Federal Department of Home Affairs FDHA Federal Office of Meteorology and Climatology MeteoSwiss Mesoscale Atmospheric Systems Radar meteorology 2 Urs Germann MeteoSwiss, Locarno-Monti With material from H Wernli, I Zawadzki, and the radar colleagues at MeteoSwiss
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Federal Department of Home Affairs FDHAFederal Office of Meteorology and Climatology MeteoSwiss
With material from H Wernli, I Zawadzki, and the radar colleagues at MeteoSwiss
2Urs Germann, MeteoSwiss
A band of bandsFrequency band meteorological application
20-300 MHz 1-15 m VHF wind profiler, ocean surface motion
400-900 MHz 0.3-0.7 m UHF wind profiler
1 GHz 0.3 m L-band boundary layer wind profile
2-4 GHz 7-15 cm S-band long-range precipitation radars
4-8 GHz 4-7 cm C-band long-range precipitation radars8-16 GHz 2-4 cm X-band precipitation radars, marine radars16-20 GHz 1-2 cm Ku-band radars
35 GHz 8.5 mm Ka-band precipitation and cloud radars
90-100 GHz 3 mm W-band cloud radars, Mie minimum
3Urs Germann, MeteoSwiss
Part 1: Radarnetworking
4Urs Germann, MeteoSwiss
Optimal wavelength for radar in the AlpsArguments for C-band compared to S-band• Better weather-to-clutter ratio ( -4 in Rayleigh approximation)• Narrower beam and lower side lobes (for given antenna size)
• Better chances to detect weak echoes in snow over the Alps• Installation and operations affordable on mountain peaks• Receiver can be mounted on antenna (better sensitivity, no dual rotary joint)
Arguments for S-band• Attenuation negligible• Larger Nyquist interval
Other networks• Radar networks in many other regions around the world:
Canada, Japan, Australia, …
• S-Korea: Exceptionally dense network thanks to thesuperposition of networks from several independentoperators. Challenge due to orography similar to Switzerland.
• CINRAD: New generation radar network in China with totally216 radars. 144 operational by end of 2012.
9Urs Germann, MeteoSwiss
Part 2: Meteorologicalprocessing
10Urs Germann, MeteoSwiss
PPI, RHI, HTI, CAPPI, HARPI, …PPI (Plan Position Indicator): Measurements froma 360° scan withconstant elevationangle, projected on the ground.
RHI (Range Height Indicator):Measurements froma scan with constantazimuth
HTI (Height Time Indicator):Measurements overtime period withantenna pointing in the vertical direction
easting
north
ing azimuth
distance on the ground
heig
ht
time
heig
ht
CAPPI (Constant AltitudePlan Position Indicator):
Horizontal cross section at a selected height level through a
volume of PPI scans. The x and y axes of the plot correspond to
easting and northing.
HARPI (Height AzimuthRange Position Indicator):
Vertical cross section at constantrange through a volume of PPI scans. The x and y axes of theplot correspond to azimuth and
height.
11Urs Germann, MeteoSwiss
PPI scan
First measurements of new Plaine Morte radarduring installation, November 2014
Doppler velocity Reflectivity Zh
12Urs Germann, MeteoSwiss
Quantitative precipitation estimation QPE
reflectivity
estimate ofprecipitation@ ground
13Urs Germann, MeteoSwiss
D6, D3, D4
Radar reflectivity Z
log
Volumetric liquid water content W
Rainfall rate R
N(D) is the drop size distributionvt(D) is the terminal fall velocity of drops
14Urs Germann, MeteoSwiss
Drop size distribution N(D)
How can we estimate R from Z if N(D) varies so much? Fortunately, we know a lot about N(D) and we can obtain reasonable estimatesof R from Z using approximations, so-called Z-R relations.
And there is dual-polarization, that helps a lot …
From microwave disdrometer
Figures: Zawadzki (McGill), GW Lee
15Urs Germann, MeteoSwiss
Deq = 2.6 mm 3.4 mm 5.8 mm
Deq = 7.4 mm 8.0 mm
Raindrops falling with their terminal velocity are oblate.
Drops can be described as rotationalellipsoids with the axis a and b
Observations in a vertical wind tunnel
(small drops are spherical, large ones are oblate)
(Pruppacher & Klett)
Shape of falling drops
baa
b
16Urs Germann, MeteoSwiss
Dual-polarization radarDual polarization radarstransmit and receive horizontally and vertically polarized electromagnetic waves.
Rain Graupel Hail
Most dual-pol radars transmit H and V simultaneously, and process H and V of the return signal in parallel in 2 separate receiver channels.
Some dual-pol radars first transmit H and receive H and V, thentransmit V and again receive H and V, but …
Dual-polarisation radarPOLDIRAD, DLR (since 1986)P Meischner, M Hagen, et al.
17Urs Germann, MeteoSwiss
Why dual-polarization radars?
• Distinguish between meteorological (hydrometeors) and non-meteorological targets (ground clutter, birds, insects, aircrafts).
• Classification of hydrometeors (large drops, small drops, ice crystals, snowflakes, melting snow, graupel, hail).
• Better data quality (redundant receive channels, hardware monitoring, …)
• Correct for attenuation of radar signal in heavy rain.• Better estimation of rainfall rates, especially in presence of large
drops.
18Urs Germann, MeteoSwiss
Dual-polarization measurables
ZH, ZV Reflectivity particle size + number (precipitation rate)
• melting hail and graupel can havehigh ZDR and high KDP
• ice dendrites can have high ZDR• (…)
21Urs Germann, MeteoSwiss
Dual-polarization hydrometeor retrieval
Hail cell in Rubigen (BE)as seen by dual-polarizationradar on Albis in operational mode (!) at 90 km distance.
Besic, Figueras, Grazioli, Gabella, Germann and Berne (2016)
22Urs Germann, MeteoSwiss
Real-time radar-raingauge mergingRadar-only
Space-time co-kriging with external driftwith data transformation and sophisticated quality control
Sideris, Gabella, Erdin, Germann (QJRMS, 2013)
Radar – raingauge integration
For demonstration purposes we have selectedan example where the radar has severlyunderestimated rainfall at ground. In most cases, errors are smaller.
23Urs Germann, MeteoSwiss
Part 3: Meteorological studies
24Urs Germann, MeteoSwiss
Stratiform precipitation and bright band
• vertical wind < ~ 1 m/s• horizontal scale ~ 100 km• stable stratification• external lifting (e.g. through fronts or orography)• in polar regions and in midlatitudes
24
25Urs Germann, MeteoSwiss
Stratiform precipitation
Figure from Zawadzki et al (McGill)
26Urs Germann, MeteoSwiss
Bright band
Within melting layer: Particles have size of snowflake, but dielectric constant ofwater. On top, some stick together and form large aggregates.
► Layer of enhanced reflectivity («bright-band»).
Below melting layer: Increase in dielectric constant and decrease in number densitydue to larger fall speed roughly have same effect but withopposite sign. (In reality more complex: break-up and more.)
► Before and after melting reflectivity is similar.