Radar Interpretation Problems Reflectivity Radar Palette Home Radar

Radar Interpretation Problems Reflectivity Radar Palette Home Radar Artifacts 1

Acknowledgments • • • Phil Chadwick’s Radar Palette Course Met. Ed courses- stole a few slides Polarimetric presentation- staff at King City Dave Patrick. . Radar and snow rates. K-stream Radar Palette Home Radar Artifacts 2

Outline Radar Interpretation Problems-Reflectivity • Attenuation – By Intervening Precipitation – Wet Radome • Radar Beam filled or not filled • Low topped precip – Drizzle • Virga • Bright Band • The sometimes invalid assumptions of the z-r relationship • Second Trip Echoes Radar Palette Home Radar Artifacts 3

Radar Sources of Error Attenuation Radar Palette Home Radar Artifacts 4

Attenuation: What lies beyond? Radar Palette Home Radar Artifacts 5

Attenuation • The All time best Canadian Example Radar Palette Home Radar Artifacts

Example of Attenuation in Precipitation Wedge of attenuation Foreground Storm Radar Palette Home Radar Artifacts

Attenuation Warm Frontal Supercell Radar Palette Home Radar Artifacts

Attenuation • • Radar Palette Home Radar Artifacts north end of derecho?

Correcting Attenuation Polarimetric Radar Palette Home Radar Artifacts 10

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Heavy Rain QPE (cont’d) Rain Accumulation (Z) Radar Palette Home Radar Artifacts 13

Heavy Rain QPE (cont’d) Rain Accumulation (ZCORR) Radar Palette Home Radar Artifacts 14

Sources of Error - Radar Dome Wetting Radar Palette Home Radar Artifacts 15

Bright Band • Melting snowflakes are large bright radar targets • Reflectivity from melting snow is larger than that of the rain below or the snow above as falling snow passes through the melting layer • Huge impact on quantitative precipitation estimates Radar Palette Home Radar Artifacts 16

Bright band shows up as ring (or partial ring) of high Z centred on the radar Bright band Radar Palette Home Radar Artifacts

Melting of Snow Flakes starts at 2. 5 km height Bright Band • Freezing Level ~2. 5 km AGL • Either associated with Rain or Freezing Rain Radar Palette Home Radar Artifacts

Melting layer signature: - Z Radar Palette Home Radar Artifacts 19

Melting layer signature: - ρHV Bright Band Ring Radar Palette Home Radar Artifacts 20

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Vertical Bright Band over WKR January 21 st, 2006 0°C snow Radar Palette Home rain Radar Artifacts 22

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Radar Beam Overshooting What real weather the radar can miss Radar Palette Home Radar Artifacts 24

PPI and CAPPI 4. 0 km CAPPI 0. 3 Degree PPI 1. 5 km CAPPI 0. 3 Degree PPI Radar Palette Home Radar Artifacts 25

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Approximate current lowest beam location BALD North East South West North Approximate proposed lowest beam location LOALA North Radar Palette Home East South West Radar Artifacts North

Radar Beam Undershooting Weather that’s not hitting the ground Radar Palette Home Radar Artifacts 29

Sources of Error - Virga Typical of strong overrunning WCB with equally strong and dry CCB Radar Palette Home Radar Artifacts 30

Cross section to the southwest Radar Palette Home Radar Artifacts 31

Low-elevation angle Doppler Z PPI scan Let’s look at the same case observed by the Doppler scans Low level reflectivities to the west of the radar – rain? Radar Palette Home Radar Artifacts 32

3. 5 Degree Doppler Z PPI scan Echoes at higher altitude precede precipitation at low Levels – virga? Radar Palette Home Radar Artifacts 33

What the radar misses close in Radar Palette Home Radar Artifacts 34

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Algorithms that fail very close to Radar…Hail • As you approach the cone of silence the top part of the storm is above the highest scan angle of the radar. • Height of MAXR and VIL values are unrealistically low. • URP hail algorithm fails. Radar Palette Home Radar Artifacts 36

June 16 th 2008 - 1900 Z Radar Palette Home Radar Artifacts 37

June 16 th 2008 - 1900 Z Radar Palette Home Radar Artifacts 38

June 16 th 2008 - 1900 Z 150. 25/3. 161/5. 75 25 1 1 2 Radar Palette Home Z 51. 16 55. 72 Zcorr 49. 52 52. 47 Zdr 2. 52 2. 67 Zdrcorr 2. 17 0. 53 Rhohv 0. 98 0. 76 Radar Artifacts 2 39

