SPC Soundings

[L.B. LaForce]

I have yet to have a thermodynamics class, but still know a lot about the subject. One thing that I have noticed for a while, but I guess really never made me question is the different ways the SPC calculates CAPE.

Take for instance this sounding from FWD:

I know all the parts to a skew-t, etc, and know that red=temp, green=dewpoint, and the light blue=wet bulb temp

I'm assuming the dashed dark red/brownish color line is the parcel path via the virtual temperature.

1. I looked up the formulas to calculate this, and its something along the lines of Tv=T(1=.61*w) where T is in Kelvins, and w, the mixing ratio is in kilograms per kilogram. So are they just taking the virtual temperature, and raising the parcel the standard way? Also, w is the mixing ratio, and not the saturation mixing ratio, correct?

2. What is the dashed red line? Virtual temperature all the way up the profile?

3. There seems to be some advantages of using MU LFC and ML LCL vs others - any reason as to why?

This somewhat came into question as I was playing with some RUC soundings. I was trying to figure out how they calculated the CAPE, as when I tried to calculate it a few ways (SB, ML - approx 100mb, or Tv) it did not quite match up to what it had. Obviously there is no way of knowing which is the correct way, and none of them are exact, but was just curious. The skew-t below I calculated using virtual temperature:

Temp at sfc: 19.8C = 293.5K

Dewpoint at sfc: 17.6C

Pressure at sfc: 969mb

w= 13.17 g/kg

293.5(1+(.61*.01317))=295.656K = Tv and using that, it gave me approx 560 CAPE

[isohume]

The virtual temperature will still cool at the MALR after it becomes saturated just like T would. In most cases, interchanging T for Tv is a decent enough approximation as SBCAPE magnitude will be large enough to account for the low-end scale comparison. However for initiating tstms, Tv will have a better handle on the potential early CAPE since it begins warmer than T...and therefore Tv should be able to identify areas more probable for tstm initiation than T alone. Tv induced CAPE is also at times a better metric for low-top convection, barotropic, and cool season elevated instability.