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: – Helicity: – 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