Second-Moment Turbulence Budgets in the Baroclinic Atmospheric Boundary Layer

Theoretical and experimental studies on the structure and statistics of turbulence in the atmospheric boundary-layer (ABL) have predominantly focused on “idealized” barotropic flows (a pressure-gradient force that is constant with height). A more realistic and common occurrence, however, are baroclinic ABL flows where the horizontal pressure gradient or equivalently the geostrophic wind velocity vector (U_g, V_g) varies with height, a concept known as thermal winds. Using the well-studied barotropic ABL as a benchmark case, the work here examines the effects of baroclinicity on the Reynolds-averaged budgets of second-order turbulence moments in the ABL. In particular, a suite of large eddy simulation (LES) cases spanning a range of stability conditions, and strength and rotation angle of the geostrophic velocity vector was designed to explore how the production, transport, and dissipation of individual velocity variances, velocity co-variances (stresses), and heat fluxes are affected. The premise is that the interplay between advection of warm or cold air, positive or negative shear, and static stability may alter the magnitude and sign of the individual budget terms and explain closure problems and challenges in field experiments or weather and climate models. Particular emphasis is placed on transport and pressure-velocity gradient or pressure-temperature gradient terms, which are explored using a reduced model of the variance and co-variance budgets. In this talk, we present the underlying eddy topology and flow structure responsible for such changes in the budgets and discuss the effect of baroclinicity on second-order closure models.