severestorminterp2005 fmi

Severe storm radar signatures: 

Severe storm radar signatures Jim LaDue Warning Decision Training Branch, NWS Norman, OK James.G.LaDue@noaa.gov

Topics: 

Topics Estimating updraft strength Mesocyclones Assessing the potential for Severe winds Large hail Tornadoes Heavy rainfall

Updraft strength estimation: 

Updraft strength estimation Upper level reflectivity core Storm reflectivity and velocity structure Low-level convergence

Elevated reflectivity core: 

Elevated reflectivity core Where does the precipitation form relative to the freezing level? How high does the precipitation extend? Link to loop

Case 1: July 10, 2003 very severe reflectivity profile: 

Case 1: July 10, 2003 very severe reflectivity profile Let’s examine the reflectivity profile of storm K0 and compare it to the Severe Hail Index as determined by the previous page

Case 1: July 10, 2003 very severe reflectivity profile: 

Case 1: July 10, 2003 very severe reflectivity profile 0109 UTC Deep, high reflectivity well above/below –20 C level 0° C -20° C Z=55dBZ VIL = 83 kg/m2 VIL density = 4.94 g/m3 MEHS = 3” Result: baseball hail

Case 1: July 10, 2003 very severe reflectivity profile: 

Case 1: July 10, 2003 very severe reflectivity profile 0109Z Values are integrated upward The Severe Hail Index integrates the vertical reflectivity profile Hail Detection Algorithm (HDA) converts this to estimated hail size 0° C -20° C Note how rapidly the SHI increases above the 0 °C level POSH=90%

Case 2: July 10, 2003 Nonsevere updraft: 

Case 2: July 10, 2003 Nonsevere updraft Let’s examine the reflectivity profile of storm F6 and compare it to the Severe Hail Index as determined by the HDA worksheet

Case 1: July 10, 2003 nonsevere reflectivity profile: 

Case 1: July 10, 2003 nonsevere reflectivity profile 0109 Z Bottom-heavy and weak reflectivity storm F6 0° C -20° C Z=55dBZ Storm parameters for cell F6 VIL = 42 kg/m2 VIL density = 2.7 g/m3 MEHS=.75”

Case 1: July 10, 2004 nonsevere reflectivity profile: 

Case 1: July 10, 2004 nonsevere reflectivity profile July 10, 2003 – Wichita Note that SHI responds exponentially less given 10-15 dBZ lower reflectivities in this storm POSH=90% Storm parameters for cell F6 VIL = 42 kg/m2 VIL density = 2.7 g/m3 MEHS=.75” Result: no severe reports

Interim summary: updraft intensity through upper-level reflectivity : 

Interim summary: updraft intensity through upper-level reflectivity Use the higher slices to detect the towering cumulus above the boundary layer echoes Look for the first strong core to develop above the freezing layer. More severe storms have higher elevated reflectivities and more top heavy vertical reflectivity profile Slightly smaller reflectivity aloft leads to large changes in expected weather

Severe updraft structural signatures: 

Severe updraft structural signatures Objective: Understand how the following signatures are an indication of, or contribute to severe updrafts in convection WERs BWERs Stormscale velocity

Nonsevere storm structure: 

Nonsevere storm structure Upper level storm top lies over low-level reflectivity maximum Lower reflectivities overall Low height of reflectivity thresholds

Severe sheared storm structure: 

Severe sheared storm structure Upper level storm top lies over low-level reflectivity gradient on the side of low-level storm relative inflow Strong echo overhanging a Weak Echo Region (WER)

Trailing mesocyclone supercell updraft storm structure: 

Trailing mesocyclone supercell updraft storm structure Upper level storm top lies over low-level WER Sometimes a BWER (Bounded Weak Echo Region) Sustained intense elevated reflectivity core A mesocyclone

Severe sheared updraft intensity – BWER detection: 

Severe sheared updraft intensity – BWER detection BWER (Bounded Weak Echo Region) 0.5° 1.5° 2.4° 3.4° BWERs difficult to detect this far out Typical BWER detection tops off near the -20° C level -20° C Needs a connection to the low-level WER

Classic severe updraft signature case: 

Classic severe updraft signature case Trailing updraft Normal width Produced a few record sized hailstones BWER = 2mi max size -20° C

