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Plotting trajectories in McIDAS-V

Posted: Mon May 18, 2015 4:02 pm
by bobc
It has been requested by multiple users to create a forum post to demonstrate the trajectory display functionality in McIDAS-V. The options for controlling the display are described in the Grid Trajectory Controls page of the User’s Guide.

Below is a step-by-step example of creating 2D gridded trajectories in McIDAS-V:

To begin, add remote gridded data source:
  1. In the Data Sources tab of the Data Explorer, open the Gridded Data > Remote tree.
  2. Using the default catalog, select a model run and click Add Source. In this example, I am using the catalog with the Realtime data from IDD > NCEP Model Data > Global Forecast System (GFS) > GFS-CONUS 80km > Latest * model run.

Add the trajectory layer. There are two methods that can be used to create trajectories; through the Grids > Grid 2D/3D Trajectory formulas, or through the derived Grid 2D/3D trajectory fields. When using the formulas, you will be prompted for the u, v, color-by field, and the vertical field if creating 3D trajectories. The derived fields will only prompt for the color-by field, as the u, v, and vertical components will be automatically selected:
  1. To create a 2D trajectories, in the Field Selector, select the 2D grid > Derived > Grid 2D Trajectory field. Note that this field name may differ dependent on the data source being worked with.
  2. If desired, subset a block of times. In the Times tab of the Field Selector, use the dropdown to change Use Default (all times included with the grid) to Use Selected. Right-click in the Times tab and choose Select Range > First 10.
  3. Click Create Display.
  4. A Field Selector window will pop up prompting for the field to color the trajectories by. Select any 2D field. In this example, I am using the GFS CONUS 80km > 2D grid > Relative humidity @ Specified height level above ground.
  5. Click OK in the Field Selector window.

This will take you to the Layer Controls. Below is a description of the Layer Controls menu items:
  1. The Levels menu item is only used when plotting 3D trajectories. This will allow for setting the initial vertical level of the trajectories.
  2. The Trajectory Initial Area items allow for defining the locations where the trajectories will originate from. Points allows for setting multiple individual locations for the trajectories to originate. ClosePolygon allows for drawing any shape in the Main Display. The trajectories will originate within the geographical bounds of the polygon. Rectangle works the same as ClosePolygon, except the shape can only be drawn as a rectangle.
  3. The Initial Area Skip Factor item sets the number of grid points to skip over in the Trajectory Initial Area region if Close Polygon or Rectangle were used. By default, this is set to 0, meaning that every grid point will be used. Setting this to 1 will plot every other trajectory, and so on.
  4. The Backward trajectory item plots back trajectories. In this case, the trajectories will be drawn in a way where the trajectories end up in the initial Trajectory Initial Area region. This is unchecked by default, meaning that the trajectories will originate from the Trajectory Initial Area region and be drawn out from there.

Create a display of the trajectories:
  1. For Trajectory Initial Area, select Rectangle.
  2. In the Main Display, use Left-Click+Drag to draw a bounding box. Draw this box in a region contained within the bounds of the data. In this example, I’m drawing a box over the continental United States.
  3. In the Layer Controls, click Create Trajectory.
  4. Use the Time Animation Widget controls in the Main Display to play a loop of the trajectories.
    Loop of full-length trajectories
    Loop of full-length trajectories

By default, the entire length of the trajectory is displayed. Dependent on the resolution of data used for the display, this can lead to a cluttered display. Adjust the length of the trajectory to only show the last two time steps in each trajectory:
  1. In the Layer Controls, go to the Times tab and click the Time Mode button.
  2. In the Time Settings window, change Start Time to Relative to End Time.
  3. Click the Offset button for Start Time and in the Set Start Offset window, set the offset to – 12 hours. Click OK.
  4. The Start Time is now set to be 12 hours less than the End Time, which is set to From Animation Time. This means that the trajectory will end at the current animation time of the loop, and start 12 hours before the animation time. In this example, the GFS 80km CONUS data is in 6 hour time steps, so the last two time steps are included in each trajectory.
    Setting trajectory length by time
    Setting trajectory length by time
  5. In the Main Display, use the Time Animation Widget to play through the loop.
    Loop of short trajectories
    Loop of short trajectories

Additional settings for the trajectories can be set through the Layout tab in the Layer Controls. These settings include:
  1. The color table enhancement used for the trajectories.
  2. The visible range for the trajectories. For example, turning this option on and setting the range to 50 to 100 will only draw trajectories when the relative humidity is between 50 and 100%.
  3. The width or thickness of the trajectory lines.

Thanks -
Bob Carp
McIDAS User Services