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 fr om 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 norther n Wisconsin. Mixed layer depths from the profiler are in good agreement wit h 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 pho tosynthesizing vegetation. An empirical relationship between z(i) and the s quare root of the cumulative surface virtual potential temperature flux is obtained (r(2) = 0.98) allowing estimates of z(i) from measurements of virt ual potential temperature flux under certain conditions. In fair-weather co nditions the residual mixed layer top was observed by the profiler on sever al nights each month. The synoptic mean vertical velocity (subsidence rate) is estimated from the temporal evolution of the residual mixed layer heigh t during the night. The influence of subsidence on the evolution of the mix ed, stable, and residual layers is discussed. The CO2 jump across the inver sion at night is also estimated from the tower measurements.