Describe the basic (e.g., horizontal radar) structure of a progressive and serial derecho.

Progressive Derecho: Characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow. Serial Derecho: Consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line.

Identify and describe the key synoptic elements of the weather setup supportive of the 3 synoptically-evident severe weather outbreaks types according to the Johns (1993) paper, including supercell outbreaks, progressive derechos and serial derechos. Be sure to compare and contrast the three types in your response.

Supercell Outbreaks: Spring season Deep, strong surface low Strong trough, negative tilt High CAPE Very strong shear Discrete supercells Tornadoes, hail Progressive Derecho: Late spring-summer season Weak to moderate surface low Zonal to weak trough Very high CAPE Moderate to strong shear Bow echoes Damaging winds Serial Derecho: Fall-early spring season Surface low: Strong, extratropical cyclone Broad trough, negative tilt Low to moderate CAPE Very strong shear Squall lines Damaging winds, embedded tornadoes

Identify and describe some key environmental factors (e.g., shear, instability like CAPE/MUCAPE, mid-level (4-6 km) system relative winds [SRW], low-level system-relative inflow (0-2 km), 0-6 km mean wind and DCAPE) associated with bow echoes that create intense winds and derechos (including both weak forcing [WF] and strong forcing [SF] types).

WF Bow Echo/Derecho: High CAPE MUCAPE -- Important in elevated environments Moderate to strong 0-6 km shear Moderate to strong mid-level shear (4-6 km SRW), aiding RIJ 0-2 km SR Inflow -- Moist inflow High DCAPE 0-6 km Mean Wind -- Zonal/NW flow; supports fast forward motion SF Bow Echo/Derecho: Moderate CAPE MUCAPE -- Often less critical Strong to very strong 0-6 km shear Moderate to strong mid-level shear (4-6 km SRW), aiding RIJ 0-2 km SR Inflow -- Strong inflow with frontal lifting Moderate DCAPE; downward momentum transport from aloft 0-6 km Mean Wind -- SW flow; synoptic-scale organization

Identify and describe the most likely location of MCCs in the continental United States (CONUS) from a climatological standpoint. Identify and describe the diurnal timing of MCCs in the CONUS.

Most likely region: Central U.S. This area frequently sits on the northwestern edge of the Gulf moisture plume LLJs from the Gulf provide a steady feed of warm, moist air at night The central U.S. also features frequent nocturnal elevated convection Diurnal timing: Formation Time : Late afternoon to early evening Peak Intensity : Midnight to early morning Dissipation : Mid to late morning Fuel Source : Nocturnal low-level jet, elevated instability

Identify and describe the mesoscale convective vortex (MCV) and its physical and dynamical origins. Explain the potential importance of the MCV in subsequent convective initiation after MCS/MCC dissipation.

Mid-level cyclonic eddy is called a mesoscale convective vortex (MCV) (slides 87, 88) 100-300 km diameter Typically centered at 3-8 km AGL Origin : From vertical stretching of vorticity due to diabatic heating in MCSs Key Process : Latent heat release in stratiform region causes mid-level warming/lowering Can last several hours post-MCS Can reinitiate convection by enhancing lift and low-level convergence May evolve into a tropical cyclone over warm oceans

Identify the locations of climatological frequency maxima of tornado activity, significant tornadoes (EF2-EF5) and violent tornadoes (EF4-EF5) in the United States. Identify and describe the significance of geographical regions called "Tornado Alley" and "Dixie Alley".

The region of highest tornado risk is from Oklahoma extending east to Alabama ( "Dixie Alley" ) Significance : More deadly tornadoes despite lower total numbers Higher population density , more nighttime tornadoes , and hilly/forested terrain reduce warning time and visibility Peak activity often occurs in late fall through early spring Significant maximum area of risk extending from central MS into northern AL "Tornado Alley" Significance : Frequent clash of warm, moist air from the Gulf with cool, dry air from the Rockies and Canada Prominent springtime tornado activity due to ideal wind shear and instability Often produces long-track , highly visible tornadoes due to flatter terrain

Define and describe the forward flank downdraft (FFD) and rear flank downdraft (RFD) of supercells, including the physical processes responsible for them. Identify likely locations of tornadoes relative to supercell structure.

