Unidata Summer Workshop - 2003

IDV Lab F

(updated to version 1.1)

Several, self-paced IDV labs are available. Each assumes different user backgrounds, addresses different problems, and highlights different IDV features. Please try the ones that seem most interesting.

Lab F Part a

  • requires no previous experience with the IDV
  • compares precipitation forecasts from 2 different NWS operational numerical models
  • utilizes the familiar contour plan view and a simple formula

The IDV (Integrated Data Viewer) is Unidata's Java program that can run on any platform. Furthermore, there are several ways to start the application.

Under Windows, Start the IDV by:

  • double clicking the IDV icon on the Windows Desktop
or
  • choose, Start, Programs, Integrated Data Viewer, IDV.

If using Sun Microsystem's Java Web Start program,

Web Start may require you to choose the application and click the, Start, button.
Web Start may require you to download the latest version before running the application.
Web Start may report the error, There is no disk in the drive, several times during start up. Just click the, Continue, button.
Do not close the, Java Web Start Application Manager, window while the IDV is running.

The IDV uses multiple windows. Position them comfortably on your desktop.

Unidata IDV - used to display data; do not close until ready to exit the IDV.
Data Selector - used to select data; do not close until ready to exit the IDV.
IDV Help Tips - used to show help tips; may be closed.
Other - windows open when needed.

The Unidata IDV contains a user's guide, which explains many IDV topics. Feel free to use it at any time.

From the, Help, menu select, User's Guide.
Maximize the, IDV User's Guide, window.

Displaying model data is a two step process. First, you select data; then, you create a display.

I. Select model data.

  • In the, Data Selector, window: click the, Add New Data Source, button.
  • In the, Data Source Selector, window: select the, Catalogs, tab.
  • The default catalog is on the UCAR server, which is named, motherlode. A list of available model datasets will be retrieved and shown in a few seconds.

Notice that the IDV retrieves data from remote servers. Thus, the IDV provides access to many data sources, and one is not limited by what is available locally.

In the, Data Source Selector, window, note that data is available from several different NCEP models. In fact, data from the GFS model are available on several different grids; each covers a different geographic region. The CONUS grid covers the United States and surrounding areas, so it is the grid to use today.

Each model is run several times each day, according to a set schedule. Each model run is identified by the day and starting hour, GMT (Greenwich Mean Time). It is initialized with observations available up to that time and makes forecasts by solving the laws of Physics. To see a list of available runs for a model, open the appropriate folder, by clicking on the key to the left of the folder icon.

  • Open the folder with GFS CONUS model data.

The first model run, Latest, will link to the most recent run of the model. You will often want to look at the most recent model forecast, so, lets use it.

  • Click the, Latest, model run.
  • Click the, Add Source, button.

The, Data Source Selector, window will close; and you must return to the, Data Selector, window.

In the, Data Selector, window the selected data source should be listed in the, Data Sources, pane. In the, Fields, pane you shuld see two folders: 2D grid and 3D grid, which contain two-dimensional and three-dimensional grids.

  • Open the, 2D grid, folder.

The next step is to select a quantity, by clicking on your choice.

  • Select, Total precipitation.

The next step is to select forecast times, in the, Times, pane.

+ For a loop of all available times, default, you do not need to do anything.
+ If you wish to select forecast times, uncheck the, Use All, box. Choose desired times by single clicking the first time; then, Shift-click the last time for a range, or Ctrl-click to add other times.

  • Select all forecast times.

II. Choose a display.

One unique feature of the IDV is the availability of different display types. Choose one by clicking the display name, then click the, Create Display, button.

  • Open the, Plan Views, folder (if needed) and Choose, Contour Plan View.
  • Click the, Create Display, button.

The display should appear soon in the, Unidata IDV, window, using a map appropriate to that data source.

Only one forecast time is displayed at a time, however you can use the loop control buttons in the upper right hand corner of the, Unidata IDV, window to control an animation.

