We discuss how synoptic-scale variability controls the transport of atmospheric water vapour by midlatitude cyclones. Idealized simulations are used to investigate quantitatively which factors determine the magnitude of cyclone moisture transport. It is demonstrated that large-scale ascent on the warm conveyor belt and shallow cumulus convection are equally important for ventilating moisture from the boundary layer into the free troposphere, and that ventilated moisture can be transported large distances eastwards and polewards by the cyclone, before being returned to the surface as precipitation.
The initial relative humidity is shown to have little effect on the ability of the cyclone to transport moisture, whilst the absolute temperature and meridional temperature gradient provide much stronger controls. Scaling arguments are presented to quantify the dependence of moisture transport on large-scale and boundary-layer parameters. It is shown that ventilation by shallow convection and warm-conveyor-belt advection vary in the same way with changes to large-scale parameters. However, shallow convective ventilation has a much stronger dependence on boundary-layer parameters than warm-conveyor-belt ventilation. Copyright © 2011 Royal Meteorological Society