Forward Flank Downdraft (FFD): The main downdraft, located on the forward (usually NE or E) flank of the storm's updraft Characterized by heavy precipitation, rain-cooled air, and often a shelf cloud Physical Processes : Precipitation drag Entrainment and evaporation Mid-level wind shear Rain-cooled air Outflow Rear Flank Downdraft (RFD): A region of dry, cool air that descends from the mid-levels of the storm, wrapping around the rear side of the rotating updraft (mesocyclone) Forms when mid-level winds outside the storm's updraft are forced downwards as they are drawn into the rotating circulation. This descending air is often drier and colder than the surrounding environment due to adiabatic warming as it descends and the cooling effect of precipitation Likely locations of tornadoes relative to supercell structure: The north side of a supercell, particularly in the area associated with the hook echo or the wall cloud --> This region marks the interaction between the main updraft and the RFD, and is often associated with the mesocyclone

Identify and describe the "dynamic pipe effect" and explain its role in a classical theory of tornadogenesis in supercells. Explain how recent observational evidence appears to reject a prominent role for the dynamic pipe effect in supercell tornadogenesis.

(slides 11-13) The "dynamic pipe effect" is a model of tornado formation in which the incipient tornado vortex builds downward from aloft owing to the effects of radial convergence of angular momentum at progressively lower heights Role in Classical Tornadogenesis Theory: A strong, sustained mid-level mesocyclone creates vertical pressure gradients and stretching in the updraft The RFD develops as a result of dynamic subsidence behind the mesocyclone The RFD helps transport mid-level cyclonic vorticity downward The updraft then stretches the vorticity vertically , completing the formation of a tornado Mobile phased array data from several tornadogenesis events showed the tornado vortex signature (TVS) was first found near the surface, and grew upwards with time -- invalidate the dynamic pipe theory

Describe an updated theory of tornadogenesis in supercells based on more recent field campaigns (e.g., various VORTEX), highlighting the role of the RFD and the strength of the cold pool in non-tornadic, weakly tornadic and significantly tornadic supercells.

(slides 16, 17) Current leading hypotheses for tornadogenesis suggest that vertical vorticity may develop as air descends within storm-scale temperature gradient of the outflow If near-surface circulation moves into region of strong ascent (updraft), now the circulation can be stretched upwards and contracted to tornado strength

Explain why a downdraft is hypothesized to be critical for the generation of significant vertical vorticity near the surface associated with the tornadogenesis process in supercells without pre-existing vertical vorticity near the ground. And, explain why an updraft likely cannot serve the same role.

(slides 16, 17) Downdraft Role: Vorticity Generation : Tilts and transports horizontal vorticity into vertical near surface Vorticity Stretching : Not the primary agent Key Location : RFD or FFD (where cold pools or gust fronts are present) Outcome : Creates conditions necessary for tornadogenesis Updraft Role: Vorticity Generation : Can't do this effectively Vorticity Stretching : Stretches existing vertical vorticity Key Location : Updraft base or mesocyclone core Outcome : Only effective after vorticity exists

Explain why a "goldilocks" RFD that is strong but not too strong and cold is likely optimal for the development of significant tornadoes in supercells.

(slide 18) Near surface circulation is more likely, BUT many supercells develop near-ground rotation, yet are NOT tornadic Colder RFD means more negative buoyancy at low-levels...low-level vertical accelerations required for stretching are inhibited! "Goldilocks" zone of temperature contrast that allows development of significant surface circulation while also allowing stretching and final contraction Colder outflow may also move quickly away from location of maximum updraft

Identify approximate ranges of SRH (both 0-1 km and 0-3 km) associated with supercells and tornadic environments.

