An analysis of secondary circirculations and their effects caused by small-scale surface inhomogeneities using large-eddy simulation

A parallelized large-eddy simulation model has been used to investigate the effects of two-dimensional, discontinuous, small-scale surface heterogeneities on the turbulence structure of the convective boundary layer. Heterogeneities had a typical size of about the boundary-layer height z(i). They were produced by a surface sensible heat flux pattern of chessboard-type and of strong amplitude as typical, e.g., for the marginal ice zone. The major objectives of this study were to determine the effects of such strong amplitude heat flux variations and to specify the influence of different speeds and directions of the background wind. Special emphasis has been given to investigate the secondary circulations induced by the heterogeneities by means of three-dimensional phase averages. Compared with earlier studies of continuous inhomogeneities, the same sized discontinuous inhomogeneities in this study show similar but stronger effects. Significant changes compared with uniform surface heating are only observed when the scale of the inhomogeneities is increased to z(i). Especially the vertical energy transport is much more vigorous and even the mean emperature profile shows a positive lapse rate within the whole mixed layer. However, the effects are not directly caused by the different shape of the inhomogeneities but can mainly be attributed to the large amplitude of the imposed heat flux, as it is typical for the partially ice covered sea during cold air outbreaks. The structure of the secondary flow is found to be very sensitive to the wavelength and shape of the inhomogeneities as well as to the heatflux amplitude, wind speed and wind direction. The main controlling parameter is the near-surface temperature distribution and the related horizontal pressure gradient perpendicular to the main flow direction. The secondary flow varies from a direct circulation with updraughts mainly above the centre of the heated regions to a more indirect circulation with updraughts beneath the centre and downdraughts above it. For background winds larger than 2.5 m s(-1) a roll-like circulation pattern is observed. From previous findings it has often been stated that moderate background winds of 5 m s(-1) eliminate all impacts of surface inhomogeneities that could potentially be produced in realistic landscapes. However, this study shows that the effects caused by increasing the wind speed strongly depend on the wind direction relative to the orientation of the inhomogeneities. Secondary circulations remain strong, even for a background wind of 7.5 m s(-1), when the wind direction is orientated along one of the two diagonals of the chessboard pattern. On the other hand, the effects of inhomogeneities are considerably reduced, even under a modest background wind of 2.5 m s(-1), if the wind direction is turned by 45 degrees. Mechanisms for the different flow regimes are discussed.