  In this study the framework of the $\tau$-$\omega$ model is used to derive
vegetation loss coefficients and canopy penetration depths from SMAP
multi-temporal retrievals of vegetation optical depth, single scattering albedo
and ICESat lidar vegetation heights. The vegetation loss coefficients serve as
a global indicator of how strong absorption and scattering processes attenuate
L-band microwave radiation. By inverting the vegetation loss coefficients,
penetration depths into the canopy can be obtained that is displayed for the
global forest reservoirs. A simple penetration index is formed combining
vegetation heights and penetration depth estimates. The distribution and level
of this index reveal that for densely forested areas the soil signal is
attenuated considerably, which can affect the accuracy of soil moisture
retrievals.
