Calibration bias evaluation and correction of S-band ground-based radar reflectivity using Ku-band space borne radar observations along the east coast of India

Abstract

The India Meteorological Department (IMD) has strategically deployed several S-band weather radars along India's coastal vicinity for weather monitoring. As radar reflectivity is critical for rainfall estimation, nowcasting, and numerical model assimilation, ensuring data quality is essential before use. The preferred approach is to minimize errors or compensate using offsets through relative (engineering) or absolute (reference-based) calibration. This study compares reflectivity measurements from four operational IMD S-band ground radars (GR) along India's east coast with Ku-band observations from the Global Precipitation Measurement (GPM) satellite's spaceborne radar (SR). The common volume sampled by the SR and GR is identified using the volume matching method, accounting for resolution, geometry, frequency scaling, and timing. The assessment is conducted to quantify the absolute calibration bias in GRs. Comparison with SR measurements reveals systematic mean differences in GR reflectivity ranging from − 9.4 to 0.3 dB across sites. The reflectivity bias varies by up to ±20 dB, driven by spatial and temporal variability, precipitation type, bright-band presence, and other meteorological factors affecting GR. Linear transformations and iterative methods account for calibration bias at each site, thereby improving results. The mean bias at each site is reduced to < |1| dB. Further, corrections are applied to compensate for the bias variability introduced by spatial-temporal averaging and by process-based diagnostic approaches, such as distinct corrections for convective, stratiform, and bright-band effects.