Severe updraft in a squall line segment: 

Severe updraft in a squall line segment Squall line moving 52 kts Storm top 26 kft AGL Is there a WER? -20° C The ‘forward jump’ in the reflectivity from 2.4 to 3.4° is more than just movement between elevation slices

Conceptual model of a more severe linear system updraft: 

Conceptual model of a more severe linear system updraft Key things to note are the Relatively nondescending RIJ Front end echo overhang with linear BWER ahead of the surface gust front Deep convergence zone

Updraft strength – Low-level convergence: 

Updraft strength – Low-level convergence Convergence parameters to affect updraft magnitude magnitude depth residence time of storm over convergence

Updraft strength – Low-level convergence depth: 

Updraft strength – Low-level convergence depth 6500ft - 2 km Assuming a steady state convergence Depth: Z = 2 km Boundary width = 1 km (one .54 nm range gate) Mean convergence over 1 km: V = 10ms-1 / 1000m = .01 s-1 Updraft strength at 2 km W = (V) Z = .01s-1 *2000m = 20 m/s 10 kts (5 m/s) 1 km Convergence V is simplified to be V/width or 10 m s-1 /1000 m

Updraft strength – Low-level convergence depth: 

Updraft strength – Low-level convergence depth 10000ft - 3 km 10 kts (5 m/s) 30 m/s 1 km Assuming a steady state convergence Depth: Z = 3 km Boundary width = 1 km (one .54 nm range gate) Mean convergence over 1 km: V = 10ms-1 / 1000m = .01 s-1 Updraft strength at 3 km W = (V) Z = .01s-1 *3000m = 30 m/s

A severe upright convective system with deep, strong convergence: 

A severe upright convective system with deep, strong convergence Deep Convergence zone What do you think the spotter’s seeing? 15 kft

Conceptual model of a less severe linear system updraft: 

Conceptual model of a less severe linear system updraft Key things to note are the Descending RIJ No deep convergence zone Shallow sloping updraft over top of cold pool with numerous discrete cells merging into a line

Visual and radar obs of strong vs. weaker convective wind events: 

Visual and radar obs of strong vs. weaker convective wind events Case: 11 August 2004 Cocoa Beach, FL How deep is this gust front?

Visual and radar obs of strong vs. weaker convective wind events: 

Visual and radar obs of strong vs. weaker convective wind events Case: 11 August 2004 Cocoa Beach, FL How deep is this gust front?

Summary: Severe updraft signatures: 

Summary: Severe updraft signatures severe updraft signatures common to all storms in order of most severe first BWER WER Intense reflectivity core, and deep relative to the –20° C level Storm top displaced over WER Deep convergence zone

Stormscale rotation: 

Stormscale rotation First, a review Which is (cyclonic convergent, anticyclonic convergent, convergent, divergent)?

Mesocyclones: 

Mesocyclones A Rankine Combined Vortex

Mesocyclone criteria: 

Mesocyclone criteria Vr Core diameter from –Vmax to +Vmax should not exceed 10 km Rotational Velocity Vr = (| –Vmax | + |Vmax |)/2 exceeds user thresholds Persistence  10 min Vertical continuity -Vmax +Vmax

mesocyclone: 

mesocyclone Rotational velocity = (|max outbound| + |max inbound|)/2 Use representative inbounds and outbounds, not the absolute maximum values Vmax = 50kt Vmin = -22kt Rotational V = 36 kt Meso diameter = 3.5 nm

Another classic supercell: 

Another classic supercell J. LaDue

Another classic: 

Another classic J. LaDue

A diversity of mesocyclone sizes: 

A diversity of mesocyclone sizes All of these were tornadic. Only the big one shows a meso hit G. stumpf Courtesy G. Stumpf

Summary: mesocyclones: 

Summary: mesocyclones Less than 5 nm, 10km in diameter Persistent Not shallow Partly occupied by updraft and downdraft Begins as mostly updraft then matures as downdraft forms

Severe weather threats: 

Severe weather threats Wind Hail Tornado Heavy rain

Severe wind threats: 

Severe wind threats Isolated downbursts Supercell wind threat Organized multicell wind threat

Pulse storm downbursts: 

Pulse storm downbursts Rapid and severe growth in updraft Descent of the reflectivity core Midlevel velocity convergence The idea is to look for clues for potential downbursts before it reaches the ground.

