87th AMS Annual Meeting

Wednesday, 17 January 2007: 11:00 AM
A preliminary back-trajectory and air mass climatology for the Shenandoah Valley (Formerly J3.16 for Applied Climatology)
214C (Henry B. Gonzalez Convention Center)
Robert E. Davis, Univ. of Virginia, Charlottesville, VA; and L. Sitka, D. M. Hondula, S. Gawtry, D. Knight, T. Lee, and J. Stenger
Poster PDF (1.1 MB)
Air quality in a region is a function of both temporally varying point and regional pollutant sources as well as changes in the atmospheric environment. In an effort to examine sources of possible air quality degradation in the Shenandoah Valley and potential air quality trends, it is first necessary to establish a background circulation and air mass climatology. Here, we report our preliminary climatological findings.

Eight first-order weather stations in and around the Shenandoah Valley are analyzed: Huntington, WV Beckley, WV, Charleston, WV, Martinsburg, WV, Richmond, VA, Roanoke, VA, Lynchburg, VA and Washington, DC. Data for three of the stations begin in 1949 while six of the eight have consistent readings from 1963-2004.

Temperature, dew point temperature, sea-level pressure, total cloud cover, and surface wind speed and direction from first order weather stations are organized into four daily observations and utilized as input into the Spatial Synoptic Classification (SSC) procedure (Sheridan, 2002). The SSC employs discriminant function analysis to generate a daily discretization of these weather parameters into six air masses (dry polar (DP), dry tropical (DT), dry moderate (DM), maritime polar (MP), maritime tropical (MT), maritime moderate (MM)), and one transition type (TR)).

For each station, air mass frequencies are examined for seasonality and temporal trends. The times series for each air mass and station are evaluated for trends and change points using the objective procedures developed by Meene and Williams (2005).

Twice-daily back trajectories are run using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model for all eight stations for the period January 1, 1997 through December 31, 2005. ETA model initialization wind fields, at 80-kilometer resolution, and a 12-hour time step are utilized. The resulting trajectory latitude, longitude, and elevation are grouped via a two-stage (hierarchical and non-hierarchical) cluster analysis to identify the natural groupings of flow patterns in and around the Shenandoah Valley.

Preliminary SSC results indicate that MT air masses are becoming more frequent in and around the Shenandoah Valley. These increases are most dominant in the summer and fall seasons. At several stations, higher MT frequencies are coupled with an associated decrease in DT frequencies. This warm season humidification should have impacts on air quality trends, particularly ozone.

Preliminary trajectory results show a strong predominance of southeasterly, southerly, and southwesterly flow during the period, particularly during the warm season. This outcome provides preliminary support and validation of the air mass trend results and evidence of higher humidity in the last few decades.

Ongoing and future research will involve specifically linking trajectories to air masses and introducing both point and regional pollutant information into the analysis. These results will provide a critical climatic context for observed air quality trends and changes in the Shenandoah Valley.

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