Predictability of mesoscale convective systems in two- and three-dimensional models

Mesoscale convective systems (MCSs) are responsible for a substantial fraction of warm season rainfall in the central United States. However, very little is known about the predictability of MCSs. To begin to address this lack of knowledge, a series of ensemble runs were performed on a 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 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 100 km on either side of the location of the MCS in the control run.

Using that standard, in two-dimensional simulations, MCSs occur roughly 70% 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. Preliminary results with the three-dimensional version of the model suggest that the two-dimensional simulations may provide a reasonable estimate of MCS predictability.

Predictability for other storm characteristics (e.g., maximum updraft, maximum surface wind, etc.) will be presented as well, along with results illustrating the challenge of distinguishing between systems that will remain linear and those that will evolve into bowing structures.