Observed and WRF-Simulated Low-Level Winds in a High-Ozone Episode during the Central California Ozone Study

Abstract

A case study is carried out for the 29 July-3 August 2000 episode of the Central California Ozone Study (CCOS), a typical summertime high-ozone event in the Central Valley of California. The focus of the study is on the low-level winds that control the transport and dispersion of pollutants in the Central Valley. An analysis of surface and wind profiler observations from the CCOS field experiment indicates a number of important low-level flows in the Central Valley: 1) the incoming low-level marine airflow through the Carquinez Strait into the Sacramento River delta, 2) the diurnal cycle of upslope-downslope flows, 3) the up-and down-valley flow in the Sacramento Valley, 4) the nocturnal low-level jet in the San Joaquin Valley, and 5) the orographically induced mesoscale eddies (the Fresno and Schultz eddies). A numerical simulation using the advanced research version of the Weather Research and Forecasting Model (WRF) reproduces the overall pattern of the observed low-level flows. The physical reasons behind the quantitative differences between the observed and simulated low-level winds are also analyzed and discussed, although not enough observations are available to diagnose thoroughly the model-error sources. In particular, hodograph analysis is applied to provide physical insight into the impact of the large-scale, upper-level winds on the locally forced low-level winds. It is found that the diurnal rotation of the observed and simulated hodographs of the local winds varies spatially in the Central Valley, resulting from the combining effect of topographically induced local forcing and the interaction between the upper-level winds and the aforementioned low-level flows. The trajectory analysis not only further confirms that WRF reproduces the observed low-level transport processes reasonably well but also shows that the simulated upper-level winds have noticeable errors. The results from this study strongly suggest that the errors in the WRF-simulated low-level winds are related not only to the errors in the model's surface conditions and atmospheric boundary layer physics but also to the errors in the upper-level forcing mostly prescribed in the model's lateral boundary conditions.

Diurnal Evolution and Annual Variability of Boundary-Layer Height and Its Correlation to Other Meteorological Variables in California’s Central Valley

“…During the summer months, stronger winds associated with a nocturnal low-level jet (LLJ) occur at or below 500 m, soon after the collapse of the boundary layer (0300-1100 UTC or 1900-0300 LT). This is in accordance with previous studies (Burk and Thompson 1996;Bao et al 2008). The LLJ begins in March-April and continues through the summer until September-October, with the height of the jet maximum near 400 m a.g.l., with little hourly variation in the jet height during the night, or from month to month.…”

“…In July, and to a lesser extent in August, an elevated, secondary local maximum occurs near 1.0 km a.g.l., well above the daytime maximum height the CBL. This could be due to the location of this site, which is situated relatively close to the ocean in a gap between the San Francisco Bay Area and the Central Valley where a thermally driven flow from the cool Pacific Ocean into the Central Valley (Bao et al 2008; Zhong et al Fig. 6, is the only site showing a different CBL depth behaviour, with the CBL depths having almost the same maximum value through the entire period between April and August.…”

Long-term particulate matter modeling for health effect studies in California – Part 1: Model performance on temporal and spatial variations

“…Mean fractional biases (MFBs) of temperature and wind are generally within ±0.15, RMSEs of temperature are around 4 • C, and RMSEs of wind are generally lower than 2.0 m s −1 , especially in the SoCAB and SJV air basins which are the focus of the current study. Relative humidity is underpredicted, consistent with findings in other studies in California (Bao, 2008;Michelson et al, 2010b). Precipitation is also underpredicted with a MFB of −76.1 % and RMSE of 2.84 mm h −1 .…”

Transport and mixing patterns over Central California during the carbonaceous aerosol and radiative effects study (CARES)

“…Seaman et al, 1995;Dabub et al, 1999;Jacobson, 2001;Bao et al, 2008;Jin et al, 2010;Huang et al, 2010;Michelson et al, 2010;Pfister et al, 2011) have improved the understanding of how meteorological processes in California affect the spatial variations and chemical transformation of pollutants. Various routine measurements and several special studies in the Central Valley of California have shown that pollutants emitted from Sacramento, California during the summer are frequently transported by the thermally-driven upslope flows that draw the urban air towards the northeast over the foothills of the Sierra Nevada (Murphy et al, 2006).…”