Drizzle, Shallow Events Martin Hagen with the help from Elena Saltikoff, Paul Joe and others Deutsches Zentrum für Luft- und Raumfahrt (DLR) Oberpfaffenhofen, Germany

Schnee, Nieselregen Reflectivity in dBZ

Min 14 dBZ Min 1 dBZ Min 4 dBZ Min 14 dBZ Reflectivity in dBZ

Drizzle (mm/h) and very few echoes 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 R 1.6 Drizzle: low cloud tops  warm rain process small droplets (< 0.4mm) z = 30 R 1.1 Z = 11 dBZ (for 0.5 mm/h) typical Z: -20 < Z < 10 dBZ rain drizzle

Drizzle Drizzle 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 ZDR Saltikoff, FMI Drizzle is round! 1 km

Minimum Detectable Signal and its impact on drizzle and 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 radar constant reflectivity reflectivity factor z: unit: mm 6 m -3 logarithmic unit: dBZ

some more radar basics received signal unit: Watt (milliWatt) received signal logarithmic representation P r = C – 20 log(r) + Zlog. unit: dBm 20 log(r) = 0 at 1 km range 14 at 5 km range typical C-band radar:20 at 10 km range C = -70 with r in km 34 at 50 km range Z in dBZ40 at 100 km range P r in dBm44 at 150 km range reflectivity from received signal Z = P r – C + 20 log(r) log. unit: dBZ

more radar basics Reflectivity of drizzle -5 dBZ (see lecture by Paul Joe) r = km P r = dBm P r = dBm minimum detectable signal -110 dBm (-103 dBm old radar) 25 km56 km P r = C - 20log(r) + Z

more radar basics radar display shows reflectivity r = km P r = dBm Z = dBZ Z= dBZ Z = P r - C + 20log(r) 25 km MDS -103 dBm 56 km MDS -110 dBm

even more radar basics stronger drizzle (clear-air echoes) reflectivity 10 dBZ r = km P r = dBm Z= dBZ don‘t forget: earth is a sphere ! 141 km MDS -103 dBm 316 km MDS -110 dBm 0.5° elev 1400 m 4000 m 500 m 2300 m

Overshoot Key Concept! 0.5 o Beam 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 Joe EC Canada

Overshooting Some simulations

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 inversion at 900 hPa (550 m GND) westerly winds below; easterly winds above

Meteosat Visible 1 Feb UTC

Shallow Snow Fall Volume scan, fixed dBZ levels

Shallow Snow Fall Rx comp. DSP levels (0.5 dB) -20 – -10 dBZ 1715 UTC

Shallow Snow Fall PARSIVEL Disdrometer measurements small particles ~ 1mm slow fall speeds (morning) higher fall speeds (afternoon/evening)

Shallow Snow Fall Micro Rain Radar vertical pointing Doppler radar

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 Z 13009KT 2400 FZDZ BR OVC002 M01/M02 = EFHK Z 17006KT 0700 DZ FG OVC001 01/00 Q1005 EFHK Z 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)