Sources of Error - ECHO TOP - Beamwidth and Beamfilling Radar Palette Home Radar Artifacts 40

Echo Tops Radar Palette Home Radar Artifacts 41

Partial Beam Filling Radar Palette Home Radar Artifacts 42

Radar beam widths and sensitivity • Not all radars are equal • In Canada • Many are retrofitted Enterprise radars with 1. 1° beam width and 4 m dishes • Some (10) are Andrews radars with 0. 65° beam width and 6 m dishes • Many have obvious issues – differences in sensitivity between each other • But many have less obvious, but significant problems that only become apparent with longterm precipitation accumulations Radar Palette Home Radar Artifacts 43

Beam Spreading At 240 km 3 km diameter Radar beam At 30 km 0. 5 km diameter Radar beam Radar Palette Home Radar Artifacts 44

Partial Beam Filling Radar Palette Home Radar Artifacts 45

One storm 2 radars with different beamwidths Radar Palette Home Radar Artifacts 46

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Second Trip Echoes to a distant target. . . first pulse second pulse Radar Palette Home returning Radar Artifacts first pulse

Second Trip Echo Pulse One Pulse Two mapped position of 2 nd trip echo actual position of echo beyond range Echo within range Radar Palette Home Radar Artifacts

El on ga ted Ra d ial Second Trip Echo Weaker Far Away Randon Doppler Changing the PRF changes Location Radar Palette Home Radar Artifacts Check the Satellite

Purple Haze Radar Palette Home Radar Artifacts 53

Reflectivity…Rain Rates and Snow Rates Snow being depicted using the Rain Z-R Relationship Radar Palette Home Snow depicted using the Snow Z-S Relationship Radar Artifacts 54

Snow Reflectivity & Snowfall Rate • Value of a depends on – whether the snow is dry or rimed – snowflake size distribution y-intercept constant • Large N 0 => large number of small flakes compared to large – radar dark matter ? ? • Small N 0 => small number of small flakes compared to large – snowflake terminal velocity • dense flakes fall twice as fast Radar Palette Home Radar Artifacts 55

Snow Reflectivity & Snowfall Rate • For a typical range of values of N 0, and dry snow falling at 100 cm/s and wet snow falling at 200 cm/s – Here is a graph of Z vs. S in mm/h Dry airy snow falling slowly; 0. 75 mm/h Rimed dense snow falling rapidly; 3 mm/h Factor of 4 difference in snow rate for same Z !! Radar Palette Home Radar Artifacts 56

Snow Reflectivity & Snowfall Rate • Plot these empirical Z-S relationships on theoretical graph … Spatial dendrites Z = 3300 S 1. 7 Standard Z = 1780 S 2. 21 Plates & columns Z = 400 S 1. 6 Standard eq’n underestimates almost all high snow rate situations Radar Palette Home Radar Artifacts 57

Snow Reflectivity & Snowfall Rate Radar Palette Home Radar Artifacts 58

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Another hypothesis ZDR vs Z scatter plots and observed precipitation (snow) • Red squares – Radar estimate close • Green – Radar estimate slightly underdone • Purple triangles – Radar low by factor of 2 -3. Radar Palette Home Radar Artifacts 60

Z and Rainrate Radar Palette Home Radar Artifacts 61

Empirical Z-R Equations • Reflectivity Z is empirically related to rain rate R in the form : Z = a. R b • In Canada we use Marshall-Palmer (stratiform rain) : 1. 6 Z = 200 R • U. S. NEXRAD : 1. 4 Z = 300 R • U. S. Tropical : 1. 2 Z = 250 R Radar Palette Home Radar Artifacts 62

• Z proportional to drop diameter to 6 th power. • If more big drops than standard DSD radar overestimates rainfall – Attenuation often cancels this out. • If more small drops Z underestimates rainfall. Radar Palette Home Radar Artifacts 63

Observed precipitation Radar Palette Home Radar Artifacts 64

Radar Accumulations Radar Palette Home Radar Artifacts 65

Conclusions • Radar returns are not always what they seem. • Before making conclusions think about – -How high the beam is above the ground – How wide is it where you are looking. – What other echoes did it have to go through to get to the storm you are interested in. – What are the chances the drop size (snowflake) distribution is different from the “standard” – Does the beam go through a melting layer – Could there be second trip echoes. • Despite all the above radar does quite a remarkable job of detecting weather. Radar Palette Home Radar Artifacts 66

THE END Radar Palette Home Radar Artifacts 67