Fog-Laden Density Staircases in the Marine Atmospheric Boundary Layer

Abstract

Abstract The formation of a layered structure in the form of vertically separated density steps (staircases) in stably stratified fluids has been reported in many laboratory and oceanic studies as well as in the terrestrial atmospheric boundary layer (ABL) to a lesser extent, with attribution to different dynamical mechanisms. This paper presents observations of layered structures in fog-laden marine ABL, where both fog and density steps appear almost simultaneously following a turbulent mixing event under nocturnal conditions. The observations were made during the C-FOG (2018) field campaign aboard a research vessel using rawinsonde launches, aided by a suite of supporting onboard instruments. This is a case of great practical interest because of the impediment by fog-laden staircases to optical and near-infrared wave propagation in the ABL due to enhanced beam jitter by density steps and beam attenuation by fog. A new mechanism is proposed to explain the genesis of density layering, wherein steps appear when fluid parcels with significant buoyancy differences (Δ𝑏) osculate in regions of weak turbulence (local length and velocity scales, 𝐿𝐻 and 𝑢𝐻, respectively) devoid of adequate inertial forces (~ 𝑢𝐻2 /𝐿𝐻) to cause fluid parcels to stir past each other. This is expressed in terms of a local bulk Richardson number criterion 𝑅𝑖= Δ𝑏𝐿𝐻/𝑢𝐻2>𝑅𝑖𝑐, where 𝑅𝑖𝑐 is a critical value. A simple laboratory experiment with an idealized (three layer) density stratification and a known turbulence source (oscillating grid) was performed to demonstrate the proposed mechanism, and through a combination of measurements and modeling it was found 𝑅𝑖𝑐 ≈ 1.5. The proposed criterion was consistent with C-FOG field observations as well as representative previous layering observations in the atmosphere and ocean due to localized turbulence mixing events caused by Kelvin-Helmholtz billowing.