Do we (need to) care about canopy radiation schemes in DGVMs? Caveats and potential impacts

Dynamic Global Vegetation Models (DGVM) are an essential part of current state-of-the-art Earth System Models. In recent years, the complexity of DGVM has increased by incorporating new important processes, like e.g. nutrient cycling and land cover dynamics while biogeophysical processes, like surface radiation have been not much further developed. Canopy radiation models are however very important for a proper estimation of absorption and reflected fluxes and are essential for a proper estimation of surface carbon, energy and water fluxes. The present study provides an overview about current implementations of canopy radiation schemes in a couple of state-of-the-art DGVMs and evaluates their accuracy in simulating canopy absorption and reflection for a variety of different surface conditions. Systematic deviations in surface albedo and fraction of absorbed photosynthetic active radiation (faPAR) are identified and potential impacts are assessed. The results show clear deviations for both, absorbed and reflected, surface solar radiation fluxes. FaPAR is typically underestimated which results in an underestimation of Gross Primary Productivity (GPP) for the investigated cases. The deviation can be as large as 25% in extreme cases. Deviations in surface albedo range between -0.15 and +0.36 with slight positive bias in the order of ~0.04. Potential radiative forcing caused by albedo deviations is estimated as -1.25 < RF < 0.8 [W/m2] caused by a neglecting the diurnal cycle of surface albedo. The present study is the first one that provides an evaluation of canopy RT schemes in different currently used DGVMs together with an assessment of the potential impact of the identified deviations. The paper illustrates that there is a general need to improve the canopy radiation schemes in DGVMs and provides different perspectives for their improvement.

LOEW Alexander;  VAN BODEGOM Peter;  WIDLOWSKI Jean-Luc;  OTTO Julliane;  QUAIFE Tristan;  PINTY Bernard;  RADDATZ Thomas; 

2014-11-28

COPERNICUS GESELLSCHAFT MBH

JRC84672

1726-4170,   

www.biogeosciences.net/11/1873/2014/,    https://publications.jrc.ec.europa.eu/repository/handle/JRC84672,   

10.5194/bg-11-1873-2014,