Supercells: 0-1 km --> 100-150 m 2 /s 2 0-3 km --> 150-300 m 2 /s 2 Tornadoes: 0-1 km --> > 150-300 + m 2 /s 2 0-3 km --> > 300 + m 2 /s 2

Identify optimal ranges of LCL and 0-1 km shear associated with significant tornadic supercells.

LCL heights within the range of 500-800 m (1600-2600 ft) --> Low LCL heights Strong 0-1 km shear

Given the updated theory of tornadogenesis, explain the likely physical and dynamical basis for the apparent empirical association of relatively low LCL and high 0-1 km shear with significant tornado producing supercells when compared to those supercells that do not produce significant tornadoes.

Low LCL: Thermodynamically favorable RFD --> stronger near-surface updrafts, less negative buoyancy, efficient stretching High 0-1 km shear: Strong horizontal vorticity --> enhanced low-level mesocyclone and vertical vorticity production 1) Horizontal vorticity is abundant (from shear) 2) The downdraft tilts and deposits vorticity near the surface (especially at the RFD boundary) 3) The updraft is strong enough to stretch that vorticity 4) The RFD is not cold enough to choke off the process

Identify the Thompson et al. Significant Tornado Parameter (STP) and explain its relationship to forecasting tornadic supercells.

The Significant Tornado Parameter (STP) is a multiple ingredient, composite index that includes effective bulk wind difference (EBWD), effective storm-relative helicity (ESRH), 100 mb mean parcel CAPE (mlCAPE), 100 mb mean parcel LCL height (mlLCL), and 100 mb mean parcel CIN (mlCIN) STP is meant to highlight areas where tornadic supercell ingredients co-exist, with larger values of STP corresponding to greater "overlap" of the ingredients Significant Tornado (SigTor or EF2-EF5): STP > 1.0

Identify, describe and if given radar observations evaluate Doppler radial velocity signatures of supercell circulations, including the mesocyclone.

Mesocyclone: Appearance on Radar : Persistent, broad-to-tight couplet Deep, rotating updraft Tornado vortex signature (TVS): Appearance on Radar : Very tight, intense couplet; often < 1 km scale Stronger and lower than mesocyclone Gust front/shelf wind: Appearance on Radar : Gradual wind shift, not tight Often misidentified; lacks tight rotation What to look for: Tight couplets of inbound/outbound velocity Check multiple elevation angles to confirm vertical continuity

Identify and describe the necessary conditions for hail formation.

(slides 36-39) 1) Strong updrafts --> Lift hailstones and keep them aloft to grow 2) Supercooled water --> Fuel for hailstone growth (via accretion) 3) Deep freezing layer --> Allows time and space for hailstone development 4) Wind shear --> Separates updraft/downdraft, supports storm organization 5) Rotating storm (supercell) --> Recirculates hailstones for additional growth

Describe the role and importance of CAPE or updraft strength in the hail formation process. Describe the importance of freezing (wet bulb zero) level height in hail occurrence on the ground.

CAPE: Large hailstones are heavy, so only strong updrafts (associated with high CAPE) can hold them up and let them grow Freezing level/Wet Bulb Zero Height in Ground-Level Occurence: If the freezing level is high, hailstones melt more on their way down --> smaller or no hail at the surface If the freezing level is low, hailstones have less time to melt, so larger hailstones can survive to reach the ground Wet bulb zero height --> determines how much hail melts --> affects if and how hail reaches the ground *High CAPE + Low Freezing Level = Ideal hailstorm conditions

Identify and explain how deep shear, CAPE in the mixed phase zone (e.g., -10 C to -30 C), and the freezing level (or wet bulb zero level) can be used to forecast for the potential of severe hail.

(slides 40-43) Deep shear: What to look for : >= 35-40 kts Organizes storm, supports persistent strong updrafts CAPE in Hail Growth Zone: What to look for : High CAPE within -10 C to -30 C layer Enhances hailstone growth in supercooled region Wet Bulb Zero Height: What to look for : < 2.7 km Reduces hail melting, better survival to surface

Describe the 3-stage hail formation process in supercell storms. Be sure to define and explain the relevance of the stagnation point, the embryo curtain, the BWER, and the hail cascade in hail production within supercells.

