterization of Orography-Induced Turbulence in a Mesobeta-Scale Model, Mon. Wea. Rev., 117, 1872-1890 Cedilnik J., 2005: Parallel suites documentation, Internal LACE report Cuxart, J., A. A. M. Holtslag, R. J. Bear, E. Bazile, A. Beljaars, A. Cheng, L. Conangla, M. Ek, F. Freedman, R. Hamdi, A. Kerstain, H. Kitagawa, G. Lenderink, D. Lewellen, J. Mailhot, T. Mauritsen, V. Perov, G. Schayes, 255
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NEWSLETTER of the 28 th EWGLAM and
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Newsletter of the 28th EWGLAM and 1
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6.2 P. Clark et al.: The Convective
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1.1 Foreword In 2006, MeteoSwiss ha
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Cornel Soci National Meteorological
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2 Consortia and ECMWF reports 12
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The COSMO Consortium in 2005-2006 T
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Data Assimilation for LM Method: Nu
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LAM ACTIVITIES IN ROMANIA Cornel SO
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