A wind profiler climatology of boundary layer structure above the boreal forest

During the Boreal Ecosystem-Atmosphere Study (BOREAS) summer 1994 campaign, a 915 MHz wind profiling radar and surface meteorological station were operated continuously from May 21 through September 20, 1994, near the old jack pine site in the southern study area. The data provided by the wind profiler are vertical profiles of wind speed and direction and virtual temperature, as well as boundary layer depth and the presence of precipitation. From this data set, we have documented the diurnal evolution of the atmospheric boundary layer and lower troposphere, especially changes in the diurnal pattern during the course of the summer growing season. Boundary layer depths are found to be surprisingly large, with summer-averaged midafternoon values nearly 1.75 km and individual daily maximum depths often greater than 3 km. Monthly averaged profiler winds show a diurnal variation in scalar wind speed in the lowest kilometer, with a minimum reached near the time of greatest surface heating and a maximum reached shortly after midnight, with the difference as large as 2.4 m s−1. We interpret this to be the result of greater coupling between the atmosphere and Earth's surface during the day and a subsequent inertially generated nocturnal low-level jet. Also evident in the summer-averaged wind profiler statistics is the semidiurnal atmospheric tide. The amplitude of the tide, approximately 0.5 m s−1, is in agreement with theory and with previous measurements, as are the times of the maxima in north and west wind components. Frequency of precipitation as measured by the radar profiler shows a clear late afternoon and early evening maxima for the first half of the summer, and little diurnal variation in the second half. Finally, mean statistics from each Intensive Field Campaign (IFC) have been compared to the longer term averages to check for the meteorological representativeness of the IFCs. Similarly, mean statistics for the Golden Days within each IFC have been compared to the IFC for representativeness.