ENVISAT/ASAR VV/HH backscattering and the radiation characteristics of subarctic boreal forest
Terhikki Manninen(1) and Aku Riihelä(1)
(1) Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland
The boreal zone land cover has very significant influence to northern hemisphere albedo and it is the main factor in northern hemisphere carbon budget. Boreal forest is also a sensitive indicator to changes in local and global climate. Especially the change in the extent of the snow cover in winter time will have a strong effect on the surface albedo. In addition, the decrease of the albedo due to lack of snow cover will enhance the decrease in the annual snow covered area.
Unfortunately large parts of the boreal forest are situated so north that in winter time the retrieval of the surface albedo with optical instruments from satellites with reasonable accuracy is limited by too large sun zenith angle values. In addition, estimation of the BRDF is problematic, when the ground is snow covered as most vegetation BRDFs are designed for summer conditions. Moreover, some of them are parameterized with NDVI, which is more related to the understory vegetation than the canopy properties in the subarctic boreal forest. Also the anisotropy of the radiation at the forest floor in winter time due to shadows causes problems for albedo retrieval using radiative transfer modelling. Obviously there is a need for additional non-optical satellite based methods for estimating the surface albedo of boreal forest, especially during winter conditions.
Recently a surface albedo model for boreal forest in winter conditions was developed. Basically only the leaf area index and the forest floor albedo are needed to estimate the total surface albedo. As the BRDF of the forest floor did not appear to have a strong effect on the total albedo, only the reflectance is actually needed. On the other hand the leaf area index correlates well with the VV/HH backscattering ratio of C-band SAR, such as ENVISAT/ASAR. Thus it seems that one should be able to estimate the boreal forest surface albedo in winter conditions reasonably using only reflectance information of snow and the VV/HH backscattering ratio.
Ground based broadband surface albedo measurements were carried out at the test site is in the immediate vicinity of the Arctic Research Centre of FMI, which is situated about 100 km north of the Arctic Circle. The subarctic boreal forest in the area is relatively sparse and the maximum tree height is less than 15 m. In August 2006 the incoming and reflected radiation was measured at about 1 m height in a grid of about 80 points with a spacing of 50 m using a portable albedometer consisting of two separate pyranometers. The leaf area index (LAI) values had been measured earlier in the summer at the same grid points. One cloud free SPOT image and a few ASAR images were available at the test site in summer 2006.
Algorithms found in literature for the relationship of broadband and spectral albedo values derived from SPOT satellite were used to derive visible and NIR albedo values of the forest bottom on the basis of the pyranometer measurements and the co-registered SPOT reflectance values in the area. The global radiation measured continuously at the meteorological mast above the canopy and a photon recollision probability based canopy radiation model was used to convert the forest bottom albedo values to the forest top albedo values. The obtained spectral forest albedo values were compared to the product of the SPOT reflectance and the VV/HH backscattering coefficient ratio of the ENVISAT ASAR. A linear relationship was found between the SPOT/ASAR and pyranometer based albedo estimates in the NIR band. The coefficient of determination for the linear regression was better than 0.7, when the fraction of diffuse radiation to global radiation was less than 0.2, i.e. in clear sky conditions when the global radiation measured at the mast represented well the global radiation at the measurement points.
The relationship between the microwave backscattering in the VV and HH polarizations and the radiation properties of the canopy were studied as well. The components related to the forest floor were less correlated with the microwave signal than the canopy related terms. These results imply that the inclusion of polarized microwave observations will likely improve the accuracy of the estimation of boreal forest albedo.