Meteorological Influence on L-Band Forest Backscatter: Evidence from the BorealScat-2 Radar Tower

Boreal forest water exchange operates at sub-daily time scales, yet these dynamics are difficult to resolve using conventional satellite radar observations. In this study, height-resolved L-band measurements from the BorealScat-2 radar tower at Svartberget, northern Sweden, were analysed together with co-located Integrated Carbon Observation System (ICOS) meteorological observations to investigate how external atmospheric forcing affects forest radar backscatter during the meteorological summer of 2024. Two related questions were addressed: (1) whether dry-day diurnal radar variability is more closely linked to incoming shortwave radiation or atmospheric demand, and (2) how rainfall responses vary with polarization and forest layer. Under dry-demand conditions, incoming shortwave radiation (SW_IN) peaked before vapour pressure deficit (VPD), while the radar response peaked later. The strongest dry-day amplitude was observed in horizontally co-polarized (HH) ground backscatter, whereas vertically co-polarized (VV) canopy backscatter responded later and with smaller amplitude. Profile-based metrics further indicated an afternoon downward redistribution of the effective scattering centre. During rainfall, the strongest positive wetting responses occurred in canopy and upper-canopy levels, especially in VV, while HH ground backscatter showed an opposite-sign response. Multi-pulse rainfall events produced substantially larger anomalies than single-pulse events. These results support two contrasting short-term response regimes in boreal L-band backscatter: a dry-demand redistribution regime and a rain-driven wetting/interception regime.