The EFI (and SOT) for CAPE and CAPE-shear start using mxcape6 and mxcapes6 parameters

Prior to IFS cycle 47r1, the EFI (and SOT) for both CAPE and CAPE-shear were based on instantaneous CAPE (ShortName=cape; parameter ID=59) and CAPE-shear (ShortName=capes; parameter ID=228044) fields, in 6-hour steps, from which 24-hour maximum values were computed (more details can be found in ECMWF Newsletter 144). For example for the nominal T+24-48h EFI forecast, four CAPE values, at T+30h, T+36h, T+42h and T+48h, were used to compute the maximum for this forecast period. In 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), are introduced (see details here). In IFS cycle 47r1, for the respective EFI and SOT computations, mxcape6 and mxcapes6 will be used instead of cape and capes respectively. This change is aiming for a better sampling in the computation of the 24-hour maxima needed for the EFI. In effect with the change we extract the maximum within 24 hourly values, instead of using 4 6-hourly values. The impacts of this change are described in detail below. 

Firstly this changes has a neutral impact in terms of EFI skill, as measured by the area under the Relative Operating Characteristic (ROC) curve.

The EFI fields using mxcape6 and mxcapes6 generally look smoother and in the 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 forecasts for 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 the case of fast moving squall lines or very convective fronts as in the example in Fig. 2. This is an issue because the cold front is not jumping in reality, but is moving continuously from west to 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. Smaller scale discretization apparent on the top centre panel for HRES (attributable to the use of values at 1h intervals) is not manifested on the EFI because it will effectively be smoothed out by frontal-timing spread within the ensemble. The plot also provides one nice example of why we might expect to see more areally-extended regions of high EFI/SOT with the new formulation (and why these should be more realistic).

Fig. 2. EFI (and SOT) for CAPE-shear as well as the CAPE-shear high-resolution forecast for a case of a fast moving cold front. Stripy fields of instantaneous CAPE-shear and EFI/SOT (see black lines) are 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.

The EFI is a climate-related product which provides information on how different are the cumulative distribution functions (CDFs) of the ensemble forecast and the model climate. So an anomalous forecast does not necessarily translate into a high-impact one. In the case of convection this means that anomalous values of CAPE do not necessarily suggest that convective hazards are likely. For example, climatological values of CAPE in the cold season over the Arctic are so low that severe convective hazards are impossible even if the forecast shows up "extreme" CAPE. To filter out anomalous but insignificant signals in the EFI, CAPE values less than 10 J.kg^-1 are set to 0 before computing both the model climate and the ensemble forecast CDFs. With cape replaced by mxcape6 and capes with mxcapes6, CAPE over 10 J.kg^-1 will happen more often and therefore this may show up on the EFI (Fig. 3). To correctly interpret the EFI one should always account for the model climate as well.

Fig. 3. EFI/SOT for CAPE and max CAPE^* in Siberia, illustrating that the latter may show up a bit more often.

Finally, as a result of this change, the model climate also changes slightly across the world (Fig. 4a). In the example in Fig. 4b the EFI suggests a bit more extreme CAPE values over NW Spain. The CDFs for a representative location there show what this signal corresponds to in terms of model climate and ensemble forecast distributions.

a) b)

Fig. 4 a) Model climate 99th percentile for cape and mxcape6^* valid for the end of April and b) an example of the EFI/SOT for cape and mxcape^* and CDFs of corresponding model climate and real-time ensemble forecasts.
 

^*Naming Conventions: We refer on this web page to "max CAPE" (or mxcape6) and "max CAPE-shear" (or mxcapes6) for distinguishing the EFI/SOT of these in cycles from 47r1, from the equivalent EFI/SOT fields used up to and including cycle 46r1, but in fact when the model changes the former literally replaces the latter. Therefore users do not see any change in the naming - e.g. in ecCharts - and there will not be any additional EFI/SOT; instead the EFI and SOT for CAPE and CAPE-shear will continue to exist but they will use instead mxcape6 and mxcapes6.