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Matthew A. Janiga and Chris D. Thorncroft NE Tropical Conference 6/25/2009

The impact of African easterly waves on the environment and characteristics of convection over West Africa. Matthew A. Janiga and Chris D. Thorncroft NE Tropical Conference 6/25/2009. Questions.

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Matthew A. Janiga and Chris D. Thorncroft NE Tropical Conference 6/25/2009

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  1. The impact of African easterly waves on the environment and characteristics of convection over West Africa Matthew A. Janiga and Chris D. Thorncroft NE Tropical Conference 6/25/2009

  2. Questions • What is the structure of fields in an African easterly wave (AEW) that are relevant to the development of moist convection in a composite wave? • What are the mechanisms that cause convection to develop where it does within an AEW? Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  3. Review • Distribution of convection and it’s lifecycle… • Enhanced cloud cluster genesis ahead of trough. • Enhanced decay behind the trough and in the ridge. • Greatest rainfall ahead of the trough. • The 3D structure of the AEW… • Low-level adiabatic forcing for ascent ahead of the trough and descent behind the trough. • Fink and Reiner (2005) suggest that moisture advection could reduce negative area in the southerly flow. Cluster Genesis/Decay ζ’ > 0 AEJ Trough R N T S R Payne and McGarry (1977) Thorncroft and Hoskins (1990) Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  4. The Basic State of West Africa PV (0.1 PVU) JJAS 04-08 θ (K) U (m/s) JJAS 04-08 AEJ + ∂q/∂y - ∂q/∂y + ∂θ/∂y 0 5N 10N 15N 20N 25N 30N 0 5N 10N 15N 20N 25N 30N 233 K Exceedance Freq. (%) JJAS 04-08 Parcel Buoyancy (K) JJAS 04-08 20N 15N 10N 5N 0 A G J C LFC 0 5N 10N 15N 20N 25N 30N 20W 10W 0 10E 20E 1 3 5 7 9 11 13 15 17 19 Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  5. Production of AEW Tracks • ECMWF Interim Reanalysis PV during June-September 2004-2008 was degraded and 650 hPa PV maxima were objectively tracked. • Disturbances lasting 2 days or longer, moving at least 10° westward, and moving through the domain were used to composite reanalysis fields, radiosonde data, and objectively tracked cloud clusters. • 291 events or 2570 AEW “snapshots.” 30N 20N 10N 0 AEW 1 AEW 2 AEW 4 AEW 3 30W 20W 10W 0 10E 20E 30E 40E 50E Berry et al. (2007) 0.5 1 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  6. AEW Tracks Track density (10-2 passages day-1 2.5 degrees-1) of AEWs. • The track density of these features is peaked over topographic features which are convectively active. • There is an increase in track density moving westward. • The most intense disturbances are also to the west. • The track density and mean intensity of the AEWs will make the composites weighted toward the character of Guinea Highland AEWs. 25N 20N 15N 10N 5N 0 30W 15W 0 15E 30E 45E 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Mean Intensity (0.1 PVU) of qualified AEWs. 25N 20N 15N 10N 5N 0 30W 15W 0 15E 30E 45E 1.25 1.75 2.25 2.75 3.25 3.75 4.25 Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  7. Production of MCS Tracks NASA Merged 10.8 μm IR 2006/8/30 6 UTC • 30min, 4km resolution IR data from JJAS 2004-2008 was used to objectively track mesoscale convective systems (MCSs). • MCSs are defined as IR clusters cooler than 233K with a minimum area of 5000km2. 233K is well correlated with the boundaries of a precipitating cloud shield. • A subset of MCSs (LL) lasting longer than 6h was produced. While they are a small percent of the number of cold cloud clusters they explain 80% of all pixels cooler than 233K in the time period. 20N 10N 0 20W 10W 0 10E 20E 30E 40E Clusters <233 K 2006/8/30 6 UTC 20N 10N 0 20W 10W 0 10E 20E 30E 40E Introduction | Methods | Convection| Reanalysis | Radiosonde | Summary

  8. AEW Relative Long-Lived (>6 h) MCS Genesis • The increased genesis ahead of the trough and decreased genesis in the ridge and southerlies is significantly different than climatology according to a bootstrap test. • 50% increase in LL MCS genesis frequency ahead of the trough. • 50% decrease in the southerlies. Actual Events (#) Dif. from Climo. (#) 16 8 0 -8 -16 16 8 0 -8 -16 Relative Latitude Relative Latitude -16 -8 0 8 16 -16 -8 0 8 16 -12 -8 -4 0 4 8 12 2 10 18 26 34 42 50 Relative Longitude Relative Longitude Introduction| Methods |Convection| Reanalysis | Radiosonde | Summary

  9. Streamfunction and V Plan Views and Cross-Sections • The low-levels are characterized by a low-level baroclinic vortex (LLV) and the bottom of the mid-level (MLV). • The bowed wind perturbations suggest that barotropic energy conversions associated with the mid-level PV reversal are occurring. • The trough tilts eastward with height below the AEJ and westward above the AEJ at both. This against the shear tilt suggest baroclinic energy conversions. LLV MLV MLV MLV LLV Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  10. Temperature Plan Views and Cross-Sections • The thermal structure of the SV can be understood as a combination of the PV anomaly and temperature advection (gradient reverses above the AEJ). • Temperature advection dominates near the NV. • The warming above 600 hPa reduces the positive area of soundings. Trough Introduction| Methods | Convection| Reanalysis| Radiosonde | Summary

  11. Specific Humidity Plan-Views and Cross-Sections • Specific humidity perturbations associated with the SV are dominated by moist convection. Those with the NV are dominated by horizontal advection. • The specific hum. perturbation is coincident with the strongest southerlies. • Climatological gradients matter too! This is also where the boundary between the Saharan air layer (SAL) and monsoon layer is located. Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  12. Buoyancy Plan Views • Buoyancy is defined as… • Tvparcel – Tvenvironment • for a pseudoadiabaticlly rising parcel. • The southerly moisture flux and vertical temperature gradient associated with the PV anomaly enhance low-level buoyancy. • Advection of dry SAL air reduces low-level buoyancy. • Moist southerlies undercutting the SAL enhances upper-level buoyancy; the PV anomaly reduces it. Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  13. Q-Vector Convergence and Omega • Adiabatic forcing for ascent associated with the SV is greatest at 800 hPa and weak above the AEJ. • The SV q-vector conv. can be scaled to ~ 1 hPa/hr. • The NV and SV ascent/descent couplets are well represented in the omega fields. • While the forcing for ascent is only at low-levels moist convection results in deep ascent. Introduction | Methods | Convection| Reanalysis | Radiosonde | Summary

  14. AMMA Campaign 2006 Radiosondes • Niamey and Parakou launched the greatest number of soundings during JAS 2006. • Radiosonde composites were generated for JAS 2006 when the objectively tracked PV anomalies passed within 5° of a station. • Niamey - 15 events • Parakou - 14 events • Composites were generated using all times at 6 hr lags relative to the event. 1961–90 Mean Annual Precip. (mm) and Radiosonde Network Parker et al., (2007) Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  15. Niamey and Parakou Radiosonde Composites • Overall agreement with reanalysis composites. • Negative area in the southerlies is also about half that of the northerlies. • A mean sounding in Niamey has more positive area but also more negative area than one in Parakou. Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

  16. Schematic of the Convective Environment of the AEW Introduction| Methods | Convection| Reanalysis | Radiosonde | Summary

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