Prior to the IFS cycle 47r1, the EFI for both CAPE and CAPE-shear has used instantaneous CAPE (ShortName=cape; parameter ID=59) and CAPE-shear (ShortName=capes; parameter ID=228044) in 6-hour steps from which 24-hour maximum values have been computed (more details can be found in ECMWF Newsletter 144). For example for T+24-48h EFI forecast, four CAPE values at T+30h, T+36h, T+42h and T+48h have been used to compute the maximum for this forecast period. With the IFS cycle 45r1 two new model output parameters, maximum CAPE in the last 6 hours (ShortName=mxcape6; parameter ID=228035) and maximum CAPE-shear in the last 6 hours (ShortName=mxcapes6; parameter ID=228036), have been introduced (see details here). In the IFS cycle 47r1 mxcape6 and mxcapes6 are replacing cape and capes respectively. This change is aiming for a better sampling in the computation of 24-hour maximum values necessary for the EFI. The impacts of this change are described in detail below.
Firstly this changes has a neutral impact in terms of the EFI skill measured by the area under the Relative Operating Characteristic (ROC).
The EFI fields using mxcape6 and mxcapes6 generally look smoother and in this case of a tornadic outbreak in the USA earlier in March 2020 shown in Fig. 1 they even fit better to the severe weather reports. Please note that the EFI shows a wide area where the environment favours convective hazards whilst the actual severe thunderstorms develop along relatively narrow bands where sufficient lift is present to initiate deep, moist convection, e.g. along cold fronts or dry lines.
Fig. 1. T+24-48h CAPE-shear EFI/SOT forecast during the tornadic outbreak on 3rd March 2020 over Tennessee, USA.
As a result of taking instantaneous cape and capes at discrete time steps the EFI can exhibit a stripy structure in a case of fast moving squall fronts as in the example in Fig. 2. Apparently this is an issue because the cold front is not jumping rather, it is moving continuously from the west to the east. Replacing cape and capes with mxcape6 and mxcapes6 removes the stripy behaviour and provides a smoother and more realistic forecast field due to better sampling.
Fig. 2. The EFI for CAPE-shear as well as CAPE-shear high-resolution forecast in a case of a fast moving cold front. Stripy fields of instantaneous CAPE -shear and the EFI is due to taking instantaneous CAPE-shear values at discrete time steps. The EFI forecast looks smoother when using the smoother fields of maximum CAPE-shear in 6-hour periods. The sequence of Air mass RGB imagery shows snapshots of the cold front approaching the south-western parts of the Iberian Peninsular.
Finally, as a result of this change, the model climate also slightly changes but this should not affect much the EFI which is a relative measure of the difference between the model climate and real-time ensemble forecast CDFs.
Model climate 99th percentile for cape and mxcape6 valid for the end of April.