A new free-convection form to estimate sensible heat and latent heat fluxes for unstable cases

Abstract

Free convection limit (FCL) approaches to estimate surface fluxes are of interest given the evidence that they may extend up to near neutral stability conditions. For measurements taken in the inertial sublayer, the formulation based on surface renewal theory and the analysis of small eddies (SRSE) to estimate the sensible heat flux (H) was extended to latent heat flux (LE) with the aim to derive their FCL approaches. For sensible heat flux (HFCL), the input requirements are traces of the fast-response (such as 10–20 Hz) air temperature and the zero-plane displacement. For latent heat flux (LEFCL), input requirements are fast response traces of water vapor density, mean temperature of the air, the available net surface energy (Rn-G, where Rn and G are the net radiation and soil heat flux, respectively) and the zero-plane displacement. Taking eddy covariance (EC) as a reference method, the performance of the FCL method was tested over a growing cotton field that involved three contrasting surfaces: partly mulched bare soil, a sparse canopy and a homogeneous canopy. Using traces at 10 Hz and 20 Hz, HFCL overestimated and underestimated the EC sensible heat flux (HEC), respectively. In general, LEFCL tended to slightly underestimate LEEC. The surface energy balance closure show that (HEC + LEEC) underestimated (Rn-G) in a range of 19% (homogeneous canopy) and 8% (sparse canopy). Given that, in general (HFCL + LEFCL) was closer to (Rn-G) than (HEC + LEEC), the FCL method may be recommended for field applications, especially when the wind speed is not available.