On the predictability of mesoscale convective systems

Building upon earlier work by two of the authors a series of ensemble runs were performed on a two-dimensional storm-scale (dx = 1 km) model. Perturbations in wind speed, relative humidity and CAPE were based on current forecast errors from the North American Model (NAM). The ensemble results thus give an indication of the predictability of mesoscale convective systems (MCSs), assuming successful convective initiation. An ensemble member simulation is considered a success when it reproduces a convective system of at least 20 km in length within 75 km on either side of the location of the MCS in the control run.

Using that standard, MCSs occur roughly 65% of the time for perturbation magnitudes consistent with 24-h forecast errors. Reducing the perturbations for all fields to one-half the 24-h error values increases the MCS success rate to over 90%. The MCS success rate is roughly equally sensitive to changes in wind, relative humidity, or CAPE perturbations. Additional measures of predictability will be presented as well, along with preliminary results of three-dimensional simulations.