Usually CAPE is computed using the radiosonde vertical profiles of temperature and humidity. I am interested in knowing the alternative method if radiosonde data is not available.
I do not believe that CAPE can be calculated only from the surface T and q. If your data has a high temporal frequency, you might be able to diagnose convection based on the derivatives, but then you probably have precipitation measurements anyway.
I am not aware of any reasonable method based only on surface observations. You would have to make some assumptions about the vertical temperature profile, but then you results would only be as good as your assumption. I would suggest you try using profiles form reanalyses or model forecasts as surrogate for the radiosondes. If that makes sense or not depends on the accuracy your application/study requires.
So how shoul I considerer the CAPE_sfc variable that comes with GFS/NCEP model? That name does not suggest me a profiles has been applied. =O
Ricardo, you must look any available documentation on the output of the GFS. There are multiple ways to calculate CAPE, but all use upper-air data. The vertical structure of the atmosphere is a fundamental property determining the potential for convection.
See: http://www.atmos.albany.edu/deas/atmclasses/atm301/CAPE.htm
The different ways in calculating CAPE involve the use of different "starting points" in lifting the parcel. The most common estimates are:
- "surface-based CAPE" which uses the properties of a parcel at the surface,
- "most unstable CAPE" based on the most unstable parcel found in the lowest 300 mb of the atmosphere
- "mean layer CAPE" uses mean conditions in the lowest 100 mb
I'll bet that the CAPE_sfc variable in your model output corresponds to the first one I listed.
Hope this helps.
Yes I am concerned with the surface based CAPE. How it can computed using surface values of temperature and humidity.
The surface based CAPE just uses the thermodynamic properties of the near-surface air to define the parcel being lifted. I think it basically comes down to calculating the virtual potential temperature from the near-surface properties, comparing that value to the current environmental profile and vertically integrating (the positive part) to get CAPE.
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