Global Estimates of L-band Vegetation Optical Depth and Soil Permittivity over Snow-covered Boreal Forests and Permafrost using SMAP Satellite Data

TL;DR

This study extends a microwave emission model to estimate vegetation optical depth and soil permittivity over snow-covered boreal forests using SMAP satellite data, providing new global datasets for winter Arctic regions.

Contribution

It introduces a closed-form solution of Maxwell's equations for L-band emission modeling and demonstrates the feasibility of retrieving VOD and permittivity from noisy satellite data.

Findings

Successful retrieval of VOD and permittivity with quantified uncertainties.

Global estimates of vegetation and soil properties over Arctic boreal forests.

Potential to improve understanding of land-atmosphere interactions and carbon fluxes.

Abstract

This article expands the tau-omega model to properly simulate L-band microwave emission of the soil-snow-vegetation continuum through a closed-form solution of Maxwell's equations, considering the intervening dry snow layer as a loss-less medium. The feasibility and uncertainty of retrieving vegetation optical depth (VOD) and ground permittivity, given the noisy L-band brightness temperatures with 1 K (1-sigma), are demonstrated through controlled numerical experiments. For moderately dense vegetation canopy and a range of 100--400 $kg.m^{-3}$ snow density, the standard deviation of the retrieval errors is 0.1 and 3.5 for VOD and ground permittivity respectively. Using L-band observations from the Soil Moisture Active Passive (SMAP) satellite, a new data set of global estimates of VOD and ground permittivity are presented over the Arctic boreal forests and permafrost areas during winter months. In the absence of dense ground-based observations of ground permittivity and VOD, the retrievals are causally validated using dependent variables including above-ground biomass, tree height, and net ecosystem exchange. Time-series analyses promise that the new data set can expand our understanding of the land-atmosphere interactions and exchange of carbon fluxes over the Arctic landscape.