Long-Term Observations Of The Dynamics Of The Continental Planetary Boundary Layer

Time series of mixed layer depth, z_i, and stable boundary layer height from March through October of 1998 are derived from a 915-MHz boundary layer profiling radar and CO₂ mixing ratio measured from a 447-m tower in northern Wisconsin. Mixed layer depths from the profiler are in good agreement with radiosonde measurements. Maximum z_i occurs in May, coincident with the maximum daytime surface sensible heat flux. Incoming radiation is higher in June and July, but a greater proportion is converted to latent heat by photosynthesizing vegetation. An empirical relationship between z_i and the square root of the cumulative surface virtual potential temperature flux is obtained (r² = 0.98) allowing estimates of z_i from measurements of virtual potential temperature flux under certain conditions. In fair-weather conditions the residual mixed layer top was observed by the profiler on several nights each month. The synoptic mean vertical velocity (subsidence rate) is estimated from the temporal evolution of the residual mixed layer height during the night. The influence of subsidence on the evolution of the mixed, stable, and residual layers is discussed. The CO₂ jump across the inversion at night is also estimated from the tower measurements.