Plant Canopy Temperature and Heat Flux Profiles: What Difference Does an Isothermal Skin Make?

Abstract:

Land surface temperature T_s plays a vital role in the determination of sensible (H) and latent heat flux, upwelling long-wave radiation, and ground heat flux. While it is widely recognized that there is a range of skin temperatures represented in even a homogeneous canopy, it is often necessary or convenient to treat the surface as isothermal. This study investigates, at the sub-canopy scale, the implications of assuming that a canopy is isothermal. The focus is on profiles within the canopy of air, foliage, and soil surface temperature, and of sensible and latent heat flux source strength.

Data from a dense grassland at the Southern Great Plains experiment in 1997 (SGP97) were used to assess the ability of a multi-layer canopy model to match measured sensible and latent heat fluxes along with radiometric surface temperatures. In its standard mode, the model solves the energy balance for each canopy layer and uses Localized Near Field (LNF) theory to model the turbulent transport. The results suggest the model captures the most important features of canopy flux generation and transport, and support its use to investigate scalar profiles within canopies. For 112 data points at SGP97, the model produced realistic temperature and sensible heat flux source profiles. In addition, it was run in a mode that seeks the isothermal (soil and foliage) skin temperature (T_i) that provides the same Hproduced by the model in its standard mode. This produces profiles of air and foliage temperature and of sensible heat source strength that differ significantly from profiles from the standard mode.

Based on these simulations, realistic canopies may have a mixture of positive and negative sensible heat flux sources at various heights, typically with large contributions from the soil surface. There is frequently a discontinuity between foliage temperatures near the soil and the actual soil surface temperature. For isothermal canopies, heat sources at all levels had the same sign and soil surface contributions tended to be small. One persistent feature of T_i is that it is highly correlated with the temperature of the foliage in the middle of the canopy (or near the height with the greatest leaf density). Model-estimated radiometric surface temperatures T_r match measured values well, but T_i tends to be cooler than T_r for positive H and warmer for negative H.