Evaluation of very high-resolution simulations with the non-hydrostatic numerical weather prediction model Lokalmodell for urban air pollution episodes in Helsinki, Oslo and Valencia

The operational numerical weather prediction model Lokalmodell LM with 7 km horizontal resolution was evaluated for simulations of the meteorological conditions during observed urban air pollution episodes. The resolution was increased to experimental 2.8 km and 1.1 km resolution by one-way interactive nesting without introducing urbanisation of physiographic parameters or parameterisations. The episodes examined are two severe winter inversion-induced episodes in Helsinki in December 1995 and Oslo in January 2003, three suspended dust episodes in spring and autumn in Helsinki and Oslo, and a late-summer photochemical episode in the Valencia area. The evaluation was basically performed against observations and radiosoundings and focused on the LM skill at forecasting the key meteorological parameters characteristic for the specific episodes. These included temperature inversions, atmospheric stability and low wind speed for the Scandinavian episodes and the development of mesoscale recirculations in the Valencia area. LM forecasts often improved due to higher model resolution especially in mountainous areas like Oslo and Valencia where features depending on topography like temperature, wind fields and mesoscale valley circulations were better described. At coastal stations especially in Helsinki, forecast gains were due to the improved physiographic parameters (land fraction, soil type, or roughness length). The Helsinki and Oslo winter inversions with extreme nocturnal inversion strengths of 18°C were not sufficiently predicted with all LM resolutions. In Helsinki, overprediction of surface temperatures and low-level wind speeds basically led to underpredicted inversion strength. In the Oslo episode, the situation was more complex involving erroneous temperature advection and mountain-induced effects for the higher resolutions. Possible explanations include the influence of the LM treatment of snow cover, sea ice and stability-dependence of transfer and diffusion coefficients. The LM simulations distinctly improved for winter daytime and nocturnal spring and autumn inversions and showed good skill at forecasting further episode-relevant meteorological parameters. The evaluation of the photochemical Valencia episode concentrated on the dominating mesoscale circulation patterns and showed that the LM succeeds well in describing all the qualitative features observed in the region. LM performance in forecasting the examined episodes thus depends on the key episode characteristics and also the season of the year with a need to improve model performance in very stable inversion conditions not only for urban simulations.

Use of Episodic Controls to Reduce Frequency and Severity of Air Pollution Events

Most common urban air pollution episodes occur during readily identifiable meteorological conditions that contribute to the problem. In contrast, most air pollution controls concentrate on permanent curtailments of emissions. The argument is put forth that episodic emission control strategies can be an effective supplement to traditional measures, at a fraction of the cost. Questions regarding the legality of episodic controls have been the main barrier to their implementation. These legal issues are discussed. In addition, the forecasting and modeling implications of episodic controls are considered. Several areas have implemented episodic air pollution controls or used accurate short-term forecasts to warn the public through health advisories of pending air pollution events. Some of these success stories are highlighted. A scheme for assisting the prediction of emissions reductions based on the degree to which episodic measures are voluntary or mandatory is proposed. This method may help resolve emission reduction credit and enforcement issues, making approval of episodic measures for state implementation plans and conformity more likely. The transportation sector has the potential to make episodic measures successful. Furthermore, some of these measures are eligible for federal transportation dollars. The potential for funding transportation-related episodic controls is explored. While episodic controls cannot replace traditional controls, they can be an incremental or short-term solution that may evolve into, or be used in conjunction with, other plans. Transportation professionals are incited to develop creative short-term solutions to the pervasive, yet short-term, problem of urban air pollution.