Evaluating the vegetation–atmosphere coupling strength of ORCHIDEE land surface model (v7266)
<p>Plant transpiration dominates terrestrial latent heat fluxes (<i>LE</i>) and plays a central role in regulating the water cycle and land surface energy budget. However, Earth system models (ESMs) currently disagree strongly on the amount of transpiration, and thus <i>LE<...
| Main Authors: | , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-12-01
|
| Series: | Geoscientific Model Development |
| Online Access: | https://gmd.copernicus.org/articles/15/9111/2022/gmd-15-9111-2022.pdf |
| Summary: | <p>Plant transpiration dominates terrestrial latent heat
fluxes (<i>LE</i>) and plays a central role in regulating the water cycle and land
surface energy budget. However, Earth system models (ESMs) currently disagree strongly on the amount of transpiration, and thus <i>LE</i>, leading to large uncertainties in simulating future climate. Therefore, it is crucial to correctly represent the mechanisms controlling the transpiration in models. At the leaf scale, transpiration is controlled by stomatal regulation, and at the canopy scale, through turbulence, which is a function of canopy structure and wind. The coupling of vegetation to the atmosphere can be characterized by the coefficient <span class="inline-formula">Ω</span>. A value of <span class="inline-formula">Ω→0</span> implies a strong coupling of vegetation and the atmosphere, leaving a dominant role to stomatal conductance in regulating water (<span class="inline-formula">H<sub>2</sub>O</span>) and carbon dioxide
(<span class="inline-formula">CO<sub>2</sub></span>) fluxes, while <span class="inline-formula">Ω→1</span> implies a complete decoupling of
leaves from the atmosphere, i.e., the transfer of <span class="inline-formula">H<sub>2</sub>O</span> and <span class="inline-formula">CO<sub>2</sub></span>
is limited by aerodynamic transport. In this study, we investigated how well
the land surface model (LSM) Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) (v7266) simulates the coupling of
vegetation to the atmosphere by using empirical daily estimates of <span class="inline-formula">Ω</span>
derived from flux measurements from 90 FLUXNET sites. Our results show that
ORCHIDEE generally captures the <span class="inline-formula">Ω</span> in forest vegetation types
(0.27 <span class="inline-formula">±</span> 0.12) compared with observation (0.26 <span class="inline-formula">±</span> 0.09) but
underestimates <span class="inline-formula">Ω</span> in grasslands (GRA) and croplands (CRO) (0.25 <span class="inline-formula">±</span> 0.15 for
model, 0.33 <span class="inline-formula">±</span> 0.17 for observation). The good model performance in
forests is due to compensation of biases in surface conductance (Gs) and
aerodynamic conductance (Ga). Calibration of key parameters controlling the
dependence of the stomatal conductance to the water vapor deficit (VPD)
improves the simulated Gs and <span class="inline-formula">Ω</span> estimates in grasslands and
croplands (0.28 <span class="inline-formula">±</span> 0.20). To assess the underlying controls of <span class="inline-formula">Ω</span>, we applied random forest (RF) models to both simulated and
observation-based <span class="inline-formula">Ω</span>. We found that large observed <span class="inline-formula">Ω</span> are
associated with periods of low wind speed, high temperature and low VPD; it is also related to sites with large leaf area index (LAI) and/or short vegetation.
The RF models applied to ORCHIDEE output generally agree with this pattern.
However, we found that the ORCHIDEE underestimated the sensitivity of <span class="inline-formula">Ω</span> to VPD when the VPD is high, overestimated the impact of the LAI on <span class="inline-formula">Ω</span>, and did not correctly simulate the temperature dependence of <span class="inline-formula">Ω</span> when temperature is high. Our results highlight the importance of observational constraints on simulating the vegetation–atmosphere coupling strength, which can help to improve predictive accuracy of water fluxes in Earth system models.</p> |
|---|---|
| ISSN: | 1991-959X 1991-9603 |