Page 1 – September 4, 2015 Radar Algorithms MSC Radar Course David Patrick Hydrometeorological and...

– April 19, 2023

Radar Algorithms

MSC Radar Course

David Patrick

Hydrometeorological and Arctic Lab

Winnipeg, MB

– April 19, 2023

Acknowledgements

• Dave Ball for inspiring this course

• Operational meteorologists across Canada for giving me ideas and input

– April 19, 2023

Outline

• Basic algorithms– PRECIP, Doppler ClogZ Reflectivity, Radial Velocity,

Storm-relative Radial Velocity

• Grid-based severe thunderstorm algorithms– SvrWx, CAPPI 7.0, CoTPPI, VIL, WDraft, Hail, BWER,

Reflectivity Gradient, MesoCyclone, Microburst, Gust

• Cell-based severe thunderstorm algorithms– Cell Identification, Cell Properties, MultiRadarMerge,

Cell Tracking, Cell View, StormAssessmentClassification, SCIT

– April 19, 2023

Basic Algorithms - PRECIP

• PRECIP product is combo of

– doppler reflectivity within 125 km

– conventional reflectivity beyond 125 km

doppler clutter suppression can reduce

signal and create artificial boundary at 125

interface

– April 19, 2023

Basic Algorithms - PRECIP

• Inner doppler scan may be under-calibrated compared to outer conventional scan giving inner circle of reduced returns

7-day Rainfall Accum7-day Rainfall Accum

– April 19, 2023

Doppler & Conventional Base Reflectivity Loop

• Doppler and conventional base level reflectivities are ~5 minutes apart in 10 minute scan cycle

• Can create a 5-minute frequency loop by alternating between the two products (not operationally available)

– April 19, 2023

Basic Algorithms Doppler vs. Conventional Reflectivity

Doppler 0.5° “ClogZ” reflectivity Conventional 0.5° reflectivity

• Sometimes doppler reflectivity has holes in the data

Note doppler data has 4x resolution of conventional data

same cell with no holes in conventional

data

– April 19, 2023

Basic Algorithms – Doppler Signal Processing

• At the radar site processor, a Signal Quality Index is used to determine if a radar bin’s radial velocity is good

• SQI is related to– spectrum width, the degree of spread in the radial velocity

elements for the radar bin; high spectrum width -> low SQI– signal to noise ratio; low signal -> low SQI

• If the SQI is too low, the radial velocity is trashed, along with reflectivity!

-VN 0 +VN

0

-10

-20

-30

-60

-50-40

-70-80

dBZ

ground clutterweather

Spectrum width is measure of spread of weather

– April 19, 2023

Basic Algorithms – Doppler Signal Processing

Reflectivity

• Doppler data from 0.5° scan

• High spectrum width causes data dropout in critical area

Radial velocity Spectrum width

• 2010Z August 20, 2009; tornado on ground in missing data zone

signal trashed

– April 19, 2023

Basic algorithms – Radial Velocity

• Canadian radars are C-band radars (except S-band McGill) that use dual pulse repetition frequencies in doppler mode

• Nyquist velocity VN is max velocity that can be unambiguously determined VN = PRF ∙ λ / 4

• All V < -VN or V > VN are “folded” into –VN to +VN range• Low PRF 892 s-1 VN = 892 ∙ 0.0534 / 4 = 11.9 m/s• High PRF 1190 s-1 VN = 1190 ∙ 0.0534 / 4 = 15.9 m/s• Low and High PRF rays are alternated around scan• Processing at radar site combines/unfolds data to give

-48 m/s <= V <= +48 m/s

– April 19, 2023

The ‘dual-

PRF’

unfolding technique

The ‘dual-

PRF’

unfolding technique


• Look at relative error V1200 – V900

• Pick off actual

velocity

– April 19, 2023


• Let’s go to SErn Newfoundland

• Strong Low approaching from SW

• Original folded velocities from alternating Low/High PRF rays

• No V < -16 m/s or V > 16 m/s

– April 19, 2023


• Unfold velocities at radar site

• Note bright green speckles (V= -36

m/s) embedded in yellow-pink area (V= +30 m/s)

– Either bad unfolding or bad data leading to bad unfolded velocities

– April 19, 2023


• Corresponding analyses

1200Z Oct 17 2009

• Surface

• 850 mb

• 700 mb

– April 19, 2023


VAD winds at 0900Z Oct 17 2009

Winds are consistently from E to SE direction

in lower 5,000 feet

– April 19, 2023


• Apply “despike” algorithm that assumes data is accurate and tries to find best unfolded velocity compared to immediate neighbours

