Yuh-Lang Lin

Large Eddy Simulation of Wake Vortices in the Convective Boundary Layer

The behavior of wake vortices in a convective boundary layer is investigated using a validated large eddy simulation model. Our results show that the vortices are largely deformed due to strong turbulent eddy motion while a sinusoidal Crow instability develops. Vortex rising is found to be caused by the updrafts (thermals) during daytime convective conditions and increases with increasing nondimensional turbulence intensity eta. In the downdraft region of the convective boundary layer, vortex sinking is found to be accelerated proportional to increasing eta, with faster speed than that in an ideal line vortex pair in an inviscid fluid. Wake vortices are also shown to be laterally transported over a significant distance due to large turbulent eddy motion. On the other hand, the decay rate of the, vortices in the convective boundary layer that increases with increasing eta, is larger in the updraft region than in the downdraft region because of stronger turbulence in the updraft region.

Cellular convection embedded in the convective planetary boundary layer surface layer

Abstract Cellular convection was first studied in the laboratory by Benard [Ann. Chim. Phys. 23 (1901) 62–144] and Rayleigh [Phil. Mag. Ser. 6 (1916) 529–546] investigated these motions from a theoretical perspective. He defined a dimensionless number, now called the Rayleigh number, which is the ratio of convective transport to molecular transport, and found that if a certain critical value is exceeded, cellular convection occurs. Mesoscale cellular convection (MCC) is a common occurrence in the planetary boundary layer. Agee [Dyn. Atmos. Oceans 10 (1987) 317–341] discussed the similarities and differences of MCC and classical Rayleigh-Benard convection. A similar cellular pattern can be seen in the convective boundary layer (CBL) surface layer. It is known that in the CBL, air near the surface converges into thermals producing updrafts. This produces a ‘spoke’ type pattern similar to the mesoscale cellular or Rayleigh-Benard convection. This paper will focus on applying Rayleigh-Benard convection criteria, using a linearized perturbation method, to the CBL surface layer produced by Large Eddy Simulation (LES). We will investigate the length scales of turbulence in the CBL surface layer and compare them to those predicted from linear theory. Similarities and differences will be discussed between the LES produced surface layer and classical Rayleigh-Benard convection theory.