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Next-generation weather radar

Vaisala introduced its first weather radar – a dual-polarised C-band unit – in 2007, to complement the company’s lightning detection systems and wind profilers. Its unique antenna structure gives good cross-polarisation performance and precipitation detection. In addition to accurate data, low maintenance costs were also prioritised in the design.

Following a market study at the beginning of the decade on the potential for a new concept in the weather radar marketplace, and after looking into what dual polarisation could offer in terms of improving the quality of the data produced by weather radar systems, Vaisala launched a project to develop the technology for a new product.

Vaisala selected as its partners the University of Helsinki, Colorado State University (CSU) and Professor Chandra Venkatachalam, who had carried out research in the field for over 20 years, and a number of other leading experts.

The result was a high-performance dual-polarised C-band Doppler weather radar, which was installed in late 2004 at the University of Helsinki to carry out cold climate research and test the concept further.

A step up

Since then, the prototype unit has been developed into a commercial product including a number of new features that mark it off from traditional radars.

Weather phenomena detected by Vaisala’s new HydroClass™-enabled weather radar.

Central to these is HydroClass™, the world’s first automatic operational hydrometeor classification software package for use with dual-polarisation radars.

The antenna and feed have also been specifically designed for dual polarisation to provide better data quality using a beam width of less than 1º and a reflector measuring 4.5 metres across. The pedestal is a lightweight design to maximise data availability and simplify maintenance. The belt drive mechanisms in the pedestal for azimuth and elevation movements are controlled by smart software to provide quick acceleration and deceleration to pinpoint the antenna.

The latest, low-maintenance technology

To speed up the development process, Vaisala selected field-proven radar component suppliers for the new C-band Doppler unit. Sigmet, now a Vaisala company, was selected to supply the signal processor and radar controller, and the IRIS radar application software. The system uses the latest state-of-the-art algorithms in its signal-processing stages, and a solid-state modulator-based magnetron for its transmitter.

As weather radars are often installed in remote unmanned locations to ensure that data is representative of large areas, low maintenance is a key priority, which is why comprehensive remote control functions are essential. As the system’s azimuth and elevation motors are brushless, they require no regular maintenance either.

Thanks to built-in test and active monitoring facilities, maintenance can be coordinated from a central site. Detailed fault reporting means that maintenance personnel can assess any problem very precisely before travelling to the site, giving extended MTTR performance and higher data availability.

Vaisala’s dual-polarised antenna on a lightweight semi-yoke pedestal.

HydroClass™

Hail from a shower like the one on the radar plot shown above.

Vaisala’s HydroClass™ software takes conventional dual polarisation technology to a new level as it utilises observations made in both horizontal and vertical polarisation modes, formatting measurements as polarisation parameters that can be added to standard radar moments.

HydroClass™ is able to identify phenomena in real time at each observation point, exploring the full radar range on a sweep-by-sweep basis, every 10 to 15 seconds. This is a major advantage over conventional methods, which analyse vertical reflectivity structures and have a latency of up to 15 minutes for full-volume scans.

Classification results are presented by labelling each bin with the hydrometeor class most compatible with the observations in question. A threshold parameter is used to specify bins for which the class is ambiguous, such as non-meteorological targets.

Key application areas for Vaisala’s new dual-polarisation weather radar units include:

  • Hail detection
  • Lightning hazard potential forecasting
  • Highway snow removal
  • Airport terminal operation
  • Rain/snow line demarcation
  • Melting height detection
  • Weather modification for hail mitigation
  • Insurance claims verification
  • Chaff detection
  • Hydrological modelling.

Why go for dual polarisation?

The reason why Vaisala was keen to go for dual polarisation was quite simple really: the technique generates better-quality data that adds a new dimension to the accuracy of weather monitoring.

Vaisala’s C-band Doppler weather radar formats its dual polarisation measurements along three parameters – differential reflectivity (Zdr), specific differential phase (Kdp), and cross-channel correlation coefficient (Phv) – which are added to standard radar moments.

The additional information this generates enables scatter elements present in the atmosphere, such as rain, hail, and snow to be identified very precisely. Non-meteorological targets, such as clutter, insects, and chaff can also be isolated from weather-specific data and discarded.

> Pauli Niska-Pudas
(Published in HighTech Finland 2008)