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Ellrod Index

Posted: Wed Feb 06, 2019 10:30 am
by Cloudy
In the Ellrod Index formula I understand that the u, v fields are wind components on isobaric surfaces, the z field is the thickness between levels, top and bottom are the isobaric surfaces, is this correct?
What does the "unit" field mean and how should I load it? Many thanks

Re: Ellrod Index

Posted: Wed Feb 06, 2019 8:21 pm
by bobc

Just for full context, the Ellrod Index function is defined in the Grid Diagnostics branch of the Jython Library:

Code: Select all

def EllrodIndex(u, v, z, top, bottom, unit):
  """ calculate the wind shear between discrete layers
  <div class=jython>
   EI = VWS X ( DEF + DIV)
  VWS = windShear(u, v, z, top, bottom, unit)*100.0
  uwind = getSliceAtLevel(u, top)
  vwind = getSliceAtLevel(v, top)
  DIV = (ddx(uwind) + ddy(vwind))* (-1.0)
  DSH = ddx(vwind) + ddy(uwind)
  DST = ddx(uwind) - ddy(vwind)
  DEF = sqrt(DSH * DSH + DST * DST)
  EI = mul(noUnit(VWS), add(noUnit(DEF), noUnit(DIV)))
  return setLevel(EI, top, unit)

To make use of this function, you'll have to create a formula. From the Main Display window, select Tools>Formulas>Create Formula to open the Formula Editor window. The Description field will be how the formula will list in the Field Selector. You can set this to be whatever you want. The Name is how the formula can be referred to with a Parameter Default, this can also be whatever you want. The Formula is where we call the function: EllrodIndex(u, v, z, top[isuser=true,default=250], bottom[isuser=true,default=300], unit[isuser=true,default=hPa])

The keys are specifying the "isuser=true" for top, bottom, and unit. "top" and "bottom" are the top and bottom vertical pressure levels you want to use. "unit" is the pressure unit you are using for the top/bottom vertical values. It's important to specify because this is what allows McIDAS-V to know if you are working with hPa or Pa units. For more information on the Formula Editor, see the Formulas page of the User's Guide.

Once I created this formula, I added a NAM 80km CONUS data source to the Field Selector, selected the formula, and clicked Create Display. In the Select Input window, I left my default values there from when I defined the formula earlier and clicked OK. In the Field Selector window, I selected the u, v, and geopotential height fields in each column. In each column, I went to the Levels tab and selected pressure values 250 through 300 (the top/bottom values I specified in the formula) and clicked OK. The values I'm seeing in my display (-1.7 to 6.6) seem to match up with what I'm seeing as realistic values from Wikipedia:

Please let me know if you have any questions about this.

Bob Carp

Re: Ellrod Index

Posted: Thu Feb 07, 2019 4:05 pm
by Cloudy
Thanks Bob, very clear explanation. I was able to follow the instructions and obtain the Ellrod Index map. Attached example of Errold Index applied to a case study of a severe turbulence event, occurred at flight from Avianca (AV965 Lima-Buenos Aires) in June 2016 with 23 passengers injured on board.
Many thanks for all