Mixing out of a layer

[Andrew Clope]

I think I'm starting to understand this and why it matters, but like everything meteorology, it seems that just when I think I'm understanding something, I find out that I have no clue and I'm just scratching the surface.

But can someone run through the mixing out of a layer and why it's good or bad? Maybe that's way too broad of a question to ask. If so I apologize.

I would think that you'd rather the layer be well mixed as it may be more consistent that way and easier to predict for? If not mixed, couldn't the moisture variable change drastically over a given period of time? Also, what determines how deeply a moisture layer will mix upwards? Is there a parameter that ultimately limits this? Or is it just the overall amount and quantity of moisture available which dictates how deeply it will mix and how deep the moisture will be?

Thanks, and sorry for the possibly very broad question!

 

[Jeff Duda]

Vertical mixing occurs because of friction and generally manifests itself via mechanical shear (vertical wind shear) and buoyant plumes in the PBL. Mixing generally only occurs in the PBL, and the depth to which the PBL mixes determines the PBL depth. The two main parameters that mix within the PBL are potential temperature and mixing ratio. You can identify particularly well mixed PBLs by their dry-adiabatic lapse rates and constant mixing ratios. Winds try to mix vertically, but because other forcings like the pressure gradient force are constantly acting, and typically in different directions with height in the PBL, you don't typically see wind speed or direction constant with height in the PBL.

I'm assuming that, by "good or bad" you're asking if mixing would promote or prohibit thunderstorm development. The true answer is: yes, no, and it doesn't matter. Typically the PBL mixes during the hottest part of the day. So a lot of times surface moisture (dewpoint) will decrease as the PBL mixes. However, the actual amount of moisture in the PBL doesn't change when it mixes, just the amount at the surface. Research has shown that mixed layer parcels are the best representation of convective processes that occur in the afternoon. So if you see a morning sounding with a ton of CAPE because the surface parcel is very moist and nearly saturated, make sure to check the MLCAPE value as well. If the moisture is skin deep, it will mix out and CAPE will be lower in the afternoon once the PBL mixes. Typically CIN will also be higher in such a case. If the PBL is well mixed, then surface-based and mixed-layer parcels will essentially have the same stability parameters (CAPE/CIN/LI etc. will be the same between them). Even if the PBL does not mix well, when a surface parcel is lifted, the parcel will entrain (i.e., collect and absorb) air with less moisture in it as it ascends through the PBL, which will keep it from converting some of the SBCAPE it has into kinetic energy.

For chasing purposes, you want to see MLCAPE as high as possible and MLCIN as low as possible to have the best chance of seeing afternoon storms. This happens when the PBL fully mixes out and there is sufficient moisture in the PBL to keep capping from being an issue.

As far as what impacts the depth of the PBL...many factors. Sensible heat flux is a big factor, and it depends on many factors, including: time of year, time of day, soil type, soil temperature, soil moisture, vegetation coverage and type. Generally the more buoyant you can get parcels coming out of the surface layer (a sub-layer of the PBL about 10-100 m deep), the deeper the PBL will mix. You need strong heating and strong sensible heat flux to get that. Latent heat flux helps for making parcels unstable, but due to the higher heat capacity of water-laden air, latent heat flux not only reduces sensible heat flux but also reduces heating in general.

 

[Andrew Clope]

Jeff,

Awesome. Thanks so much for taking the time to do that. Answered my question perfectly.

 

[David Williams]

Wow Jeff, great answer. Are you a professor? If not, you really should be. I've noticed that you and Skip always give the most thorough answers. It's good to have you on ST. Also, speaking of questions, I had a couple about your model data website. Should I pm you about those or ask in a new thread?