Composite Method Results Artificial Cases April 2008 Jason Nachamkin Naval Research Laboratory Monterey, CA

Geometric Cases CASE I Composite Centered on All Predicted Events Composite Centered on All Observed Events Grid centered on ‘forecast’ Grid centered on ‘observation’ Compositing displays average distributions for predicted and observed events. Composites of single cases are very simple. The only difference between the forecast-based and observation-based composite is the location of the grid origin. Composites for all geometric cases are centered on the 50 mm(?) ovoid. Composite grids are 501x501 points in size.

Geometric Cases CBD Conditional bias difference (CBD) = bias of forecast-based composite minus bias of obs-based composite. Case I generally performed best, though poor scores existed close to the event center. Case III had the worst score at intermediate distances from the event. Case V scored best close in, but worst at large distances. Errors weighted lower with increasing distance - larger averaging area is more generous.

Perturbed Cases CASE III Composite Centered on All Predicted Events Composite Centered on All Observed Events mm hr-1 Composited all events with precip rates of 11 mm/hr or greater (convective cores), minimum event size = 50 grid points, composite grid size = 201x201 points. Each case contained 7 events ranging in size from 51 to 530 points. Covariance drops very rapidly with distance as indicated by the small area of coherent averages. Lower thresholds produce larger coherent areas.

Geometric Cases CBD Case I performed the best at all box sizes. Case V performed the worst at large distance. As displacement distance increased, error rapidly maxed out close to the events due to their small size.