Mass Flux Response Explained

[David Williams]

Can anyone give me a theoretical explanation of mass flux response? I've found equations for calculating mass flux, but I'd like to know exactly what it is. I have some basic ideas, but I'm hoping to get a vivid explanation from some of our great ST members.

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[Jeff Duda]

Can you give an example of when this term was used? I do not use the term "mass flux response". Perhaps you are thinking of something like "moisture flux convergence"?

 

[Royce Sheibal]

It's been a long time since I did any physics or fluid dynamics, but that's where I've heard that exact term used before. So obviously we all know what mass flux is - delta mass per unit area over time (IE delta flow in a pipe), but using "response" with mass flux means we're probably talking about mass flux due to a change in surface viscosity, temperature, outside factors, other weather stuff, etc. I've seen it used when talking about mass flux over cities in response to climate change, etc. I never really liked dynamics so Duda can keep it for himself.

 

[Warren Faidley]

I do know it's impossible to measure without a calibrated Flux Capacitor.

 

[David Williams]

Unfortunately, I don't remember where I read it, but I think it was in reference to large-scale processes. Instead of mass flux being used in convective, storm-scale terms, it sounded like it was in reference to large environmental or mesoscale terms.

This isn't a perfect example, but in this article it sounds like mass flux is a response to large-scale environmental shifts. So, perhaps my question is can mass flux refer to both storm-scale convective processes and large-scale mesoscale or even synoptic scale processes? Which also begs the question, is mass flux a process or a product of the storm-scale?

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[Jeff Duda]

Unfortunately, I don't remember where I read it, but I think it was in reference to large-scale processes. Instead of mass flux being used in convective, storm-scale terms, it sounded like it was in reference to large environmental or mesoscale terms.

Royce's response is pretty helpful

It's been a long time since I did any physics or fluid dynamics, but that's where I've heard that exact term used before. So obviously we all know what mass flux is - delta mass per unit area over time (IE delta flow in a pipe), ...

Mass flux is a generic term describing material movement aided by wind. In the case of the atmosphere, there is no "window" or "pipe" through which the mass is moving, so we just consider an arbitrary square or rectangle of unit area (1 m^2) and reference mass flux per unit area using that arbitrary window. The window can be located anywhere it helps you. Mass flux is formulated as the product of wind and some other material object, such as air mass (i.e., density) or water mass (e.g., mixing ratio). So dry air mass flux could be |V|*rho (if considering the full 3-dimensional flux), or if considering vertical mass flux only (as is usually done for cumulus convection and is usually referred to as "updraft mass flux") then w*rho.

Mass flux can occur on any scale of motion, including large-scale/synoptic motion and storm-scale motion. It really just comes down to what problem you are considering.

The "response" part of this is just a statement of conservation of mass. In meteorology we use the continuity equation to express conservation of air mass. There are also conservation equations for water species (mixing ratios of both vapor and condensate). The response is just how the atmosphere responds to the activity of air advecting material around. For example, in a thunderstorm updraft air is rapidly accelerating upward, thus promoting high updraft mass flux. The atmosphere responds below by replacing the lost air, i.e., convergence and mass replacement near the ground. It also responds aloft by ejecting the air that has been packed in, i.e., divergence.

 

[David Williams]

Great! Makes total sense. Thanks Jeff and Royce... and Warren for the BttF reference.

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[Paul Knightley]

Just as an aside to the fuller answers above, perhaps you read about 'mass response' in an SPC (or other) discussion, when an upper trough is approaching a region (e.g. the Plains). In this case, 'mass response' is indicating what how the low-level atmosphere is responding the the effects of the approaching upper trough, i.e. winds becoming southerly and advecting moisture northwards as a lee trough forms just east of the Rockies. If not, please ignore me!

 

[David Williams]

That makes sense Paul. Thanks for mentioning that.

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