AOS 101 Severe Weather April 1/3

Lifting Condensation Level (LCL) Level at which a parcel lifted from the surface would reach saturation (i.e. level where temperature = dewpoint) Last week: DALR → T decreases at 10oC/km In addition, dewpoint (Td) decreases at 2oC/km LCL will coincide with how high above the ground the cloud bottom is.

Level of Free Convection (LFC) Level at which a parcel raised from the surface would become warmer than the environment. Above this level air is able to freely convect, or ascend unabated to the tropopause. Equilibrium Level (EL) Level at which parcel is no longer warmer than the environment Usually near the tropopause where environmental lapse rate is near 0oC/km Coincides with height of cloud top.

EL LFC LCL Tparcel Tenv SATURATED Td T 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 20 30 TEMPERATURE (oC)

Ingredients for Thunderstorms Instability Lifting mechanism Shear Moisture

1. INSTABILITY In an unstable atmosphere, lifted parcels of air will be warmer than its surroundings. In this situation, a buoyancy force acts on the object, accelerating it upward.

Buoyancy Force The larger the difference in Tparcel and Tenv , the larger the force and acceleration (F = ma) More buoyancy leads to stronger updrafts, up to 50 m s-1

CAPE (Convective Available Potential Energy) A measure of how buoyant parcels will be as they ascend in a thunderstorm cloud. Higher CAPE means stronger updrafts and more intense thunderstorms. Is equal to the area between parcel path and environmental temperature curve when the parcel is warmer (between the LFC and EL).

CAPE EL LFC SATURATED 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 20 10 20 30 TEMPERATURE (oC)

2. LIFTING MECHANISM On the typical summer day, the atmosphere will be conditionally unstable. Stable for unsaturated parcels Unstable for saturated parcels Surface (unsaturated) parcels will not be able to rise on their own. Some mechanism must raise the parcel until it reaches saturation (LCL) and then past a level at which it is warmer than its surroundings (LFC).

Tp=14oC LFC Tp=20oC LCL Tp=30oC, Td=22oC FREELY CONVECT 2 km: Te=14oC Tp=14oC LFC SATURATED LIFTING MECHANISM 1 km: Te=22oC Tp=20oC LCL UNSATURATED 0 km: Te=30oC Tp=30oC, Td=22oC

Examples Convergence of winds Orography (upslope) Intense surface heating Outflow boundary from storm in the vicinity

3. SHEAR Difference in winds with height 2 types: Directional shear (wind changes direction with height). Speed shear (same direction of winds, but speed increases with height). 2 km 1 km 3 km sfc DIRECTIONAL 3 km 2 km 1 km sfc SPEED

Air mass thunderstorm CUMULUS MATURE DISSIPATING COOL DOWNDRAFT WARM MOIST UPDRAFT LIGHT RAIN HEAVY RAIN

Why shear is needed… With no shear, downdrafts cut off source of moisture (updraft) Vertical shear displaces downdraft from updraft, allowing t’storm to continually replenish moisture Storms last longer and become more intense.

4. MOISTURE Thunderstorms need plentiful source of moisture to drive circulation High moisture → High dewpoint → Low dewpoint depression → Low LCL → Low LFC → Less lifting needed for free convection (also more CAPE).

CAPE EL LFC LCL Td T 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 20 TEMPERATURE 30oC DEWPOINT 14oC 8 CAPE HEIGHT (km) 6 LFC 4 2 LCL Td T -40 -30 -20 -10 10 20 30 TEMPERATURE (oC)

CAPE EL LFC LCL Td 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 20 30 TEMPERATURE 30oC DEWPOINT 22oC 8 CAPE HEIGHT (km) 6 4 LFC 2 LCL Td -40 -30 -20 -10 10 20 30 TEMPERATURE (oC)

Severe Weather Criteria Wind in excess of 58 mph Hail larger than .75 inches Tornado

WIND 58 MPH wind gust or higher Strong winds are created by downdrafts which strike the ground and spread out Downburst, Microburst Can exceed 120 mph (weak tornado strength)

HAIL .75 inch hail (nickel size) or greater Strong updrafts keep ice chucks aloft so that more water freezes to them Record hailstone: 7.0 inches (volleyball size) Aurora, Neb. June 22, 2003

TORNADO Can have winds up to 318 mph Moore, OK – May 3, 1999 Stay on the ground for an hour or more Record: 1924 Tri-State Tornado (3.5 hours) Track for tens of miles Record: Tri-state, 219 miles Up to a mile wide Record: 2.5 miles – Hallam, NE – May 22, 2004

Ranked using the Fajita Scale Assessors look at damage, than match to wind speeds F0 = weakest, F5 = strongest Only 1% are F4 or F5 Last F5: Greenville, KS 2007 (first since 1999)

Types of Thuderstorms Airmass (usually not severe) Multi-cell Squall line (bow echo, derecho) Supercell

Multicell Groups of cells moving in a line Outflow of one storm provides lifting mechanism for the next cell in the line Can “train” over one area for hours Flash Flooding

Squall Line (Bow Echo, Derecho) Continuous line of storms moving quickly Typical if speed shear but no directional Outflow in front of storm lifts air upwards Can persist for over a day and travel 1000 km or more Very strong winds (120+ mph)

Supercell Most severe of all storms. Produce most strong tornadoes (F2+) and large hail (2”+) Need directional shear, winds turning clockwise with height. Hook echo (TVS) Mesocyclone Overshooting top

Mesocyclone

Overshooting Top