Pleim, Jonathan and Robert Gilliam, US Environmental
Protection Agency
Recent evaluation
studies of the WRF-CMAQ meteorology and air quality modeling system have shown
a persistent tendency to overpredict ground-level concentrations of locally
emitted pollutants such as CO and NOx in developed areas. Emissions of these chemical species are
predominantly from mobile sources and typically peak during the morning and
evening rush hours which correspond to transition times for PBL
development. We have hypothesized
that model overpredictions, especially during the evening transition time, are
due in part to inadequate representation of urban heat island effects in the
Pleim-Xiu land surface model (PX LSM).
Comparisons to tethersonde observations made during the evening and
overnight at the University of Houston for several nights in September 2006
show that the lowest 200 m stay neutral until 9 PM LT while the model has developed a stable potential
temperature inversion.
Modification to the PX LSM, such as scaling the surface heat capacity
according to the fractional coverage of impervious surfaces from the National
Land Cover Data (NLCD) and increasing the roughness length for the 4 developed
land-use categories greatly improve the modeling of the nocturnal boundary
layer in urban areas and reduce the overprediction of emitted pollutants.