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Testing the Effect of Domain Size on RegCM3’s Precipitation Output Over Southern Africa

Testing the Effect of Domain Size on RegCM3’s Precipitation Output Over Southern Africa. Anne Barrable ¹ Mary-Jane Kgatuke² Chris Lennard¹. 1- University of Cape Town ; 2- South African Weather Service. Aims and Objectives.

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Testing the Effect of Domain Size on RegCM3’s Precipitation Output Over Southern Africa

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  1. Testing the Effect of Domain Size on RegCM3’s Precipitation Output Over Southern Africa Anne Barrable¹ Mary-Jane Kgatuke² Chris Lennard¹ 1- University of Cape Town ; 2- South African Weather Service

  2. Aims and Objectives • Establish the effect of different domain size and location on RegCM3 output. • Particularly examining precipitation in an agriculturally sensitive region.

  3. Methods • Define 3 different domains over southern Africa including Madagascar. • Two domains chosen to overlap a common region, which formed the 3rd domain…

  4. SA Domain SA + Madagascar Common Domain Grid points: 43x32 Grid points: 64x44 Grid points: 18x18

  5. Methods • Define 3 different domains over southern Africa. • Examined February 1995, 1996, 1997. • 11th February anomalously wet day in study region –compared model between domains.

  6. Model Parameters • 60km grid. • Lambert Conformal projection. • NCEP reanalysis data. • Grell convection scheme using Fritsch & Chappell closure.

  7. Results • Examined surface model precipitation data. • Daily precipitation averages.Comparison over common domain. • 11th February 1996.Comparison over common domain.

  8. 1995 1996 1997 ?

  9. Tropical Cyclone Josie was present in the Mozambique Channel for 5 days (9-14 Feb 1997)

  10. Daily average Feb 1995 SA Domain Common Domain SA + Madagascar

  11. Daily average Feb 1996 SA Domain Common Domain SA + Madagascar

  12. Daily average Feb 1997 SA Domain Common Domain SA + Madagascar

  13. SA Domain 11th February 1996 ♣ ♦ ♣ ♥ ♠ ♠ Bloemfontein (11.5mm) ♣ Polokwane (10.4mm) ♦ Johannesburg (65.0mm) ♥ Van Reenen (74.2mm) ♦ ♠ ♥ ♣ Common Domain ♦ ♠ ♥ SA + Madagascar

  14. General Conclusions • Summer and winter rainfall regions identified. • 1996 wetter year corresponds to known La Nina year. • Different domains display different spatial precipitation characteristics. • Although not presented here, temperature (surface & 2m) showed a cold bias (3°). • Choice of domain location and size crucial w.r.t. topography AND synoptic systems.

  15. Caveats and Model “Hitches” • Common domain too small; nudged by forcing data and did not allow the model to run freely. • Running smaller grid sizes (30km & 20km) did not work: • 2 segmentation faults (even with time step at 20 seconds); • 2 computer freezes; • 1 floating point exception error….Gave up! • Stretched grid for smaller domains?

  16. Caveats and Model “Hitches” cont… • Regrid for different grid size comparisons (in GrADS; already available in Ferret). • Map projection difficult to implement with precipitation. • Map possibly shifted, particularly in SA-Madagascar domain.

  17. Acknowledgments • Special thanks to Bill Gutowski for meaningful discussions of model results. • Thanks to Filippo Giorgi, his team, and the ICTP for allowing us the opportunity to learn and run RegCM”3”.

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