Lufft R2S (Radar Rain Sensor)

• Dimensions: (diameter x length) 90mm x 220mm
• Weight: approximately 4.5 kilograms
• Black aluminum sensor housing
• Heated plastic dome
• Easy installation

• Maintenance free precipitation sensor
• Differentiation between rain / snow
• Determination of quantity (1mm, 0.1mm, 0.01mm)
• Interface: RS485 and 2 digital outputs
• Can be configured for replacing tipping bucket systems

• Storage temperature: -40°C to +70°C
• Relative humidity: 0 to 100% RH
• Operating temperature: -40°C to +60°C
• Relative humidity: 0 to 100% RH

• Sensor can be provided with shield if required distances not met.

• There is an 8-pole screw-in connector on the underside of the device.
• This serves to connect the power supply and the interfaces using the associated connection cable.

• 1 negative power supply
• 2 positive power supply
• 3 RS485_A
• 4 RS485_B
• 5 not assigned
• 6 Uout1
• 7 GND reference potential for the digital outputs
• 8 Uout2

• Data bits: 8
• Stop bit: 1
• Parity: none
• Settable baud rates: 1200, 2400, 4800, 9600, 14400, 19200, 28800, 57600
• 19200 is factory setting and baud rate for firmware update

• Power supply: 20 to 30 VDC; typically 24 VDC
• Power consumption: < 100 mA (heating off)
• Heating duty 30VA
• Protection class: III (SELV)

• The Uout1 and Uout2 digital outputs are short-circuit proof high side switches (12V) with integrated pull-down resistors.
• Possible configurations for Uout1 are:
  • Tipping bucket simulation with 1mm, 0.1mm, or .01mm resolution
  • Length of the output pulse for this simulation is typically 50 milliseconds

• Type of precipitation is transmitted on output Uout2 in the form of different frequencies.

• If accumulated precipitation quantity is greater than 0.01mm, the frequency signal is transmitted for 2 minutes.
• The output of the frequency signal is maintained if a precipitation quantity greater than or equal to 0.01mm is measured within 2 minutes.

• Factory setting
  • Device ID: 1
  • Baud rate: 19200
  • RS485 protocol: binary
• ID must be changed if several R2S devices are operated in a UMB network.

• Drizzle detection:
  • When activated, measurement takes place with greater sensitivity.
  • Identifies water droplets with a diameter of 0.3mm.
  • The only disadvantage is that the high sensitivity may cause a slightly higher water quantity to be measured.

• Hail detection:
  • If hail detection is activated, the side shield must be installed in all cases.
  • The measurement signal reacts to movements (e.g. trucks) of up to 72 km/h
  • Movements are interpreted as precipitation.
  • Since the fall speed is identical, very large water droplets may be interpreted as small hail

• Evaporation per day:
  • Simulate the natural evaporation of a tipping bucket
  • A defined value is deducted from the rainfall quantity every minute.
  • This is set at 0.24mm per day in the delivered condition.

• Rainfall, Snowfall, and Hail correction factor:
• The water quantity is assessed with this factor.

• The three particle types (rain, snow and hail) are added together and the precipitation type is assessed every minute.
• Requests 1-4 are carried out.
• They only take place if previous conditions are unfulfilled.
• The factors cannot be changed.

1) Number of hail particles per minute > 40% (Hail factor)

• Precipitation type = Hail

2) Number of rain particles > 90% (Freezing Rain factor) and ambient temperature ⇐ 0°C

• Precipitation type = Freezing rain

3) Number of rain particles >20 % (Sleet factor) and ambient temperature in the range from –5°C to 4°C:

• Precipitation type = Sleet

4) Number of rain particles > 50% (Rain factor) :

• Precipitation type = Rain

If none of the 4 conditions is met but particles were measured, the precipitation type is snow.

• Adjustment values:
  • The range of the rain sensor measurement spectrum is from 130Hz (drizzle) to 1600 Hz (heavy rain).
  • This range is divided into 23 zones, which can be individually corrected with factors from 0.1 to 10.

