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Metsfan

Difference between MLCAPE and MUCAPE

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Hey all, 

 

I need help with the difference with these parameters and there difference in significance. The SPC description is not helping. 

 

MUCAPE the way I understand it is the instability of a parcel needed to get to it's Level of Free Convection

 

MLCAPE  the way I understand it is the instability a parcel has when it has reached it's Level of Free Convection. 

 

Thanks guys. 

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MUCAPE is Most Unstable CAPE. It is most commonly used to assess elevated instability and is useful in forecasting nocturnal thunderstorms when a low level inversion may be present. It can also be useful in forecasting thunderstorms that may occur out ahead of a warm front where low level inversions also tend to be present. Although there are different algorithms to calculate it, MUCAPE typically is found by lifting every layer in the lowest 300 mb of the troposphere and finding the CAPE for each. The layer that produces the highest CAPE value is the MUCAPE.

 

Here's a good tutorial on MUCAPE: http://theweatherprediction.com/habyhints2/634/

 

MLCAPE is Mixed Layer CAPE. It is most commonly used to assess instability during the part of the day when the atmosphere is well mixed (i.e. the afternoon) and may provide a more realistic picture of daytime instability than surface based CAPE as it will typically look at the temperature and moisture conditions in the lowest 100 mb of the troposphere. MLCAPE is calculated by averaging temperature and moisture variables in the lowest 100 mb and lifting that layer.

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MUCAPE is Most Unstable CAPE. It is most commonly used to assess elevated instability and is useful in forecasting nocturnal thunderstorms when a low level inversion may be present. It can also be useful in forecasting thunderstorms that may occur out ahead of a warm front where low level inversions also tend to be present. Although there are different algorithms to calculate it, MUCAPE typically is found by lifting every layer in the lowest 300 mb of the troposphere and finding the CAPE for each. The layer that produces the highest CAPE value is the MUCAPE.

 

Here's a good tutorial on MUCAPE: http://theweatherprediction.com/habyhints2/634/

 

MLCAPE is Mixed Layer CAPE. It is most commonly used to assess instability during the part of the day when the atmosphere is well mixed (i.e. the afternoon) and may provide a more realistic picture of daytime instability than surface based CAPE as it will typically look at the temperature and moisture conditions in the lowest 100 mb of the troposphere. MLCAPE is calculated by averaging temperature and moisture variables in the lowest 100 mb and lifting that layer.

Nice explanation.

 

I have a few more questions

 

1) Can we use MUCAPE during the afternoon with no inversion?

 

For this example this meteorologist mentioned MUCAPE with the morning sounding 

 

http://ryanhanrahan....xed-layer-cape/

 

but then went on to MLCAPE. 

 

 

 

2) Does the remnant MUCAPE(elevated instability) become MLCAPE when the cap breaks? 

 

3) Shouldn't MLCAPE be the most unstable because of the accessibility for a parcel to reach the available energy, because of the steep low and mid level lapse rates?

 

4) What makes MUCAPE more unstable then MLCAPE?

 

Thanks for answering these additional questions. 

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Nice explanation.

 

I have a few more questions

 

1) Can we use MUCAPE during the afternoon with no inversion?

 

For this example this meteorologist mentioned MUCAPE with the morning sounding 

 

http://ryanhanrahan....xed-layer-cape/

 

but then went on to MLCAPE. 

 

 

 

2) Does the remnant MUCAPE(elevated instability) become MLCAPE when the cap breaks? 

 

3) Shouldn't MLCAPE be the most unstable because of the accessibility for a parcel to reach the available energy, because of the steep low and mid level lapse rates?

 

4) What makes MUCAPE more unstable then MLCAPE?

 

Thanks for answering these additional questions. 

 

1: You can always use MUCAPE. Typically, without an inversion, the most-unstable CAPE will be equal to the surface-based CAPE, since surface parcels will have the highest instability. MUCAPE is typically used for overnight/elevated convection just because it is in those instances where the SBCAPE can be most misleading, so to speak.

