Chapter 8: Organization of isolated deep convection a brief review the distinction between the 3 storm types is largely controlled by wind shear
8.1 The role of wind shear bulk Richardson number: weak shear strong shear Fig. 8.1 Fig. 8.2
8.1 The role of wind shear no shear strong shear quicktime movies:
8.1 The role of wind shear Weisman: convective storm matrix: buoyancy-shear dependencies. COMET-MetEd module blue contour: v ’=-0.2K near surface red contour: w (10 m s -1 ) at 4 km green: q r +q s +q g > 1 g kg -1 at 1 km arrows: storm-relative flow weak shear strong shear Wilhelmson-Klemp (1982) sounding (CAPE=2200 J kg -1 ) Fig. 8.3 no shear strong shear
Brief history of thunderstorm field research ’48-’49: Thunderstorm Project (Byers & Braham) ’55: creation of the NSSL to develop weather radars and other instruments to better observe thunderstorms (Kessler) ’72-’76: NHRE (hail, hail suppression) ’78: NIMROD (microbursts) (Fujita) ’79: SESAME ’82: CCOPE ’84: JAWS ’87: PRESTORM (squall lines, MCSs) ’90: COHMEX ’95,’97: VORTEX (tornadoes) ’02: IHOP (convective initiation, low-level jet) ’04: BAMEX 07: COPS ’09-’10: VORTEX-II
The Thunderstorm Project Early field project: summer 1946 in Florida, July 1947 in Ohio Justified in part by need for wx information for the expanding aviation industry Ten military aircraft, P61C (“Black Widow”), five each mission, spaced at 5000’ intervals Used new radar developments from WW-II (first use of 5 cm C- band radars) First meso-net (people recording wx at 5 min intervals during IOPs) In-flight data obtained from photographs of instrument panels focused on determining kinematic and thermal structure and evolution of thunderstorms
The Thunderstorm Project : thunderstorm stages References: –the project report: “The Thunderstorm” –Byers and Braham, 1948: Thunderstorm structure and circulation. J. Meteorol., 5, 71-86 Thunderstorm described as composed of a number of relatively independent cells Each cell evolves through stages: –“cumulus” stage –mature stage –dissipating stage
The cumulus stage: Updrafts throughout, ~ 5 m/s max (15 m/s peak); no downdrafts Cell sizes: 2-6 km Updraft increases with height but diameter remains about constant ( entrainment). LL convergence Positively buoyant throughout Graupel and rain in-cloud 15-30 min in duration Wind, temperature, and hydrometeors
Surface convergence pattern measured at the time of first formation of cumulus clouds:
The mature stage: Rain first reaches the ground; heaviest rain and strongest turbulence in this stage Downdraft forms from above the FL Updrafts also remain strong, most intense higher in cell Strong surface divergence forms below the heaviest rain, and the cloud outflow forms a gust front at the surface Both positive and negative buoyancy is present ( v ’~ 2 K) Wind, temperature, and hydrometeors
Surface wind measurements show outflow below the region of radar echo echo >30 dB New convergence line ??
The dissipating stage: LL divergence Downdrafts weaken, turbulence becomes less intense, and precipitation decreases to light rain. Lasts about 30 min Wind, temperature, and hydrometeors
the Thunderstorm Project The 3 storm stages have since been interpreted as characteristic of airmass thunderstorms Byers and Braham recognize the importance of wind shear: –“strong shear prolongs the mature stage by separating the precipitating region with downdrafts from the updraft region” They also estimate entrainment: –estimated from mass balance: 100% in 2 km –estimated from soundings around storms: 100% in 5 km –discrepancy probably arose from downward motion of mixtures after entrainment, making the former estimate more reliable
8.2 Airmass Thunderstorms Scattered, small, short-lived, 3 stages Environment has little CAPE, but also little CIN, and little wind shear They are usually triggered along shallow convergence zones (BL forcing) Rarely produce extreme winds and/or hail, but may be vigorous with intense lightning
Mature airmass thunderstorms over the Pacific seen by the Space Shuttle
height (100s of ft) Schematic of the evolution of an airmass storm, as seen by radar The reason why an airmass thunderstorms is so shortlived is that there is little wind shear, therefore the rainy downdraft quickly undercuts and chokes off the updraft. Photo by Moller
8.3 Multicell Thunderstorms Multicell storms can occur in a cluster, or be organized as one line. Individual cells are short-lived like any air-mass thunderstorm, but the multicell cluster is long-lived, due to the ability of old cells to trigger new cells. The key to the long life of the multicell is the interaction of the gust front with the ambient LL shear gust front shelf cloud above gust front U env
Multicell storms were recognized by Byers and Braham (the Thunderstorm Project, 1948-49) Byers and Braham recognized the importance of cold pool building by decaying cells in the triggering of new cells.