• Doppler radars measure velocity by estimating the frequency-shift produced by an ensemble of moving targets.
• Doppler radars also provide information about the total power returned and about the spectrum width of the precipitation particles within the pulse volume.
• The reflected signal is the result of the energy from the transmitted pulse interacting with precipitation (snow, ice pellets, hail, and rain) particles.
• A small portion of the power is then returned to the radar and analyzed to determine an estimate of the rain or snow rate.
• The relationship between the size and power return is highly non-linear.
  • An example is a very small, spherical drop.
  • If you double the size of a the drop, you increase the reflected power return by a factor of 64.
  • If you triple the size of the drop, you increase the reflected power return by a factor of 729.
  • Polarimetric radars are designed to eliminate this problem.

Precipitation Rates

• Example of two scenarios with identical rain rates:
  • Rain water concentrated in a very small number of large drops.
  • Rain water concentrated in a very large number of small drops.
  • Reflected power returned to the radar is heavily weighted towards the largest drops.
  • If only using the returned power to estimate rain rate, you might end up with either a significant overestimation or a significant underestimation of the rain rate.
• Radar power returned from irregular shaped mixtures of precipitation types can get quite complicated.
• The rainfall rate (R) is a product of the mass content and the fall velocity in a radar volume.
• Precipitation rate depends on particle size distributions.
• The natural variability in drop-size distributions is an important source of uncertainty in radar measurements of precipitation.

• Precipitation is usually measured by using the Z-R relation:
  • Z = AR^b
    • A and b are constants.
    • This relationship is not unique.
    • Many empirical relations have been developed.
    • Typical values for the index and exponent are A = 200, b = 1.60

Radar Equation

• Pr = (C |K|² Z)/r²

• |K|² is the refractive index factor of the target
• r is the slant range from the radar to the target (meters)
• Z is the radar reflectivity factor (usually taken as the equivalent reflectivity factor Ze when the target characteristics are not well known), in mm⁶ m^-3.
• C is the radar constant.

Attenuation

• Attenuation by hydrometeors can result from both absorption and scattering.
• It is dependent on the shape, size, number and composition of the particles.
• Attenuation is dependent on wavelength.
• At 10 cm wavelengths, the attenuation is rather small while at 3 cm it is quite significant.
• Wavelengths below 5 cm are not recommended for good precipitation measurement except for short-range applications.
• The attenuation is dependent on water mass of the target.
• Ice particles attenuate much less than liquid particles.
• Snow or ice particles (or a hailstone) can grow to a size much larger than a raindrop.

Radar Wavelength

• The larger the wavelength, the greater the cost of the radar system.
• This is due both to an increase in the amount of material and to the difficulty in meeting tolerances over a greater size.
• Bands of weather radar interest include S, C, X and K.
• The sensitivity or ability of the radar to detect a target is strongly dependent on the wavelength.
• For the same antenna, the target detectability increases with decreasing wavelength.
• The shorter wavelengths provide better sensitivity.
• The disadvantage is that the smaller wavelengths have much larger attenuation.

• The Surface Transportation Weather Research Center (STWRC), along with the North Dakota Department of Transportation, established the STWRC Road Weather Field Research Facility (RWFRF) in late 2006.
• The purpose of this site is to understand better the physics associated with the interactions between the atmosphere and the roadway.

• All observations are presented in 1-minute intervals.
• Evaluates the performance of the precipitation sensor.
• Provides monitoring of precipitation events.

• Road Weather Field Research Topics Online:
  • Overview of the facility
  • Field Site Data
  • 24 hour Observations
    • http://stwrc.und.edu/timeplot/rwfrf.html
  • Pavement Condition Model Validation
  • Lufft R2S Observed Precipitation
  • RWFRF Camera Images

• Service and maintenance is carried out by a trained specialist.
• The recommended service interval is 12 months.
• The device must be disconnected from the power supply.

• Estimated cost for Lufft R2S system:
• $3000-4000

• The device is calibrated in the factory.
• The recommended calibration interval is 24 months.
• An onsite calibration service is available on enquiry.
• Testing of the signal processor with known artificially generated signals.
• Doppler calibration includes verification and adjustment of phase stability using fixed targets or artificial signals.
• The presence or absence of echoes from fixed ground targets may also serve as a crude check of transmitter or receiver performance.

• Lufft R2S Precipitation Sensor Manual. 2006.
• RADAR for Meteorologists. Rhinehart. 2004.
• Surface Transportation Weather Research Center. http://stwrc.und.edu/. 2008.