Instructions for using the HTML sounding display with interactive parcel trajectories

The sounding consists of a Skew-T plot, a hodograph, and a wind plot. Wind directions are always reported with respect to true north. The following thermodynamic indices are also shown, if available:

• These values are taken directly from model output, if available:
  • CAPE (Convective Available Potential Energy) in J/Kg. For the RAP, we use the most unstable CAPE (MUCAPE) using the parcel with highest theta-e in lowest 300 mb.
  • CIn (Convective inhibition) in J/Kg. For the RAP, we use the most unstable CIN (MUCIN) using the parcel with highest theta-e in lowest 300 mb
  • PW (Precipitable water) in Kg/m^2
  • Helic (storm-relative helicity) in m^2/s^2 (not currently available)
• These values are calculated directly from the sounding data by the display software
  • TT (Total Totals index)
  • KI (K index)
  • SI (Showalter index)
  • LI (Lifted index) of the mixed layer (assumed to be the lowest 100 mb of the sounding)
  • LCL (Lifted Condensation Level) of the mixed layer (assumed to be the lowest 100 mb of the sounding)
• These values are calculated when the user specifies a parcel (see paragraph below)
  • iCAPE (interactive CAPE) - approximate CAPE of the parcel
  • iCIn (interactive CIn) - approximate CIn of the parcel
  • iLCL (interactive Lifted Condensation Level) of the parcel

A parcel may be specified by clicking with the mouse at any point, or by releasing your finger on a touch screen. The parcel will ascend from that point (using the environmental dewpoint at that pressure), and regions of CAPE and CIn will be indicated. The Lifted Condensation Level (cloud base) for the parcel is indicated by a short horizontal black line on the parcel trajectory. Holding down the shift key while clicking with the mouse will bring up a selection window that will allow you to select the initial parcel dewpoint, temperature, and pressure. Click or press on a sounding's button to remove the parcel trajectory. Thanks to Tom Whittaker of the University of Wisconsin for providing the core of the code that performs this task.

(Against Tom's advice, and to facilitate stability calculations using AMDAR soundings that often lack humidity data, iCAPE and iCIn are calculated using the temperature difference between the parcel and the sounding, rather than the virtual temperture difference. According to Doswell and Rasumssen (1994, Weather and Forecasting, 9, pp 625-629), this may cause errors > 30% for values of CAPE below 500 J/Kg and similar errors in CIn, but smaller error for larger values.)

The hodograph shows wind direction (with respect to true north), speed in kts, and height in km.

The sounding has two default altitude ranges, one extending up to 150 mb and the other to 0.5 mb. Each of these ranges can be zoomed by dragging the mouse diagonally across the portion of the Skew-T plot desired. Unzoom by clicking a 'scale' button. (To drag, hold the mouse button down while moving the mouse. When you release the mouse button, the zoomed region will fill the plot.) (The zoomed region will have the same aspect ratio as the main plot, so the red rectangle that appears may not follow the mouse quite the way you think it should.)

On a touch screen, you may zoom the plot using two fingers in the normal way. However, you may need to wait about a second after using any single-finger touch for the multi-touch zooming (and roaming) functionality to work. See touch help for more details.

Other functions are as follows:

Move the mouse or your finger along the sounding to see the following printed at the level of the mouse point (from left to right):

• The pressure in mb, and the pressure altitude (the altitude in a standard atmospherethat corresponds to this pressure, in km and feet. (For AMDAR data, the time, bearing and range from the airport are also shown.)
• The dewpoint in Celsius (and also Farenheit below 750 mb).
• The temperature in Celsius (and also Farenheit below 750 mb).
• The wind direction (with respect to true north) and speed in knots.
• A highlighted wind barb.
• A highlighted portion of the wind speed curve.
• A highlighted portion of the hodograph

In addition, the following values are indicated below the cursor (reading counter-clockwise from the cursor).

• T: the temperature in Celsius (and Farenheit).
• Theta: the potential temperature.
• The pressure altitude (the altitude in a standard atmospherethat corresponds to this pressure, in ft.)
• The pressure in millibars.

Click on the Load Sounding(s) button to pop up the choose sounding window. In this window you can

• Enter a 3- or 4-letter METAR, RAOB, or AMDAR-airports site name.
• OR, enter a latitude and longitude (in decimal, with west and south being negative), separated by a comma.
• OR, enter a WMO ID for a metar site or RAOB.
• OR, enter a series of WMO IDs for metar sites or RAOBs, separated by commas.
• The program searches for the location of the desired site as follows:
  • If a 'lat,lon' is entered that is used
  • If an name or WMO ID is entered, the program first searches a list of world-wide METAR sites. If no match is found, a list of world-wide RAOB sites is searched, first for the site name or WMO ID, and then for the name with a 'C' prepended (to catch Canadian RAOB sites by name).
  • A METAR and RAOB lists are maintained locally, and are generally not up-to-date with the latest location changes.
• Enter a starting valid time by choosing a year, month, day, and hour (UTC).
• Enter the number of hours of soundings you would like to display. Model soundings are generally available for the past 16 hours, and for intermittent hours up to 36 hours into the future. In addition, for Bak20, dev, and dev2 models, analyses, and 1, 3, 6, 9h forecasts at 0 and 12 UTC, are available RAOB sites since 23 Feb 2006. Also, for dev and dev2 models, analysis soundings for the last 60 days are available at selected airports. If you enter two numbers separated by a comma, the second number will be taken as the hour increment. For instance, "10,2" will give you soundings for every other hour for the next 10 hours.
  
  Remember that RAOBs are only available at 0 UTC and 12 UTC.
  
  
• Click on an initial data source. You can load different kinds of soundings from the java display as well. (AMDAR data are restricted to certain users, per GSD's agreement with the airlines that provide the data.) Soundings will appear one after another on the plot as soon as they're ready, and a button with each sounding's information will appear below the plot.

Once a sounding is loaded, you can use its button to (re)display it.

• Click on or touch a button to clear the display, and display one sounding. It generally takes less than a second for an already-loaded sounding to reappear.
• Shift-click one or more buttons to add additional soundings to the plot. (Shift-click means hold down the shift key while clicking the mouse button.)
• Control-click a sounding's button to remove the sounding and its button.

The sounding information shown on each button consists of the site name, and a code:

• code R indicates a RAOB sounding
• code P indicates a Profiler sounding
• code A indicates a sounding from a model analysis
• code Fn indicates a sounding from a model n-hour forecast
• For instance "DEN(F9)" indicates that the sounding data are from a model forecast based on an analysis from 9 hours before the indicated valid time.
• codes Up and Dn indicate AMDAR ascent or descent soundings, respectively.

These instructions are copied verbatim from NOAA's GSD Sounding web site.