47r1 - Changes in CIN

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 on the right). 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 a 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 CIN output from the model, 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. Please note that the CAPE computation will not change with the cycle 47r1 due to implications that such a change may have to the users, e.g. for the re-forecasts and the EFI. Instead a set of new CAPE and CIN parameters are under preparation including most-unstable and mixed-layer CAPE/CIN which all will use virtual temperature correction. These will become available later with one of the upcoming IFS cycles.

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 all the parcels. 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 deep moist convection.

The plot on the right shows a Skew T - log p diagram. The dashed black curve represents the temperature curve for a mixed-layer parcel lifted from the lowest 30 hPa layer as it is taken in the model. The red area between the level of free convection (LFC) and the equilibrium level (EL) is proportional to CAPE whilst the blue area under the LFC is proportional to CIN. Apparently, this is a quite unstable profile with some CIN which has to be overcome for the parcel to reach LFC. In the table CAPE and CIN values are displayed using different options for computing buoyancy. The CIN prior to cycle 47r1 suggests that while the environment is very unstable (CAPE of the order of few thousands of J.kg^-1 ) the CIN is so large that practically no thunderstorms could be initiated. In fact CIN is much lower as the thermodynamic diagram shows (only few tens of J.kg^-1). It's worth noting that if we use the environmental equivalent potential temperature instead of saturation equivalent potential temperature, we will end up with massive overestimation of CAPE whilst the CIN will be fine. This behaviour is due to the fact that CAPE is usually above the LCL where the air is saturated and the use of  saturation θ_e instead of θ_v is a reasonable approximation. In contrast, the CIN is chiefly in the boundary layer under LFC and the same approximation is generally not valid. With the IFS cycle 47r1 CIN will change significantly across the Globe with values in accordance with the parcel's theory. 

Skew T - log p diagram showing a lowest 30-hPa mixed layer parcel curve (dashed black curve), the environmental temperature curve (red curve) and environmental dew point curve (blue curve). CAPE is proportional to the area in red, and CIN - to the area in blue under the LFC. CAPE and CIN computations are also shown according to the parcel theory and approximations used in the IFS. The change in the CIN computation is also highlighted. In this case the CIN computation prior to cycle 47r1 massively overestimates the CIN while with 47r1 its value is much closer to the parcel theory. For reference see the table below.

Table 1. CAPE and CIN values for the parcel shown on the Skew T - log p diagram computed with different approximations. Please note that CAPE is lower and CIN higher in the case when virtual temperature corrections are not used. The last row shows the IFS CAPE and CIN prior to cycle 47r1 - apparently, CIN is massive (shown in the red cell) compared to parcel theory estimation. With cycle 47r1 forecasters will get the values in green - no change for CAPE for the time being whilst CIN will generally change significantly providing the best estimate according to the parcel's theory.