Sensitivity Tests of Precipitation and T2m to Soil Moisture Using NCEP WRF Ensemble

1. Sensitivity Tests of Precipitation and T2m to Soil Moisture Using NCEP WRF Ensemble Jun Du, George Gayno and Kenneth Mitchell EMC/NCEP and SAIC

2. outline • Sensitivity study • Basic physical mechanism understanding • A soil moisture perturbation design • Ensemble system improvement (to be done) • Summary

3. Under dispersive issue in warm season precipitation and near-surface variables such as T2m

4. Ensemble spread of 12h-apcp from NCEP 21-member SREF (Jun 2006)

5. ( from NMC/CMA, Y. Li)

6. (from NMC/CMA, Y. Li)

7. 3 WRF_NMM (NCEP, 40km) and 3 WRF_ARW (NCAR, 45km)

8. 0-10cm Soil moisture diff (nam-gfs) NOAH Diff ~0.1 Nam is wetter than gfs in general (except For SE US and Greenland) NOAH

9. Sensitivity of 3h-apcp forecast to soil moisture difference within WRF_NMM With NAM soil moisture (NOAH LSM) With GFS soil moisture (HOAH LSM)

10. Difference in forecast caused by different soil moisture IC can not be neglected!

11. Sensitivity of 3h-apcp forecast to soil moisture difference within WRF_ARW With NAM soil moisture (NOAH LSM) With GFS soil moisture (NOAH LSM)

12. Difference in forecast caused by different soil moisture IC can not be neglected!

13. Within WRF_NMM model: Impact on T2m is significant! With nam soil moisture (NMM) T2m diff (namSM – gfsSM, NMM) With gfs soil moisture (NMM)

14. Within WRF_ARW model: Impact on T2m is significant. With nam soil moisture (ARW) T2m diff (namSM – gfsSM, ARW) With gfs soil moisture (ARW)

15. (3h-apcp) Impact comparison between “IC diff” and “soil moisture diff” within WRF_NMM [note: (a) <0.1” contour not plotted, (b) it’s only 3h accumulation] Difference caused by different ICs Difference caused by different soil moisture Impact from diff soil moisture is comparable to that from diff ICs within NMM!

16. (3h-apcp) Impact comparison between “IC diff” and “soil moisture diff” within WRF_ARW Difference caused by different ICs Difference caused by different soil moisture Impact from diff soil moisture is comparable to that from diff ICs within ARW!

17. 3h-apcp Impact comparison between “model difference” and “soil moisture difference” Difference caused by different models Difference caused by soil moisture diff (nmm) Impact from soil moisture diff is slightly less than but still comparable in some areas to that even from model diff (NMM vs. ARW)!

18. 3h-apcp Impact comparison between “model difference” and “soil moisture difference” Difference caused by different models Difference caused by soil moisture diff (arw) Impact from soil moisture diff is slightly less than but still comparable in some areas to that even from model diff (NMM vs. ARW)!

19. T2m -- impact comparison (NMM): Soil moisture impact on T2m is bigger Than that of IC and of model difference! different IC (NMM; namSM) Diff soil moisture (nam – gfs; NMM) different models (nmm vs. arw; namSM)

20. T2m -- impact comparison (ARW): Soil moisture impact on T2m is comparable To that of IC but smaller than model diff Different IC (ARW, namSM) Diff soil moisture (nam – gfs; ARW) Diff models (nmm vs. arw; gfsSM)

21. Physical mechanism: soil moisture  evapotranspiration partitioning of net available surface thermal energy between latent heat and sensible heat flux  convection and local circulation (a) dry case: increased sensible heat will have an earlier initiation of convection (e.g. afternoon thunderstorm); (b) wet case: in contrast to “dry case”, reduced sensible heat will delay initiation convection (e.g. late night thunderstorm); (c) perturb soil moisture  local thermal gradient  local circulation  precipitation

23. 0-10cm Soil moisture diff (max-min) Wet ~0.5 Diff ~0.3-0.4 Dry ~0.1

24. With WRF_NMM model Local early morning Local afternoon T2m diff (wet – dry, NMM) T2m diff (wet – dry, NMM)

25. Afternoon (dry) Afternoon (wet) Late night (wet) Late night (dry) (Aligo, Gallus and Segal, 2006 WAF)

26. Wet soil moisture  more rainfall In general especially in mid-high Latitudes than dry soil situation Wet soil moisture Diff (wet – dry) with ARW Dry soil moisture

27. Response of 3h-apcp forecast to IC difference depends on soil moisture content: More sensitive in wet condition than in dry condition (WRF_NMM) Wet soil moisture condition Dry soil moisture condition

28. T2m is more sensitive to IC in “dry soil” than in “wet soil” condition (WRF_NMM model) Wet soil moisture condition Dry soil moisture condition

29. Response of 3h-apcp forecast to model difference depends on soil moisture content: More sensitive in wet condition than in dry condition Wet soil moisture condition Dry soil moisture condition

30. Similar sensitivity of T2m to model difference is observed under wet vs. dry tests Wet soil moisture condition Dry soil moisture condition

31. Sensitivity of T2m to model diff depends on which soil moisture initial states to use With GFS soil moisture (dryer) With NAM soil moisture (wetter)

32. Original soil moisture perturbation design for N members • mem1 = S1 (GFS) • mem2 = S2 (NAM) • Mem3 = (S1+S2)/2 • Mem4-N=min(S1,S2)+random(i)*|S1-S2|; (i=1,N; 0.0<random<1.0) S1(gfs) ----S(i)-----S3-----S(i)-----S2(nam) (for 4 layers: 0-10, 10-40, 40-100, 100-200cm)

33. Modified soil moisture perturbation design for N members • mem1 = S1 (NAM-0.05) • mem2 = S2 (NAM+0.05) • Mem3 = (S1+S2)/2=NAM • Mem4-N=min(S1,S2)+random(i)*|S1-S2|; (i=1,N; 0.0<random<1.0) S1(namL)--S(i)--S3(nam)--S(i)--S2(namU) (constrain: Smin<S(i)<Smax) (for 4 layers: 0-10, 10-40, 40-100, 100-200cm)

34. Hopefully an Improved ensemble system after adding soil moisture perturbations (plan to use pre-operation MIST to do a longer time period of experiment)

35. Perturb SM only with 5-member WRF_NMM ensemble (T2m spread)

36. Perturb SM only with 5-member WRF_ARW ensemble (3h-apcp spread)

37. Summary • Impact on precipitation and T2m forecast caused by soil moisture uncertainty cannot be neglected. It might be comparable to that caused by IC uncertainty and to, in a varying degree, that caused by different models in these two cases. Therefore, soil moisture perturbation needs to be considered in mesoscale ensemblingalthough the degree of sensitivity may vary with model used. More careful study is surely needed. • In general, wet soil moisture produces more rainfall than dry soil moisture especially in high/mid-latitudes. [Others also showed that dry soil leads early afternoon thunderstorms while wet soil late night thunderstorms.] Precipitation becomes more sensitive to IC and physics when soil moisture is wet than dry. (more evapotranspiration  more moist air?) • Soil moisture directly impacts T2m by changing surface thermal energy balance. Perturbing soil moisture has larger impact on T2m than on precip. T2m seems to be more sensitive to IC in “dry soil” than in “wet soil”. (more sensible heat?) • A simple but realistic soil moisture perturbation generating method is proposed. Hopefully, future test result will show that adding perturbation to soil moisture improves ensemble performance (so keep your finger crossed)