Titan's planetary boundary layer structure at the Huygens landing site

Abstract

[1] Huygens Atmospheric Structure Instrument (HASI) for the first time performed an in situ measurement of the thermal structure in Titan's atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer (PBL). The vertical potential temperature profile reveals the presence of a weakly convective PBL, with a surface layer thickness of 10 m and an outer layer with a depth of 300 m. With a mean eddy diffusivity of only $7.4 Â 10 À3 m 2 s À1, the turbulence in the PBL is weak. The turbulent heat flux in the surface layer was upward but tiny ($0.02 W m À2 ), indicating that the ground surface was marginally warmer than the air. The surface heat flux is too small to cause a diurnal variation of the PBL except in the lowest few meters, so the observed profile may be a nearly steady state feature within this season at the landing site. In the surface layer the mean wind speed is likely to be less than 0.1 m s À1 . Given the tiny surface heat flux, the buoyant production of turbulence is very weak. The PBL structure reveals that the weather condition at the time and place of the Huygens landing resembles a calm, overcast day on Earth but is clearly different from the PBL typical for the polar night.

Titan's Tropical Storms in an Evolving Atmosphere

“…These altitudes are separated by stable regions, or steps, at 0.35-0.5 km and 0.7-1 km, defined by 9 and 5 measurement points. Below 0.3 km, the boundary layer, dry convection presently occurs (Tokano et al 2006a). Convection cells, with rising air interspersed with subsidence, measured as the probe drifted ∼200 m horizontally in the lower 2 km of its descent, could cause this signature.…”

Storms, polar deposits and the methane cycle in Titan's atmosphere

“…Temperatures follow those of an adiabatically rising parcel (i.e. the dry lapse rate) below 300 m; this lower region thus defines the current planetary boundary layer (Tokano et al 2006b). A couple of additional altitude regions below 2 km approach the dry lapse rate, suggesting that dry convection has occurred up to 2 km altitude in the past .…”

The dynamics of Titan's troposphere

“…This bias regularly disappears in numerical models by virtue of viscosity. This might imply that the tiny turbulence in Titan's convective PBL (Tokano et al 2006b) is somehow conducive to the formation of equatorial surface westerlies.…”

“…This corresponds to the altitude range in which evidence of a cloud layer made of solid methane is found in the humidity data measured by Huygens (Tokano et al 2006a). Turbulence reduces towards the surface, and precise temperature and pressure measurements by HASI reveal the presence of only a weakly convective PBL in the lowest 300 m (Tokano et al 2006b). The derived eddy diffusion coefficient is less than 10 K2 m 2 s…”

“…The horizontal probe motion tracked by the descent imager/spectral radiometer is consistent with the zonal wind profile of DWE (Tomasko et al 2005;Karkoschka et al 2007). The surface wind was constrained to much less than 0.2 m s K1 on the basis of the cooling behaviour of the Huygens probe after the landing as well as the thermal profile in the planetary boundary layer (PBL; Tokano et al 2006b). …”