What is the blue line ("Variable 10xKz") on the Stability Plot?
Kz is the vertical mixing coefficient (times 10 hence 10x) at the top of the surface layer (about 75 m). It has units of m2/s. When divided by the turbulent length scale, which could be proportional to the depth of the boundary layer in the daytime, the result is a turbulent mixing velocity. For instance, if Kz were 100 and the PBL depth was 1000, then the mixing velocity would be 0.1 m/s, suggesting that it would take about 3 hours (1000 m / 0.1 m/s) for the PBL to become well mixed.