drizzle, shallow events martin hagen with the help from elena saltikoff, paul joe and others...
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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)