Multicell Thunderstorms Shelf Cloud often indicates rising air over the gust front. New cells develop in front of the storm. Gust front maintained by the cool downdrafts. Gust front is typically several miles in front of the thunderstorm Gust front appears like a mesoscale cold front. Outflow boundary is the remnant of a gust front.
The sequence on the right shows individual cells and their place in the evolution of a multicellular system. Ludlam Fig. 8.10 Role of cell lifecycle in multicell storms
young cell old cell Photo by Doswell Photo by Moller Hobbs and Rangno 1985 (small multicell Cb over Cascades)
single-cell vs multicell storms: effect of LL shear balance between baroclinic & ambient horizontal vorticity leads to deeper ascent – more likley above the LFC (Rotunno, Klemp, Wilhelmson 1987, known as the RKW theory) shear no shear
5 km updraft (color) -1K ’ (contour) h, solenoidal h, ambient 0-1 km multicell simulations
multicell simulations: cluster migration towards region with higher CAPE
8.4 Supercell Thunderstorms Fig. 8.16 Supercell thunderstorms are defined as having a sustained deep- tropospheric updraft ~coincident with a mid-level vorticity maximum –They are typically ‘severe’ (strong horizontal wind gusts, large hail, flash flood, and/or tornadoes) They are rare (<1% in US, <5% in Southern Plains in May), long-lived They are easily identifiable on radar –Mesocyclone (sometimes TVS) –elongated anvil (to the east), often with a V notch –a hook-shaped flanking line (@ south side for right movers) –bounded weak-echo region (BWER) –reflectivity often suggests hail presence They form under strong shear –see right: composite hodograph –based on 413 soundings –near cyclonic supercells Fig. 8.15 storm motion
Supercell Thunderstorms occur most frequently in the southern Great Plains in spring. compared to single cells, supercells are: –longer-lived –larger –organized with separate up- and downdrafts.
Mesocyclone & hook echo storm motion to the ENE (70°) radar to the south 3 May 1999 Moore OK F5 tornado: reflectivity animation radial velocity animation Fig. 8.18
Fig. 8.15 storm motion storm-relative flow in a supercell composite hodo from ~400 soundings near supercell storms Fig. 8.20 young supercell mature supercell Fig. 8.23: sfc pressure perturbations (contours – mb), -1K cold pool, rain water @ 1 km (green colors), and updraft @ 1 km (pink) interpret this inflow low using Bernouilli eqn v 2 +p’=constant
the bounded weak echo region (BWER) Fig. 8.21 in textbook RHI Fig. 8.22
How does the BWER form ? As the storm intensifies, the updraft becomes stronger and more erect. The result are: –the development of mid-level echo overhang (WER) – a tighter reflectivity gradient (hail is most common just north of the WER) – a shift in cloud top position (right above the WER) These are strong indicators of a dangerously severe storm.
Base scan (0.5°) RHI 16.5 km echo tops NWSE BWER on radar: range height indicator (RHI) displays (source: WSR-88D Operations Training Manual)
south to north west to east BWER using horizontal & vertical slices (e.g., in soloii) Fig. 8.19
fallspeed of hail as function of diameter D BWER & the hail cascade
Where do we go from here? covered in 2011: Section 8.4 Supercell dynamics: COMET/METEDCOMET/METED –Supercell rotation 8.4.3: origin of mid-level rotation 8.4.4: solenoidal vorticity and the mesocyclone –8.4.5: storm splitting & supercell propagation –homework #3: Weisman: convective storm matrix: buoyancy-shear dependencies. COMET-MetEd module Weisman: convective storm matrix: buoyancy-shear dependencies. COMET-MetEd module not covered in 2011: 9. Mesoscale organization: –Mesoscale Convective Systems: Squall Lines and Bow Echoes (webcast)Mesoscale Convective Systems: Squall Lines and Bow Echoes –MCSs: BAMEX Science OverviewBAMEX Science Overview –MCV dynamics (Fritsch 1996) not covered in 2011: 10. Severe weather hazards: –severe weather & storm environment –tornado dynamics –derechoes: straight line winds
Storm classification summary variables: buoyancy and shear profiles