Airwatch – Wind tracjectory analysis

1. Background

As the day progresses, the winds at your monitoring station change with time. If the winds were actually to be the same at every place, rather than varying due to terrain features, then plotting the path raversed by a parcel of air would be simple: compute the distance travelled at the measured speed and in the measured direction
between each recorded reading; plot these paths as a progressive vector diagram on a map. The path is called a ‘trajectory’. A forward trajectory is one where the air parcel moves forward in time away from the monitoring station. It tells you where the air parcel is going. A backward trajectory shows where the air has come from. So a backward trajectory can tell us where measured air pollution might be coming from — this is really useful and is used a lot by EPA complaints investigators.

Of course the real circumstances are more complicated than our simple picture: the wind varies from place to place at any time due to geographic features; pollutants are often emitted from tall chimneys at heights where
the wind is different in speed and direction to that measured at the surface; and the origin of a pollution event can become confused due to interference from several sources. So using wind measurements from more than one monitoring station can improve the predictions a lot. Nevertheless, the trajectory constructed from data from a single monitoring station can give a good first estimate of a parcel trajectory.

2. Analysing trajectories

In the AIRWATCH kit we supply a computer program, TRAJ.EXE, that can calculate forward and backward trajectories and display them on the PC screen. The trajectory program takes as input, data you have saved from the weather station.

You can obtain a copy of the computer program (requires a basic PC with VGA screen to run) and its various input files. Set it up in its own directory on the computer.

A picture of the screen from the running program is shown here. It is from a study of data at Caversham monitoring station in eastern Perth. You can see the outline of the coast, Rottnest Island, the Swan River, and the Perth central business district on the map. Wind data from a record over two days are plotted, centred on Caversham at 1500 hr on the first day. The trajectory makes two loops as the wind changes from easterlies to a sea breeze and back, showing that pollutants can be brought back over Perth a day after they are emitted.

Using the software, you can:

• Zoom in and out by pressing <+> or <-> keys or by clicking with the mouse on the box labelled 'Zoom': a left mouse click zooms in, a right mouse click zooms out.
  
• Plot a forward, backward or forward+backward trajectory by pressing <t> successively, or by left clicking on the word ‘Trajectory’ with the PC mouse.
  
• Change the centre time, that is the start time for forward trajectories, which is the end time for backward trajectories. Press <c> or left click on the word ‘Centre time’ and enter a new time in 24 hour clock time. To change the centre time day, press <d> or left click on the words ‘Centre Day’. Each time you do so, the day is incremented by one to the maximum of the data set. It then resets back to the first day in the data set. Choose a meaningful time for the data set you are plotting.
  
• See how pollutants emitted from the location of the monitoring station would spread further downwind. This is done by drawing circles at each observation hour. The diameter of the circle is proportional to the rate of dilution of pollutants in the atmosphere. This rate of spread is controlled by the ‘Plume Sigma’ parameter, and this can be changed by pressing <m> or left clicking on the word ‘Sigma’ and typing in a new number. Numbers in the range 0 to 0.3 are realistic; the higher the number, the more rapidly does the pollutant spread in the air. For sunny light wind conditions, Sigma » 0.3; for overcast windy conditions, Sigma » 0.15; for clear night conditions, Sigma » 0.08. See p101: Lyons, T. and B. Scott (1990) ‘Principles of Air Pollution Meteorology’, Belhaven Press, ISBN 1 85293 079 9.
  
• The time that the pollutant begins to spread can also be changed. It is initially set to the centre time. The day is always the same as the day for the centre time. Press <s> or left click on the word ‘Spread time’ and enter a new time in the range of the data.
  
• If you have a HP printer or another type that understands HP PCL, then pressing <p> or left clicking on the box ‘Print’ will give a printout of the screen. If you have some other sort of printer connected to the computer, at best you will get garbage, at worst the computer will ‘hang’. You can print to any printer by running the trajectory program full screen in a ‘DOS shell’ in Windows, pressing the <PrintScreen> key, switching to the Paintbrush program in Windows, pasting the picture of the screen into it and printing from there.

3. Setting up the trajectory software

The program and example files for Sydney and Perth are available here as ZIP files. As well as the operating program, the archive contains a file called SOURCE.ZIP that includes a copy of the source code in Turbo Pascal. You do not need to look at this; it is included for reference. Unzip the files to a working directory. The various files are:

GO.BAT, GO.PIF (a file for MS Windows), TRAJ.EXE — these are the program files. Execute or double-click on GO.BAT is one way to run the trajectory program.

TRAJ.DAT is a text file that contains the weather data from the WeatherLink software.

TRAJ.CST is a text file containing grid information about the geography of the study region: such as coastline, water bodies, main roads or other features.

TRAJ.TWN is a text file that contains coordinates and labels for your air monitoring station, suburb names, and other points of interest.

