Diagnosing LE

Despite its omnipresence in atmospheric models, the Penman-Monteith (PM) equation often fails to represent the latent heat (LE) flux accurately. Deviations of several tens of % between modelled and observed LE flux are not an exception. The original PM equation assumed a constant stomatal resistance in time, but most current atmospheric models implement a varying resistance that depends on atmospheric conditions such as radiation, temperature and vapor pressure, while more recent models account for plant physiological stomata control.

In this study, we present a diagnosis of LE fluxes modelled based on the Penman-Monteith equation combined with a fixed, an environmentally driven and a plant physiology driven stomatal conductance model versus observed LE fluxes by Eddy-Covariance. The analysis covers a decade of observations for a grass and three years for a forest site in the Netherlands. We identify atmospheric conditions where the model and observations most strongly disagree and evaluate the contribution of varying stomatal resistance models in reproducing flux observations. We demonstrate that implementing models that account for varying stomatal conductance in response to atmospheric and soil conditions does not help to improve LE model estimates for these two datasets. We investigate the role of aerodynamic versus stomatal conductance in controlling LE flux as well as the effects of diurnal effects of radiation, VPD and stomatal conductance response and how they differ between the grass and forest sites. The aim is to provide suggestions for conceptual improvements that can help resolve some of the shortcomings in the PM-based LE flux estimation.