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Lecture 2a Severe Thunderstorm Primer Synoptic Laboratory II – Mesoscale Professor Tripoli Severe Weather Definitions I) World Meteorological Organization (WMO) A) General – – – – – – – Heavy rain Strong wind/ wind gusts Hail Lightning tornadoes flash floods extreme temperature B) Specific Events – – – – Snow storms dust/sand storms sea swell/ tsunamis/ storm surge extended area of fog for transport (aviation especially) Severe Weather (National Weather Service) • Weather that “poses a threat to life and/or property” • May include: – – – – – – – – – Heavy snow Freezing rain High winds Flash flooding River flooding Thunderstorms Tornadoes Tropical storms Hurricanes Severe Thunderstorm • National Weather Service Definition – Thunderstorm having at least one of: • Large hail (1 inch in or 2.5 cm in diameter or larger) • Damaging winds (at least 58 mph or 55 kts)) • Tornado (visible funnel cloud reaching the surface) – Not included: • Lightning • flooding Basic conditions for Thunderstorms • Potential Conditional Instability – Environmental lapse rate is greater than the moist adiabatic rate and less than the dry adiabatic lapse rate – Sufficient lifting of air parcels from low levels will result in free convection (LFC), i.e. positive CAPE – Some Convective inhibition <50 J/kg – Existence of a Theta_e minimum usually ~3 km above the surface, or ~700 hPa Basic Conditions for Severe Thunderstorms 1. Enhance basic conditions for thunderstorms – significant CAPE (strength of updraft ) – Significant Theta_e minimum in vertical (strength of downdraft) – Moderate Cap (distribute to CAPE to a limited number of isolated storms) Basic Conditions for Severe Thunderstorms 2. Vertical Wind Shear – – – – Significant environmentally-supplied vorticity/PV on scale of thunderstorms Inertially stabilizes storm structure Organizes updraft/downdraft structure of storm Strength of shear required is proportional to CAPE, i.e. matching important • Turbulence tempered by a relationship between static stability and wind shear such as a Richardson number (static stability/wind shear or thermal forcing/ inertial forcing) • Goldilocks Ri: – `Too much shear blows storm apart! – Too little shear insignificant compared to thermodynamic forcing – Certain shears “just right” – Straight line versus curved hodograph • Curved hodograph implies more organization of specific updraft/downdraft structures – Thermal wind: low level veering (backing) shear in NH (SH) suggestive of warm air advection • Low level WAA preferable to CAPE forcing – Tornadogenesis very dependent on curved wind shear profiles – Bottom Line: Severe weather season is during the spring and Fall when there exists strong jet streams and CAPE simultaneously Basic Conditions for Severe Thunderstorms 3. Helicity – Critical for supercells and tornadoes – Helicity density: h = V iV ; z =Ñ ´ V – Helicity: 3km H= ò h dz o – Helicity layers : • 0-1 km • 0-3 km Basic Conditions for Severe Thunderstorms 4. Large Scale dynamic forcing (QG PVA, low PV at outflow level) – Thunderstorm has to form full 3D vertical circulation – Downdraft helpful at low levels providing an “energy releasing” downdraft, Kind of like a biker pushing a pedal down on one side (updraft CAPE release) and lifting up on the other (downdraft evaporationally enhanced cooling) – Large scale QG forcing or low inertial stability, ie reduced “outflow resistance” helpful also • Required vertical motion for QG adjustment can be manifested as cumulus clouds, ie the sum total of cumulus updrafts ARE the QG lifting • Upward cumulus mass flux will build outflow along isentropic surface of cumulus updraft theta_e • This will force air down in compensation if not part of a large circulation, and that will require energy-stealing WORK • Energy consumed related to outflow resistance, which is proportional to inertial stability, ie vorticity or PV along outflow surface • Low PV lessens work, and so increases potential strength of convection • Lowest resistance toward anticyclonic side of jet stream, ie poleward if jet core is poleward Basic Conditions for Severe Thunderstorms 5. Low Freezing Level – Particular condition formation of hail – Almost all thunderstorms have hail since riming of ice is what causes lightning – For hail to reach surface it cannot melt • Hail must be sufficiently large not to melt (CAPE wind shear induced organization) • Freezing level sufficiently low Basic Conditions for Severe Weather • Composite of predicted or observed (nowcasting) environmental dynamical and thermodynamical state • Assessment of dynamical and thermodynamical forcing, ie QG forcing, local mesoscale circulations • Composite diagrams • Composite indices