Life Cycle of Warm-Season Midlatitude Convection

1. Life Cycle of Warm-Season Midlatitude Convection Stan Trier NCAR (MMM Division)

2. Outline • Diurnal Cycle of Convection • Rainfall Episodes - Phase Coherence - Latitudinal Corridors • Propagating Nocturnal Convection (Model Composite Study) - Statistics - Evolving structure and propagation mechanism - Environmental characteristics

3. Amplitude and Phase of U.S. Diurnal Cycle of Thunderstorm Occurrence 12 LST 18 6 From Wallace and Hobbs (1977) Atmospheric Science: An Introductory Survey 0

4. Hourly Average Rainfall Frequency (June-August 1996-2004) On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller

5. Time/Frequency Diagram of United States Warm-Season Convection (1996-2004) On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller

6. NOAA/CMORPH Rain Rate Boreal Summer - JJAS 2004 mm/hr Courtesy of Steve Nesbitt, presented at Warm Season Rainfall Workshop (9 June 2006)

7. From TRMM Tropics-wide observations: • Over ocean, all types of precipitation features produce the most rainfall at night around 6 AM, mainly controlled by MCSs • Over land, the total rainfall peaks in the afternoon when the atmosphere is least stable, however MCS rainfall peaks later at night, around midnight, due to their longer life cycle Nesbitt and Zipser (2003), Mon. Wea. Rev.

8. June 20-24 1998 Example of Coherent Rainfall Episodes Latitudinal Corridor Propagating with Intermittency Time (day/hr UTC) Stationary Locally Forced Continuous Propagation 115W 95W 75W 30N 36N 42N 48N Latitude Longitude On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller

10. Radar+Sat Sat Only Documented Locations of Long-Lived Coherent Precipitation Episodes Courtesy of John Tuttle, presented at Warm Season Rainfall Workshop (9 June 2006)

11. Study Domains & Period 2.0 Average Elevation 0-20N (km) 1.0 0 1.2 Average Elevation 35S - 20S (km) 0.6 0 20N May - Aug Main Focus May - August 5-year (1999 to 2003) 2-year Sep-Oct: 1999, 2003 2-year Nov–Dec: 1999, 2003 Meteosat-7 IR, 30min 0 Sep-Oct 20S Nov - Dec 40E 20W 0 20E Courtesy of Arlene Laing, presented at Warm Season Rainfall Workshop (9 June 2006)

12. Change in phase likely due to mesoscale convective vortex formation Courtesy of Arlene Laing Tropical N. Africa: 16 – 30 June 2003 253K 233K 213K 16 18 20 22 24 26 28 30

13. LATITUDE-TIME LATITUDE – PRESSURE CONVECTION MEAN ZONAL WIND (20W-35E) JUNE 2003 AEJ Shear Mean Latitude of convection with zonal wind shear (associated with AEJ) S N Courtesy of Arlene Laing

14. LATITUDE-TIME LATITUDE - PRESSURE CONVECTION MEAN ZONAL WIND (20W-35E) AUGUST 2003 TEJ AEJ Shear W’ly S Mean Latitude of convection with W’ly to E’ly shear (monsoon) N Courtesy of Arlene Laing

15. Comparing Continents Courtesy of Arlene Laing

16. Span vs Duration for Four Continents Europe, 1999-2003 Tropical N. Africa, 1999-2003 East Asia, 1998-2001 US Mainland, 1997-2000 Courtesy of Arlene Laing

17. Common Features of Episodes • Global phenomenon (on all continents with deep convec) • Genesis along and immediately downstream of significant topography • At least moderate vertical shear (10 m/s) in environment • Most frequent and longest-lived at height of warm season • Movement at speeds greater than synoptic disturbances (e.g., baroclinic waves) or low-middle tropospheric steering flow

18. Candidate Mechanisms for Long-Lived Coherent Propagating Convective Episodes • Density currents • Trapped gravity waves • Gravity-inertia waves in the free troposphere • Balanced circulations associated with and/or modified by convection (e.g., MCVs) Discussed by Carbone et al. (2002) J. Atmos. Sci

19. 0 6 12 18 24 Corridors of Precipitation July-Aug 1998-2002 Radar + RUC Analysis CAPE/Shear (600-900 mb) 900 mb Winds Radar 300 mb Winds/Heights Initiates during the night in the central plains Locally forced TIME (UTC) Propagating convection Initiates at time of max solar heating over higher terrain 110 100 90 80 LONGITUDE WEST From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.

