Mlcape, mucape, sbcape...

Discussion in 'Advanced weather & chasing' started by Andrea Griffa, Mar 9, 2005.

  1. Andrea Griffa

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    I know that every kind of cape is calculated taking a different type of parcel: so we may obtain Mixed layer cape, most unstable cape and surfaced based cape. I know as well that Mlcape is the most important kind of cape in thunderstorm forecast, because it better represents the real situation of low troposphere.
    But how I can calculate these features on sounding?
     
  2. Jeff Snyder

    Jeff Snyder Member

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    Not necessarily... Typically, MLCAPE likely is a better estimate of the CAPE available since most parcels aren't indefinately thin (e.g. using the T/td at a level would mean that the entire updraft is characterized, initially, by that T/td, which isn't true in most cases). Air in an updraft will originate from a "layer"... By convention, it seems that MLCAPE tends to use the lower 100mb layer NOw, what happens in situations where the parcel layer is less than 100mb? Take the possibility that a storm only draws the lowest 25mb layer into it's updraft... In this case MLCAPE (as typically defined using lowest 100mb layer) can significantly underestimate or completely mask available potential instability. A good example of this is the 4-20-04 IL/IN tornado event...

    I'm not sure that we really know what determines the "thickness" of the parcel layer. I can assume it is a function of low-level lapse rates and forcing depth, but that would be an assumption.

    As for calculating CAPE... unless you do it computationally, it's not a practical calculation. CAPE is a measure of the buoyancy integrated over the depth/height between the LFC and the EL. As such, there really isn't a feasible/practical way to measure the CAPE without the aid of a computer. That said, if you look at enough soundings, you may be able to eye-ball CAPE to a decent degree... Even so, the various distributations of CAPE on various soundings (e.g. tall thin CAPE profile vs. short fat profile) complicates this further...
     
  3. Andrea Griffa

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    Thanks, Jeff for considerations, but do you want to say 250mb or 25mb?

    Besides, as to your words there are no ways to make difference from mlcape and sbcape seeing a sounding...When you see a sounding, you see the cape area between LFC and EL: this is sbcape if I'm right.
    I wanna say: How I can differentiate a mixed layer parcel from a surface based cape from sounding?
     
  4. Glen Romine

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    Probably meant 25 mb - as in a fairly thin layer (~300 m [900 ft] around sea level pressure).
    Soundings with the idealized parcel lines drawn over them can be made however the programmer chooses - could be MLCAPE or SBCAPE. That said, there are a great number of ways to calculate the CAPE - and most sites fail to mention how they get the number. The SPC mesoanalyses use the virtual temperature for calculating CAPE (both types) - and this is generally considered more accurate. The appropriate depth of mixing (meaning for the mixed layer depth, a parcel's properties is the average potential temperature and mixing ratio for the lowest xx mb and is then lifted) is best related to the depth of the boundary layer for surface-based convection - but for elevated convection the depth of the real inflow can be tricky - so MUCAPE (most unstable CAPE, found by treating each height as if it were the surface and lifting it to determine the CAPE) is often used - but a better alternative called effective CAPE is coming into use. See this other thread on effective xxxx products:

    http://www.stormtrack.org/forum/viewtopic.php?t=5329

    After all this blabbering about CAPE - it is probably only most applicable to a supercell (storm with a strong mesocyclone) for the parcel theory used in calculating the CAPE - as soundings released into updrafts have shown. Ordinary convection has too much mixing with environment air as the parcels rise - yeilding subtantially less buoyancy. Also, most released soundings should not be literally interpretted as representative of a larger scale environment - there is a great deal of mesoscale to microscale variability of typically unknown magnitude in the real atmosphere. You'll often find vigorous convection and the nearest sounding show little or no buoyancy. Convection also modifies the local environment - soundings near convection even 15 minutes apart can show substantial differences.

    Glen
     
  5. Andrew Revering

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    I want to just step back and try to simplify things here in an effort to answer the question real basically.... more operationally speaking.

    I think the question is... how do you "see" the difference between SBCAPE and MLCAPE on a sounding.

    The difference is this:

    SBCAPE:

    Raise a parcel using the surface temperature and surface dew point. Do not mix.


    MLCAPE:

    'average out' the temperature and dew point in the lowest 100mb of the sounding, then raise the parcel.

    When you draw the two parcel traces you typically will find that the surface based parcel trace will be farther to the right of the mixed layer parcel trace. This extra space between the parcel trace and the environmental temperature is the extra CAPE that you're adding to the SBCAPE value that you don't have with the MLCAPE value.

    How do you raise a parcel? You didnt' ask that I don't think, but I'll throw it in here for good measure.

    Follow a mixing ratio line up from the dew point. Either the surface dew point for the surface based calculation, or raised from the mixed or averaged dew point from the ML calculation. Follow the dry adiabat line up from the temperature, again either the surface temp or the mixed temp depending on what you're doing.

    Where these two lines intersect (mixing ratio and dry adibat) is the LCL or cloud base. Your parcel trace follows up the dry adiabat from the surface or mixed temperature to the LCL height. Then it follows up the moist adiabat to the top of the sounding.
     

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