Derecho Composite Parameter (DCP)

The DCP was developed to identify environments considered favorable for cold pool “driven” wind events through four primary mechanisms: 1) Cold pool production [DCAPE], 2) Ability to sustain strong storms along the leading edge of gust front [MUCAPE], 3) Organization potential for any ensuing convection [0-6 km shear], and 4) Sufficient flow within the ambient environment to favor development along downstream portion of the gust front [0-6 km mean wind]. Normalized values were developed for each parameter using the 51 observed proximity soundings near "Weak Forcing" derechos, which were compared to values from 31 proxy soundings from WF non-derecho MCSs. It was found that DCAPE > 980 J/Kg and MUCAPE > 2000 K/kg were common (25th percentile), while sfc-6 km shear > 20 kt and sfc-6 km mean wind > 16 kt were uncommon (75th percentile) in the non-derecho dataset. Though the Evans and Doswell (2001) dataset suggests the two kinematic parameters alone could be used to discriminate the derechos from non-derecho cases, it would be necessary to assume favorable thermodynamics exist. In order to remove this assumption, all four parameters were collected into a single composite parameter as follows:

DCP = (DCAPE/980) * (MUCAPE/2000) * (0-6 shear/20 kt) * (0-6 mean wind/16 kt)

The ability of the DCP to discriminate between non-derecho and derecho MCSs can be inferred from the figure below, which shows complete separation of the interquartile ranges (25th – 75th percentiles) of the DCP between the non-derecho and derecho MCS data sets.

For more information, see the complete 2005 Summer Program Plan and/or Evans and Doswell (2001).