2 research outputs found
Multicomponent radiatively driven stellar winds IV. On the helium decoupling in the wind of sigma Ori E
We study the possibility of the helium decoupling in the stellar wind of
sigma Ori E. To obtain reliable wind parameters for this star we first
calculate an NLTE wind model and derive wind mass-loss rate and terminal
velocity. Using corresponding force multipliers we study the possibility of
helium decoupling. We find that helium decoupling is not possible for realistic
values of helium charge (calculated from NLTE wind models). Helium decoupling
seems only possible for a very low helium charge. The reason for this behaviour
is the strong coupling between helium and hydrogen. We also find that
frictional heating becomes important in the outer parts of the wind of sigma
Ori E due to the collisions between some heavier elements and the passive
components -- hydrogen and helium. For a metallicity ten times lower than the
solar one both hydrogen and helium decouple from the metals and may fall back
onto the stellar surface. However, this does not explain the observed chemical
peculiarity since both these components decouple together from the absorbing
ions. Although we do not include the effects of the magnetic field into our
models, we argue that the presence of a magnetic field will likely not
significantly modify the derived results because in such case model equations
describe the motion parallel to the magnetic field.Comment: 11 pages; accepted for publication in A&
A vertically discretised canopy description for ORCHIDEE (SVN r2290) and the modifications to the energy, water and carbon fluxes
Since 70% of global forests are managed and forests impact the global carbon cycle and the energy exchange with the overlying atmosphere, forest management has the potential to mitigate climate change. Yet, none of the land surface models used in Earth system models, and therefore none of today’s predictions of future climate, account for the interactions between climate and forest management.
We addressed this gap in modelling capability by developing and parametrizing a version of the land surface model ORCHIDEE to simulate the biogeochemical and biophysical effects of forest management. The most significant changes between the new branch called ORCHIDEE-CAN (SVN r2290) and the trunk version of ORCHIDEE (SVN r2243) are the allometric-based allocation of carbon to leaf, root, wood, fruit and reserve pools; the transmittance, absorbance and reflectance of radiation within the canopy; and the vertical discretisation of the energy budget calculations. In addition, conceptual changes were introduced towards a better process representation for the interaction of radiation with snow, the hydraulic architecture of plants, the representation of forest management
and a numerical solution for the photosynthesis formalism of Farquhar, von Caemmerer and Berry.
For consistency reasons, these changes were extensively linked throughout the code. Parametrization was revisited after introducing twelve new parameter sets that represent specific tree species or genera rather than a group of often distantly related or even unrelated species, as is the case in widely used plant functional types. Performance of the new model was compared against the trunk and validated
against independent spatially explicit data for basal area, tree height, canopy strucure, GPP, albedo and evapotranspiration over Europe. For all tested variables ORCHIDEE-CAN outperformed the trunk regarding its ability to reproduce large-scale spatial patterns as well as their inter-annual variability over Europe. Depending on the data stream, ORCHIDEE-CAN had a 67% to 92% chance to reproduce the spatial and temporal variability of the validation data.JRC.H.5-Land Resources Managemen