Vicarious use of calibration scans from the operational FMI weather radar network

The Finnish operational weather radar network has 12 C-band polarimetric radars covering the country. Altough primarily scanning at low elevation angles, the operational scanning schedule for the weather radars contains a vertical ‘birdbath scan’ which is repeated every 15 minutes. The ‘birdbath’ scan is designed for calibrating differential reflectivity (ZDR) and the antenna rotates 360 degrees in azimuth while remaining vertically-pointing. All pulses within the scanning time (< 30 s for a full rotation) are then averaged and processed as normal to provide one vertical profile with the three Doppler moments (Z, v, and spectral width) and the polarimetric variables ZDR and cross-correlation coefficient rho_hv. ZDR at vertical is not useful for meterological purposes but the remaining variables can be utilised in the same manner as for a vertically-pointing Doppler (cloud radar (Frech et al., 2026). It should be noted that the spectral width is broadened due to the rotation. Rho_hv is an excellent indicator of the melting level and can be employed in the same way as LDR or slanted-LDR is used within the Cloudnet processing scheme, and hence the collection of vertical ‘birdbath’ scans can be concatenated to provide very similar output to vertically-pointing Doppler cloud radars and used for satellite and model evaluation in the same way.

Although the effective temporal resolution is rather coarse, this data has the advantage of no Doppler aliasing and minimal attenuation, even in heavy rain. The sensitivity appears sufficient to capture the majority of ice clouds (although maybe missing some liquid layers) including ice clouds above strong precipitation which would be missed by Doppler cloud radars operating at Ka- or W-band. After adjusting for the frequency difference (for both Z and v), these observations have exceptional potential to complement the satellite cal/val and model evaluation activities being performed by cloud radar networks.

 

Frech, M., S. Kneifel, P. Ockenfuss, and M. Gergely, 2026: Exploring the Untapped Potential of Operational Weather Radars for Vertical Profiling of Precipitation and Clouds. Bull. Amer. Meteor. Soc., 107, E127–E141, https://doi.org/10.1175/BAMS-D-24-0113.1.