|HABYTIME MINI LECTURE 49: MOST UNSTABLE CAPE
METEOROLOGIST JEFF HABY
Most unstable CAPE is the parcel trajectory that produces the largest value of CAPE. When first learning about CAPE generation, often the CAPE
that will be determined is using a parcel of air lifted from the surface since it is relatively easy to locate the surface dewpoint and
temperature and lift the parcel adiabatically throughout the troposphere. This method is good for a first approximation but it is not as
realistic as determining the average dewpoint in the lowest 100 mb and the average temperature in the lowest 100 mb, plot these points
50 mb above the surface and then lift the parcel adiabatically to produce CAPE. This is a more realistic approach since there is
significant mixing of air in the boundary layer and the parcel that is being lifted is a large volume of air that can take up a
significant depth of the boundary layer. Also, the dewpoint and temperature values right at the surface tend to be contaminated
by physical processes that are not adiabatic.
To find the MU CAPE (Most Unstable CAPE), a computer program will lift a parcel from a large number of pressure surfaces (i.e. from the
surface, from 975 mb, from 950 mb, from 900 mb, and everything in-between and more). The trajectory that produces the maximum amount
of CAPE is the MU CAPE. For example, suppose lifting from the surface generates 2,000 J/kg of CAPE, lifting from 950 mb generates
3,000 J/kg of CAPE, lifting from 900 mb generates 4,000 J/kg of CAPE and lifting from 850 mb produces 2,800 J/kg of CAPE. If only
these values are used, then the MU CAPE would be the 4,000 J/kg generated from lifting the parcel from 900 mb. The MU CAPE can give a
forecaster an idea of the maximum amount of CAPE that could be generated at a particular moment in time based on the sounding
and model data. Of course, for MU CAPE to materialize the lifting would have to start from the pressure level the MU CAPE was
generated from. This may or may not happen. It is generally up to the forecaster to determine at what range of pressure levels
the lifting is most likely to begin from.
The diagram below shows an example of CAPE being generated by lifting from a variety of pressure levels. For simplicity it is assumed
in the diagram that the troposphere is saturated and thus lifting immediately starts at the wet adiabatic lapse rate. This diagram
gives a sense of how dramatically the CAPE can change depending on what pressure level the lifting starts from. The CAPE is the
shaded region where the parcel temperature is warmer than (to the right of) the actual temperature sounding profile. Note the
hatched area is larger if lifting occurs from 900 mb as compared to lifting from 950 mb or the surface.