Drizzle, Shallow Events

Martin Hagen

with the help fromElena Saltikoff, Paul Joe and others

Deutsches Zentrum für Luft- und Raumfahrt (DLR)Oberpfaffenhofen, Germany

Schnee, Nieselregen

Reflectivity in dBZ 1 19 28 37 46 55

Min 14 dBZ

Min 1 dBZ

Min 1 dBZ

Min 4 dBZ

Min 14 dBZReflectivity in dBZ 1 19 28 37 46 55

Drizzle (mm/h) and very fewechoes

Germany Example

Lang, DWD

5-6°C

Drizzle ,,

Unusual widespread drizzle from cloud echoes aloft. At surface only few echoes above 1dBZ. Note: change in threshold for DWD, see more drizzle!

Hamburg

Germany Example 2

Lang, DWD

Drizzle in Finland!

Saltikoff, FMI

1. Why was drizzle observed in Finland but not Germany?

2. Why is the drizzle observed only around the radar?

3. Why is the reflectivity pattern stronger near the radar and decreases away from the radar?

4. Why is there a range limit to see drizzle?

Reflectivity Factor Drizzle

Drizzle:

• rain rate < 0.5 mm/h

• Marshall-Palmer: 20 dBZ (rain !)z = 200 R1.6

• Drizzle: low cloud tops warm rain process

small droplets (< 0.4mm)z = 30 R1.1

Z = 11 dBZ (for 0.5 mm/h)

• typical Z: -20 < Z < 10 dBZ

rain

drizzle

DrizzleDrizzle is due to warm rain process. Slow growth which results in small drops (0.1 mm, 1 mm/h)

Note: Colour scales are different!

dBZ

dBZ

ZDR

Saltikoff, FMI

Drizzle is round!

1 km

Minimum Detectable Signal

and its impact on drizzleand clear-air observations (insects)

Slide from Paul Joe’s presentation

some radar basics

The radar measures the reflected power scattered by a remote object:

Radar equation for volume scatters: i scattering cross section

of a particle

i

Vol22

20

22t

r r (2)ln1024

h g p = p

radar constant

reflectivity

D K = 6i4

5

i2

reflectivity factor z: unit: mm6 m-3

logarithmic unit: dBZ

some more radar basics

• received signal

unit: Watt (milliWatt)

• received signal logarithmic representation

Pr = C – 20 log(r) + Z log. unit: dBm

20 log(r) = 0 at 1 km range14 at 5 km range

typical C-band radar: 20 at 10 km rangeC = -70 with r in km 34 at 50 km range

Z in dBZ 40 at 100 km range Pr in dBm 44 at 150 km range

• reflectivity from received signalZ = Pr – C + 20 log(r) log. unit: dBZ

z r

c = p

2r

more radar basics

• Reflectivity of drizzle -5 dBZ (see lecture by Paul Joe)

r = 1 5 10 50 100 150 kmPr = -70-0-5 -70-14-5 -70-20-5 -70-34-5 -70-40-5 -70-44-5 dBmPr = -75 -89 -95 -109 -115 -119 dBm

• minimum detectable signal -110 dBm (-103 dBm old radar)

25 km 56 kmPr = C - 20log(r) + Z

more radar basics

• radar display shows reflectivity

r = 1 5 10 50 100 150 kmPr = -75 -89 -95 -109 -115 -119 dBm

Z = -75+70+0 -89+70+14 -95+70+20 -109+70+34 -115+70+40 -119+70+44 dBZZ= -5 -5 -5 -5 -5 -5 dBZ

Z = Pr - C + 20log(r)

25 kmMDS -103 dBm

56 kmMDS -110 dBm

even more radar basics

• stronger drizzle (clear-air echoes) reflectivity 10 dBZ

r = 1 5 10 50 100 150 kmPr = -60 -74 -80 -94 -100 -104 dBm

Z= 10 10 10 10 10 10 dBZ

• don‘t forget: earth is a sphere !

141 kmMDS -103 dBm

316 kmMDS -110 dBm

0.5° elev

1400 m

4000 m

500 m

2300 m

OvershootKey Concept!

0.5oBeam totally overshoots the weather beyond this range! No detection at all!

Shallow Weather

The weather is detected but the beam is not filled beyond this range, so reflectivities are quantitatively underestimated from this range and beyond

Note: the lower the beam the longer the range for detection ability!

Paul JoeEC Canada

Overshooting

Somesimulations

Vertical Profile of Snow Function of Range

1. Snow originates aloft but grows as it falls.

2. The same vertical profile as observed by radar at increasing range due to beam filling, beam broadening (smoothing) and Earth curvature (can’t see lowest levels)!

Shallow Snow Fall

• METAR Munich airport

• Snow grains SG -SG

Shallow Snow Fall

• Munich sounding

• sharp inversionat 900 hPa(550 m GND)

• westerly windsbelow;easterly windsabove

Meteosat Visible 1 Feb. 2011 0930 UTC

Shallow Snow Fall

• Volumescan, fixed dBZ levels

Shallow Snow Fall

• Rx comp.DSP levels(0.5 dB)

-20 – -10 dBZ

1715 UTC

Shallow Snow Fall

PARSIVEL Disdrometermeasurements

small particles ~ 1mm

slow fall speeds(morning)

higher fall speeds(afternoon/evening)

Shallow Snow Fall

Micro RainRadar

verticalpointingDopplerradar

Freezing Drizzle Case

Monday 17 Jan 2011

provided by Elena Saltikoff, FMI

24h Microphysical 09 UTC

Ice crystal clouds

water clouds

SWCs

Vantaa Metars: Visibility + weather

• EFHK 170520Z 13009KT 2400 FZDZ BR OVC002 M01/M02 =

• EFHK 170850Z 17006KT 0700 DZ FG OVC001 01/00 Q1005

• EFHK 171020Z 19007KT 0300 DZ FG VV002 01/00 Q1005=

• (southerly winds, freezing drizzle, mist and fog, overcast layer of low cloud, around 0°C)

Radar 06, 09 and 12 UTC

RHI North of Anjalankoski

RHI south of Vantaa (Hydroclass)

Jokioinen sounding

Summary drizzle, clear-air echoes

• minimum detectable signal reduces the distance where weak signals can be received

• overshooting beam reduces the distance where shallow targets (drizzle, snow, clear-air echoes) can be detected

• minimum dBZ values on display reduce the ability to see weak echoes (drizzle, clear-air echoes)