/ APPLICATION NOTE

Attenuation Correction of C-Band Weather Radars

For decades, Doppler weather radar has been the tool for meteorologists to obtain high resolution observations of precipitation. Quantitative estimates of precipitation (QPE) allow meteorologists to gather the cumulative rainfall measurements that aid authorities in everything from aviation safety to hydropower optimization. However, there remain challenges in obtaining sufficient accuracy – one of them being attenuation of radar signals in heavy precipitation. Attenuation is the gradual loss of power as the radar energy travels through rainfall. This causes errors when converting the amount of received energy to rainfall rates. Historically, different measures have been attempted to correct this.

Vaisala Dual Polarization worse estimation on gates further more expensive S-band weather radar. C-Band Weather Radar down the range. Given that X-band With the advent of dual polarization Surpasses S-Band and C-band wavelengths both technology, Vaisala has been able to experience attenuation, regions of successfully correct for the attenuation Classical gate-to-gate algorithms for the world with heavy precipitation in the C-band radar echoes, resulting attenuation correction were tried on were previously left with only the in quantitative precipitation estimates conventional radars with fairly poor limited option of deploying the that rival S-band in accuracy in actual results. An overestimation on the larger, more energy-intensive, and field testing. closer range gates resulted in an even Weather radar is designed to observe hydrometeors and other atmospheric scatterers by emitting and receiving radio frequency (RF) signals. These signals are first transmitted from the antenna as collimated pulses, propagated through the atmosphere, and scattered back from the hydrometeors to the radar receiver. Technically, the choice of radar frequency (the wavelength) is a compromise between optimizing the sensitivity of the radar for identifying the atmospheric targets and limiting the degree of attenuation (the loss of sensitivity) along the signal path. Radars utilizing shorter wavelengths Figure 1. A single ray comparison of C-band reflectivities (observed and corrected have a clear advantage in terms for attenuation) versus S-band reflectivity, towards the intense rain at 40 degrees of signal sensitivity. Conversely, azimuth. these same wavelengths experience significant attenuation effects generated by the hydrometeors whenever the conventional measure received signals in horizontal and along their signal trajectory. The of radar reflectivity factor (echo vertical channels. The measured Φ level of attenuation increases sharply dp power, Z ) is applied. In the past, contains a component which evolves as the wavelength shortens. For a h there was only one practical solution smoothly in range. The evolution given intensity of rainfall, signals to mitigate this specific issue – of Φ is a reference measure of at the X-band (3.2 cm) attenuate dp invest in longer-wavelength, more attenuation. In this approach, the 30 times more than signals at the expensive S-band weather radar quantitative rainfall information largely unattenuated S-band systems. delivered by the smooth phase (10 cm). The phenomenon remains measurements are combined with relatively moderate at the C-band Today however, advances in the reflectivity data. As a result, (5.4 cm) with an attenuation factor of polarimetric weather radar the observations are corrected for less than four compared to S-band technology can provide attenuation bias while their highest (Doviak and Zrnic 1993). For a meteorologists with greater spatial resolution is conserved. given wavelength, the strength of sensitivity at significantly lower attenuation grows proportionally to cost using C-band weather radar A significant precipitation event at the rainfall intensity. with attenuation correction. By mid-latitudes (34° 38.8’N, 86° 46.3’W) calculating the differential phase was analyzed using the observations In practice, intense rain often leads shift between the vertical and from the ARMOR polarimetric to a complete loss of the X-band horizontal orthogonal signal planes, C-band radar in Huntsville, Alabama, signal, severely limiting the range of we can precisely estimate the USA (PETERSEN et al., 2007). The radar usability, while the moderately attenuation of the returning independent observations made attenuated C-band radar signals C-band echoes. at the proximate NEXRAD S-band can still penetrate through even radar (NEXRAD 2007) were used the most intense rain. However, for comparison as an unattenuated the measurement of C-band echo An Operational Solution reference. The weather event was a power is strongly biased. Clearly, Put to the Test prefrontal squall line approaching attenuation has been one of the The polarimetric method is based Huntsville from the northwest. The major mechanisms responsible for on the differential phase (Φ ) which dp extended line structure was clearly deteriorating quantitative estimates is the power weighted average visible in the S-band data, while the of rainfall intensity (R) in heavy rain phase difference between the maximum reflectivities exceeded 58 In the sweep display, the quality attenuation with dual polarimetric dBZ. The fields of differential phase and high spatial resolution of the algorithms. This shorter wavelength were observed at the polarimetric reflectivity fields are found to be weather radar performs on par with radar and evolved significantly preserved within all the radar S-band in the same condition testing along the rays passing through the ranges, including significant regions at a fraction of the CAPEX (Capital intense regions of precipitation. The of non-meteorological echoes from Expenditure) and with significant effects of attenuation in the C-band the southwest to southeast. savings on annual OPEX (Operational were evident in the reflectivity fields Expenditure). beyond the intense precipitation The Vaisala Weather Launched in 2007 with the market- to the northeast and west when Radar Advantage leading Sigmet signal processing compared with the corresponding As a market leader with over 70 software, the Vaisala C-band Weather S-band observations. years experience in meteorology, Radar has been an instant success, The impact of the attenuation Vaisala was eager to take up the securing over half the market correction on the reflectivity fields challenge of providing high quality share in new C-band weather radar of the case study is quantified in weather data at a more competitive deployments. The first commercial Figure 1. Attenuation effects up to price. Vaisala has successfully installation was in Estonia in 2008, 20 dB are resolved in the C-band accomplished this by developing an with over 30 installations now in reflectivity data, rendering them advanced dual polarization C-band place all around the world. comparable to S-band observations. Weather Radar that corrects the Figure 2. Left panel, corrected reflectivity fields at C-band, and right panel, S-band measurement at the same time. The arrow is pointing to the C-band radar location versus S-band radar location.

Similarly, there are considerable C-Band Outperforms in Reference Cost and Reliability OPEX savings to be achieved with several key design features. DOVIAK, R.J., ZRNIC’ D.S. 1993: Vaisala C-band Weather Radar Self-calibration, remote real-time Doppler Radar and Weather operates on a shorter wavelength, Observations, 562p. 2nd ed., monitoring, and detailed fault 5.5–5.7 GHz, which means it has Academic, San Diego, CA. reporting will result in fewer service several intrinsic advantages over visits over the lifetime of operation. NEXRAD 2007: Data inventory S-band. The radar uses a 250 kW The magnetron tube also has a http://www.ncdc.noaa.gov/ magnetron transmitter, which is longer lifecycle in the C-band due to nexradinv/ significantly smaller in size, weight, its lower energy usage. Since many and energy usage. The difference in PETERSEN, W.A., KNUPP K.R., radar stations are located in remote antenna weight can be as much as CECIL D.J., MECIKALSKI J.R., areas for coverage optimization, half that of S-band. In real financial The University of Alabama, service and maintenance visits terms, the CAPEX investment for Huntsville THOR Center incur sizeable costs – hence, greater Instrumentation: Research and C-band is 33–50% less compared to reliability results in significant operational collaboration, the standard S-band weather radar. savings in the total lifetime cost. 2007, AMS 33rd Conference on Radar Meteorology, Cairns, Australia

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