• This is what you see on the fcst desk

Data is smoothed but area of towards-radar velocities is increased

With despikeWithout despike

– April 19, 2023


• Try unfolding velocities at forecast office using velocity info from a wider neighbourhood (under development)

– April 19, 2023

Basic Algorithms – Extra Info Long Range Radial Velocity

Note!• When looking at LongRange Velocity product, velocity folding occurs at just 16 m/s, not the usual 48 m/s

• Look for light blue and orange adjacent to each other

– April 19, 2023

Basic Algorithms – Storm Relative Velocity

couplet embedded in away velocities

couplet showing away and towards motion relative to storm

• Subtract off storm motion vector from all radial velocity bins to view motion in storm-based frame of reference

Radar-based radial velocityStorm-based radial velocity

Subtract 250/40Subtract 250/40km/hkm/h from all bins from all bins

– April 19, 2023

Grid-based Svr Wx AlgorithmsSVRWX

• Based on work done by US Air Force & NSSL in 1960s and updated in Canada in 1980s

• Height of 40 dBZ echo is correlated to severe weather occurrence (can be 45 dBZ in parts of Canada)

– Not strongly correlated to tornadoes in high shear environments

• Evidence of strong deep updraft with significant moisture/hail

– April 19, 2023

• SvrWx values:– 40 dBZ @ 5.5 km => level 1 blue– 40 dBZ @ 8.5 km => level 2 green– 40 dBZ @ 10.5 km => level 3 yellow– 40 dBZ @ 12.0 km => level 4 red


Increasing probability of severe weather

– April 19, 2023


• CAPPI 1.5 km shows large

swaths of strong echoes

• SvrWx highlights

sig convection

– April 19, 2023

Grid-based Svr Wx AlgorithmsCAPPI 7.0

• CAPPI 7.0 km has traditionally been used to highlight sig cvctn

• But its use depends on airmass temperature

• Poor in cool airmasses

– April 19, 2023

Grid-based Svr Wx AlgorithmsCoTPPI -20°C

• Try using Constant Temperature PPI sfc instead of Constant Altitude PPI

• -20°C sfc slides up and down with airmass

• Use model data for temps

not operationally available

– April 19, 2023

• Take Z for a radar bin volume, convert to liquid, and integrate thru vertical column

• Shows heavy rain potential, large hail, ~ wet microburst potential• Note: VIL Density (VIL divided by depth of echoes) will partially account for a distant cell that is substantially below the lowest scan angle

Grid-based Svr Wx AlgorithmsVIL

– April 19, 2023

• Developed by S.R. Stewart, 1991

• WDraft = 3.6 * SQRT (20.628571 x VIL - 3.125 ET**2) km/h

• Algorithm looks only at instantaneous VIL and EchoTop, not rate of change of these parameters

Grid-based Svr Wx AlgorithmsWDRAFT

– April 19, 2023

• Big VIL packed into a relatively low echo top gives high WDraft

Grid-based Svr Wx AlgorithmsWDRAFT

– April 19, 2023

Grid-based Svr Wx AlgorithmsHail

• Uses an algorithm developed in Southeastern Australia

• Height of 50 dBZ and freezing level are empirically correlated to hail diameter

• VIL and freezing level are also empirically correlated to hail diameter

• Given hgt 50 dBZ, VIL and freezing level, calculate hail diameter using both methods and choose the larger of the two.

– April 19, 2023


Observed Hail Size

vs. Height

50 dBZ & Freezing

Level

– April 19, 2023


Observed Hail Size

vs. VIL &

Freezing Level

– April 19, 2023

• Tends to over-forecast hail diameter

• But in this case, 7.1 cm was good


– April 19, 2023


Dauphin hailstorm Aug 9, 2007

– April 19, 2023

• Use a reflectivity-to-hail energy relation and integrate this energy with height above the freezing level

• Using a large dataset, this integrated energy is statistically related to

– MESH (Maximum Expected Size of Hail)– POSH (Probability of Severe Hail)

