The Surface Heat Imbalance I: Ground Heat Flux Lags

Observations obtained in 2016, 2017 and 2018 at an east Tennessee agricultural field site yield evaluations of the delay from changes in net radiation to the detection of their consequences in ground heat flux. The delay causes an erroneous assessment of the surface heat budget if it is ignored, enough to explain much of the imbalance detected in earlier studies of the heat balance at other sites. Using observations with 5-min time resolution, lag partial correlation analyses reveal delays of typically 10 to 15 min. Observations indicate that such lags are mostly associated with transfer of heat through the vegetation biomass and the layer of crop residues blanketing the soil. The response time of ground heat flux plates is a contributing factor. Precipitation is a major modifier, influencing the water content of the thermal blanket as well as the water content of the soil and its thermal diffusivity. The average errors associated with delays appear sufficient to explain much of the surface heat imbalance commonly reported. No evidence is found for a consistent error in the ground heat flux measurements made using flux plates; the error usually associated with them is dominated by the thermal lag syndrome now addressed and with their own response time. The analysis reveals that standard micrometeorological time resolution, with run times typically of 30 min, is inadequate to reveal the syndrome now reported. The 5–min averages of the present dataset appear adequate but not optimal. It is recommended that data acquisition and archiving should provide a time resolution of a few minutes or less. The lag syndrome and its consequences are expected to apply to forests in much the same way as for the present agricultural application, but this has yet to be tested.