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Lamb LWT2 description method developed by P.James, Hadley Centre

By Sylvie Jourdain Météo-rance. Lamb LWT2 description method developed by P.James, Hadley Centre. Catalogue description. Abreviation: LWT2 Method: Lamb Weather Types/Empirical Classification Parameters: SLP 26 types

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Lamb LWT2 description method developed by P.James, Hadley Centre

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  1. By Sylvie Jourdain Météo-rance Lamb LWT2 descriptionmethod developed by P.James, Hadley Centre

  2. Catalogue description • Abreviation: LWT2 • Method: Lamb Weather Types/Empirical Classification • Parameters: SLP • 26 types • The LWT2 Method is a modified and improved version of the objective Jenkinson-Collison (JC) system for classifying daily MSLP fields into 26 flow categories, indicating flow direction and vorticity. • The LWT2 grid can be placed anywhere. The vorticity and flow strength thresholds (of JC) are modified dynamically so that exactly 33% of days (in ERA40) fall into each of the three vorticity classes Ax, Ux and Cx.

  3. Calculation description Original method developped over the British Isles, with 16 grid points, grid size : 30 degrees of latitude. • Calculation of southerly flow (S°, westerly flow (W), total flow (F) , southerly shear vorticity, westerly shear vorticity and total shear vorticity (Z) with 16 values of mean sea lavel air pressure • Test on Z / F or Z/2F values to determine A U or C • Direction determined by Tan-1(W/S) (A, NE, E ,S, SW,W, NW, N) New method LWT2 : • New ratio for Z/F adjusted systematically to force the type frequencies to be evenly- tempered • Do thresholds depend on the domain ? Values of the thresholds?

  4. Problem of the Grid size Normally typical north-south extent for Lamb Weather Types grids : 24-30 degrees For COST 733 Grid size depends on the domain size and forced for COST 733, 16 points grid array 1x1° Sub- domains : 12-18 degrees latitude Large domain : 46 degrees larger than normal

  5. References • P.D. Jones , M.Hulme and K.R.Briffa, 1993, A comparison of Lamb circulation types with an objective classification scheme • P.M. James, 2006, Second-Generation Lamb Weather Types - A new generic classification method with evenly-tempered type frequencies • COST 733 catalogue description

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