Energy fluxes at a subtropical mountain cloud forest

Results are presented of a 5-day micrometeorological experiment to study the fluxes of radiation and sensible and latent heat above a homogeneous Chamaecyparis forest in northeastern Taiwan at 1650 m a.s.l. The site exhibits a well pronounced diurnal cycle of winds, with valley winds from the SE during daytime, advecting warm and moist air masses and frequently leading to foggy conditions, and with mountain winds from the NW during the nights, typically advecting dryer and slightly cooler air masses, leading to the dissipation of fog. The latent and sensible heat fluxes are high during the days, with Bowen ratios above one and below one (but above zero) occuring with the same frequency of about 38%, respectively, indicating that neither sensible nor latent heat is dominant over the other. During foggy conditions, the short wave radiation is strongly reduced, and the long wave radiation is balanced. Sensible and latent heat fluxes are also strongly limited during foggy conditions. The radiation budget indicates a net gain of energy through radiation in the order of 90 W m−2. The energy balance (as calculated from the one-dimensional approach) exhibits negative values particularly after the onset of the valley winds. This indicates that the advection of warm air plays an important role in the energy balance of the ecosystem. The total net energy flux appears to be slightly negative (−10 W m−2), further supporting the role of advection. During some of the nights, the Bowen ratios were negative with positive (upward) latent heat fluxes and negative fluxes of sensible heat. These conditions occurred during advection of relatively warm and dry air. The energy fluxes were balanced during these conditions, so that the energy for evaporation of water virtually originated from the sensible heat flux.