Near the bottom of the, Unidata IDV, window is a legend bar for each display. To remove a display, if you made a mistake and want to start over, right click the middle of the legend bar, and choose, View --> Remove Display. To customize a display, right click the middle of the legend bar, and choose, Control Window.

Next, you are going to overlay a similar forecast from the Eta model.

To carefully compare forecasts, it is necesary to customize the map. The state boundaries need to be present and in an appropriate color.

  • In the, Unidata IDV, window, right click on the middle of the, Background Maps, legend bar; choose, Control Window.
  • In the, Background Maps, window: check the, Hi-Res US, box.
  • In the, Background Maps, window: click the color square at the far right of, Hi-Res US.
  • In the, Set Map Color, window: choose the, Swatches, tab; click the small color box in the second column, seventh row; click the, Ok, button.
  • In the, Background Maps, window: check the, World Coastlines, box; set the color of this map to the same color as the, Hi-Res US; click the, Ok, button.
  • In the, Background Maps, window: either close or minimize the window.

The data would be easier to visualize if the two sets of contours, one for each model, were in different colors. In addition, the 0 contour should be omitted, and the interval between contours should be larger.

To customize a set of contours

  • Open the, Control Window, for that display.
  • In the control window: click the, Set, button; enter the following values - Interval: 5; Base: 5; Minimum: 5; click the, OK, button. (Note: precipitation is in millimeters.)
  • In the control window: click the button next to, Color Table; choose: Solid and a color. Use green for the eta precipitation, and red for the GFS precipitation.
  • Click the, Close, button.
  • Repeat for the other model.

To hide one set of contours in order to better see the other set.

  • Left click near the center of that display's legend bar to toggle that display on and off.

To zoom and pan around the display: use the navigation buttons on the left edge of the, Unidata IDV, window.

A few questions to consider.

  • Do the differences between the GFS and ETA precipitation forecasts increase with time?
  • Does one model predict higher amounts of precipitation than the other, or are they about the same?
  • Does one model predict precipitation to move faster than the other, or are they about the same?

Lab F Part b

  • requires some experience with the IDV
  • compares precipitation forecasts from 2 different NWS operational numerical models
  • utilizes a bundle file and a simple formula

The IDV can take a snapshot of its current state, by creating a bundle file. This is done from the, Unidata IDV, window: choose the, File, menu; choose, Save As; choose a filename and directory; and click the, Save, button. This has many uses, one of which is to allow others to easily create the same display.

If you did not follow the instructions for the first part of the lab, you can quickly catch up by opening a bundle file.

In the, Data Selector, window click the, Add new data source, button.
In the, Data Source Selector, window choose the, URLs, tab; in the entry box beside, URL:, type or copy, http://apollo.lsc.vsc.edu/~fingerhutb/UserWshop2003/eta_gfs_pcp.xidv; click the, Add Source, button.
Shortly, the correct data will be displayed.

Another way to compare the precipitation forecasts from two models is to actually subtract one model's values from the other, and display the resulting field. This is done by first creating a formula, and then defining the variables in the formula.

  • Hide the two precipitation displays created earlier.

To create a formula to subtract two fields:

  • In the, Unidata IDV, window: choose the, Edit, menu; select, Formulas; select, Create Formula.
  • In the, Formula Editor, window, enter the following data
    • Name: difference
    • Formula: P1-P2
    • Description: difference of two fields
    • Group: model formulas
  • click the, Add Formula, button.

To choose data for a formula,

  • In the, Data Selector, window: open the, model formulas, folder; click, difference of two fields.
  • In the, Displays, pane: open the, Plan Views, folder; click, Contour Plan View; click the, Create Display, button.
  • In the, Field Selector, window: in the, P1, pane - find and choose the ETA precipitation.
  • In the, Field Selector, window: in the, P2, pane - find and choose the GFS precipitation.
  • In the, Field Selector,: click the, OK, button.
  • After the display appears, adjust the map, contour interval, and color table as needed.

The ETA model forecasts do not extend as far into the future as those from the GFS model. As a result, the last few frames of the animation will have only the GFS precipitation forecast - there is no ETA precipitaton forecast nor a difference between the two models.