Stage 1: Embryo Formation Small particles like graupel or frozen raindrops (called hail embryos) form in the lower to mid-levels of the updraft Embryo Curtain: Broad region of initial graupel/hail embryo formation in forward flank --> Provides seed particles for hail growth Stage 2: Hailstone Growth Embryos get entrained into the strong updraft and carried upward into the supercooled liquid water region between -10 C and -30 C (the hail growth zone) Stagnation Point: Location where horizontal flow stops and air is lifted into the updraft --> Efficiently injects embryos into the hail growth zone BWER: Radar-indicated weak reflectivity core within strong updraft --> Indicates intense vertical motion and likely large hail growth area Stage 3: Hail Fallout Eventually, the hailstones become too heavy for the updraft to support, and they begin to fall toward the surface Hail Cascade: Region where mature hailstones fall out of the updraft --> Main area of surface hail impacts; can be hazardous

Identify the physical mechanisms responsible for severe straight-line wind events within convective storms.

Evaporative Cooling: Evaporation chills and sink air --> Initiates and strengthens downdraft Precipitation Loading: Weight of rain/hail pulls air down --> Adds mass to downdraft Rear-Inflow Jet (RIJ): Jet of air punches into rear of storm --> Enhances mid-level downdrafts Cold Pool & Gust Front: Outflow spreads rapidly along surface --> Drives damaging surface winds Downburst (Wet/Dry): Concentrated burst of wind hitting the surface --> Can cause tornado-like damage

Define downburst, microburst, wet microburst, and dry microburst.

Downburst - An area of strong, often damaging, winds produced by one or more convective downdrafts Microburst - A convective downdraft (downburst) that covers an area less than 4 km along a side with peak winds that last 2-5 minutes and has a differential Doppler velocity of 10 ms -1 Wet Microburst - A type of microburst that is accompanied by heavy precipitation reaching the surface Dry Microburst - A type of microburst that is characterized by little to no measurable precipitation reaching the ground

Describe key wind pattern differences between straight line and tornadic wind events. Explain why damage patterns associated with each type of event might be confused with each other, unless care is taken.

Straight-Line Winds: Wind Direction : Uniform (generally blows in one direction) Source : Downdraft and outflow Speed : Can exceed 100 mph (especially in derechos) Duration : Often brief but widespread Wind Convergence : Winds diverge outward from impact point Tornadic Winds: Wind Direction : Rotational Source : Mesocyclone or low-level mesovortex Speed : Ranges from weak to > 200 mph Duration : Usually more localized and short-lived Wind Convergence : Winds converge inward toward rotation center Why Damage Can Be Confused: Both can uproot trees and snap large limbs, tear off roofs and damage structures, and create localized zones of intense damage. Microbursts can cause intense, focused damage that resembles a weak tornado Tornadoes can embed within bow echoes, making it difficult to isolate straight-line vs. tornadic damage without a close survey Eyewitness reports can be misleading

Describe the role of lapse rate from cloud base to surface and rain water content or radar reflectivity on the formation of severe straight-line wind events. Explain how the presence of ice and melting might alter this relationship.

Steep lapse rate (dry below cloud): Enhances evaporative cooling and accelerates downdrafts High radar reflectivity/rain: Indicates heavy precipitation that contributes to precipitation loading Presence of ice/melting layer: Cooling from melting and evaporation strengthens downdrafts

Explain how downdraft CAPE (DCAPE) might be used to forecast the potential for downbursts or severe straight-line winds. Alternatively, evaluate the relative downburst threat when given different values of DCAPE. Identify the physical mechanism associated with downbursts that is measured by DCAPE (review DCAPE).

