Inclusion of an Improved Parameterisation of the Wet Deposition Process in an Atmospheric Transport Model

Abstract

Current pollution models describe the wet deposition of chemical species from the atmosphere using the concentration of the species, its scavenging coefficient and the rainfall rate. However, over the British Isles (BI), the spatial pattern of rainfall intensity often varies on a scale smaller than the spacing between rainfall stations, especially in mountainous areas. Indeed, it is well known that the rainfall rates occuring over mountainous and hilly regions are significantly greater than those over the surrounding low level ground. For the BI conditions, we must take into account the orographic enhancement of precipitation and hence wet deposition from the seeder-feeder effect, whereby orographically induced or enhanced clouds over high ground (feeder clouds) allow precipitation falling from higher-level clouds (seeder clouds) to grow rapidly, giving greater precipitation amounts than over neighbouring low ground (Bergeron, 1965). The quantitative effects of orographic enhancement over the BI are very dependent on flow direction. A model of directional orographic enhancement of precipitation is developed which incorporates cloudwater generation by orographic and synoptic-scale lifting and the conversion of cloudwater to rainwater (Weston and Roy, 1994). The directional orographic precipitation given by this model is normalised to the BI orographic observed precipitation and incorporated in FRAME through a new parameterisation of the scavenging coefficients.