The existing files are for Caversham, east of Perth. Edit the files in your working directory using a simple text editor from DOS or Notepad from Windows. Never edit the original files!

4. Obtaining a data set suitable for trajectory analysis

The easiest way is to use the recommended weather station to produce a data file in the correct format automatically. However, the requirements are so simple that almost any time series of wind speed and direction data can be used.

The Weather Monitor II weather station and WeatherLink software needs to be set up to report metres per second for wind speed to produce data files of the right kind for the trajectory analysis. The steps in generating a suitable data file are

1. Measure the winds for a day or two.
2. Save the data to disc and then using the <Print> button on the WeatherLink menu, create an Export File.
3. Move the Export File to the directory where TRAJ.EXE is located and overwrite the file named TRAJ.DAT.
4. The structure of the TRAJ.DAT file should then look something like:

"packedDate","packedTime","tin","tout","hitout",
…


(- these are headings for data, comma-delimited)


date, time, …,windspeed, wind direction, …


(- date, hhmm, …, m/s, compass points=0..16)


(- continued to End Of File)

Here is a part of the file that comes with the trajectory software for Perth (there is no initial blank line):

(Note for those trying to use other data sources to generate trajectories: The heading MUST contain the words

packedDate,packedTime,wsp,wdir

at the least. The words may be in any order, separated by commas and may be in quotes or not. Other headings may be included, such as "zmix". These are ignored by the trajectory program. Following on separate lines below the headings, the data MUST be in the same order as the headings, again separated by commas. Quotes are optional.

"packeddate" is a simple string representing the date of the observations.
"packetime" is the time of day as an integer in the form "hhmm", eg "130" is 1:30am, "1715" is 5:15pm.

"wsp" is wind speed, a decimal number in m/s.
"wdir" is wind direction, a decimal number [0…16), ie the number of 22.5degree sectors from North. eg "14.7" is 330degrees from North (WSW).
Look again at the example given earlier.
End of Note.)

Before running the trajectory program, be sure two other files are also present in the same directory as TRAJ.EXE: TRAJ.CST and TRAJ.TWN. These may need to be changed by you to be relevant to your area.

5. Creating display files for your study area

TRAJ.CST defines the coordinates of geographic features to be plotted on the grid map. For example, the coastline of Perth is an obvious feature to plot.

Find a suitable map of your area which covers at least 120 km × 120 km. Why so big? Well, if the wind speed is on average 5 m/s and the wind direction is steady for 10 hours, a parcel of air will travel 180 km, so the grid map must be large. The location of the weather station/air monitoring equipment should be near the centre of the map so that the zoom feature of the program works correctly. Decide the area that your grid will cover (eg 100 km × 100 km) and pencil a set of grid lines on the map at a manageable spacing, say 5,000 m or 10,000 m. You then need to construct a table of x,y points of the different features, measured in metres from the bottom left hand corner of the map (x,y point 0 0 )

All the points are joined up by a line until End of File or a 0 0 pair is read. If the latter, a new line is started. The structure of the file TRAJ.CST should look like:

Title - title of feature outline (discarded): A
X Y - coords of first point in m.
... - (0 0 breaks joining line)
... - continued to End Of File.

Here is a part of the file included with the program for Perth. It defines Rottnest Island and part of another outline (there is no initial blank line and the heading is necessary but not used):

Perth Coast
55725 37500
57500 35725
59275 37500
59425 40000
59425 42500
57500 44425
56475 45000
55000 46475
53525 45000
55000 43525
55575 42500
55575 40000
55725 37500
00000 00000
33700 62500
35000 61200
37500 60625

TRAJ.TWN defines the coordinates of the region and of points of interest to use for the trajectory. Note that whichever place is listed first in the list of names, this will be used as the centre for the trajectories. So, list the location of the monitoring station first. The structure of the file should look like:

File: TRAJ.TWN

Here is the complete file included with the program for Perth (there is no initial blank line and the heading is necessary and is used to label the screen display):

Caversham data
0 0 125000 125000 50000
3
CAVERSHAM
82500 77500
PERTH
72500 67500
Rottnest
40000 62500

Look at the example files that come with the software using a text editor to confirm that you understand the requirements for the trajectory program. The Perth example gives files that strictly comply with the requirements. The Sydney example includes files that are also acceptable.

If you have problems getting the program to work in your locality, follow these steps:

• Use the example program and files as supplied. Does the program work?
• Change only the data file of winds (TRAJ.DAT). Do the wind trajectories look reasonable, even though the coastline and labels are probably not right?
• Change the labels file (TRAJ.TWN). Is the size of the region adequate? Do the labels display correctly?
• Change the coastline file (TRAJ.CST). Does the program now display the correct coastline or other features you require? If so, you are finished!

Contact: peter.manins@csiro.au

Return to The Airwatch Index

Modified: 17 May 2002