Simulation and classification of range-folding echo using radar long-range observation and vertical profile of reflectivity

A weather radar typically observes the position and intensity of precipitation particles by repeatedly transmitting pulse-type electromagnetic waves and receiving the signals backscattered by the particles in the atmosphere. The time of reception is determined by the pulse repetition frequency(PRF), which in turn determines the maximum observation range (maximum radius). Signals that return from targets farther than the maximum radius are considered to have returned with the next transmission signal and are thus processed as signals with a distance equal to the maximum radius minus the actual distance, resulting in "range-folding". Setting a longer maximum observation radius reduces the effect of range-folding, but creates a dilemma of increasing the time interval between data. In practice, a maximum radius of 200 to 300 km is often used for volume scan strategies. Range-folding can occur not only due to precipitation particles, but also due to terrain or abnormal propagation, and echoes caused by range-folding of precipitation are difficult to distinguish from actual precipitation due to their similar moving trend and radar variable characteristics.
This study developed a method to classify echoes caused by range-folding in real-time using simulated range-folding reflectivity from long-range observation data with twice the observation range of the radar's volume observation data. The method involves: 1) performing single, low-altitude, long-range observations at 480 km just prior to every 240 km volume observation, 2) simulating the reflectivity of the 480 km observation as the range-folding reflectivity in the 240 km observation using the radar equation, and 3) classifying the echoes by comparing the simulated range-folding reflectivity with the actual 240 km observation reflectivity. To improve the classification performance when precipitation echoes and range-folding echoes overlap, a comparison condition for the dual-polarization variables was also applied. To simulate range-folding at higher elevation angles, the method was designed to correct the reflectivity at 480 km observation using seasonal vertical profiles to match the reflectivity corresponding to the higher elevation angle observation.
Additionally, because KMA weather radars use a dual Pulse Repetition Frequency (PRF), range-folding occurs twice, so this study simulated range-folding reflectivity for two maximum radii corresponding to the two PRFs and used their averaged reflectivity for comparison. This technology has enabled the removal of echoes caused not only by precipitation, but also by any clutters by abnormal refraction, and has been applied in the current operational system, resulting in a significant reduction in false detection rates in radar precipitation observations.