Beyond surface fluxes: Observational and computational needs of multilayer canopy models – A walnut orchard test case

Land surface models simulate fluxes exchanged between the land and atmosphere in weather and climate models. The prevailing modeling paradigm uses a big-leaf canopy parameterization that is not vertically-resolved. Multilayer canopy models have received interest over the past several years as a means to improve surface fluxes and enable new science. We present results from a comparison of the Community Land Model (CLM) multilayer canopy model (CLM-ml v2) and observations of air temperature, specific humidity, wind speed, and fluxes (net radiation, sensible heat, latent heat, momentum) at multiple heights in and above a walnut orchard during the Canopy Horizontal Array Turbulence Study (CHATS). The dataset provides a benchmark with which to test multilayer models. Above-canopy sensible heat, latent heat, and momentum fluxes are well simulated under a range of atmospheric regimes spanning strongly unstable, weakly unstable, near-neutral, weakly stable, and strongly stable, as are vertical profiles of fluxes within the canopy. Vertical profiles of wind speed closely match the observations under all stability regimes. Vertical profiles of air temperature and specific humidity are well simulated except for strongly stable conditions, when the first-order turbulence closure cannot represent within-canopy non-local vertical mixing that would otherwise transport the cool air produced by radiative cooling of the upper canopy downward to the lower canopy. Our model–data comparison highlights the potential of multilayer models to simulate the surface air space. The multilayer canopy model is simpler and more consistent with theory than is the CLM big-leaf canopy model, and it modernizes the canopy physics for theoretical and computational advances compared with CLM's outdated ad-hoc parameterizations. Nonetheless, our analysis points to further modeling needs and identifies observations central to model testing. Measurements of within-canopy micrometeorology and leaf gas exchange are needed in addition to above-canopy fluxes.