 

2: I'm not really sure how to answer this question, but I think the answer is no. Usually when referencing the cap "breaking", people are referring to surface parcels breaching the CIN and ascending past through LFC. Elevated instability, seen via the MUCAPE, can exist above the cap and generate convection regardless of any cap.

 

3 & 4: MUCAPE and MLCAPE are different ideas addressing different issues.

 

MUCAPE is used when there may be inversions limiting the extent of surface-based instability, but where instability may exist above this inversion. It only seeks to locate the maximum CAPE possible lifting any parcel.

 

MLCAPE, on the other hand, is the CAPE found when the lifted parcel has the conditions of the average lowest 100 mb of the atmosphere. This is done to obtain more realistic CAPE values, as due to low-level mixing a parcel will typical not have the conditions of the lowest (ground) level but will be representative of the lowest ~100mb or so of the atmosphere. Often, temperatures will be superadiabatic right at ground level (and therefore deceptively high compared to the T a parcel would have), and moisture pooling will often exist in the lowest data point, whereas a parcel would be much more thoroughly mixed. 

 

Therefore, MUCAPE isn't inherently more unstable than MLCAPE.... it depends on the atmospheric situation at play. As per #3, MLCAPE can be the most unstable if the atmospheric conditions allow for it, but it isn't always the case. Check out this sounding showing elevated instability:

 

1_2.png

 

As you can see, the parcel ascending from 940mb has quite a bit of positive buoyancy and CAPE (and would be considered MUCAPE, as it is not surface-based and is the parcel with the most CAPE out of any in the sounding). In contrast, the MLCAPE, taken by using the average parcel conditions in the lowest ~100mb, would be T=~15C and Td=~4C, and would not result in very much MLCAPE, and certainly less so than the MUCAPE. 

 

Keep in mind that MUCAPE will lift parcels from anywhere in the sounding to find the one that is the most unstable, while MLCAPE is essentially surface-based CAPE, except that the "surface-based" parcel has the average conditions in the lowest ~100mb as opposed to the true "surface" values. 

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In the context of this thread it may be good to note that there are apparently two different ways to calculate MUCAPE.

 

1. Lift each parcel level in the search layer (usually lowest 300mb) and perform the standard CAPE calculation. The level yielding the highest CAPE value will be the LPL and it's value is the MUCAPE.

 

2. Determine the wetbulb potential temperature at each level in the search layer. The level yielding the highest theta-w value is the LPL. Then perform a standard CAPE calculation. This value is the MUCAPE. The visual method is to plot the wetbulb temperature profile and then eyeball which level yields a Tw value that is furthest to the right in relation to the moist adiabats.

 

The RAOB Program uses method #2 and I believe (someone correct me if I'm wrong) the SPC mesoanalysis also uses this method. In most cases method #1 and #2 will yield the same result. However, if there is an elevated inversion with a high relative humidity and that inversion is very deep then method #2 may yield a value that could much be lower than that from method #1.

 

So why does method #2 even exist? I don't know really. Having written a program that generates soundings myself I can tell you that method #1 is computationally more expensive because the CAPE calculation must be performed at every level whereas with method #2 it is only performed at one level. The CAPE calculation itself is computationally expensive whereas theta-w may not be if the moist adiabat reference points are already present. So it's possible method #2 exists to speed up the generation of gridded products. I could be completely wrong though...just a guess on my part.

 

Can any of you experts shed some light on this?

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Correct.  My RAOB Program does use Method #2 for MUCAPE calculation ( highest theta-w ).  I just added Option #1 in the new RAOB 6.9 program.  This option calculates CAPE at 1 mb intervals for the entire lower 300 mb layer (or whatever layer you identify) and picks the level with the highest CAPE value.  However, it is very time consuming -- taking 2 to 3 extra seconds for an average PC. 

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