Analysis of leaf heat balance affected by operation of a frost protective fan in tea fields

Under climate change, frosts in agricultural fields often bring serious damage to crops. Accordingly, it is keenly desired to improve frost protection methods based on the quantitative evaluation of their effects. The method for quantifying the thermal effect brought by operation of frost protection methods was newly developed based on analysis of leaf heat balance, where leaf boundary layer conductance was continuously determined in field conditions by applying artificial model leaves and a mathematical model. Reliability of the newly developed method was demonstrated in a tea field under the operation of a frost protective fan. The frost protective fan operating at the upper layer of a thermal inversion with strong radiative cooling enhanced convective heat transfer with increased the leaf boundary layer conductance and the ambient air temperature by supplying leaves with warm air. As a result, sensible and latent heat fluxes were significantly increased, which brought a rise of leaf temperature. This thermal effect brought by the fan operation was remarkably fluctuated due to the fan oscillation and advection from the surrounding field, while increasing gradually until just before sunrise when the lowest leaf temperature was observed. Thus, the newly developed method based on the heat balance analysis can be expected to contribute to the improvement of frost protection methods.