115 Canopy conductance subject to minute-scale variations in insolation: validation of canopy conductance parameterizations using innovative observations

Wednesday, 22 June 2016
Alta-Deer Valley (Sheraton Salt Lake City Hotel)
Arian van Westreenen, Wageningen University, Wageningen, Netherlands; and A. F. Moene, B. van Kesteren, and O. K. Hartogensis

The canopy conductance (gc) is a key-variable for the energy and carbon exchange in atmospheric models, i.e. in their land-surface schemes that represent land-atmosphere interactions. For meteorological purposes, commonly used parameterizations are the fully empirical Jarvis-Stewart (JS) and the A-gs parameterizations. The latter, in part based on plant-physiological insights, relates the net assimilation rate (An) to gs, the stomatal conductance. These parameterizations were originally developed for typical time scales of 30-60 minutes. However, nowadays atmospheric models run on increasingly finer spatial and temporal resolutions. At the same time, the skill of the gc parameterizations at these short time scales remains unknown.

This study aims to validate both parameterizations for gc at short time scales (∼ minutes). To gain more insight in the A-gs parameterization, its sub models for the internal CO2 concentration (Ci) and the net assimilation rate (An) are examined as well. Data from an experiment over growing winter-wheat have been used for validation purposes. Turbulent fluxes with 1-min time resolution were obtained with an innovative method that combines scintillometer measurements with scalar turbulence measurements. Hence, we can focus on non-stationary conditions, mainly caused by rapid changes in incoming short wave radiation.

We found that the JS- and the A-gs parameterizations had difficulty in simulating the observed responses of gc to rapid variations in insolation. Particularly in the morning hours, before 9 UTC (which is 2 hours before local noon), the observed gc increased with cloud cover, in contrast to the decrease predicted by both parameterizations. Concerning the sub models of the A-gs parameterization, we found that the variability of An was reproduced well, but the predicted magnitude of the variations was too small. Ci was not reproduced realistically. Moreover, our results suggest that the assumption of Ci being a function of only VPD and the CO2 concentration at the leaf surface does not hold for insolation lower than 500 W mâˆ'2. We found that the representation of Ci is crucial in explaining the limited skill of the A-gs parameterization under rapidly varying insolation.

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