Grid-based Svr Wx Algorithms U.S. NWS MESH & POSH Algorithms

– April 19, 2023

• Hail kinetic energy E

– where W(dBZ) is a weight based on reflectivity; dBZL=40, dBZU=50

0 for dBZ <= dBZL

W(dBZ) = dBZ – dBZL for dBZL < dBZ < dBZU

dBZU – dBZL

1 for dBZ >= dBZU

)(10105 084.06 dBZWE


– April 19, 2023

• Severe Hail Index SHI

– where H0 is height of fzlvl, HT is height of storm top, WT(H) is a weight based on temperature, and Hm20 (hgt -20C) All heights AboveRadarLevel

0 for H <= H0

WT(H) = H – H0 for H0 < H < Hm20

Hm20 – H0

1 for H >= Hm20

dHEHWSHITH

H

T 0

)(1.0


– April 19, 2023

– where WT is warning threshold

if WT < 20, set to 20

SHIMESH 54.2

50ln29 WT

SHIPOSH

1215.57 0 HWT


– April 19, 2023

• URP Hail has +ve bias


• U.S. NWS MESH no bias but underfcsts large hail; overfcsts small hail

• U.S. NWS POSH if >70% svr hail likely

– April 19, 2023

• Find upside down cup patterns in the reflectivity volume scan, as evidence of strong updraft with rotation

• Go out and up from each radar bin, looking for higher values

• URP looks out in 8 directions, but up in only 5 (let’s make it 9)

Grid-based Svr Wx AlgorithmsBWER (Bounded Weak Echo Region)

– April 19, 2023

• Do a double pass over the data

• First pass– In these 13 (try 17) directions, go out a max of 10

radar bins (try 10 km), looking for dBZ values that are at least 8 dBZ greater than the centre radar bin, and >= 40 dBZ in value (try giving partial credit to

6+ dBZ gradients)– Count the number of “hits”– Weight all directions the same (try requiring vertical

direction hit)


– April 19, 2023

Example:

CAPPI 1.5 km

XSM

0030Z July 30 2005


Rocky Mountains

– April 19, 2023


BWER algorithm checks out a long way in the azimuthal direction

– April 19, 2023

• Second pass– Go thru the first pass results, traversing the same

13 (17) directions, and subtract one “hit” every time you come across a radar bin that has a lower hit count than your centre radar bin

– This gives a measure of boundedness of the centre radar bin you’re examining

– Look thru the volume at all the radar bins with a non-zero hit count, and for each column of radar bins, save the height of the highest one (try saving

volume as well)


– April 19, 2023


A final hit count of 1 or 2 is left

oops

– April 19, 2023

Usual way to plot BWER is in white on CELL View


Giant BWER

– April 19, 2023

• Calculates the reflectivity gradient for any specified areal product, e.g. CAPPI 1.5 km dBZ or CAPPI 3.0 km dBZ

• Sharp mid-level gradients on upwind side of CB are correlated to rear-flank downdraft, mesocyclone development

Grid-based Svr Wx AlgorithmsReflectivity Gradient

– April 19, 2023

3 km CAPPI dBZ 3 km dBZ Gradient

July 24 2000

2230Z

Brunkild MB

Hook echo

Grid-based Svr Wx AlgorithmsReflectivity Gradient

– April 19, 2023

• Doppler data is available for the 0.5, 1.5 and 3.5 degree elevation angles at every 0.5 km in range out to 112.5 km, and every 0.5 degrees azimuth around the radar; 225 x 720 bins

• Rays alternate between low PRF and high PRF• Choose only high PRF (VN=16 m/s) rays• For every range bin away from the radar out to 225

bins, go around 360 degrees, looking for aziumthal shear across every 2nd bin, i.e. every high PRF ray

• Note zones of strong connected bin-to-bin (gate-to-gate) azimuthal shear over the 3 scan angles and group them into “features”.

Grid-based Svr Wx AlgorithmsMesocyclone

– April 19, 2023

• If feature has too little– area,– shear less than min shear threshold,– higher shear but weak momentum, or– dBZ

▪ then trash.

• These thresholds are configurable.


– April 19, 2023

• Assigning levels– Level 1 => 1 meso detected at 1.5 or 3.5 degrees– Level 2 => 2 mesos detected at 1.5 & 3.5 degrees– Level 3 => mesos detected at all 3 levels OR meso detected

at lowest 0.5 degree level– Level 4 => a level 3 that has sufficient momentum and max

gate-to-gate shear– Level 5 => a level 4 that is at least 5 km across in the radial

direction?

