Evaluation of the timing and strength of MM5 and Eta surface trough passages over the eastern Pacific

Abstract

The timing and strength of surface troughs forecast by the Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) and the National Centers for Environmental Prediction's (NCEP's) Eta Model are evaluated over the eastern Pacific during the 1997-2000 cool seasons (Sep-Mar). The troughs are verified by comparing observations from eight offshore buoys with 3-48-h model forecasts. An automated approach is used to identify trough events by setting thresholds for 3-hourly cyclonic wind shifts and pressure rises. The mean timing errors (biases) for surface troughs in the 36-km resolution MM5 domain are 1-2 h early (negative) to the north of 42°N and near the coast, and 0.5-1.0 h late (positive) over the offshore waters to the south of 42°N. The mean absolute timing errors range from 2-3 h for the southern offshore sites to 4-5 h over northern offshore waters as well as near steep coastal terrain. For stronger trough events, the MM5 trough mean errors increase to 2-3 h early near the coast and decrease to 0.2-1 h early offshore to the north of 42°N. Early MM5 troughs have surface winds that are 5%-15% too strong and 3-h pressure rises that are 15%-40% too weak. The 48- and 32-km Eta model forecasts interpolated to a 80-km grid were verified separately for the same trough events as the MM5. The Eta mean timing errors are similar to those of the MM5, except that the Eta early biases are not as large offshore. The slower troughs in the Eta are associated with 5%-10% weaker surface winds than in the MM5. The MM5 has smaller mean absolute timing errors than the Eta to the west of 130°W (by 0.25-0.5 h) and greater errors than the Eta near the coast (by 0.25- 0.75 h). The early and late events in the MM5 and Eta are episodic, with clusters of early and late trough events lasting approximately 2-4 weeks. A composite study of the large-scale flow using the NCEP reanalysis grids illustrates that the early events are associated with more significant large-scale troughing over the north-central Pacific and ridging over the western United States than the late events or climatology. The early events south of 42°N have significantly stronger mid- and upper-level flow over the central Pacific compared to climatology. At the surface, the late events often involve a well-defined trough or cyclone moving eastward, while the early events are generally associated with weaker troughs rotating northeastward toward the Pacific Northwest.