In the IFS cycle 47r1 the following two changes will be implemented for the Convective Inhibition (CIN):
- CIN will be computed with a completely revised code using virtual temperature correction;
- CIN and Convective Available Potential energy (CAPE) will both refer to the same most unstable air parcel.
Details of these to changes are provided below.
In the parcel theory, CAPE and CIN are computed as vertical integrals of Buoyancy expressed in terms of virtual temperature (or virtual potential temperature) difference between the lifted air parcel and its environment (see the figure below). In the IFS, for computational efficiency, CAPE and CIN are approximated with the difference between the equivalent potential temperature of the parcel, which is conserved during pseudoadiabatic ascent (the condensate is removed immediately from the parcel as it forms), and the environmental saturation equivalent potential temperature, which is function of environmental temperature only (for details see this technical memorandum). This approximation provides a relatively good estimate for CAPE but it can massively overestimate CIN. In practical terms, this means that the model output can suggest no chance of thunderstorm initiation whilst according to the parcel theory CIN is so small that thunderstorm initiation is very likely. To correct this deficiency in the model CIN, its computation is completely changed with the IFS cycle 47r1. In the new computational code, CIN is estimated with the difference between virtual potential temperatures of the parcel and the environment exactly as it is in the parcel theory. This will provide an estimate for CIN which is much more in line with the parcel theory and forecast practices. Please note that in the CAPE and CIN provided from the IFS surface parcel is not considered. Instead, for all the model levels in the lowest 60 hPa mixed layer parameters are used. This is in line with the notion that the updraught in thunderstorms will probably involve a deeper layer (e.g. 50 hPa deep) near the surface rather than just the surface air parcel.
Besides, now CAPE and CIN will both represent the most unstable parcel. Prior to cycle 47r1, CAPE represented the maximum value retained from all the air parcels departing from each model level from the surface up to 350 hPa in the atmosphere. CIN respectively represented the minimum values retained from the same parcel. As a result CIN and CAPE could represent different air parcels which makes their interpretation more difficult. With cycle 47r1 both CAPE and CIN refer to the same most unstable parcel which improves their usability for diagnosing seep moist convection.