• Assigning circle diameter– Seems that the circle diameter is related to the diameter of

the circulation in the azimuthal direction … 10 times? diameter


– April 19, 2023

• Strong mesocyclone NW of Kitchener/ Waterloo

• Tornado on ground at time

WSO Kitchener

Hwy 401


– April 19, 2023

• Messy High Precipitation supercell pattern, with convective arms

WSO Kitchener

Hwy 401


– April 19, 2023

• Strong mesocyclone near Brunkild, SW of Winnipeg

• Wall cloud observed at time

Winnipeg

XWL


– April 19, 2023

• Classic supercell

Winnipeg

XWL


– April 19, 2023

• Remember doppler data has 225 range bins, 0.5 km resolution, and 720 azimuth bins, 0.5 degree resolution

• Rays alternate between low PRF and high PRF

• Choose only high PRF (VN=16 m/s) rays

• For the lowest scan angle, for every 2nd azimuth ray, i.e. every high PRF ray … go out along all the radial to just 113 bins, i.e. 57 km, looking for divergent radial shear.

• Note zones of radial divergent shear and group them into “features”.

Grid-based Svr Wx AlgorithmsMicroburst

– April 19, 2023

• If feature has too little– area,– shear less than min shear threshold,– higher shear but weak momentum, or– dBZ

▪ then trash.• These thresholds are configurable.• Assign Level 1 to all features.


– April 19, 2023

• Microburst algorithm seems to be too “sensitive”, especially near radar

• Maybe we can “fine-tune” it

WSO Kitchener

Hwy 401

WSO


– April 19, 2023

• Microbursts, maybe yes, maybe no

Winnipeg

XWL


– April 19, 2023

• Yes, doppler data has 225 range bins, 0.5 km resolution, and 720 azimuth bins, 0.5 degree resolution

• For the lowest scan angle, for every 2nd azimuth ray, i.e. every high PRF ray … go out along all the radial to 226 bins, i.e. 113 km, looking for convergent radial shear.

• Note zones of radial convergent shear and group them into “features”, i.e. shear lines.

Grid-based Svr Wx AlgorithmsGust (shear line)

– April 19, 2023

• If feature has too little– area,– shear less than min convergent shear threshold,– higher shear but weak momentum, or– dBZ

▪ then trash.• These thresholds are configurable.• Assign Level 1 to all features.


– April 19, 2023

• Lines of strong convergent shear detected approaching radar

WHK

multiple gust fronts painted dark yellow


– April 19, 2023

• Definition of cell is very simple – Area of echoes is called a cell if the 45 dBZ MaxR

above 2 km covers at least 2 radar bins in radial direction and 2 radar bins in the azimuthal direction, i.e. >= 4 bins

– Ground clutter MaxR returns can’t be allowed to contaminate cell identification

– Mask them out by making a blue sky MaxR image and then blacking out all future MaxR values from these areas

– 45 dBZ level and number of contiguous azimuthal radar bins is configurable, but not number of radial bins

Cell-based Svr Wx AlgorithmsCell ID

– April 19, 2023

A plot of URP cells on the 7 km CAPPI product

URP Cell ID tends to get overly excited close in to the radar

Cell-based Svr Wx AlgorithmsCell ID

WSO radar

– April 19, 2023

• Instead of keying on MaxR, try using volume of strong echoes and measures of AP likelihood to limit “fake” echoes

• Identify thunderstorm cells by adjusting the following thresholds

– Core dBZ threshold, eg. 45– Minimum core volume, eg. 50 km**3– Temperature threshold, eg. -20C– Outer dBZ threshold, eg. 30– Minimum outer volume of outer dBZ colder than

Temperature threshold, 5 km**3

• Also assess likelihood that “cell” is AP by looking at– Texture, spin, model MUCAPE, model AP risk

Cell-based Svr Wx AlgorithmsCell ID possible improvements

– April 19, 2023

Current Cell ID

37 cells

Many insignificant cells near radar


– April 19, 2023

Proposed New Cell

ID

14 cells


– April 19, 2023

• Calculates and stores a number of properties for the cell which are available to any program for use

• Construct best-fit ellipse for MaxR footprint of the cell• Within and in immediate vicinity of the ellipse,

calculate …– Average & max values and location of CAPPI 1.5,

EchoTop, MaxZ, 45dBZEchoTop, Hail, VIL, VILD, WDraft, LoLvlZGrad, MidLvlZGrad, BWER, Meso, Micro, (try Lightning)

