Mesoscale Convective Systems Associated with African Easterly Waves

1. Mesoscale Convective Systems Associated with African Easterly Waves Niamey Bamako 30th AMS Conference on Hurricanes and Tropical Meteorology Matthew A. Janiga and Chris D. Thorncroft University at Albany, SUNY 500 km

2. What do we know about the AEW-MCS Relationship? 12.5°-20°N Squall Line Generation • Fink and Reiner (2003) found that squall line generation occurred NW of the mid-level AEW vortex and south of the northern AEW low-level vortex. • Dissipation occurred in the ridge. NE Latitude NW 5°-12.5°N Mid-level Vortex Longitude Adapted from Fink and Reiner (2003)

3. What is responsible for this relationship? Based on Berry and Thorncroft (2005) • The NW squall line initiation maximum is related to a westerly low-level jet (LLJ) with high equivalent potential temperature (θe) (Berry and Thorncroft, 2005). • The westerly LLJ is associated with the northern low-level vortex. SHL – Saharan heat low LLJ – Low-level jet ITD – Intertropical discontinuity AEJ – Africa easterly jet (at ~600 hPa) X – Vorticity maximum or trough

4. What is still not understood? a) The Properties of the Precipitation Features… • The size, intensity, and vertical development of the precipitation features (PFs). • The vertical structure of convective and stratiform rain. • These properties are crucial to diagnosing heating rates and the upscale effect of convection on the AEWs. b) How AEW Structure Controls this Relationship… • The vertical distribution of moisture and temperature. • This is important for forecasting intense events.

5. Convection as Viewed by the TRMM Platform

6. Data Used to Determine AEW Phase, Amplitude, and Structure TRMM 3B42 is a continuous gridded rainrate dataset (Huffman et al., 2007). 3-hrly observations at 0.25° grid-spacing. Utilizes passive microwave, precipitation radar (PR), geostationary infrared (IR), and rain gauge data. Wavenumber-frequency filtered TRMM 3B42 is used to determine AEW phase and amplitude. The NCEP Climate Forecast System Reanalysis (CFSR) (Saha et al., 2010) is one of the latest-generation reanalyses. Analyses have a horizontal grid-spacing of 0.5° and 29 vertical levels between 1000-50 hPa. Analyses are composited by AEW phase to diagnose the thermodynamic and kinematic AEW structure.

7. Climatological Rain Rate The plot shows the rainrate from averaging all the TRMM PR passes. The unconditional rainrate includes zeros. The PR samples each point about every 3 days.

8. Climatological Rain Rate Intensity Sahelian Belt Congo In this plot the averaging is conditional, it excludes 0 mm hr-1 rain rates. This indicates the rain rate intensity.

9. Identification of Precip. Features (PFs) • Precipitation features (PFs) are defined as contiguous regions of rain rate > 0 mm hr-1from the TRMM PR (no size criteria are applied). • Using the rain rate and reflectivity profiles for each PF, a variety of criteria can be calculated: • Rain rate: PF size, volumetric rain, convective/stratiform ratios. • Reflectivity: Echo tops, the area > 20 dBZ above 10 km. • TRMM Microwave Imager (TMI): Minimum 85 gHZ polarization corrected temperature. • Since small PFs are unimportant, we focus on the total rainfall from PFs with characteristics.

10. Volumetric Rain Contributions from PFs • We can determine the relative importance of certain PF characteristics by calculating their contribution to the volumetric rain fall in each region. • To summarize the distribution of PF contributions, the 50% divide is calculated (see Liu 2011). MCS Sub-MCS 50% 50% 50% of the rainfall comes from PFs larger than 21,500 km2. The percent number of PFs larger than this is much smaller. 10,000 km2 marks the approximate separation of “MCS” from “Sub-MCS” rainfall.

11. Volumetric Rain Contribution: System Size 10000 The “50/50 split” of the volumetric rain bins at each 1.0°x1.0° grid box. The regions with high rain rates are dominated by contributions from large PFs while the cold tongue and Egypt are dominated by very small systems.

12. Volumetric Rain Contribution: Echo Tops • Add 85 gHZ PCT • There is an axis of intense convection stretching from Sudan westwards toward Senegal. • A second region of intense convection is found on the slopes where the Great Escarpment meets the Congo Basin.

13. Climatological CAPE and Shear

14. AEW Phase Partitioning using Wavenumber-Frequency Filtered Rain TRMM PR Binning Schematic INC 1.5σ NOT USED MIN MAX DEC

15. AEW Activity Measured by Filtered 3B42 AEW Composite Domain 5-15°N, 10°W-20°E is used to construct the AEW composites. This region has convection ahead of the trough and a similar amplitude (e.g. Kiladis et al. 2006).

