Measured microwave radiative transfer properties of a deciduous forest canopy

A field experiment was performed with an L- and X-band radiometer operating at 1.4 GHz and 11.4 GHz in a deciduous forest in Jülich (Germany) from September to November 2004. The radiometers were installed on the ground being directed upwards through the canopy. In this position downwelling microwave radiation was measured during the defoliation of the forest with a time resolution of 4 h. Simultaneously we measured the air and soil temperatures.Based on these data, the transmissivities of the forest canopy were estimated at different foliation states. Typical L-band transmissivities determined for the foliated and the defoliated states were 0.41 and 0.46, confirming the semi-transparency of the canopy at low microwave frequencies. Due to the anisotropic crown structure the L-band brightness temperatures were slightly horizontally polarized to the same degree for any state of the vegetation. From this we conclude that branches are the prevailing emitters of the canopy whereas leaves and trunks are less relevant for L-band observations. In the X-band, the canopy was opaque in the foliated state and became semi-transparent in the course of defoliation indicating that leaves are an important radiation source at this wavelength.Our results provide the experimental evidence to quantify the radiation properties of a deciduous forest canopy. This is crucial for interpreting the microwave radiation emitted from forested areas measured from space-borne microwave sensing platforms.