While one cannot expect from a global NWP model to forecast individual convective cells, it can be used to determine whether the environment is favourable for development of deep moist convection (DMC). To facilitate forecasting severe convection two new EFI parameters have been added: CAPE and CAPESHEAR (or CAPESHEAR Parameter CSP).
- CAPE combines two of the necessary ingredients of DMC: instability and moisture.
- CAPESHEAR is computed according to:
and is targeted at organised DMC such as supercell convection as the deep layer wind shear helps organising convection into long-lived cells.
Practical remarks
- The EFI for both CAPE and CAPESHEAR shows where severe convection is likely if it initiates. This means that in general high EFI values cover areas where DMC does not develop for example either because the lift mechanism is missing or the capping inversion (CIN) is too strong. To help determine whether DMC will initiate or not one can use the probability forecast for precipitation in conjunction with the EFI for CAPE and CAPESHEAR for example.
- The EFI shows the extremity of a given parameter compared to the model climate. Hence high values of the EFI may appear in the areas where no severe convection develops just because the values of the convective parameters in the model climate are too low. For example this can happen over the continents in winter.
More information about CAPE and CAPESHEAR parameters can be found in the ECMWF Newsletter No.144 at http://www.ecmwf.int/sites/default/files/NL-144.pdf.
Further details and practical information about the EFI for CAPE and CAPESHEAR are available in the help pages below the EFI web charts at http://www.ecmwf.int/en/forecasts/charts/catalogue?f[0]=im_field_chart_type%3A481&f[1]=im_field_product_type%3A1042.