How DCAPE is Calculated: 1) Taking a parcel from just below cloud base 2) Bringing it down dry adiabatically while accounting for evaporative cooling in a dry sub-cloud layer 3) Integrating the parcel's boundary deficit (how much cooler it is than the environment) all the way to the surface Physical Mechanism Represented: DCAPE measures negative buoyancy created by cooling due to evaporation and melting, which drives downbursts How DCAPE Relates to Downburst Threat: DCAPE < 500 J/kg --> Low threat; Weak or no downdraft potential DCAPE 500-1000 J/kg --> Moderate threat; Possible strong wind gusts DCAPE > 1000 J/kg --> High threat; Favorable for severe downbursts/damaging straight-line winds

Given various combinations of lapse rate, precipitation type and amount (e.g., radar reflectivity), evaluate the potential for intense downdrafts and severe straight-line winds.

Lapse Rate (Cloud Base --> Surface): Steeper lapse rates (e.g., >= 8.5 C/km) promote stronger negative buoyancy via evaporative cooling Precipitation Amount (Radar Reflectivity); High reflectivity ( >= 50 dBZ) implies heavy rain/hail, increasing downdraft speed via precipitation loading Precipitation Type: Hail or melting ice promotes additional cooling (latent heat absorption), enhancing downdrafts Sub-cloud Layer Dryness: Dry layers allow more evaporation --> stronger cooling --> stronger downdraft acceleration

Be able to describe the main atmospheric ingredients for flash flooding, specifically the conditions that a forecaster can monitor for flash flood events.

(slide 6) Slow storm system motion (increases duration (D)) --> Weak mean steering level winds or back-building (propagation cancels advection) Large low-level water vapor concentration in presence of strong updrafts (increases vertical water vapor flux [w*r v ] and hence instantaneous rain rate (R)) Large environmental relative humidity (increases precipitation efficiency (E) and hence R) Significant cloud depth below freezing level (increases E and hence R) Perhaps weak vertical wind shear (likely increases E and hence R)

Forecasting Mesoscale Processes Final Exam Study Guide

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Josie Nelson

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FLASHCARD QUESTION

Front

Given the linear perturbation term for supercells, sketch and describe (physically and mathematically) the linear dynamical effects of a strong updraft at mid-levels in a convective cell interacting with moderate-to-strong environmental shear that is either a) unidirectional, b) curved/veering or c) curved/backing. In your sketch, be sure to label perturbation low- and high-pressure centers and the sense of the linear perturbation pressure terms. Diagnose any induced perturbation pressure gradient force and associated vertical accelerations. Discuss implications for supercell storm development and propagation (e.g., right vs. left moving storms).

Back

a) Unidirectional

(slide 57)

Result: You get a low (high) perturbation pressure induced downshear (upshear) of the updraft

--> These pressures induce upward (downward) accelerations downshear (upshear) of updraft

Implication: Provided shear + buoyancy is strong enough, PGF will be strong enough to lift parcels to LFC downshear of current updraft

--> New cell growth occurs on downshear side and is suppressed on upshear side associated with dynamical force

--> Cell propagates downshear

b) Curved/Veering

(slide 58)

Result: With veering hodograph, vertical perturbation PGF is upward directed on south side or right flank of supercell

--> Veering (CW) curvature --> preferred growth on right flank --> hence, right, deviate motion relative to typical eastward cell motion (0-6 km flow)

c) Curved/Backing

A backing (CCW) hodograph results in upward PGF on north side or left flank

Preferred growth on left flank --> hence, left deviate motion relative to typical eastward cell motion

FLASHCARD QUESTION

Front

Identify Doppler radar (reflectivity and radial velocity) signatures associated with downbursts and tornadoes within bow echo storms.

Back

Downbursts:

Look for a prominent "rear inflow notch (RIN) (region of lower reflectivity behind the main convective line, often co-located with the RIJ)

Tornadoes:

A "hook echo" is a signature of mesocyclonic rotation (hook-shaped extension of high reflectivity, usually on the right-ear flank of the storm)

Look for the "tornado vortex signature" (TVVS) (a tight area of rotating winds on the velocity product). You'll see a small area with red and green velocities wrapping together, indicating strong rotation

FLASHCARD QUESTION

Front

Identify, describe and explain the underlying dynamical processes of the three (3) primary mesovortex genesis hypotheses. Be sure to compare and contrast them. For example, identify the primary take-aways from each hypothesis and identify how these are similar and different for each.

