Shallow Boundary Layers and Suppressed Vertical Mixing in the Very Stable Boundary Layer with a Weak LLJ

Under weak-wind, very-stable conditions previous studies have found that the depth of the very stable boundary layer (vSBL) is often only 10-30 meters. In this study we use data from the 60-m tower, the Tethered Lifting System (TLS), and the High-Resolution Doppler Lidar (HRDL), deployed during the September 1999 campaign of the Cooperative Atmospheric-Surface Exchange Study (CASES-99), for two purposes: (1) to verify this finding and (2) to show the turbulent mixing in a layer immediately above this shallow boundary layer (BL) was so strongly suppressed as to be negligible. Evidence for negligible turbulence and mixing consists not only of suppressed finescale-fluctuation amplitudes, but also of the temperature T becoming nearly constant at the 40-55 m levels AGL for several hours in the middle of each night considered. The constancy of T indicates that the very cold air at the surface was not being mixed upward to levels above the ~25 m depth of the shallow BL.

The significance of this quiescent layer above the shallow BL is that the atmosphere above the shallow BL was essentially detached from the surface. Corroboration of this behavior was also found in time series of ground-level ozone (O₃) concentrations, which remained above 20 ppb on nights with stronger winds, due to downward mixing of O₃ from aloft, but became zero on light-wind, very stable nights, as a result of the lack of vertical mixing combined with near-surface sink activity (deposition and chemical reaction).

Implications of the complete decoupling of the atmosphere above the shallow BL from the surface are many. For example, the appropriate way to parameterize the surface layer in numerical models under very stable conditions would be to shut off vertical mixing processes between the surface and the atmosphere. Practically, this would mean modeling this lower boundary as a free-slip, thermally insulated layer, in which source or sink activity between the surface and the atmosphere was set to 0.