Assessing the Feasibility of Insect Density Quantification by Single-Polarization Weather Radar

Operational single-polarization weather radars offer the potential for large-scale, continuous monitoring of migratory insect density. However, whether single-polarization reflectivity can be quantitatively interpreted as insect density remains unresolved due to the complex and variable scattering properties of insect populations and the lack of systematic feasibility assessment. In this study, we conducted a long-term joint-observation experiment combining an operational S-band weather radar with a nearby Ku-band high-resolution fully polarimetric entomological radar providing independent local insect density measurements. Analysis across three migration seasons reveals two major constraints on reflectivity-based insect quantification. First, bird-insect co-occurrence introduces substantial positive bias in reflectivity-based retrievals. Second, even under insect-dominated conditions, strong temporal variability in effective mean radar cross section (RCS) reduces the stability of the reflectivity-density relationship, with stronger variability observed during spring and autumn than summer. A local empirical reference RCS was derived from lower error insect-dominated observations and evaluated over the complete insect-dominated dataset, showing that fixed-RCS retrieval becomes less reliable when effective mean RCS varies substantially over time. These results indicate that single-polarization insect density retrieval is constrained by biological echo composition and temporal RCS variability, highlighting the need for improved biological classification and RCS calibration approaches.