Convective Available Potential Energy

Datasets and Usage
1

Hi everyone!

Would someone know the following?

  1. What time of day does the CAPE (Convective Available Potential Energy) data represent? Are they late afternoon?

  2. Does the CAPE data use virtual temperature? (specifically, how is it calculated?)

In advance, I thank you for your consideration.

Respectfully,

Raphael

2

Hi Raphael,

  • Timing:
    CAPE values are instantaneous, meaning they represent the state of the atmosphere at the given forecast time step (e.g. step = 12 of the 00 Forecast is an estimate for 12 UTC). They’re not daily or afternoon averages.
    If you’re after a more representative measure of peak instability, you can use mxcape6, which gives the maximum CAPE over the previous 6 hours, derived from hourly forecast data.

  • Types of CAPE available:
    In addition to the standard surface-based CAPE, we now also provide:

    • MUCAPE – Most Unstable CAPE, based on the parcel with the highest potential energy in the lowest part of the atmosphere.

    • Mixed-layer CAPE variants MLCAPE50– averaged over different depths to better capture situations where the surface is stable but nearby layers are not.

  • Calculation method:
    CAPE is computed using virtual potential temperature, which includes a virtual-temperature correction to account for the buoyancy effect of water vapour (since cycle 48r1).

So in short: CAPE is instantaneous, mxcape6 provides a short-term maximum, and the newer MUCAPE and mixed-layer CAPE fields are usually more robust indicators of convective potential.

You can find further details in the IFS Documentation – Part IV: Physical Processes under “Computation of Convective Indices.”

I hope this helps!

Emma