Deep Convection Ordinary Cells Multicell storms Supercells Ordinary
- Slides: 21
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/thun der 0. html
Ordinary Cells Results 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 Cells 2) 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 Cells 3) 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 FFD RFD updraft
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?
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