Multi-Cellular Thunderstorms

Ø  Multi-cell Thunderstorms

̶        Dominated by gust front processes

̶        Add more shear (from single cell thunderstorms)

̶        Flanking line towers become other storms

Ø  Multi-cell Cluster

̶        Series of evolving cells where new updraft growth is on a preferred flank

̶        Mean wind of the cloud barring layer

̶        New cells more where the low level convergence is the strongest

̶        Deviates (Verb)

̶        Deviant (adjective)

̶        In moderate wind shear environments, multi-cell storms may take a few hours to evolve into a supercell storm

̶        In a squall line new squalls form on the continuous gust front (leading edge)

Ø  Line Echo Wave Patterns (LEWP)

̶        Series of bow echoes

̶        A wavy appearing squall line

̶        Can result in an enhanced severe threat at the leading edge of the bow or in the “crest” of the waves

̶        Sometimes can get a small vortex within the line (tornado)

̶        Not a supercell tornado, it’s a QLCS tornado

Ø  QLCS (Quasi Linear Convective System)

̶        Somewhat a line

̶        Can produce tornados (usually weak)

Ø  Supercell:

̶        Storm with a  Mesocyclone

Ø  Mesocyclone:

̶        Strong and persistent

Ø  Squall Line Cross Section

̶        Compare with super cell schematic

̶        Updrafts are on the leading edge of the cold pool, hence tornado threat is small after passage of gust front although hail will now occur

̶        Squall lines have well-developed cold pools

̶        Surface based: Warm air from the ground is going straight up

Ø  Cross Section of multi-cell

̶        Continuous gust front

Ø  Outflow Boundary (OFB)

Ø  OFB on Composite radar

̶        Land spouts

Supercell Thunderstorms

Ø  Forward and rear flank downdrafts are rapping around the updraft which is occlusion

Ø  Occlusion (pg. 162 – Figure 9.7)

Ø  New updraft will form on the triple point of the occlusion

Ø  V-Notch

̶        Strong updraft

̶        Goes around the updraft

̶        Inflow notch

Ø  Beaver’s Tail

̶        Flat cloud that lines up on the forward flank gust front

̶        Connected to the rain free base

Ø  Tail Cloud

̶        Attached to wall cloud

Ø  SCUD – Scattered Cumulus Under Deck

̶        Cooler air from the forward flank is drawn into the storm and reaches saturation below the cloud’s LCL

̶        Often the wall will lean toward the precipitation

̶        Strong motions are often apparent to the spotter

Ø  HP (High Precipitation)

̶        These storms are difficult to spot for two primary reasons:

1.    Obscuration and misplacement of important features and safety. The best place to spot the tornado is usually in its path

2.    Originally, HP supercells were considered rare. In reality, perhaps half of all supercells are HP

̶        HP supercells are also prolific tornado producers, much more than originally thought.

̶        These are usually big and scary storms.

̶        Great deal more precipitation in the RFD with rain actually falling through the rain free base (not really a rain free base)

̶        2 reasons they are EASY to find

1.    Visual Vault

2.    Beaver’s Tail

̶        Beaver’s tail is INFLOW into the storm

Ø  LP (Low Precipitation)

̶        Storm without a rear flank downdraft

̶        These storms are easy to identify marked by light precipitation in the main downdraft

̶        LP’s tornadic potential is somewhat limited

̶        They do produce damaging hail and can change modes throughout their lifetime

̶        Prolific producers of large hail

̶        IF tornadoes do form the storm may be becoming more like a supercell

Thunderstorm: Variables & Ingredients

Ø  Variables needed for Severe thunderstorms:

1.    Moisture

2.    Instability

3.    Lift

4.    Wind shear

Ø  Bob, from Texas

̶        Launches weather balloons (radiosonde)

̶        Radioing back temperature, dewpoint, etc…

̶        Thermodynamic diagrams

Ø  Wind shear

1.    Speed Shear

o   Winds increasing speed with height

2.    Directional Shear

o   Winds changing direction with height

Ø  Development

̶        Cumulus Humilius

̶        Cumulus Congestus

̶        Towering Cumulus – not precipitating

̶        Cumulonimbus (Cb) – precipitating

Ø  Texture

̶        More “cauliflower” the stronger the updraft

̶        “rock hard towers” implies that most of the cloud is in the liquid phase

̶        Updraft liquid weakens or reaches high in the troposphere = liquid freezes = giving cloud a “glaciated” texture (considered fairly weak)

Ø  Anvil

̶        Crisp

̶        Fuzzy

Ø  Vertical Shear

̶        Increases longevity and organization

̶        Strong shear = storm-scale rotation by tilting horizontal vorticity into vertical vorticity

̶        Too much shear = the storm cannot organize (“orphan anvils”) = CAPE is too weak and shear is too strong

Ø  Flanking Line

̶        Flanking line leading into the main updraft

̶        Main cell SW is tilted due to the environment shear

Ø  Boundaries (pg. 307)

̶        Describes fronts

̶        The leading edge of thunderstorm outflow

̶        Leading edge of the sea breeze

̶        Any other lines marking the junction of 2 airmasses

Ø  Creating Boundaries

̶        Differential heating of air either over surfaces with different properties, such as water, and lands, forests and fields, urban and rural landscapes, or over surfaces heated differently  (land over cloudy versus clear skies)

Ø  Occlusion = Cold air rapping around a cove

Ø  WER = Not a lot of precipitation/ at all

Ø  Anvil à Sinus Cloud à Made from ice crystals

Ø  More evaporation = High LCL’s = Relative humidity is lower towards the ground

Ø  LCL = Helps indicate the relative humidity of the sub-cloud layer

Ø  Wet Bulb Zero = Sleet = Frozen Rain

Ø  Verga = Rain that evaporate before hitting the ground

Ø  BRN (Bulk Richardson’s Number): CAPE is too weak and the shear is too strong

̶        Sweet spot: 10-45 BRN

                 BRN = CAPE / Shear

Ø  What 3 influences does dry air have on severe weather? 

̶        More evaporation = Stronger downdraft

̶        Dryer air in Mid-level of atmosphere tends to promote large hail growth

̶        Connectivity unstable (will learn in unit 2)

Ø  Single Cell Thunderstorms

̶        Single cell storms are dominated by buoyancy processes

̶        Sometimes called “air mass” t-storms, these storms are poorly organized and pose relatively little threat to the public (lightning and hail)

̶        Typical of afternoon thunderstorms

̶        Updrafts form in relatively random locations

̶        The dominant forcing feature is instability since they form in a low-shear

̶        Goes through the cycle within 30-60mins

̶        Severe weather threats minimal

̶        Pulse Severe Storm

Ø  Severe single cell thunderstorm

̶        Forms in a low shear environment

̶        Taller updraft/More instability

̶        More intense reflectivity/More intense core

̶        Longer lasting

̶        Precipitation takes longer to descend to the ground/Stronger updraft

̶        Vertical Integrated Liquid (VIL) is larger

̶        “Popcorn Severe”

Ø  What are the differences with ordinary thunderstorm and a “pulse” severe thunderstorm?

1.    Taller updraft/More instability

2.    More intense reflectivity/More intense core

3.    Longer lasting

4.    Precipitation takes longer to descend to the ground/Stronger updraft

̶        Both form in a low shear environment

Ø  Land Spouts

̶        Single cell thunderstorm can create them

̶        Horizontal shear causing vertical vorticity that stretches and causes a tornado

̶        Tend to have a double vortex (thin core and translucent on the outside)

̶        Usually weak, not always