Response of vegetation optical depth across multiple microwave frequencies to global vegetation dynamics

The multi-frequency Vegetation Optical Depth (VOD), retrieved from microwave measurements, is crucial for monitoring vegetation water status and biomass at regional to global scales. However, the sensitivity of VOD retrieved from different microwave frequencies and polarizations is not well understood. This study aims to characterize the effects of microwave frequencies, polarization and acquisition time on the relationship between VOD and above-ground biomass (AGB) and leaf area index (LAI), using a newly developed VOD product retrieved at L-, C-, X-, and Ku-bands, analyzed on a consistent 9 km spatial grid, a scale not previously explored in global multi-frequency comparisons. The results indicate that the L-VOD product shows the strongest correlations with AGB across diverse biomes, with minimal saturation effects and high spatial consistency (R > 0.83). This is due to L-band microwaves having a deeper emission, meaning that L-band brightness temperature (TB) contains information on both the vegetation canopy and tree trunks. Conversely, VOD at higher frequencies (C-, X-, and Ku-bands) exhibits stronger correlations with LAI and can better capture the vegetation seasonal variations caused by leaf development, indicating the dominant role of the vegetation canopy. Moreover, the study highlights that polarization differences are most pronounced for high-frequency VOD retrieved from TB observations with an ascending orbit (13:30), whereas horizontal polarization generally provides more accurate assessments of vegetation biomass and leaf area dynamics across all frequencies. These findings confirm the effects of microwave frequencies on the VOD sensitivity to different vegetation vertical layers and highlight the impacts of polarization and acquisition time on satellite-based VOD retrieval. This study offers new insights into optimizing multi-frequency VOD retrievals and practical guidance for selecting the optimal microwave frequencies in global ecological and climate monitoring.