PNNL

Abstract

An EPA-recommended diagnostic wind model (CALMET) was evaluated during a typical lake-breeze event in the Chicago region. We focused on the performance of CALMET in terms of simulating winds that were highly variable in space and time. The reference winds were generated by the PSU/NCAR MM5 assimilating system, with which CALMET results were compared. Statistical evaluations were conducted to quantify overall errors in wind speed and direction over the domain. Within the atmospheric boundary layer (ABL), relative errors in (layer-averaged) wind speed were about 25% to 40% during the simulation period; wind direction errors generally ranged from 6 to 20 deg. Above the ABL, the errors became larger due to lack of upper air stations in the studied domain. Analyses implied that model errors were dependent on time due to time-dependent spatial variability in winds. Trajectory analyses were made to examine the likely spatial dependence of model errors within the domain, suggesting that the quality of CALMET winds in local areas depended on their locations with respect to the lake-breeze front position. Large errors usually occurred near the front area, where observations cannot resolve the spatial variability of wind, or in the fringe of the domain, where observations are lacking. We also compared results simulated using different datasets and model options. Model errors tended to be reduced with data sampled from more stations or from more uniformly-distributed stations. Suggestions are offered for further improving or interpreting CALMET results under complex wind conditions in the Chicago region, which may also apply to other regions.