Pulse storm downbursts: 

Pulse storm downbursts Is this the time a warning should be issued?

Pulse storm downbursts: 

Pulse storm downbursts As an aside 300 m

Pulse storm downbursts: 

Pulse storm downbursts As an aside

Pulse storm downbursts: 

Pulse storm downbursts This time height trend of reflectivity shows the descending core hitting the ground just after 0006 UTC. Updraft phase

Pulse storm downbursts: 

Pulse storm downbursts 2356 UTC Updraft begins to build a core aloft

Pulse storm downbursts: 

Pulse storm downbursts 0002 UTC

Pulse storm downbursts: 

Pulse storm downbursts 0008 UTC Midlevel convergence signifies downdraft commencing

Pulse storm downbursts: 

Pulse storm downbursts 0014 UTC

Pulse storm downbursts: 

Pulse storm downbursts 0019 UTC Downdraft impacts the ground

Pulse storm downbursts: 

Pulse storm downbursts 0024 UTC

Pulse storm downbursts: 

Pulse storm downbursts 0029 UTC

Visual and radar obs of a strong wind event: 

Visual and radar obs of a strong wind event Case: 12 August 2004 Cocoa Beach, FL Downburst precursor Mid altitude convergence is strong: A downburst precursor

Visual and radar obs of strong wind event: 

Visual and radar obs of strong wind event 5 minutes later…

Pulse storm downbursts: 

Pulse storm downbursts This event shows that VIL, height of Max reflectivity and storm top did not give lead time to downburst. Tracking the descent of the core gave a better lead time Monitoring updraft growth might give even better lead time The stronger the elevated core, the stronger the initial updraft

Supercell severe wind threat: 

Supercell severe wind threat The most severe winds are in the Rear Flank Downdraft (RFD) surrounding the mesocyclone. Detected by deep convergence zone. Pulse storm downburst mechanisms also occur

Supercell wind events: 

Supercell wind events Can produce a large number of the most damaging wind events without tornadoes Most common with HP supercells

Supercell wind events: 

Supercell wind events 0.5° 1.5° 2.4° 3.4° These high wind events often have a very deep convergence zone, extending 2 km or more. Deep convergence zone 15 kft

Organized multicell convective wind events: 

Organized multicell convective wind events Squall lines Bow echoes

Squall line heading toward radar: 

Squall line heading toward radar The worst winds are pointed directly at the radar. And the radar is close so that the low-level winds can be sampled.

Squall line not heading toward radar: 

Squall line not heading toward radar Where do you think the strongest winds in this squall line will hit in the next hour?

Squall line not heading toward radar: 

Squall line not heading toward radar

Squall line not heading toward radar: 

Squall line not heading toward radar We must use other techniques to estimate wind severity Speed of motion Estimated strength of updraft Near storm environment True wind Tangential wind component

Look for Mid Altitude Radial Convergence Zone (MARC): 

Look for Mid Altitude Radial Convergence Zone (MARC) MARC signatures found here at 4 km AGL MARC signatures are small: < 15 km long > 25 m/s convergence

Bow echoes: 

Bow echoes Narrow bow echoes are typically more severe than wide ones given everything else being equal.

Bow echoes: 

Bow echoes Example of narrow bow echoes and very severe winds

Hail potential: 

Hail potential Radar cannot directly detect hail One big hailstone sends back the same energy as 1000s of regular raindrops Either scenario could take place in a radar volume Thus we have to infer the presence of hail from other clues

Favorable hail clues: 

Favorable hail clues Environmental Dry air aloft, moist below, large instability Enough wind shear for supercells Fairly low freezing level (wet bulb) 7500-10000’ Storm structure Intense reflectivity core (>55 dBZ) above the –20 C level Strong updrafts with a WER or BWER Storm rotation (supercells) Updraft persistence

Favorable hail clues: 

Favorable hail clues Intense elevated core Know how high your elevation slices are to your 0° and –20° C heights at the storm location. Look for high reflectivity (>55 dBZ) LRM products at 24 – 33 kft and especially the 33-60 kft level. -20 C 0 C

Favorable hail clues: 

Favorable hail clues Bounded Weak Echo Region (BWER) Intense updraft forms a hole in the reflectivity core. Typical BWER heights 0.5° 1.5° 2.4° 3.4° BWERs not typically seen this far out BWER

Favorable hail clues: 

Favorable hail clues Weak Echo Region (WER) Intense updraft also levitates a large region of core. Look for high over low reflectivities on the inflow side of a storm WER typically from sfc to 15-20 kft. 0.5° 1.5° 2.4° 3.4° WER B A A B Watch out for anvil WERs. They are not updrafts.