Back

#1 -- Trapp and Weisman (2003):

(slides 56-58)

Two stages

1) "Early QLCS stage"

*Tilting of environmental vorticity by convective updrafts

Dynamical Focus: Local convective updrafts

Main Process: Tilting of ambient horizontal vorticity

Result: Weak pockets of vertical vorticity

2) "Mature QLCS stage"

*Stretching and amplification of vertical vorticity by system-scale convergence zones

Dynamical Focus: System-scale lifting/convergence

Main Process: Stretching of vertical vorticity

Result: Strong, organized mesovortices

#2 -- Atkins and St. Laurent (2009b)

(First Mechanism)

Two source regions for air parcels entering the mesovortex

1) Cold pool, either via RIJ or convective downdrafts

2) Storm-relative inflow

Step 1:

Physical Process: Horizontal vorticity exists due to vertical wind shear

Source: Environmental

Step 2:

Physical Process: Tilt that vorticity downward via updrafts and downdrafts

Source: Gust front dynamics

Step 3:

Physical Process: Vertical vorticity is generated at low levels

Source: From shear via tilting

Step 4:

Physical Process: Stretching intensifies vertical vorticity

Source: From convergence/updraft zone

#3 -- Atkins and St. Laurent (2009b)

(Second Mechanism)

Similar to Trapp and Weisman (2003)

Step 1: Baroclinic generation -- Gust front produces horizontal vorticity due to density and pressure gradients

Step 2: Upward tilting -- Convective updrafts tilt this vorticity into the vertical

Step 3: Stretching -- Convergence along gust front intensifies the vertical spin into mesovortices

FLASHCARD QUESTION

Front

Explain why we cannot necessarily reject the role of shearing instability in the formation of mesovortices.

Back

Wheatley and Trapp (2008) do cite horizontal shearing instability as a potential source of meso-vortex generation in their cool-season QLCS case!

This highlights that in certain environments, especially where baroclinic or shear-driven processes dominate, instability in the low-level shear profile may play a significant role in generating vertical vorticity.

FLASHCARD QUESTION

Front

Describe the individual and potentially combined role of the descending rear in-flow jet (RIJ) & mesovortices in the production of damaging wind in bow echoes within a QLCS. What is a key feature in the strongest damaging winds associated with bow echoes and mesovortices?

Back

Important:

Bow echoes are formed through the intensification of a RIJ, which is driven by a pressure gradient induced by buoyancy due to the upshear-tilted updraft.

Meso-vortices create intense horizontal pressure gradients due to (ζ’)² relationship (the non-linear p' term!)

Result:

Meso-vortices without a RIJ tended to be much weaker or non-damaging than those associated with a RIJ

*The most damaging winds with bow echoes/meso-vortices tend to occur where the effects of the descending RIJ and the meso-vortex overlap

FLASHCARD QUESTION

Front

List a general (simple) definition of a derecho and explain the key elements of one formal definition of a derecho (note: it's not necessary to list all of the details but emphasize the key ideas).

Back

A derecho is a widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo

Key Elements:

Widespread wind damage

Long-lived event

Violent wind speeds

Convectively induced

Fast-moving band of storms

Bow echo structure

FLASHCARD QUESTION

Front

Describe in words and graphically the warm and cool season climatology of derecho frequency in the CONUS.

Back

(starts on slide 71)

Warm Season:

Most common type of derecho in the CONUS

High instability

Moderate shear

Progressive, localized structure

Triggered by surface heating, cold pools

Cool Season:

Low to moderate instability

Strong shear

Serial structure, large-scale squall lines

Triggered by synoptic systems (fronts, jet streaks)

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