Radar profiling

Radar profiling uses a nadir-pointing radar in orbit to measure the vertical structure of precipitation and cloud particles. It is the three-dimensional reference for how rain and snow are distributed with height,^[1][2] and it gives direct profiles of cloud vertical structure and of ice and water cloud layering.^[3][4]

Requirements. Radar frequency sets the target. Precipitation radars use Ku band near 13.6 GHz or Ka band near 35.5 GHz, as on the Dual-frequency Precipitation Radar (DPR) of the Global Precipitation Measurement (GPM) mission,^[5] while cloud radars use W band near 94 GHz.^[3][6] Vertical resolution is 250 m on DPR (250 or 500 m on its Ka radar),^[1] 250 m on the Tomorrow.io Ka-band radar,^[7] and 500 m, sampled every 100 m, on the Cloud Profiling Radar (CPR) of EarthCARE (Earth Cloud, Aerosol and Radiation Explorer).^[6]

Limits. Coverage is the first limit. DPR scans a 245 km swath,^[1] and the cloud radars sample a single curtain along the ground track,^[6] so the method samples sparsely and does not map cloud cover. Sensitivity is the second. DPR's rain-rate sensitivity is 0.5 mm/h at Ku and 0.2 mm/h at Ka,^[8] and the Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) listed 0.7 mm/h.^[9] Surface clutter hides the lowest part of the DPR profile, which the retrieval estimates from a clutter-free bottom found for each ray.^[10] The Tomorrow.io radar has a sensitivity of about 8.2 dBZ (decibels of radar reflectivity), which limits it to precipitation,^[11] whereas the EarthCARE CPR is rated better than -35 dBZ with 10 km along-track integration.^[6] Before launch, CloudSat's expected minimum detectable signal of -28 dBZ was projected to detect 90 to 95 percent of radiatively significant ice clouds and up to 80 percent of water clouds.^[4] Rain attenuation is the third limit: at 10 mm/h it is about 0.44 to 0.52 dB/km at 13 to 14 GHz, about 2.7 dB/km at 35 GHz and about 6.4 dB/km at 90 GHz,^[12] so W-band radars saturate in heavy rain and DPR treats the Ka-to-Ku path attenuation ratio as nominally 6.^[10]

Implementation. The GPM Core Observatory, launched on 27 February 2014, carries DPR, built by NEC Toshiba Space Systems for the Japan Aerospace Exploration Agency (JAXA) and the National Institute of Information and Communications Technology (NICT);^[5][1] it is the reference standard for the GPM constellation radiometers.^[2] The EarthCARE CPR, developed by JAXA and NICT, uses 94.05 GHz and adds vertical Doppler velocity with accuracy better than 1.3 m/s at 10 km integration.^[6] As of 21 October 2026, two earlier radars have ended: TRMM PR, a 13.8 GHz Ku-band radar, ended science operations on 15 April 2015,^[9][13] and the 94 GHz CloudSat radar, the first cloud radar in space, was turned off permanently in December 2023.^[3]

Other radar methods. The WMO OSCAR (Observing Systems Capability Analysis and Review) database types cloud and precipitation radar, with 11 instruments, separately from imaging synthetic aperture radar (47), radar altimeters (23) and radar scatterometers (13).^[14]