Vertically Integrated Liquid: 

Vertically Integrated Liquid Integrates what the radar thinks is liquid water in the vertical Not a reliable hail indicator, no set thresholds Does show location of the ‘biggest storm’

VIL: 

VIL Hail is loosely associated with VIL but the threshold changes with season and location

VIL density: 

VIL density VIL is normalized by echo top height in meters and then multiplied by 1000 to yield a density of g/m3 Attempts to reduce effects of different environments on a consistent large hail threshold -20 C 0 C VIL = 47.5 kg/m2 ET = 9.1 km VIL = 70 kg/m2 ET = 13.4 km

VIL density: 

VIL density Warning performance statistics show a VIL density ~ 3.28 g/m3 performs well as a large hail threshold in multiple CWAs. However… Cerniglia and Snyder, 2002 – ER Tech memo

VIL density: 

VIL density VIL density does not perform well in estimating severe hail size Edwards and Thompson, 1998

Other ordinary cell hail considerations: 

Other ordinary cell hail considerations Reflectivities > 60 dBZ indicate a high likelihood of hail Cannot discriminate hail size The Hail Detection Algorithm tends to overestimate the Probability of Severe Hail (POSH) in weakly sheared storms over low terrain Hail potential increases as the freezing level approaches the ground or vice versa (i.e. topography)

Tornado potential: 

Tornado potential A tornado vortex signature Strong gate-to-gate shear Prefer to see this for at least two slices The bottom should be on the lowest slice or within 600 m AGL I prefer to see this persist for a couple scans But some situations will not allow me to wait. Due to beam spreading, my maximum TVS range is about 60 nm. After that, I’m only seeing mesocyclones.

Tornado vortex signature: 

Tornado vortex signature Shear = |outbound + inbound| in adjacent gates Anywhere from 35 to more than 140 kts depending on range and severity

Occurrence of tornado with LLDV: 

Occurrence of tornado with LLDV TVS low-level gate-to-gate velocity difference, LLDV (m/s) FAR = green line POD = red line HSS = black line Inset = POD vs FAR LLDV m/s

Occurrence of tornado with MDV: 

Occurrence of tornado with MDV TVS Maximum gate-to-gate velocity difference, MDV (m/s) FAR = green line POD = red line HSS = black line Inset = POD vs FAR MDV m/s

A descending TVS: 

A descending TVS 50% of are associated with supercells (from Trapp et al., 1999) Offers greater lead time Trapp et al., 1999

Nondescending Tornado Signatures: 

Nondescending Tornado Signatures 80% of squall lines 50% of supercells (from Trapp et al., 1999)

What is the TVS seeing?: 

What is the TVS seeing?

What is the TVS seeing?: 

What is the TVS seeing? 1. Flanking line 1. 1.

What is the TVS seeing?: 

What is the TVS seeing? 1. Flanking line 1. 1. 2. Dry slot and hook 2. 2.

What is the TVS seeing?: 

What is the TVS seeing? 1. Flanking line 1. 1. 2. Dry slot and hook 2. 2. TVS TVS TVS Tornado

Example of a TVS in polar vs. cartesian coordinates: 

Example of a TVS in polar vs. cartesian coordinates NIDS velocity 1 km boxes Full resolution SRM 0.2 km gates June 13, 1998 OKC

Squall line vortices: 29 June 1998: 

Squall line vortices: 29 June 1998 Adapted from Pryzbylinski (2002)

Time height comparisons: 

Time-height Vr trace Core #2. Time height comparisons Core #2 was more intense Vr=30 m/s (60 kts) Nondescending Vr with time indicates low-level vorticity becoming stretched upward

Mesocyclone strength: isolated vs linear convection : 

Comparison of circulation characteristics between Przyblynski et al. 2001 data set and Burgess et al. (1982) data set. Larger mesocyclone diameters in linear systems than with isolated cell mesocyclones Weaker Vr with linear systems Mesocyclone strength: isolated vs linear convection L = surface to 8200 ft.

