The Structure and Evolution of Titan’s Daytime Planetary Boundary Layer

The structure and evolution of Titan’s daytime planetary boundary layer (PBL) are investigated through large eddy simulation (LES) modeling.  The PBL is the interface between the surface and the free atmosphere through which energy, mass, and momentum are exchanged via turbulent eddies.  The sounding from the Huygens probe provided the only direct, vertically resolved measurement of the structure of the PBL at a single moment in time. How the observed structures develop and evolve remain uncertain, and the turbulent exchange processes are challenging to constrain from the single profile. LES techniques provide a mechanism for understanding the observed structure and dynamics of the PBL, better constraining turbulent atmosphere-surface exchange, and improving the parameterization of the PBL in larger-scale models.  Results from LES studies forced by diurnally-varying radiation are presented for Titan.  The development of three distinct PBL layers are noted: 1) a near-surface layer dominated by frictional dissipation; 2) a mixed-layer of near neutral stability; and 3) a relatively deep entrainment layer capping the top of the PBL.  The three layers are similar in character to what is often observed in the Earth’s convective PBL. The interpretation of the modeled structures and PBL evolution in the current LES study differs significantly from previous mechanisms inferred from GCM studies and shows important differences from prior work that lacked diurnally-varying radiative forcing.