Improving LPRM C-Band VOD Retrievals Using Stratified Temporal Optimization of the Single-Scattering Albedo

Vegetation Optical Depth (VOD) characterizes the attenuation of soil-emitted microwave radiation as it propagates through the vegetation layer. VOD is retrieved in the microwave domain, making it less susceptible to saturation effects and atmospheric conditions. Hence, VOD is used in a wide range of applications, including drought monitoring, fire risk analyses, carbon-flux modelling, or studies on ecosystem resilience and tipping dynamics.

The Land Parameter Retrieval Model (LPRM) is based on a tau-omega radiative transfer model, which simultaneously retrieves soil moisture and VOD via a sequential, refining search, maximizing the agreement between modeled and observed brightness temperatures. The single scattering albedo ω expresses the fraction between scattered and absorbed radiation intercepted by vegetation and is treated in LPRM conventionally as a global (time-invariant) parameter. Recent studies, however, have shown that ω exhibits systematic temporal variability, suggesting that the assumption of global constancy may not be adequate in LPRM.

This study proposes an extension of LPRM in which C-band brightness temperature observations from the Advanced Microwave Scanning Radiometer 2 (AMSR2) mission are optimized in a two-level procedure for boreal deciduous forests (BDF). Local and global model parameters are solved separately by fixing corresponding parameters through large weights. In a first step, all global model parameters are optimized for the full time series while keeping the local parameter ω fixed. In a second stage, the time series is stratified into temporal windows, within each ω_j is solved independently, keeping global parameters fixed, and allowing for seasonal alignment of ω.

This leads to the following hypothesis: Allowing the single-scattering albedo ω to vary temporally, via a stratified two-step optimization procedure, improves C-band VOD retrievals, specifically in terms of (a) correlations between LAI and C-band VOD; and (b) agreement between modeled and observed brightness temperatures, when being compared against retrieval scenarios in which ω is treated constant.

The findings of this study aim to provide insights into the seasonality of ω, and to assess whether the conventional assumption of constant ω is sufficient or if future studies should treat it as time-variant model parameter.