Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.1 A short course on radar meteorology Tobias Otto

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.2 Radar Meteorology Reference: AMS Glossary Radar: Radio Detection And Ranging An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy. Meteorology: The study of the physics, chemistry, and dynamics of the Earth's atmosphere. Radar Meteorology: The study of the atmosphere and weather using radar as the means of observation and measurement.

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.3 Microwaves HF3- 30 MHz VHF MHz UHF MHz L1-2 GHz S2-4GHz C4-8GHz X8-12GHz Ku12-18GHz K18-27GHz Ka27-40GHz V40-75GHz W75-110GHz mm GHz KNMI Windprofiler IDRA Cloud Radar WLAN mobile phones Reference: Radar-frequency band nomenclature (IEEE Std ) KNMI Weather Radar PARSAX

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.4 T. Otto electromagnetic waves frequency≈ 2.4 GHz wavelength≈ 12 cm scattering absorption Microwaves and Water

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.5 PARSAX on top of EWI faculty T. Otto received power time radar Water and Radar

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.6 Information in microwaves 1.Amplitude can be related to the strength of precipitation / rain rate Electromagnetic waves are transversal waves, i.e. their direction of oscillation is orthogonal to their direction of propagation. A A x y z

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.7 Information in microwaves Electromagnetic waves are transversal waves, i.e. their direction of oscillation is orthogonal to their direction of propagation. x y z 1.Amplitude can be related to the strength of precipitation / rain rate 2. Frequency Doppler shift, relative radial velocity of the precipitation

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.8 Information in microwaves Electromagnetic waves are transversal waves, i.e. their direction of oscillation is orthogonal to their direction of propagation. x y z 1.Amplitude can be related to the strength of precipitation / rain rate 2. Frequency Doppler shift, relative radial velocity of the precipitation 3.Phase can be used to measure refractive index variations y

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.9 Information in microwaves Electromagnetic waves are transversal waves, i.e. their direction of oscillation is orthogonal to their direction of propagation. x y z 1.Amplitude can be related to the strength of precipitation / rain rate 2. Frequency Doppler shift, relative radial velocity of the precipitation 3.Phase can be used to measure refractive index variations 4. Polarisation hydrometeor / rain drop shape

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.10 Radar Meteorology Reference: AMS Glossary Radar: Radio Detection And Ranging An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy. Meteorology: The study of the physics, chemistry, and dynamics of the Earth's atmosphere. Radar Meteorology: The study of the atmosphere and weather using radar as the means of observation and measurement.

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.11 Radar Equation for a Point Target PtPt PrPr GtGt GrGr transmitter receiver antennae range R target σ P t.. transmitted power (W) G t.. antenna gain on transmit R.. range (m) σ.. radar cross section (m 2 ) G r.. antenna gain on receive P r.. received power (W) backscattered power density at receiving antenna isotropic antenna power density incident on the target effective area of the receiving antenna

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.12 Radar Equation for a Point Target PtPt PrPr GtGt GrGr transmitter receiver antennae range R target σ P t.. transmitted power (W) G t.. antenna gain on transmit R.. range (m) σ.. radar cross section (m 2 ) G r.. antenna gain on receive P r.. received power (W) radar constant target characteristics free-space propagation

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.13 From Point to Volume Targets -the radar equation for a point target needs to be customised and expanded to fit the needs of each radar application (e.g. moving target indication, synthetic aperture radar, and also meterological radar) -radar has a limited spatial resolution, it does not observe single targets (i.e. raindrops, ice crystals etc.), instead it always measures a volume filled| with a lot of targets  volume (distributed) target instead of a point target -to account for this, the radar cross section is replaced with the sum of the radar cross sections of all scatterers in the resolution volume V (range-bin):

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.14 Radar Resolution Volume range R radar resolution volume (range-bin) c..speed of light B..bandwidth of the transmitted signal (the bandwith of a rectangular pulse is the inverse its duration B=1/ τ )  Δr is typically between 3m - 300m, and the antenna beam-width is between 0.5° - 2° for weather radars θ antenna beam-width

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.15 Range Resolution of a Pulse Radar

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.16 Range Resolution of a Pulse Radar e.g. pulse duration 1 µs 300 m f0f0

