Nocturnal Thunderstorms Lately

[Mike Johnston]

Late last night, as I was closing up shop on my laptop I was somewhat surprised to see the radar lit up like a Christmas tree at midnight over S Kansas and NE OK. It peaked my interest enough to read some papers and articles on nocturnal thunderstorms. At least in the plains, great emphasis on initiation at night seems to be placed on the strengthening of the low level jet. Evidently, at night, sometimes cooling occurs more quickly on the western high plains than the eastern plains, causing some changes in pressure gradients at 850mb, causing an E to W flow - which becomes a southerly flow because of the earth's curvature.

Now, this is where my understanding ends. OK, there is sometimes stronger southerly flow at 850mb at night - so what's the connection to convective initiation? Is it more the direction of the flow at work, advecting moisture from more east/southerly locations, or, is it more the greater speed of the flow enhancing crossover w/ height, shear etc?

Believe me, it isn't a question to suggest thoughts of chasing at night. At least for me, I don't like what I can't see. However, understanding what's at work here may help me understand what has been missing during daylight hours lately.

 

[Kiel Ortega]

The low level jet is usually responsible for isentropic lift. Isentropes are lines of equal potential temperature. Isentropic maps (I usually use the one's on DuPage's weather page, though I have my own that create dynamically) show height or pressure fields contoured on isentropic surfaces. This is an easy way to see whether you have rising or sinking motion on the synotpic scale. Where you have lift, you can get clouds and precip (like wide spread stratiform precip...think of a good warm front). For convective cases, you can assume, you just lift a parcel a little more which helps any capping issue and then you get a storm. That's why LLJ storms are elevated because the boundary layer is beginning to stabilize and parcels originate downstream.

Hopefully that helps and didn't screw up the explanation somewhere.

 

[Aaron Kennedy]

A brief run through...

http://twister.caps.ou.edu/MM2005/Chapter2.3.pdf

Aaron