Vertical turbulence structure and second-moment budgets in convection withrotation: A large-eddy simulation study

The structure of the instantaneous flow fields and turbulence statistics an d the second-order moment budgets in convection affected by rotation are an alysed using a large-eddy simulation (LES) dataset. Three archetypes of con vective flows driven by the surface buoyancy flux are generated. One is the reference case of the non-rotating convective boundary layer (CBL) growing into a quiescent stably stratified fluid. The other two are CBLs affected by rotation. In the geophysical turbulence context, these non-rotating and rotating CBLs mimic an early stage and a mature stage, respectively, of the vertical mixing phase of open-ocean deep convection. Instantaneous flow structures reveal strong localization of the buoyancy an omalies and the non-hydrostatic pressure anomalies near the surface in rota ting CBLs and their dilution as one moves towards the CBL outer edge. These anomalies are associated with the localized cyclonic vortices which are th e centres of intense vertical motions. Most of the cyclones never reach the outer edge of the CBL. Increasing rotation results in less mixing, reducing the entrainment flux a t the CBL outer edge and maintaining a negative buoyancy gradient throughou t the CBL. The effect of counter-gradient transport, which occurs in free c onvection, is largely reduced. The vertical-velocity variance and the layer -averaged turbulence kinetic energy are reduced by rotation, while the vari ances of buoyancy and pressure are enhanced. The vertical velocity and buoy ancy fields are positively skewed in both rotating and free convection. The buoyancy skewness is considerably larger in the bulk of the rotating CBL t han in the non-rotating CBL, reflecting strong localization of positive buo yancy anomalies. The pressure transport term in the turbulence kinetic-energy budget becomes more important as the rotation rate increases, whereas the contribution of the third-order transport term is reduced. All terms in the buoyancy varia nce budget grow in amplitude as the rotation rate increases. The mean-gradi ent term and the turbulent transport term are both gains that are offset by a loss to dissipation in the bulk of the CBL. This is different from free convection where the buoyancy variance budget in mid CBL is maintained main ly by turbulent transport since the mean-gradient term is small there. The budget of the vertical buoyancy Aux in convection with rotation is strongly dominated by the pressure-gradient/buoyancy covariance and the buoyancy pr oduction terms. Evaluation of closures for the turbulence energy dissipation against the LE S data supports the idea of imposing a limitation on the dissipation length scale due to the background rotation. This limitation is required to accou nt for the reduced turbulence energy in convection with rotation. A similar limitation on the length scale for the dissipation of buoyancy variance is not found to be important. Analysis of parametrized budgets of the third-o rder moments reveals the dominance of the direct effects of buoyancy. These effects are enhanced with increasing rotation rate. They must be included in parametrizations for the third-order moments. The conventional downgradi ent approximations neglecting the buoyancy effects would greatly underestim ate the turbulent transport.