Utilities: graphing wind and wind stress data

Graphing utility for wind and wind stress computation

This utility plots self-generated or imported wind data and calculates and displays wind stress.
See end of page for instructions and explanatory notes.

This page combines JavaScipt with a Java applet. See the Utilities entry page for details on browser requirements.

Instructions: The utility comes with two demonstration data sets. The WESTERLIES data are from a research voyage south of Australia, the TRADES data from a voyage in the tropical eastern Indian Ocean. During both voyages the wind was recorded every 6 minutes. The length of both time series is 60 hours.

To use the demonstration data set the appropriate checkbox and press DEFAULT DATA, followed by SUBMIT. The first ten data pairs will be displayed in the window. Move to other points in the time series by setting the value number and pressing the <> button. If desired, the default data can be edited in the data windows before the SUBMIT button is pressed.

To enter own data press OWN DATA and follow the instructions that will appear in the window. Select the entire window text and replace it with your data, either by typing the data directly into the window as time, east component, north component separated by commas and one group per line, or by pasting data in from a spreadsheet or text processor. When all data in the window are complete press SUBMIT. This produces a display of the data as they are accepted by the programme. Press RESTORE DATA to return to the data in their original form if further editing is required. RESET clears the entry field.

To plot the data displayed in the window click on the type of graph you want to generate (components, speed and direction, etc.). This changes the data display to the appropriate variables. Reset both graph windows, press TRANSFER DATA TO APPLET, followed by GRAPH. If a graph becomes too "busy" (shows too many overlapping curves) it is possible to switch off one of the two variables (east vs north component or speed vs direction). Use the check buttons at the bottom right of the graphing panel to select the variable you wish to plot, reset both graph windows and press TRANSFER DATA TO APPLET, followed by GRAPH.

To edit a graph edit the data in the number windows, press RESET in the applet and repeat the plotting procedure.

A running average over the time series can be calculated when data are displayed in the number windows. Set the averaging interval and press CALCULATE AVERAGE. To maintain alignment with the time series the averaging interval is automatically set to the nearest (higher) odd number. The smallest acceptable averaging interval is over 5 data points, the largest is over the total number of data points minus 10. To display the average together with the original time series, press RESET in both graph windows, then TRANSFER DATA TO APPLET and graph the data again. To display the data without the average set the averaging interval to zero and proceed as if you were averaging. Note: When averaging the wind stress settings (components vs. speed/direction) override the wind settings.

The variables are colour-coded as follows:

upper data window:
wind velocity

 east component      averaged east component
north component      averaged north component

wind speed           averaged wind speed
wind direction       averaged wind direction

lower data window:
wind stress

 east component      averaged east component
                     east component from averaged wind
north component      averaged north component
                     north component from averaged wind

stress magnitude     averaged stress magnitude
                     stress magnitude from averaged wind

(see notes)

NOTES:

The time is given in arbitrary units as a simple a data count. (The sampling interval for the default data is 6 minutes.) Wind components and wind speed are in m s^-1, wind stress is in N m^-2. Units for data supplied by the user are defined by the user.
The wind stress components τ_x and τ_y are calculated from the equation τ = c_a ρ_a |u| u, where ρ_a = 1.2 kg m_-3 is the density of air, c_a = 0.0015 a dimensionless drag coefficient, u the wind speed and |u| its magnitude.
Wind stress magnitude is calculated from wind magnitude as τ = c_a ρ_a |u|² .
In the speed and direction plot direction is in degrees divided by 10. This allows a reasonable scale for both variables.
On slow machines it may be advisable to reduce computation time by copying a small part of the default data and pasting it in as "own data" before pressing SUBMIT.

This utility is closely related to the material presented in Exercise 3 of Advanced Exercises in Physical Oceanography, which explains why the calculation of wind stress from averaged wind underestimates the true stress and gives detailed explanations of the proper use of wind stress components and magnitude.

Last updated 3 May 2000. Contact address:


Use own data or set the checkboxes: Trades Westerlies
wind time
stress
xmin: xmax: ymin: ymax:
xmin: xmax: ymin: ymax: over values Graph: variable 1 variable 2