Impact of Tropical Easterly Waves during the North American Monsoon (NAM) using a Mesoscale Model

1. Impact of Tropical Easterly Waves during the North American Monsoon (NAM) using a Mesoscale Model Jennifer L. Adams CIMMS/University of Oklahoma Dr. David Stensrud NOAA/National Severe Storms Laboratory October 27, 2005

2. What is the NAM? • Distinct shift in mid-level winds accompanied by an increase in rainfall • Occurs over NW Mexico and SW United States • Onset usually in July and decays in September • Great deal of variability

3. NAM Moisture Source • Moisture source current consensus • low-level moisture  Gulf of California (GoC) • mid-level moisture  Gulf of Mexico (GoM) • Transport of low-level moisture by gulf surges (one way) • Induced by passage of tropical easterly waves (TEWs) over GoC and/or outflow boundaries/gust fronts

4. Gulf surges Hales (1972) and Brenner (1974) • Cooler temps, increased dewpoints, pressure rise, southerly wind • Increase in convection • Shallow vertical extent • Loss of definition upon entering desert SW Adams and Comrie (1997)

5. Motivation • NAM predictability very low • TEWs influential to strength of NAM • Inverse relationship between NAM and U.S. central plains rainfall

6. Goals • Explore impact of TEWs on the NAM • gulf surges • NAM region rainfall • Control run of MM5 compared to simulation where TEWs are removed

7. Model Description • Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) • Model domain (350x180x23) at 25 km grid spacing Puerto Penasco

8. MM5 Parameterization Schemes • Kain-Fritsch convective scheme (Kain and Fritsch 1990) • MRF PBL scheme (Hong and Pan 1996) • Simple water and ice microphysics (Dudhia 1989) • Global terrain dataset – 10 minute resolution (25 USGS land use categories) • Rapid Radiative Transfer Model for radiation • 5-layer soil model (Dudhia 1996) • Model initialization: NCEP/NCAR reanalysis data • supply boundary conditions every 6-h • GoC SSTs set to constant 29.0ºC

9. Methodology • Four one-month periods • July 1990, July 1992, August 1988, August 1986 • ECMWF reanalysis data and CPC precipitation analysis • Varying number of TEWs and rainfall amounts

10. ECMWF Hövmoller Diagrams (850 mb) July 1990 July 1992 August 1986 August 1988

11. Methodology • Harmonic analysis to remove TEWs from boundary conditions • Reed et al. (1977) – TEWs average wavelength 2500 km, propagation speed of 8 m/s, and average period of 3.5 days • TEWs with periods of approx. 3.5-7.5 days identified – amplitudes replaced with value of zero south of 30ºN • T, q, u, v, ght, and slp

13. Harmonics

14. Harmonic Amplitudes (~40˚W) August 1988 Harmonic E Harmonic E TEW No-TEW

15. MM5 Hövmoller Diagrams (700 mb) TEW No TEW

16. Results • 18 surges over 4 months examined • 17 induced by TEW/tropical storm • Varying degrees of strength and frequency

17. Surge Criteria • Used time-series data at Puerto Penasco, Mexico as a “first pass” to ID surge events • Surges occur when: • winds shift to southerly • maximum daily dewpoint exceeding 65ºF for at least 2 days • peak wind speeds greater than 5 m/s • decrease in daily max temp of greater than 5ºF from the previous day

18. August 1986 Time-series

19. August 1986 Time-series

20. August 1986 Results • 6 surges in the control run • all show up in the time-series data at Puerto Penasco • 5 induced by TEWs and 1 initiated by a tropical storm (Howard?) • 2 TEWs possibly contained in the model initial conditions

21. TEW passage (18Z Aug 26) No TEW TEW

22. Pre-surge (18Z Aug 26) TEW No TEW

23. Surge onset (06Z Aug 27) TEW No TEW

24. Surge (12Z Aug 28) TEW No TEW

25. Post-surge (12Z Aug 29) TEW No TEW

26. Surge summary • TEW passage 12 hours prior to surge onset • Entire GoC shifts to southerly winds • 10 of 18 surges (most common) • Surge virtually absent from no-TEW simulation

27. TEWs and NAM rainfall • Absence of TEWs has impact on precipitation amounts over the NAM region • Many areas receive more rainfall when TEWs are present • Influences overall extent of NAM precipitation

28. August 1988 Rainfall Differences (TEW-no TEW)

29. Central Plains Rainfall Differences (TEW-no TEW) August 1988

30. Meridional Moisture Flux

31. Rainfall Differences (00Z Aug 17-06Z Aug 23) August 1988

32. Rainfall Differences -- 12Z Aug 19 - 00Z Aug 25 August 1986

33. Meridional Moisture Flux

34. Precipitable Water

35. Mid-latitude forcing -- 12Z Aug 20, 1986 TEW No TEW

36. Adding TEWs • July 1992 --> weak monsoon season July 1990 --> strong monsoon season • Removed waves from July 1992 boundary conditions • Inserted July 1990 TEWs into July 1992 boundary conditions

37. MM5 Hövmoller Diagrams Hybrid July 1992

38. 12Z July 19 Hybrid run TEW passage

39. Surge (00Z July 20) Hybrid July 1992

40. Hybrid - July 1992 TEW Run

41. Conclusions • Harmonic analysis successfully removes TEWs from the model boundary conditions • MM5 reproduces surges over the GoC • full gulf, partial gulf, and SMO • NAM shows great deal of interannual variability • Surges impacted by absence of TEWs

42. Conclusions • Reduction of surge events in the no-TEW run reduced rainfall amounts over the NAM region • Absence of TEWs increases precipitation over the central United States • CAPE • mid-latitude forcing • Adding waves enhances NAM • more distinct surge events • increase in rainfall over core monsoon region

43. Harmonic Analysis • Since the model data used to create the boundary conditions are equally spaced in time and contain no missing values, the model data can be represented exactly as a series of n points in time by summing a series of n/2 harmonic functions….