Diurnal Cycles of L-Band Tomographic SAR Backscatter in a Boreal Forest During Summer: Observations by the Borealscat Tower Radar

The BorealScat tower radar experiment, in Remningstorp, Sweden, acquired tomographic L-band backscatter time series over a boreal forest stand consisting of Norway spruce (Picea abies). Data from summer 2018 is analysed in this study, where diurnal cycles with variations up to more than 1 dB are identified for HH and VV polarizations. The HH cycle has its minimum at night, its maximum at noon and mainly occurs in the middle layers of the forest. Conversely, the VV cycle has its maximum at night, its minimum at noon and is most apparent in the upper and lower forest layers, especially during June and July. The cycles are compared with vapour-pressure deficit (VPD), an indicator of favourable transpiration conditions, and they are seen to co-vary closely. The VV cycle compares well to that seen for P-band in a previous study and is speculated to originate from stem water content variations. The HH cycle cannot be directly connected to stem water content, but is indicated to follow a currently unknown transpiration related phenomena.