Above-ground thermal energy storage rates, trunk heat fluxes and surface energy balance in a central Amazonian rainforest

The thermal (physical) energy storage rates (ESR) of the above-ground components of a “terra firme” forest site in central Amazonia, Brazil, were determined for a set of typical days during the dry season of 2003 and the wet season of 2004, and their relevance to the surface energy balance was studied. To obtain the ESR for the above-ground part of the biomass (trunks, branches and litter), the temperatures of one trunk of a dominant tree species of the forest site were measured at three height levels and several radial depths. To compute the ESR in the other parts of the above-ground biomass (twigs, leaves and small size components such as palms and lianas) and the ESR atmospheric fraction, the temperature of the air was measured at four different heights inside and above the forest. The air, trunks and other biomass components contributed 35%, 40% and 25% to the total daily ESR of the forest respectively. For the hourly values during the morning, the total ESR was usually observed to be between 30 and 70 W m−2, but with maxima that can exceed 90 W m−2; during rain events minima as low as −200 W m−2 were observed. During the night and at dawn and dusk, the total ESR may constitute a sizable fraction of the net radiation, on some occasions equaling, or even exceeding it. On a daily basis, values of the total ESR were observed to be between −40% and 5% of the net radiation, depending on the weather conditions during the day. Further, the inclusion of the ESR to compute the surface energy balance improves its closure, mainly during the daylight period and, particularly, during the morning hours; but the best closure occurs for the days in which the daily balance of the total ESR is positive. Finally, simplified expressions are presented for those components of the ESR which do not depend on trunk temperature.