Tuesday, 13 August 2002: 8:30 AM
Operational ensemble cloud model forecasts: Some preliminary results
From 15 July through 30 Spetember of 2001, an ensemble of cloud scale models was run for the SPC on a daily basis using the NSSL beowulf cluster. Each ensemble run consisted of 78 members whose initial conditions were derived from the 20 km RUC, the 22 km Eta, and the 22 km EtaKF models. Each ensemble was valid over a 160 x 160 km region from 1630 UTC through 0130 UTC (a period of 9 h). The ensembles were used primarily to provide severe weather guidance. To that end, storm lifetimes greater than 60 min and/or a sustained correlation of at least 0.5 between mid-level updrafts and positive vorticity were considered severe weather indicators. Storms generated by the ensemble were also treated as an indicator that general convection would occur within the ensemble region, while the lack of storms was treated as an indicator that no convection would occur. The presence of cloud-to-ground lightning was used as an indicator of observed convection. Manual determination of cell lifetimes is used to validate the forecast storm lifetimes.
Objective skill scores are presented for severe weather and general convection indicators. Along the way, some interesting characteristics of the mesoscale models used to initialize the ensemble were revealed and will also be discussed.