While a global NWP model cannot be expected to forecast individual convective cells, such a model can tell the user whether the broadscale environment is favourable for development of deep moist convection (DMC). So to assist with the forecasting of severe convection two new EFI parameters have been added: CAPE and CAPESHEAR (=CSP, for "CAPESHEAR Parameter").
- CAPE combines the effects of two of the necessary ingredients of DMC: instability and moisture. The higher the CAPE the more unstable the air mass is.
- CSP is computed according to:
where deep-layer wind shear
is computed between levels l1=925 hPa and l2=500 hPa, and the second term is proportional to the maximum vertical velocity in convective updraughts as computed using simple parcel theory:
. CSP is targeted at forecasting organised DMC, such as supercell convection, as the deep-layer wind shear helps to organise convection into long-lived cells. CSP nits are m2/s2 .
Practical remarks
- The EFI fields for both CAPE and CAPESHEAR show where severe convection is likely if it initiates. This means that in general high EFI values include also areas where DMC does not develop, for example because a capping inversion, as denoted by convective inhibition (CIN) is too strong. To help determine whether DMC will initiate or not one can employ also the probability forecast for precipitation, for example, in conjunction with the EFI for CAPE and CSP (Figure 1).
- The EFI shows the extremity of a given parameter compared to the model climate. Hence high values of the EFI may appear in areas where no severe convection develops because even the more extreme values of the convective parameters in the model climate are too low. For example this can happen over the continents in winter. Conversely a large CSP EFI would be particularly concerning if seen in an area and at a time where organised DMC was relatively common.
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 also available in the help section below the EFI web charts, at sub-section entitled "EFI for CAPE and CAPESHEAR" at http://www.ecmwf.int/en/forecasts/charts/catalogue?f[0]=im_field_chart_type%3A481&f[1]=im_field_product_type%3A1042.
Figure 1. EFI forecasts for A) CSP; B) CAPE; C) CSP where probability of precipitation above 1mm/24h is greater than 5%; D) CAPE where probability of precipitation above 1mm/24h is greater than 5%. The EFI signal disappears over a large part of Germany except the northern areas after filtering out EFI values where probability of precipitation above 1 mm/24h is less than 5%. Severe convection developed just over the northern parts of Germany whilst it stayed dry and sunny over the rest of the country.