Handout (4.2 MB)
The Hubbard Brook Experimental Forest (HBEF) is an 8,700 acre, temperate mixed deciduous and conifer forest located in North Woodstock, New Hampshire, USA. The HBEF is also a mountain-valley system. It has a heterogenous canopy roughly 20 m AGL and sparse undergrowth. From 25 May - 16 June 2022 and 18 May - 9 June 2023, two eddy-covariance systems gathered 3-D wind speed and direction, water vapor and CO2 concentrations, temperature, and relative humidity at the top of a 30-m flux tower and on the tower at 6 m AGL. Synoptic-scale maps were acquired from the NWS WPC’s Surface Analysis Archive of United States (CONUS) Analyses to compare with the flux tower data.
Nighttime above- and below-canopy winds often followed westerly mountain flows, while daytime above-canopy winds were often driven by synoptic-scale winds. Two coupling metrics, one using the standard deviation of vertical velocity (σw) and one using friction velocity (u*), were used to delineate coupled and decoupled regimes. A below-canopy σw coupling threshold value was found to be 0.105 m/s. Daytime and nighttime below-canopy u* coupling threshold values were found to be 0.080 m/s and 0.090 m/s respectively. Times with values above these thresholds were considered coupled. Both u* and σw agreed that coupling occurs more frequently during the day. However, the σw metric suggests a secondary peak at night, with brief decoupling during the transition periods of sunrise and sunset. Further research and additional data are needed to increase certainty in our results.

