On the Nature of the Transition from Roll to Cellular Organization in the Convective Boundary Layer

Observational and numerical studies of the convective boundary layer (CBL) have revealed that when surface heat fluxes are strong and mean wind shear is weak, convective updrafts tend to organize into open cells, similar to what is found in turbulent Rayleigh-Bénard convection. Conversely, when heat fluxes are weak and mean shear is strong, convection tends to organize into horizontal rolls aligned with the mean wind. The organization of these large-scale coherent structures in the CBL has a number for implications for the initiation of deep moist convection, errors in in situ measurements of turbulent fluxes, and for turbulence modelling. While some studies have suggested that the transition from rolls to cells occurs rapidly at some critical value of -z_i/L (where z_i is the ABL depth and L is the Obukhov length), other studies have reported transitional structures or the coexistence of rolls and cells.

Using a suite of ten large eddy simulations spanning a range of stabilities from -z_i/L = 7.2 to 48.9, we systematically investigate the roll to cell transition in the CBL and explore the physical mechanisms responsible. Mean vertical profiles including velocity variances, heat and momentum transport efficiencies, and integral lengthscales are found to vary smoothly with increasing -z_i/L and support the existence of a gradual transition from rolls to cells. Two-point correlations of vertical velocity and instantaneous snapshots of the velocity and temperature fields reveal the existence of a number of transitional structures between rolls and cells. Quadrant analysis, correlations between streamwise velocity and temperature, and components of the fluctuating vorticity furthermore suggest that, as -z_i/L increases, coherent structures in the surface layer transition from hairpin vortex packets to buoyant thermals, thereby disrupting the process leading to horizontal roll formation.