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.17 Range Resolution of a Pulse Radar target 1 response target 2 response target 3 response Now we sample the backscattered signal. each sample consists of the sum of the backscattered signals of a volume with the length c· τ /2 for a pulse radar, the optimum sampling rate of the backscattered signal is 2/ τ (Hz)

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.18 Radar Equation for Volume Targets R θ/2 r tan(α) ≈ α (rad) for small α 2 volume reduction factor due to Gaussian antenna beam pattern

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.19 Radar Cross Section σ P backscattered S incident.. backscattered power (W).. incident power density (W/m 2 ) Depends on: -frequency and polarisation of the electromagnetic wave -scattering geometry / angle -electromagnetic properties of the scatterer -target shape  hydrometeors can be approximated as spheres

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.20 Radar Cross Section σ Monostatic radar cross section of a conducting sphere: a.. radius of the sphere.. wavelength Rayleigh region: a << Resonance / Mie region: Optical region: a >> Figure: D. Pozar, “Microwave Engineering”, 2 nd edition, Wiley. normalised radar cross section electrical size

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.21 Radar Cross Section σ  hydrometeors are small compared to the wavelengths used in weather radar observations: weather radar wavelength 9cm  max. 6mm raindrop diameter  Rayleigh scattering approximation can be applied; radar cross section for dielectric spheres: D |K| 2.. hydrometeor diameter.. wavelength.. dielectric factor depending on the material of the scatterer

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.22 Radar Equation for Weather Radar radar constantradar reflectivity factor z, purely a property of the observed precipitation

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.23 Radar Reflectivity Factor z To measure the reflectivity by weather radars, we need to: -precisely know the radar constant C (radar calibration), -measure the mean received power P r, -measure the range R, -and apply the radar equation for weather radars: Summary: Radar observations of the atmosphere mainly contain contributions from hydrometeors which are Rayleigh scatterers at radar frequencies. This allowed us to introduce the radar reflectivity factor z, a parameter that - only depends on the hydrometeor microphysics and is independent on radar parameters such as the radar frequency, -i.e. the reflectivity within the same radar resolution volume measured by different radars should be equal

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.24 Summary of the assumptions made so far In the derivation of the radar equation for weather radars, the following assumptions are implied: the hydrometeors are homogeneously distributed within the range-bin the hydrometeors are dielectric spheres made up of the same material with diameters small compared to the radar wavelength multiple scattering among the hydrometeors is negligible incoherent scattering (hydrometeors exhibit random motion) the main-lobe of the radar antenna beam pattern can be approximated by a Gaussian function far-field of the radar antenna, using linear polarisation so far, we neglected the propagation term (attenuation)

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.25 Radar Meteorology Reference: AMS Glossary Radar: Radio Detection And Ranging An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy. Meteorology: The study of the physics, chemistry, and dynamics of the Earth's atmosphere. Radar Meteorology: The study of the atmosphere and weather using radar as the means of observation and measurement.

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.26 Radar Reflectivity Factor z  spans over a large range; to compress it into a smaller range of numbers, a logarithmic scale is preferred 1 m 3 one raindrop D = 1mm equivalent to 1mm 6 m -3 = 0 dBZ raindrop diameter#/m 3 Zwater volume per cubic meter 1 mm dBZ mm 3 4 mm136 dBZ33.5 mm 3 Knowing the reflectivity alone does not help too much. It is also important to know the drop size distribution.

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.27 Raindrop-Size Distribution N(D) where N(D) is the raindrop-size distribution that tells us how many drops of each diameter D are contained in a unit volume, i.e. 1m 3. Often, the raindrop-size distribution is assumed to be exponential: concentration (m -3 mm -1 ) slope parameter (mm -1 ) Marshall and Palmer (1948): N 0 = 8000 m -3 mm -1 Λ= 4.1·R with the rainfall rate R (mm/h)

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.28 Reflectivity – Rainfall Rate Relations reflectivity (mm 6 m -3 ) liquid water content (mm 3 m -3 ) rainfall rate (mm h -1 )  the reflectivity measured by weather radars can be related to the liquid water content as well as to the rainfall rate: power-law relationship the coefficients a and b vary due to changes in the raindrop-size distribution or in the terminal fall velocity. Often used as a first approximation is a = 200 and b = 1.6 terminal fall velocity raindrop volume

