A large-eddy simulation study of the influence of subsidence on the stably stratified atmospheric boundary layer

Abstract

The influence of the large-scale subsidence rate, S, on the stably stratified atmospheric boundary layer (ABL) over the Arctic Ocean snow/ice pack during clear-sky, winter conditions is investigated using a large-eddy simulation model. Simulations of two 24-h periods are conducted while varying S between 0, 0.001 and 0.002 ms-1, and the resulting quasi-equilibrium ABL structures and evolutions are examined. Simulations conducted with S = 0 yield a boundary layer that is deeper, more strongly mixed and cools more rapidly than the observations. Simulations conducted with S > 0 yield improved agreement with the observations in the ABL height, potential temperature gradients and bulk heating rates. We also demonstrate that S > 0 limits the continuous growth of the ABL observed during quasi-steady conditions, leading to the formation of a nearly steady ABL of approximately uniform depth and temperature. Subsidence reduces the magnitudes of the stresses, as well as the implied eddy-diffusivity coefficients for momentum and heat, while increasing the vertical heat fluxes considerably. Subsidence is also observed to increases the Richardson number to values in excess of unity well below the ABL top.