Deep Convection

•Ordinary Cells•Multicell storms•Supercells

Ordinary Cells•Cb – ordinary cells are the most basic form of convection•Have been studied and documented since the late 1800’s:

Ordinary Cells•The first well-known experiment on thunderstorms was “The Thunderstorm Project” in the mid 40’s

–Occurred over Florida in summer–Collected surface, aircraft, sounding data

•Results of the experiment were published by Byers and Braham in 1949 in a book called “The Thunderstorm.”

•Check out: http://www.history.noaa.gov/stories_tales/thunder0.html

Ordinary CellsResults from the Thunderstorm

Project describe the evolution of an ordinary cell in three stages:

1) Cumulus Stage:

• developing Cu is dominated by updraft (< 10 m/s)

• Precip develops and is suspended by updraft

Ordinary Cells2) Mature Stage:

• Downdraft has now developed

• The downdraft is produced by precip loading and evaporative cooling

• Precip reaches the ground

• Leading edge of downdraft produces a gust front

Ordinary Cells3) Dissipating Stage:

• The cell is dominated by downdraft – is weak

• Light precip at the ground

Ordinary Cells•Life span is about 30-50 minutes•Form in weakly sheared, convectively unstable environments•Move at speed of mean environmental flow from 0-5,7 km•Can produce rain, hail, high winds, rarely tornadoes

Multi Cell Storms

•Can be thought of as a collection of ordinary cells in various phases of their life cycle:

From Houze (93)

Multi Cell Storms

•Note the air flow patterns, gust front, precip locations

•New storm development occurs on flank of gust front where convergence is maximized with low level storm relative ambient flow

Multi Cell Storms•New storm development occurs on flank of gust front where convergence is maximized with low level storm relative ambient flow

•Hence, cell motion (Vc) may be different than the system motion (Vs)

•Therefore, multicell storms may not propagate in the direction of the mean 0-5,7 km ambient flow

Multi Cell Storms

•Cell motion versus system motion:

Multi Cell Storms

•Form in larger sheared environments than ordinary cells

•The shear allows the updraft and downdraft to be separated

•Therefore, they can last for hours at a time

•Can produce copious rain, hail, high winds, some tornadoes on the gust front

From Atkins et al. (04, mwr)

Supercells

•Can be long-lived – 12 hrs at a time

•Have a single, quasi- steady rotating updraft

•Exhibit deviant motion from the mean flow, either to the left or right, mostly to the right

•Can produce, hail, high winds, significant tornadoes

•Can be very dangerous

Supercells•Early radar observations from Lemon and Doswell (79, mwr)•Note:

–Forward flank downdraft and gust front–Rear flank downdraft and gust front–Up draft location–Tornado location–Hook

Tornado location:

Supercells•Early radar observations from Lemon and Doswell (79, mwr)

•Note: FFD, updraft, and RFD

RFD

updraft

FFD

Supercells•Early modeling results from Klemp and Rotunno (83, JAS)

•Note the similarities with the Lemon and Doswell observations

Supercells

•Most always contain a bounded weak echo region (BWER)

Supercells

•Most always contain a bounded weak echo region (BWER)

Supercells•Show deviant motion, both to the left and right of the mean flow

•They have also been observed to split:

Supercells

•Modeled storms have also been observed to split:

So, the following questions naturally arise:

–Given observations of the environment, which convective storm mode, or structure, should you anticipate?

•Ordinary cells•Multi cells•Supercells

–What environmental parameters should you look at?•Instability•Vertical wind shear

–What physical processes are responsible for the generation and evolution of the three aforementioned storm types?