GROWING-SEASON BOUNDARY-LAYER CLIMATE AND SURFACE EXCHANGES IN A SUB-ARCTIC LICHEN WOODLAND

Between June and August 1990, observations were made at two surface mi crometeorological towers near Schefferville Quebec (54 degrees 52N, 66 degrees 40.5W), one in a fen and one in the subarctic lichen woodland , and at four surface climatological stations. Data from these surface stations were supplemented by regular radiosonde launches. Supporting measurements of radiative components and soil temperatures allowed he at and moisture balances to be obtained at two sites. The overall surf ace meteorological experiment design and results of micrometeorologica l observations made on a 30-m tower in the lichen woodland are present ed here. Seasonal variation in the heat and water vapor transport char acteristics illustrate the marked effect of the late summer climatolog ical shift in air mass type. During the first half of the summer, aver age valley sidewalls only 100 m high are sufficient to channel winds a long the valley in the entire convective boundary layer. Channeling ef fects at the surface, known for some time at the long-term climate sta tion in Schefferville, are observed both at ridge top and in the valle y, possibly the response of the flow to the NW-SE orientation of valle ys in the region. Diurnal surface temperature amplitude at ridg0e top (approximate to 10 degrees C) was found to be half that observed in th e valley. Relatively large differences in precipitation among these st ations and the climatological station at Schefferville airport were ob served and attributed to the local topography. Eddy correlation observ ations of the heat, moisture and momentum transports were obtained fro m a 30-m tower above a sparse (approximate to 616 stems/ha) black spru ce lichen woodland. Properties of the turbulent surface boundary layer agree well with previous wind tunnel studies over idealized rough sur faces. Daytime Bowen ratios of 2.5-3 are larger than those reported in previous studies. Surface layer flux data quality was assessed by loo king at the surface layer heat balance. Diurnal and seasonal scale hea t budget imbalances were found. We suggest that unmeasured surface hea t storage may be responsible for some of the observed imbalance. The p resence of the unexplained residual in this and other studies of energ y balance over forests casts a note of caution on the interpretation o f energy balance components obtained using heat residual methods.