A few questions to reconsider.

  • Do the differences between the GFS and ETA precipitation forecasts increase with time?
  • Does one model predict higher amounts of precipitation than the other, or are they about the same?
  • Does one model predict precipitation to move faster than the other, or are they about the same?

Lab F Part c

  • requires some experience with the IDV
  • examines stability associated with precipitation forecasts from NWS's Eta model
  • utilizes bundle files and skew-T display

Forecasters need to examine the change of temperature with height to determine the intensity of a precipitation event. Typically, vertical soundings of temperature, moisture, and horizontal wind are plotted on a special thermodynamic diagram, a skew-T.

To prepare for this part of the lab:

  • if you have not yet created any data displays, click here for instructions to do so;
  • remove all displays except the Eta precipitation contours.
    • step backwards to the last frame of the loop, 14, with only GFS forecast data;
    • hide one display at a time to determine which is the GFS display;
    • remove the GFS display

To display a forecast vertical sounding,

  • In the, Data Selector, window: select the Eta model run.
  • In the, Fields, pane: open, 3D grid; open, derived; choose, Sounding Data (with true winds).
  • In the, Displays pane: open, Soundings; choose, Grid Skew-T.
  • Click the, Create display, button.

The display should appear soon in a new window, Sounding Data. No data will be plotted in the, Sounding Data, window if the sounding location is outside the Eta domain, which is a little bigger than North America. The location of the sounding is shown by a solid (colored) rectangle in the, Unidata IDV, window. To change the location of the sounding use the left mouse button and slowly drag the colored rectangle to a new location. As you do so, the sounding in the SkewT window should change.

There will be one sounding shown, but you can loop through 10 soundings, one for each forecast time. Furthermore, you can coordinate the day/time of the sounding loop and the precipitation loop - in each display's control window, choose the, View, menu and then check the, Share, box.

The SkewT display is too complex to fully describe here. For this lab, you need to know that the thick red curve shows the temperature at different pressure levels; temperature generally increases from left to right; and pressure increases top to bottom, as it does in the atmosphere. For more information please see the user's guide.

To determine the intensity of rain events, it helps to compare the temperature of a rising surface parcel of air with the temperature of air at the same level.

  • In the, Sounding Data, window: check the box (near the bottom) to show the, Parcel Path.
A cream colored curve is added to the display that shows the temperature that a surface parcel of air would have at different pressure levels. If the parcel becomes warmer than the environmental temperature (at the same level), then tall clouds that are capable of heavy/strong showers are possible. If the parcel does not become warmer than the environmental temperature, then shallow clouds that are capable of light to moderate steady rain are possible.

  • Examine several soundings, each in the center of a different region where precipitation is forecast.

Can you find regions where the rain is likely to be steady?
Can you find regions where the rain is likely to be showers?

Lab F Part d

  • requires some experience with the IDV
  • examines the proximity of the jet stream to forecast precipitation areas using Eta data
  • utilizes bundle files, isosurface display, vertical cross section display, and interactive IDV displays embedded in an HTML page

Although a sounding can tell a forecaster how the wind changes with height directly over head, it can not tell the forecaster where the jet stream is relative to a station: e.g., directly over head, to the north, or to the south. This is important because certain types of severe weather are more likely near the jet stream.

Next, the jet stream will be displayed as an isosurface, or surface of constant wind speed. Furthermore, this display will be embedded as an interactive IDV display within an HTML document.

To open an HTML document in the IDV's browser,

  • In the, Data Selector, window click the, Add New Data Source, button.
  • In the, Data Source Selector, window choose the, URLs, tab; next to, URL:, enter, http://apollo.lsc.vsc.edu/~fingerhutb/UserWshop2003/W2003_IDV_LabFd.html; click the, Add Source, button.
  • After the document is displayed, follow the instructions shown in that window.

A few questions to consider.

  • Which states should have precipitation from shallow clouds?
  • Which states should have precipitation from tall clouds?
  • Which states should have precipitation from tall clouds and are under the jet stream?