Trajectories are aerial maps showing the path taken by an air parcel. These estimates of air movement are important to better understand the behavior and potential impact of air pollution.
- Where Did the Air Come From and Where is it Going?
- How to Read the Trajectory Maps
- How the Trajectories Were Created
Two types of trajectories are presented for each location: forward and backward. Choose a location from the table below.
|Beaumont||El Paso||San Antonio|
|Big Bend National Park||Houston||Victoria|
These trajectories were obtained from the National Oceanic and Atmospheric Administration's Air Resources Laboratory and present an estimate of air movement based on one model. A very different picture of air movement can be gained by using other models or by putting different information into the same model. Compare these trajectories with those available at CAPITA, Ozone Transport Assessment Group .
Where Did the Air Come From and Where Is It Going?
The maps available from this page illustrate estimates of the general path air traveled over 48 hours to arrive at a particular Texas location and where it is going immediately afterward. The images are calculated every day for 2100 hours Zulu (Z) or Coordinated Universal Time (UTC), which is 3 p.m. Central Standard Time (or 2 p.m. Mountain Standard Time). They are posted to the TCEQ's Internet Web server the following hour. Trajectories are available for the past 30 days.
These estimates of air movement are important to better understand the behavior of air pollution. The meteorological dynamics that cause air to rise or fall, and that determine its path can affect air quality by carrying air pollutants many miles from their sources.
How to Read the Trajectory Maps
Each backward trajectory map shows an aerial (plan) view of the path taken by an air parcel, and a vertical view of its movement at different altitudes, before arriving at 164 feet (50 meters) above the earth's surface at a particular location. Similarly, the forward trajectories show the same information starting at 164 feet for the ensuing 36 hours. The forward trajectories start at 2100Z and end at 0900Z (3 a.m. Central Standard Time) the second calendar day after the trajectory started.
The aerial view traces the path the air parcel followed over 48 hours (36 hours for forward trajectories). Tick marks and the numbers 00, 06, 12 and 18 from the 24-hour clock are used along each trajectory to indicate the position of the air parcel over the 48-hour or 36-hour period. The vertical view at the bottom of each map shows the height of the air measured at these times by corresponding tick marks.
The tick marks, except for the first and last marks, indicate six-hour increments. The first tick mark along the trajectory from the air's final destination marks the parcel of air's location three hours prior to its arrival. The last tick mark on the trajectory shows where it was three hours into its journey. The height of the air is measured in hecto pascals (HPA). Eight hundred and fifty HPA is about 5,000 feet.
How the Trajectories Were Created
The trajectories were calculated using the HYSPLIT trajectory model written by Roland Draxler. (For a discussion about one application of the model and the algorithms used, see Draxler, R. R., 1996: Trajectory Optimization for Balloon Flight Planning. Weather and Forecasting, No.11, Pages 111-114.)
The input for the HYSPLIT model is the output from the eta model, which is run at the National Centers for Environmental Prediction (NCEP--formerly known as National Meteorological Center). Trajectory and weather forecasting models have error. Clearly, any error in the eta weather forecasting model is passed along into the HYSPLIT trajectory model. Moreover, as the input parameters to HYSPLIT vary, significantly different trajectories are generated.