Modeling the impacts of diffuse light fraction on photosynthesis in ORCHIDEE (v5453) land surface model
<p>Aerosol- and cloud-induced changes in diffuse light have important impacts on the global land carbon cycle, as they alter light distribution and photosynthesis in vegetation canopies. However, this effect remains poorly represented or evaluated in current land surface models. Here, we add a...
Main Authors: | , , , , , , , , , , , , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2020-11-01
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Series: | Geoscientific Model Development |
Online Access: | https://gmd.copernicus.org/articles/13/5401/2020/gmd-13-5401-2020.pdf |
Summary: | <p>Aerosol- and cloud-induced changes in diffuse light have
important impacts on the global land carbon cycle, as they alter light
distribution and photosynthesis in vegetation canopies. However, this effect
remains poorly represented or evaluated in current land surface models. Here,
we add a light partitioning module and a new canopy light transmission
module to the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) land surface model (trunk version, v5453) and use the
revised model, ORCHIDEE_DF, to estimate the fraction of
diffuse light and its effect on gross primary production (GPP) in a
multilayer canopy. We evaluate the new parameterizations using flux
observations from 159 eddy covariance sites over the globe. Our results show
that, compared with the original model, ORCHIDEE_DF improves the
GPP simulation under sunny conditions and captures the observed higher
photosynthesis under cloudier conditions in most plant functional types
(PFTs). Our results also indicate that the larger GPP under cloudy
conditions compared with sunny conditions is mainly driven by increased
diffuse light in the morning and in the afternoon as well as by a decreased vapor
pressure deficit (VPD) and decreased air temperature at midday. The observations show that the
strongest positive effects of diffuse light on photosynthesis are found in
the range from 5 to 20 <span class="inline-formula"><sup>∘</sup></span>C and at a VPD < 1 kPa. This effect is found to
decrease when the VPD becomes too large or the temperature falls outside of the abovementioned range, which is
likely due to the increasing stomatal resistance to leaf CO<span class="inline-formula"><sub>2</sub></span> uptake.
ORCHIDEE_DF underestimates the diffuse light effect at low
temperature in all PFTs and overestimates this effect at high temperature
and at a high VPD in grasslands and croplands. The new model has the potential to
better investigate the impact of large-scale aerosol changes and long-term
changes in cloudiness on the terrestrial carbon budget, both in the
historical period and in the context of future air quality policies and/or
climate engineering.</p> |
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ISSN: | 1991-959X 1991-9603 |