An Overview of Convective Available Potential Energy and Convective Inhibition provided by NWP models for operational forecasting

This document presents a review of the computation of Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) from NWP model data conducted for ECMWF by the European Severe Storms Laboratory (ESSL). Various ways of computing CAPE and CIN are illustrated in selected meteorological situations. The most important differences arise from the choice of the air parcel for which CAPE and CIN are calculated, but other assumptions are also important. The forecaster survey at the ESSL Testbed 2019 among 38 participants, suggests that forecasters prefer using both CAPE/CIN for a near-surface mixed-layer parcel and for the most unstable parcel in the profile, but not for a parcel originating from the lowest model level. CAPE and CIN provided by the Integrated Forecasting system (IFS) are compared to those provided in other operational NWP models (ARPEGE, AROME, UM, HiRLAM, ICON, COSMO). The
implemented calculations differ regarding their intended use (for forecasters, model-internal use),
choice of parcel (most unstable only, or also for a near-surface mixed-layer), whether entrainment
is assumed in the parcel, whether the process is reversible or pseudo-adiabatic ascent, and whether
the latent heat of freezing and disposition are accounted for. ESSL provided a code of the
computation of various types of CAPE and CIN which was tested with the IFS model data. Results
of the testing have been summarised and ESSL provided recommendations on the computation of
CAPE and CIN and their subsequent use. CAPE and CIN for the near-surface 50 hPa mixed-layer
parcel and for the most unstable parcel contain some independent information that is valuable to
forecasters. In parcel calculations, it is recommended to implement pseudo-adiabatic ascent to ensure that an underestimation of instability is prevented (which could occur if reversible ascent is implemented) and without accounting for latent heat of freezing and disposition. Future ECMWF’s plans of revising the CAPE and CIN model output are outlined at the end of this paper.