16. Partitioning AEWs into Phases Using TD Filtered TRMM 3B42 Inc. Min. Dec. Max. Kelvin and TD signals contoured every 1σ. Phases masked for amplitude >1.5σ

17. Variation of Rainfall and Stratiform Ratios by Wave Phase The TD waves show a clear transition from convective to stratiform rainfall between the leading and trailing edges of the convective envelope. Variation in stratiform fractions are due to changes in spatial coverage AND conditional intensity. Trough Ridge S Ridge N Black solid = Total rainrate Blue solid = Stratiform rainrate Red solid = Convective rainrate Black dashed = % stratiform rainrate

18. Relative Importance of Intensity and Fractional Areal Coverage Conditional Rain Rates Blue solid = Stratiform rainrate Red solid = Convective rainrate Fractional Area Coverage Blue solid = Stratiform coverage Red solid = Convective coverage

19. Wave-Phase CFADs (Convective) Comp. 7:INC 8 Shows a composite CFAD for all phases and the differences between each phase. CFADs are normalized for each phase. The main difference is a tendency towards increased shallow-warm rain in the suppressed phase and deep convection in the enhanced phase. 3: MAX 2 1: MAX 4 6 5: MAX

20. Wave-Phase CFADs (Stratiform) Comp. 7:INC 8 3: MAX 2 1: MAX The main difference is a tendency towards increased reflectivity in the enhanced phase and reduced reflectivity in the suppressed phase. 4 6 5: MAX

21. Wave-Phase Rainfall Contribution A 2D histogram of the rainfall contributions as a function of PF size / echo top and wave-phase. Most of the rainfall from large/deep PFs occurs in the convective envelope. The distributions are slightly shifted toward small/less-deep systems in the suppressed phases.

22. Modification of the Diurnal Cycle Phase 1 Enhanced Phase 5 Suppressed In the enhanced phase, rain rate is somewhat constant except for a reduction at 1200 LT. In the suppressed phase, rain rate peaks at 1800 LT and quickly drops to 25% this amount between 0600-1200 LT.

23. Vortex Structure Trough Ridge The cold-core AEWs have troughs and ridges which peak between 600-700 hPa. There is a quadrapole of temperature anomalies. This is because the temperature gradient reverses above 700 hPa. Reduced Stability Increased Stability

24. U and V Structure The cold-core AEWs have troughs and ridges which peak between 600-700 hPa. The AEJ strengthens ahead of the trough and weakens behind it (could be inflow to stratiform precip.). Surface westerlies are enhanced ahead of the trough. Reduced Shear Enhanced Shear

25. Vertical Motion Structure Shallow - Deep - Stratiform The vertical motion and divergence profiles agree with TRMM showing a transition from shallow to deep convective and then to stratiform precip.

26. Instability and Moisture Moist Dry Mid-level moisture is greater in the northerlies. This is closely related to the profiles of θe. Temperature matters in the low-levels. Low CAPE High CAPE

27. Summary Illustration of the AEW-Convection Relationship • AEWs increase the vertical extent, size, and rainfall intensity of convective systems. • They also support convection through the unfavorable portions of the diurnal cycle. • Shear, instability, lift, and moisture are all increased in the northerlies.

28. Illustration

29. Future Work • Kinematic properties of convection using velocity data from Bamako and Niamey radars. • The vertical profiles of convective & stratiform heating rates using TRMM estimated Q1-QR. • How does the convection-AEW phase relationship (as determined by TRMM) vary by region and season? • How do these results compare to other waves like Kelvin waves?

30. Questions?

31. MCSs associated with the “Helene AEW” of 2006

32. Life-Cycle of the Niamey MCS Sep. 7, 1200Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) Triggering occurs near the Jos Plateau ahead of the AEW trough.

33. Life-Cycle of the Niamey MCS Sep. 7, 1800Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) In the late afternoon convection expands in coverage.

34. Life-Cycle of the Niamey MCS Sep. 8, 0000Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) Most of the other convection weakens but the Niamey MCS strengthens as the nocturnal LLJ develops.

35. Life-Cycle of the Niamey MCS Sep. 8, 0600Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) More intensification occurs through 0600Z when the LLJ reaches its peak intensity.

36. Life-Cycle of the Niamey MCS Sep. 8, 1200Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) As the LLJ weakens at 1200Z the MCS begins to dissipate.

37. 925 hPa θv (K, shaded), streamfunction (x106 m2s-1, contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1, pattern). NV MCS Sep. 8, 0600Z 2006 • 925 hPa θe (K, shaded), streamfunction (x106 m2s-1, contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1, pattern). NV MCS

38. Niamey Radar 150 km

39. Niamey Radar 150 km

40. Niamey Radar 150 km

41. Niamey Radar 150 km

42. Niamey Radar 150 km

43. Niamey Radar 150 km

44. Niamey Radar 150 km

45. Niamey Radar 150 km

46. Niamey Radar 150 km

47. Life-Cycle of the Bamako MCS Sep. 9, 1800Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) Triggering occurs in the late afternoon.

48. Life-Cycle of the Bamako MCS Sep. 10, 0000Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) Intensification occurs as the LLJ begins to develop.

49. Life-Cycle of the Bamako MCS Sep. 10, 0600Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) Continued intensification occurs up through 0600Z when the LLJ reaches its peak intensity.

50. Life-Cycle of the Bamako MCS Sep. 10, 1200Z 2006 • IR (shaded) and 700 hPa Streamfunction • (x106 m2s-1, contours) The MCS begins to weaken at 1200Z as the LLJ begins to weaken.