Nonmesocyclonic tornadoes: 

Nonmesocyclonic tornadoes Considerations Favored with steep 0-3 km lapse rates 0-3 km CAPE Boundary with strong vertical vorticity Slow moving boundary Difficult to see on radar

Tornadoes in weak shear environments: 

Tornadoes in weak shear environments Start with strong boundary with developing CU Boundary shear starts to roll into misocyclones A B C

Tornadoes in weak shear environments: 

Tornadoes in weak shear environments A B C CU updrafts grow Misocyclones A and B grow and move to the right while C weakens

Tornadoes in weak shear environments: 

Tornadoes in weak shear environments A B C TCU continue to grow. Elevated core may form Misocyclone B phases with one updraft forming a tornado Misocyclone A remains unattached, only dust devils form

Pre-existing vertical vorticity:a case: 

Pre-existing vertical vorticity: a case Storms moving with cold front Outflow boundary moving down front Rapid updraft growth on intersection 0° C -20° C

Heavy rain producing storms: 

Heavy rain producing storms Rain rate is dependent on Updraft strength X moisture content X efficiency Total rainfall is dependent on Rain rate Motion Size of storm along the motion track Let’s talk about a certain type of storm with high efficiency Warm Rain dominated storm

Low topped heavy rainfall convection09 June 2004: 

Low topped heavy rainfall convection 09 June 2004 Very humid airmass Lots of shear and low-level CAPE Not a lot of upper level CAPE

Low topped heavy rainfall convection09 June 2004: 

Low topped heavy rainfall convection 09 June 2004 Reflectivity dominated by numerous small drops at observers location

An earlier warm rain dominated supercell on 09 June 2004: 

An earlier warm rain dominated supercell on 09 June 2004

Cross Section through Warm-Rain Supercell: 

Cross Section through Warm-Rain Supercell Notice the reflectivity drop off above the freezing level.

Why spotters are still needed: 

Why spotters are still needed

Summary: 

Summary Updraft strength Stronger updrafts have any one of these features Higher reflectivity at higher altitudes relative to the equilibrium level Strong echo overhang, a BWER Very strong and deep convergence zone Not all of these must be present for a severe storm but if more exist, the more confidence you have of identifying a severe storm updraft

Summary continued: 

Summary continued Mesocyclones Must be persistent Extend through > 2 km depth Have time continuity < 10 km wide No minimum rotational velocity threshold Mature mesocyclones can be divided between updraft and downdraft

Severe hazards: 

Severe hazards Severe winds The strongest Individual downdrafts often follow strongest updraft signatures Accompanied by Mid Altitude Radial Convergence (MARC) Often follow a deep convergence zone Occur with small vortices such as mesocyclones Keep in mind the favorable environments (DCAPE, shear)

Severe hazards: 

Severe hazards Large hail Intense upper-level updraft (-10 to -30 C layer) Deep, intense reflectivities WER BWER (especially large ones) Updraft persistence (indicated by a mesocyclone)

Severe hazards: 

Severe hazards Tornadoes Mesocyclonic Strengthening rotational velocity with strong low-level updraft signatures Onset of a tornado vortex signature (TVS) Onset of a hook Squall line Front inflow notch Vortex rotational velocity increasing in intensity Deep convergence zone Nonmesocyclonic Pre-existing source of vertical vorticity Young but strong updraft Favorable environment

Severe hazards: 

Severe hazards Heavy rain Strong updraft is needed for inefficient storms Weak updraft is enough if it is efficient Large moisture Slow motion Large reflectivity core Watch out for low topped warm rain events

resources: 

resources General radar interpretation - OKFIRST http://okfirst.ocs.ou.edu/train/materials/radar.html NSSL mesocyclone and tornado case study page http://www.nssl.noaa.gov/wrd/swat/Cases/cases_pix.html NOAA radar page http://weather.noaa.gov/radar/ The Warning Decision Training Branch http://www.wdtb.noaa.gov/