– Cell location, velocity – (try storing other properties: attenuation, AP likelihood,

model FZLVL, model mean wind velocity)

Cell-based Svr Wx AlgorithmsCell Properties

– April 19, 2023

Cell-based Svr Wx AlgorithmsMultiRadarMerge

• Decide which radar will track and assess a given thunderstorm cell

• With URP, the radar that sees the greatest MaxR for the cell is the winner

– Poor choice if winner radar is a “hot” radar that only sees the top of the cell

• Try instead, go with the radar that sees the greatest depth of the cell, i.e. the one that sees the lowest down in the cell

– April 19, 2023

• Uses ThunderstormIdentificationTrackingAnalysisNowcasting method by Dixon,Wiener 1993

• From current cell velocity, generate a forecast of cell position at T+10 minutes

• Use “Hungarian” algorithm to pair up observed cells with forecast cells.

• Assign a weight to the strength of each potential match of forecast cell to observed cell.

– The greater the weight, the better the match

• Then the Hungarian algorithm will decide which combination of pairs will give highest total weight

• Hungarian algorithm will always pair up the max possible matches– Given 10 fcst and 12 obs, there will be 10 pairs and 2 new obs cells

• Current URP only breaks a match if cell speed would be > 35 m/s

Cell-based Svr Wx AlgorithmsCell Tracker

– April 19, 2023

• Calculate and use 30 minute (configurable) mean cell velocity when tracking, instead of latest 10 minute motion

– If cell < 30 min. old, use timespan of cell

• Knowledge of mean velocity provides– more consistent tracking– input into Warnings

• Generate a forecast of cell position, volume, area & other cell attributes at T+10 minutes

– For a new cell, instead of giving it no velocity, push it along with 0-8 km mean wind vector

• Assign weights for possible matches using distance, volume & area• Need rules to disallow unreasonable matches

– Cell track “matches” that result in motions that deviate “too much” from their neighbours or from the mean flow are disallowed

• Rules need fine-tuning in squall line situations where “cell” centroid moves erratically with addition and shedding of individual cells

Cell-based Svr Wx AlgorithmsCell Tracker possible improvements

– April 19, 2023

Current URP found & tracked 253 cells

between Jun 23 2200Z – Jun 24 0400Z 2007

– April 19, 2023

New prototype URP found & tracked 90 cells

between Jun 23 2200Z – Jun 24 0400Z 2007

– April 19, 2023

Cell-based Svr Wx AlgorithmsCell View (conventional scan panel)Assemble various cell-centred products in a panel

ZZ

Time series

1.5

3.0

7.0

9.0MaxRMaxR

ETET

3.0 3.0 GradGrad

1.5 1.5 GradGrad

SvrSvrWxWx

VILVIL

BWERBWER

HailHail

– April 19, 2023

Cell-based Svr Wx AlgorithmsCell View (doppler scan panel)

Assemble various cell-centred products in a panel

Time series

VV ZZ WW

0.3°

0.5°

1.5°

3.5°

– April 19, 2023

Cell-based Svr Wx AlgorithmsStorm Assessment/Classification &

SCIT

• The Storm Assessment & Classification algorithm produces

– Storm rank– Storm class, SC, SST, MST, WST

• The Storm Cell Identification Table contains a list of cell properties for each identified cell

• Any properties calculated by the Cell Properties algorithm can be displayed

• Here is the SCIT for the operational URP

– April 19, 2023

Rank weight is an even weighting of the following 7 attributes:

• BWER, Meso, Hail, WDraft, VILDensity, MaxZ, ETop45dBZ

• generally maxes out around 10

Green, yellow, red thresholds are all configurable.

Cell-based Svr Wx AlgorithmsSCIT

– April 19, 2023

Location VIL Density

WDraft (km/h)

EchoTop45 (km)

Total Ltg FlashesMax Z

Pct of CG Flashes that are +ve

Core Volume < 0C (km^3)

Core Volume (km^3)

BWER Volume (km^3)

Hail (cm)

Ave Meso Shear

Rank Weight

Primary Radar

BWER Height (km)

Max Amp (Kamp)

Flash Density (km^-2)

Flash Tendency (min^-1)

Mean Velocity (dir’n / km/h)

Cell-based Svr Wx AlgorithmsSCIT possible improvements

We could potentially add more attributes …

– April 19, 2023

Radar Algorithms

And finally we’re at

The End

Thank you

Any questions?

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