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.29 Summing-up -electromagnetic waves and their interaction with precipitation -radar principle -radar equation for a point target -extension of this radar equation to be applicable for volume targets and for weather radars -the reflectivity was introduced, how it is determined from weather radar measurements, and its relation to rainfall rate (Z-R relations) But, so far we only considered the backscattered received power which is related to the amplitude of an electromagnetic wave, what about frequency, phase and polarisation? … more after a short break …

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.30 2 nd Part - Outline  radar geometry and displays  Doppler effect  polarisation in weather radars

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.31 2 nd Part - Outline  radar geometry and displays  Doppler effect  polarisation in weather radars

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.32 Radar Display Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra PPI (plan-position indicator): RHI (range-height indicator):

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.33 2 nd Part - Outline  radar geometry and displays  Doppler effect  polarisation in weather radars

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.34 Doppler Effect target f d..frequency shift caused by the moving target v r.. relative radial velocity of the target with respect to the radar λ..radar wavelength T s..sweep time

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.35 Weather Radar Measurement Example (PPI) Data: IDRA (TU Delft), Jordi Figueras i Ventura Reflectivity (dBZ)Doppler velocity (ms -1 )

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.36 Combining Reflectivity and Doppler Velocity Doppler Velocity Power Mean Doppler Velocity Doppler Width Figures: Christine Unal Doppler Processing (Power Spectrum): Mapping the backscattered power of one radar resolution volume into the Doppler velocity domain.

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.37 2 nd Part - Outline  radar geometry and displays  Doppler effect  polarisation in weather radars

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.38 Can Polarimetry add Information  yes, because hydrometeors are not spheres - ice particles - hail - raindrops Beard, K.V. and C. Chuang: A New Model for the Equilibrium Shape of Raindrops, Journal of the Atmospheric Sciences, vol. 44, pp – 1524, June

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.39 Measurement Principle Z hh (dBZ) Z vh (dBZ) Z hv (dBZ) Z vv (dBZ) Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra transmit receive

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.40 Measurement Principle Z hh (dBZ) Z vh (dBZ) Z hv (dBZ) Z vv (dBZ) Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra transmit receive - = Z dr differential reflectivity

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.41 Hydrometeor Classification Differential ReflectivityReflectivity Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra rain melting layeraggregates (snow)ice crystals

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.42 Measurement Principle Z hh (dBZ) Z vh (dBZ) Z hv (dBZ) Z vv (dBZ) Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra transmit receive - = LDR (dB) linear depolar- isation ratio

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.43 Linear Depolarisation Ratio Reflectivity Data: POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra melting layerground clutter

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.44 Estimation of raindrop-size distribution concentration (m -3 mm -1 ) slope parameter (mm -1 ) 1.the differential reflectivity Z dr depends only on the slope parameter Λ, so Λ can be directly estimated from Z dr 2.once that the slope parameter is known, the concentration N 0 can be estimated in a second step from the reflectivity Z hh Data: IDRA (TU Delft), Jordi Figueras i Ventura

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.45 Summing-up -Doppler capabilities allow an insight into the dynamics and turbulence of precipitation -Polarisation can be successfully applied for weather observation because hydrometeors are not spherical, main applications are: -discrimination of different hydrometeor types, -suppression of unwanted radar echoes, i.e. clutter, -improving rainfall rate estimation, -estimation of raindrop-size distribution, -attenuation correction, -…-…

Remote Sensing of the Environment (RSE) ATMOS ATMOS Delft University of Technology Tobias Otto: A short course on radar meteorology.46 webhttp://atmos.weblog.tudelft.nl (for information on PARSAX) referencesR. E. Rinehart, “Radar for Meteorologists”, Rinehart Publications, 5 th edition, R. J. Doviak and D. S. Zrnić, “Doppler Radar and Weather Observations”, Academic Press, 2 nd edition, V. N. Bringi and V. Chandrasekar, “Polarimetric Doppler Weather Radar: Principles and Applications”, Cambridge University Press, 1 st edition, A short course on radar meteorology Tobias Otto