20. 22 LST Surface Potential Temp/Winds/Reflectivity In situ or weakly propagating Rapidly propagating

21. 900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar 900 mb Winds 300 mb Winds/Hgts Days with strong LLJ (>12 ms-1) 45 days out of 310 - - + + From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.

22. 900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar 900 mb Winds 300 mb Winds/Hgts Days with weak/no LLJ (<5 ms-1) 32/310 - + - From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.

23. 900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar 900 mb Winds 300 mb Winds/Hgts Days with persistent corridors lasting 4 or more days + 0 + From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.

24. Diurnal Frequency Diagrams of Convection 3-10 July 2003 Longitude vs Time Rainfall Frequency 0 3 0% 6 9 9 12 15 19 18 21 28 Time (UTC hour) 0 3 37 6 47 9 12 56 15 18 65 21 0 105W 100 95 90 85W Longitude From Carbone et al. (2002; JAS)

25. July 3-10, 2003 500 hPa Height

26. Differing Regimes for Organized Convection Quasi-Stationary E-W Front Pattern Translating Synoptic Cold Front Pattern • Supports both MCSs and long narrower linear convection • Convection primarily afternoon and early evening • Large CAPE both along and ahead (south and east) of frontal • zone • “Classic MCS pattern” (e.g., week-long BAMEX Case) • Convection primarily nocturnal and early morning • Large CAPE confined to frontal zone (restricts scale • of convection)

27. 7-Day Simulations Using WRF (00Z 3 July to 00Z 10 July 2003) • Initial and Boundary Conditions Obtained from ETA Analyses (D t = 3h) • Yonsei University PBL Scheme with Noah LSM • Long and Shortwave Radiation Parameterization • 4-km Simulation: - Central US Regional Domain (625 x 445 x 35) - Explicit Convection (No Cumulus Parameterization) - Lin et al. (1983) based Microphysical Scheme

28. Comparison of Simulated and Observed Precipitation Episodes From Trier, Davis, Ahijevych, Weisman, and Bryan (2006), To appear in J. Atmos. Sci.

29. Rainstreak Phase Speed Statistics (03-10 July 2003) Frequency Zonal Phase Speed (m/s)

31. Composite System-Relative Flow, Theta (Contours), Theta-e (Colors) Intensifying Stage (Early Evening) Mature Stage (Overnight) Height (km AGL) Distance (km) Distance (km) Weakening Stage (Around Sunrise) • Five Cases • 40-km Along-Line Average Height (km AGL) Distance (km)

33. Rainstreak Propagation • Rainstreak movement cannot be explained by advection by mean • environmental flow through storm depth

34. Rainstreak Propagation (cont.)

35. Rainstreak Propagation (cont.) • Estimates of rain streak zonal phase speed based on mature stage cold-pool • negative buoyancy (left) are systematically high • Similar estimates based on the 16-km deep integrated buoyancy anomaly • (right) are much closer to observed rain streak zonal phase speeds

36. Composites of the Mesoscale Environment for Mature Stage

37. Composite Vertical Cross Sections of the Mesoscale Environment

38. Forward Trajectory Analysis for a Strong Frontal Case Example Trajectories from NE Trajectories from SW 3.0 3.0 Height (km MSL) Height (km MSL) 2.0 2.0 1.0 1.0 0.0 0.0 18 21 03 21 03 00 00 100 100 Relative Humidity (%) 80 80 60 60 40 40 Relative Humidity (%) 20 20 21 03 18 21 03 00 00 Time (hr UTC) Time (hr UTC)

39. Diurnal Frequency and Composite Mesoscale Environment of Propagating Convection 850 hPa Temperature/Winds

40. Some Remaining Questions • Are mechanisms for nocturnal propagation (a major component of long-lived episodes) similar on other continents? - e.g., poleward low-level jets (many continents, not Africa) • Initiation of many major episodes in central U.S. tied to both topography and mobile short waves. Are they related? • What governs intermittency (redevelopment along approximate same phase line in next heating cycle)? - amplification or refocusing free-tropospheric disturbance by convection? - density current dynamics/trapped gravity waves?