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Orographic Enhancement of sulphur and nitrogen wet deposition in Models-3

Orographic Enhancement of sulphur and nitrogen wet deposition in Models-3. Ray Wright & Paul Sutton RWE npower Windmill Hill Business Park, Swindon, UK This work is funded by the Joint Environment Programme. Introduction.

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Orographic Enhancement of sulphur and nitrogen wet deposition in Models-3

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  1. Orographic Enhancement of sulphur and nitrogen wet deposition in Models-3 Ray Wright & Paul Sutton RWE npower Windmill Hill Business Park, Swindon, UK This work is funded by the Joint Environment Programme

  2. Introduction • Requirement for accurate modelling of: pollutant gases and particles, and acid deposition, in accordance with increased industrial activity and the consequent increased need for regulation – from all industrial, domestic, agricultural and natural sources. • Models-3 (“CMAQ”) has been in use for several years and has been show to reproduce acid deposition measurements in the UK to an acceptable extent • Room for further improvement?

  3. Does M3 give a good representation of sulphate and nitrate deposition? • Previous JEP work involving monitoring network indicated good agreement between measurements and model predictions (Cocks, Lucas Rodgers & Teasdale, The performance of Models-3 for Deposition and Atmospheric Concentrations over a year, 2003)

  4. Annual comparison: precipitation

  5. Annual comparison: wet deposition of S

  6. Annual comparison: wet deposition of oxidised N

  7. Models-3 compared with measurement Annual means are good, individual sites good, except for ammonia

  8. Terrain heights

  9. UK rainfall 1999 from UM (non-convective)

  10. Locations of monitoring sites

  11. Does M3 give a good representation of sulphate and nitrate deposition? • From comparison with the existing monitoring network - YES • But there could be precipitation on higher sites that M3 is missing (No sites in Snowdonia, Scotland)

  12. Models-3 seems to have all the needed processes, but does it? • Observations show enhanced wet deposition of sulphate and nitrate on high ground (Great Dun Fell campaign) • Not all of it is due to higher precipitation • High conversion rate to sulphate and nitrate in cloud. • Seeder-feeder process? • Occult deposition process? • What’s in Models-3?

  13. Seeder-feeder and occult deposition

  14. Concentration (Nitrate aerosol) If precipitation is high in bottom layer, due to seeder-feeder mechanism, then we should get high S & N deposition

  15. Investigated Models-3 “resolved cloud” module • Contrary to Models-3 documentation, the layer heights are not being used to apportion rain water to layers (e.g deeper layers should contain more water) • Tried code modification with layer heights put in • A cloud-presence threshold is in Models-3. If cloud water is below threshold, no chemistry is carried out (also discovered by Mueller & Cook, TVA, Alabama, USA. They noticed that M3 does not reproduce cloud cover fractions well) • Tried code modification by applying threshold to the summation of Qr over all layers

  16. Results of tests B007 – Original code case B009 – Layer height variation applied B010 – Threshold code change applied B011 – Both changes applied Domain is UK 6km, for JEP studies

  17. Initial conclusions from tests • Sulphate and nitrate deposition increased by taking layer heights into account • Applying threshold to summation of rain water over all layers results in substantial general increase in sulphate and nitrate deposition • Need to check effect of (just) lowering threshold (As Mueller and Cook did)

  18. What about the seeder-feeder mechanism? • Seeder-feeder mechanism is represented in MM5 and UM rainfall calculations (according to consultation) • Difference in MM5 and UM output fields may result in Models-3 NOT seeing heightened precipitation in bottom layer due to fixed apportioning. UM only outputs Q(Liquid) • Q(Cloud) = 0.85 x Q(Liquid) and Q(Rain) = 0.15 x Q(Liquid) • When Seeder-Feeder is operating, Q(Cloud) will be smaller and Q(Rain) will be larger in the bottom layer (This info does not end up in Models-3 unless we use MM5, WRF, etc), so, maybe, M3 would underestimate precipitation in bottom layer in this case.

  19. What about occult deposition? • Other work has shown that up to 50% of sulphate and nitrate deposition on high ground can come from occult deposition (Wrzesinsky et al. 2001, Thalmann et al. 2002) • Water deposition due to wetting by fog accounts for only ~ 10% of total deposition on high ground, BUT • Cloud (fog) water S & N concentrations much greater than in rain water (x5 or more) • So, occult deposition can account for 50% of deposition, depending on vegetation type • We can add this process to Models-3, internally.

  20. Conclusions • Cloud & rainfall in Models-3 not optimally handled, but can be addressed • Seeder-feeder mechanism in meteorological models, but may not fully appear in Models-3, depending on which model is used. • Occult deposition not represented, but could be introduced. • N.B. need more measurement sites on high ground to check.

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