Implementation of an optimal stomatal conductance scheme in the Australian Community Climate Earth Systems Simulator (ACCESS1.3b)

J Kala,B E Medlyn,Y.-P Wang,A J Pitman,G Abramowitz,R Lorenz,M G De Kauwe,Y.-S Lin

doi:10.5194/gmd-8-3877-2015

Abstract

Abstract. We implement a new stomatal conductance scheme, based on the optimality approach, within the Community Atmosphere Biosphere Land Exchange (CABLEv2.0.1) land surface model. Coupled land–atmosphere simulations are then performed using CABLEv2.0.1 within the Australian Community Climate and Earth Systems Simulator (ACCESSv1.3b) with prescribed sea surface temperatures. As in most land surface models, the default stomatal conductance scheme only accounts for differences in model parameters in relation to the photosynthetic pathway but not in relation to plant functional types. The new scheme allows model parameters to vary by plant functional type, based on a global synthesis of observations of stomatal conductance under different climate regimes over a wide range of species. We show that the new scheme reduces the latent heat flux from the land surface over the boreal forests during the Northern Hemisphere summer by 0.5–1.0 mm day−1. This leads to warmer daily maximum and minimum temperatures by up to 1.0 °C and warmer extreme maximum temperatures by up to 1.5 °C. These changes generally improve the climate model's climatology of warm extremes and improve existing biases by 10–20 %. The bias in minimum temperatures is however degraded but, overall, this is outweighed by the improvement in maximum temperatures as there is a net improvement in the diurnal temperature range in this region. In other regions such as parts of South and North America where ACCESSv1.3b has known large positive biases in both maximum and minimum temperatures (~ 5 to 10 °C), the new scheme degrades this bias by up to 1 °C. We conclude that, although several large biases remain in ACCESSv1.3b for temperature extremes, the improvements in the global climate model over large parts of the boreal forests during the Northern Hemisphere summer which result from the new stomatal scheme, constrained by a global synthesis of experimental data, provide a valuable advance in the long-term development of the ACCESS modelling system.

Highlights

Stomata control the exchange of water vapour and carbon between the vegetation and the atmosphere
The inherent biases within ACCESS1.3b remain and the net effect of the MED gs scheme is a small overall increase in the biases associated with TMIN
Having examined the influence of the new gs scheme on TMAX and TMIN, we focus on two extreme temperature indices based on TMAX and TMIN, namely, the warmest TMAX of the year (TXx, Fig. 4) and diurnal temperature range (DTR) (Fig. 5)

Summary

Introduction

Stomata control the exchange of water vapour and carbon between the vegetation and the atmosphere. The coupling of the energy, water and carbon exchange at the leaf level was a profoundly important step in the development of land surface models (LSMs; Sellers et al, 1996). Studies at the point scale have illustrated the potential impact of stomatal behaviour on local meteorology (e.g. Jacobs and De Bruin, 1992; Raupach, 1998; Huntingford and Monteith, 1998). Other studies suggest that physiological adaptations of subtropical vegetation to increasing CO2 could account for reductions in the annual transpiration flux of up to ∼ 60 W m−2 in some regions (de Boer et al, 2011) Given these large impacts, there is currently an urgent need to explore accurate representations of stomatal behaviour suitable for implementation in LSMs within general climate models (Huntingford et al, 2015)

Methods

Results

Discussion

Conclusion