Parametrisation of the orographic enhancement of precipitation and deposition in a long-term, long-range transport model

Orographic enhancement of wet deposition arising from the 'seeder-feeder' e ffect is, by necessity, highly parametrised in long-range transport models of acid deposition that are long-term (i.e. annual average) and spatially r esolved at tens of kilometres. Here, we describe a mechanistic approach to the incorporation of these mechanisms into such a model. The model formulat ion required the following: precipitation rate by direction and quantificat ion of the fractions that are orographic and non-orographic; treatment of t he fast oxidation of sulfur dioxide in clouds; the directionality of the se eder-feeder process; and a quantitative basis for increasing wet deposition factors to account for the seeder-feeder process. The directionality of no n-orographic precipitation was determined from meteorological data at 47 si tes across the UK. Orographic precipitation varies on a much finer scale th an can be interpolated from measurements, and thus a modelling approach was adopted. The directionality of the seeder-feeder effect was taken from mea surements. The enhancement factor of the orographic component of precipitat ion, assumed to represent feeder-rain, was determined from a review of meas urements. Fast oxidation of sulfur dioxide is an observed phenomenon in cap -cloud, but limited in duration. An adjustment was made to the sulfur dioxi de oxidation rate in the model in locations where cap-cloud was assumed to be present. The results from the model were compared with UK deposition bud gets and enhanced wet deposition maps. The revised parametrisation underest imated the UK wet deposition budgets of oxidised N and S, but spatial patte rns of deposition were improved for much of the UK. It was concluded that t his was a satisfactory outcome given the constraints of the statistical app roach of weighting of deposition at receptors utilising straight line traje ctories. The sensitivity of the model to directional constraints of seeder- feeder enhancement was tested and it was concluded that a fairly narrow con straint resulted in similar estimations to a broader one, and the broader c onstraint was thus adopted as frontal conditions which result in the proces s arrive from a fairly broad band of directions. When enhancement was allow ed to occur from all directions, UK wet deposition of oxidised N and S was increased by 10%. The sensitivity to the enhancement factor on wet depositi on was tested and found to be relatively robust. An increase in the enhance ment factor from 2 to 6 resulted in increases in UK wet deposition of oxidi sed N and S of 9 and 6%, respectively.