Deviations from Equilibrium in Daytime Atmospheric Boundary Layer Turbulence arising from Nonstationary Mesoscale Forcing

LES of the ``canonical'' daytime atmospheric boundary layer (ABL) over flat topography is developed as an equilibrium ABL with steady surface heat flux, $Q_{0}$ and steady unidirectional ``geostrophic'' wind vector $V_{g}$ above a capping inversion. A strong inversion layer in daytime ABL acts as a ``lid'' that sharply separates 3D ``microscale'' ABL turbulence at the O(10) m scale from the quasi-2D ``mesoscale'' turbulent weather eddies (O(100) km scale). While ``canonical'' ABL is equilibrium, quasi-stationary and characterized statistically by the ratio of boundary layer depth ($z_{i})$ to Obukhov length scale ($-L)$, the real mesoscale influences ($U_{g}$ and $Q_{0})$ that force a true daytime ABL are nonstationary at both diurnal and sub-diurnal time scales. We study the consequences of this non-stationarity on ABL dynamics by forcing ABL LES with realistic WRF simulations over flat Kansas terrain. Considering horizontal homogeneity, we relate the mesoscale and geostrophic winds, $U_{g}$ and $V_{g}$, and systematically study the ABL turbulence response to non-steady variations in $Q_{0}$ and $U_{g}$. We observe significant deviations from equilibrium, that manifest in many ways, such as the formation of ``roll'' eddies purely from changes in mesoscale wind direction that are normally associated with increased surface heat flux.