18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
The use of a 10km ensemble to improve warm season MCS rainfall prediction
William A. Gallus Jr., Iowa State University, Ames, IA; and M. Segal and I. Jankov
A 10 km version of the NCEP Eta model has been run for over 20 cases where convective systems occurred during the warm season in the Upper Midwest. These cases were chosen because mesoscale features present at model initialization time appeared to play a role in subsequent heavy rainfall development.

A 14 member ensemble of the 10 km simulations was created by varying the convective parameterization (Betts-Miller-Janjic, Kain-Fritsch, no scheme, both schemes alternating), the convective time step, and by adjusting the initial conditions to better represent mesoscale features such as cold pools, outflow boundaries, or moist tongues. In addition, more extensive variation of initial conditions was investigated by creating some ensemble members whose initial perturbations from a mean state varied by a small amount from the control runs.

The high-resolution results indicate that ensemble spread is rather small for adjustments to better represent mesoscale conditions. Changes in moist physics result in much larger spread. For these difficult to forecast convective systems, the ensemble guidance may be of limited value because the verifying scenario often differs from all members of the ensemble. Standard measures of ensemble skill will be presented and compared with deterministic forecasts for the events.

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