Fictitious pressure gradients arising from reductions to sea level over central Australia

When two nearby stations are at similar heights above sea level, a fictitious increment to the gradient of sea-level pressure may be introduced into the analysis by the different below-ground column-mean virtual temperatures used for reduction to sea level at each station. When the reduction method utilises a monthly climatological mean surface (screen) temperature and a standard lapse rate at each station, as the Australian method does, the fictitious increment is almost constant during a calendar month for a given pair of stations, and can be calculated in advance. Knowledge of these increments may be helpful both for analysis and for quality monitoring of sea level pressures. Using two years of observations over central Australia (20°- 27°S, 128°-137°E), it is shown that the largest systematic fictitious increments to the pressure differences between station pairs are between 0.7 and 0.9 hPa in most months. The geostrophic wind equivalent of these largest increments is of order 5 m s-1. The increments usually make the pressure gradients too strong, but may produce gradient reversals on some individual days. There is also some correspondence between the spatial pattern of the fictitious increments, and the stations whose sea level pressures appear consistently higher or lower than expected from neighbours using cross-validation.