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Atmospheric Research 55 2000 139–157 www.elsevier.comrlocateratmos
Fine scale spatial variations in wet deposition
D.W.F. Inglis , T.W. Choularton
Department of Physics, Institute of Science and Technology, UniÕersity of Manchester, P.O. Box 88, Manchester, M60 1QD, UK
Received 15 December 1999; received in revised form 10 July 2000; accepted 25 July 2000
Abstract
Results are presented from an experiment designed to investigate the processes that cause Ž
. small-scale sub-20 km variations in wet deposition to upland terrain in the UK. Bulk rain and
orographic cloud collections were made at a network of sites of typical separation 1–3 km. Detailed analysis of two frontal rain events that occurred in westerly and southwesterly flows
was aided by the use of a computer model of wet deposition. The accuracy of model predictions of inter-site variations of rain amount and ion deposition in rain is supported by the data except in the
lee of higher terrain. This suggests that the model realistically simulates most of the important physical processes which cause fine scale spatial variation in wet deposition. The role of the
upstream temperature profile in modifying the deposition through its effect on the position and properties of the orographic cloud is considered. Again, the data support the predictions of the
model. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Wet deposition; Orographic rainfall; Seeder–feeder effect; Sub-grid scale variations
1. Introduction
In mountainous terrain, the formation of orographic cap clouds is common. Cloud droplets are efficiently scavenged by rain falling from aloft and are deposited to the
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surface. This is the seeder–feeder effect, first described by Bader and Roach 1977 . The cap clouds often contain high concentrations of dissolved ions and thus, the wet
deposition of ions to the surface can be enhanced. This was first discussed by Carruthers Ž
. Ž
. and Choularton 1984 . Experiments in Cumbria by Fowler et al. 1988 , Choularton et
Corresponding author. 0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S 0 1 6 9 - 8 0 9 5 0 0 0 0 0 6 2 - 4
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al. 1988 and Dore et al. 1992a observed and quantified the enhancement. These experiments lead to the adoption of a simple scheme to predict and map orographic
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enhancement nationally Dore et al., 1992b; RGAR, 1990 . The central assumption was that the concentration of ions in the scavenged water is on average twice that in the
Ž unperturbed rain. More recent experiments have observed single rain events Inglis et
. Ž
al., 1995a,b and monitored rain and orographic cloud over long time scales Fowler et .
al., 1995 . This work has confirmed the dominance of the scavenging process in determining deposition to upland terrain and verified the simple mapping scheme
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RGAR, 1997 . However, the parameterisation only predicts average deposition to large grid squares
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of terrain 20 = 20 km . Sub-grid scale variation is important in terms of damage to soils and ecosystems which typically have characteristics which vary on length scales which
are considerably smaller than 20 km. Ultimately, the aim is to improve the resolution of wet deposition predictions to length scales close to 1 km. They can then be directly
compared to critical load calculations that are carried out on a length scale similar to that
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of the current soil type and land use maps CLAG, 1994 . Ž
. Raper and Lee 1996 made weekly collections of rain from a network of 10 sites in
the south Pennines for 1 year. Sub-grid scale variations were observed in the average concentrations of calcium, hydrogen, and sulphate.
Here, we report results from an experiment specifically designed to address the mechanisms that cause sub-grid scale variation in wet deposition to upland terrain. The
terrain and site separation are selected to allow the detailed evolution of wet deposition processes to be observed as the boundary layer encounters an extended region of
elevated terrain. The monitoring equipment deployed is simple, but full consideration of associated uncertainties is made and carried through to comparison to computer simula-
tions allowing firm conclusions to be reached.
2. The field experiment: Saddleworth Moor, 1993
