How Well Can Land-Surface Models Represent the Diurnal Cycle of Turbulent Heat Fluxes?

Maik Renner,Martin Best,Bart Nijssen,Axel Kleidon,Marvin Heidkamp,Gab Abramowitz,Martyn Clark

doi:10.1175/jhm-d-20-0034.1

Abstract

AbstractThe diurnal cycle of solar radiation represents the strongest energetic forcing and dominates the exchange of heat and mass of the land surface with the atmosphere. This diurnal heat redistribution represents a core of land–atmosphere coupling that should be accurately represented in land surface models (LSMs), which are critical parts of weather and climate models. We employ a diagnostic model evaluation approach using a signature-based metric that describes the diurnal variation of heat fluxes. The metric is obtained by decomposing the diurnal variation of surface heat fluxes into their direct response and the phase lag to incoming solar radiation. We employ the output of 13 different LSMs driven with meteorological forcing of 20 FLUXNET sites (PLUMBER dataset). All LSMs show a poor representation of the evaporative fraction and thus the diurnal magnitude of the sensible and latent heat flux under cloud-free conditions. In addition, we find that the diurnal phase of both heat fluxes is poorly represented. The best performing model only reproduces 33% of the evaluated evaporative conditions across the sites. The poor performance of the diurnal cycle of turbulent heat exchange appears to be linked to how models solve for the surface energy balance and redistribute heat into the subsurface. We conclude that a systematic evaluation of diurnal signatures is likely to help to improve the simulated diurnal cycle, better represent land–atmosphere interactions, and therefore improve simulations of the near-surface climate.

Highlights

Introduction aBackground and motivationLand surface models simulate distinct diurnal cycles of turbulent heat fluxes, but they show systematic deviations from observations, which were reported in early (HendersonSellers et al 1995; Chen et al 1997) and more recent model intercomparison studies (Holtslag et al 2013; Best et al 2015)
This emphasizes the strong control of solar radiation for land– atmosphere exchange and highlights the presence of heat storage effects that are captured by the phase lags
We focused on the simulated diurnal cycle of the turbulent heat fluxes and used the phase lag to solar radiation as a metric to quantify the model’s ability to resolve a signature that is characteristic of the diurnal cycle

Summary

Introduction

Land surface models simulate distinct diurnal cycles of turbulent heat fluxes, but they show systematic deviations from observations, which were reported in early (HendersonSellers et al 1995; Chen et al 1997) and more recent model intercomparison studies (Holtslag et al 2013; Best et al 2015). Best et al (2015) used observational meteorological forcing to drive and evaluate state-of-the-art models at 20 different flux towers. A striking finding of Best et al (2015) was that simple linear regression models with solar radiation as a predictor variable outcompeted all land surface models when evaluated. Denotes content that is immediately available upon publication as open access. The response of turbulent heat fluxes to solar radiation at the subdaily time scale represents a signature of heat redistribution.

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