Rear inflow jets/wake lows

[Joel Wright]

On mature and decaying MCC's, rear inflow jets are often brought down to the surface in association with wake low pressure systems. Do these jets always form in association with wake low pressures, or are they a totally seperate entity? Or is it the inflow jet that induces the lower pressure in the wake of the MCC? I would assume the wake low is what is responsible for the strong inflow in the rear.

Quite often in strong derechoes there are very strong rear inflow jets (sometimes easily seen on radar), and there seems to be no apparent wake low present.

I was just wondering if and how these two may be related.

 

[Chris Nuttall]

Here's my understanding. In a squall line, a pool of warm air forms aloft over a cold pool at the surface producing lower pressures at mid levels and higher pressure at the surface (under the line...not behind it, that's the wake low). The flow diverges at the surface and converges in the mid-levels. This converging flow ishthe rear-inflow jet and is brought down to the surface the rear-to-front descending flow and pushes the precip and cold pool ahead of the line...hence the bow echo.

In terms of horizontal vortcity, there are horizontal buoyancy gradients generated by the back edge of warm and cold pools, which generate a vertically stacked vorticity couplet which in turn generates the rear-inflow jet.

There are several factors contributing to RIJ strength including the stength of the buyoancy gradients, which is related to the relative warm of the front-to-rear ascending flow and the coolness of the surface cold pool. Strength of the wind shear in the low-levels is also a factor determining RIJ strength.

From expereience, it's also quite common to see the RIJ and bow echo develop along with a comma-head or "bookend vortex" type of feature just north of the bow.

And yes, OU people, I'm reading this from my Mesoscale notes.

 

[Glen Romine]

Well, I wouldn't question that you were getting that from your note Chris - I had a hard time following your description, and suspect others may have as well. Perhaps it would be helpful to have a second description - arguably more or less vague. A disclaimer is that this is not my area of expertise - mileage may vary.

Start with a squall line, present in an atmosphere with vertical wind shear that is oriented 90 degrees to the squall line. For example, a north-south oriented line of storms, and winds at the surface being easterly to westerly in the middle atmosphere. This arrangement promotes the vertical wind shear to be tilted into the vertical by the gust front on the leading edge of the squall line - and this then promotes the development of a circulation pair - the nothern rotating counter-clockwise, southern clockwise, such that the area between the two circulations has an enhanced westerly flow. This enhanced westerly flow results in more midlevel dry air penetrating into the back side of the squall line - and that leads to more evaporational cooling - leading to locally colder air - which then sinks toward the surface. In time, this leads to a large cold pool at the surface - deepest where the most agressive cooling is taking place - and where cold air is deeper, the surface pressure is higher. This local high pressure then acts to try and spread the cold air out - most agressively from the center of the cold pool - and this surges the cold pool out near the center of the squall line fastest. Also, the enhanced westerly flow from the circulations results in faster storm motions in the center of the line than on the ends - and this aids in the line bowing out in the center, and the development of a growing area of general rain behind the leading line of storms. It is then that more focused rear inflow jets can start to develop - often in focused channels (which can appear as weaker reflectivity regions penetrating into the back of the stratiform rain region), where dry air is able to make more agressive penetrations into the stratiform rain. The lower echos associated with these comes from the local evaporation of the rain - which leds to cooling, and sinking which leads to subsidence warming and more evaporation potential, but more applicable here is this mid-level warming results in lower surface pressure beneath it - a wake low. As this sinking air builds up speed - and gets closer to the ground, it splats into the top of the surface cold pool - and locally thins it out, and since it has warmed from sinking, and warm air is lighter than cold air, leads to the development of a low pressure atop the cold pool - and that aids in acceleration of even more dry air into the back of the system - enhancing the process. Sometimes the sinking air from the rear inflow jet is able to reach down to the surface - and can result in substantial wind damage - but more often it fails to penetrate through the cold pool.

Glen

 

[Chris Nuttall]

LOL.

That's okay, Glen. I figured that would happen. I still haven't quite mastered the art of explaining things in "not-quite-so" technical terms. I just happened to be looking up something in my notes at that time, so it was easy to find and post.

When looking at radar, there is a signature called MARC, or Mid-Altitude Radial Convergence. It's often a pretty good indication of the the development/presence of a strong rear-inflow jet. Thus, it will also be important to then watch for the evolution of some kind of damaging wind event (i.e., bow echo, derecho formation, etc.).

 

[Joel Wright]

Thanks for the replies.

That all sounds very logical. It's fascinating that so many variables contribute to the different features. And it all usually begins with just an ordinary thunderstorm or two.