F1. An index of maximal land-atmosphere coupling and its use in estimating evaporative fraction across a range of Ameriflux sites
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Salvucci, Guido D.; Gentine, Pierre
Examination of heat and moisture fluxes at a wide range of Ameriflux sites reveals what appears to be an emergent property of the relative humidity profile, possibly due to strong coupling of the land-atmosphere system. Specifically, the magnitude of the gradient of relative humidity profile near the surface (i.e. at a nominal source height Zo+d) appears to be minimized over the course of a diurnal cycle, possibly reflecting a tendency toward thermodynamic equilibration between the land surface and boundary layer. The analysis leading to this observation is conducted as follows: Temperature and specific humidity profiles are calculated to be consistent with a simple Penman-Monteith model of evaporation, i.e. they: 1) follow simple law-of-the wall scaling (log((z-d)/Zo) consistent with the observed screen height windspeed, temperature and humidity; 2) reflect excess water vapor resistance due to stomatal and/or soil moisture limitations as expressed with a canopy resistance term; and 3) yield fluxes that are in energy balance with observed net radiation and ground heat flux. With rough estimates of the roughness and displacement heights (herein based on observed vegetation height), the preceding analysis yields a relative humidity profile, at each measurement time (e.g. half hour), for any given value of canopy resistance. At each Ameriflux site (here we focus on the Vaira Ranch, California, and the Duke Forest, North Carolina, as end-members), we calculate a set of 48 half-hourly RH profiles for each of 100 values of canopy resistance. We then calculate the sum of the 48 squared gradients of RH, evaluated at the source (Zo+d), for each canopy resistance. We find that the canopy resistance that yields the smallest mean squared gradient on a given day tends to be a close estimate of the canopy resistance that best reproduces the observed evaporation and sensible heat fluxes (and thus evaporative fraction). The ability to use this apparent intrinsic property of the coupled land-atmosphere system as a means to estimate evaporative fraction is particularly dramatic at the Vaira Ranch site, which undergoes a major seasonal dry down. In the analysis, the screen height temperature and humidity are observed, while the surface temperature and humidity are estimated by varying the canopy resistance. However, we interpret that the resulting minimization of near surface RH differences is more likely a rapid response of the boundary layer temperature and humidity to more slowly varying surface conductance, as opposed to a response of the surface to the atmosphere. In other words, we interpret the "selected" canopy resistance parameter as being the value most likely to have yielded the observed air temperature and humidity. Possible feedbacks that could yield the observed RH behavior are being systematically explored in a simplified coupled diffusion and radiative transfer model, and will be discussed.
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Internet-First University Press
Evaporation; Land-Atmosphere Interaction; Non parametric estimation of evaporation; Bouchet-Morton Complementary Evaporation; Relative Humidity