Lifted Indices
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Reanalysis output includes two lifted indices. the surface lifted
index (LFTX) and the best (4-layer) lifted index (4LFTX).

The surface lifted index is computed by finding the temperature of
an air parcel lifted from the lowest layer to 500 mb and subtracting
the ambient 500 mb temperature.

The best lifted index is computed by lifting the parcel from the
layer with the warmest equivalent potential temperature to 500 mb 
and subtracting the ambient 500 mb temperature.

For computation, the vertical profile was converted to six layers
of depth of 30 mb starting from the surface.