The impact of African easterly waves on the environment and characteristics of convection over West Africa Matthew A. Janiga and Chris D. Thorncroft University at Albany 3 rd AMMA Conference 7/24/2009

 We composited mesoscale convective system genesis locations, reanalysis data, and radiosonde data onto objectively tracked African easterly waves (AEWs).  We will address the question why does convection develop where it does within an AEW? Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Objectives

 Distribution of convection and it’s lifecycle… –Enhanced cloud cluster genesis ahead of trough. –Enhanced decay behind the trough and in the ridge. –The greatest rainfall is ahead of the trough.  Fink and Reiner (2005) suggest that there is a secondary genesis maximum in the southerlies north of 12.5°N. Review – Distribution of Convection R N T S R Cloud Cluster Genesis Payne and McGarry (1977) Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary

 Low-level adiabatic forcing for ascent ahead of the trough and descent behind the trough (small arrows).  Thorncroft and Hoskins (1990) propose that the low-level forcing ahead of the trough would trigger deep moist convection (big arrow).  There is also advection of dry air in the northerlies and moist air in the southerlies which modifies the distribution of negative area. Review – Structure of the AEW ζ ’ > 0 AEJ Thorncroft and Hoskins (1990) Trough Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Deep Convection

 ECMWF Interim Reanalysis 650 hPa PV during June-September was degraded and maxima were objectively tracked.  Disturbances lasting 2 days or longer, moving at least 10° westward, and moving through the domain ( N, 15W-20E) were used to composite reanalysis fields, radiosonde data, and objectively tracked cloud clusters.  291 events or 2570 AEW “snapshots.” Production of AEW Tracks AEW 4 Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary AEW 3 AEW 2 30W 20W 10W 0 10E 20E 30E 40E 50E N 20N 10N /8/30 06 UTC 650 hPa PV, streamfunction, objective troughs and jet (Berry et al., 2007), and objective AEW PV maxima. AEW 1

AEW Tracks Track density (10 -2 passages day degrees -1 ) of AEWs. Mean Intensity (0.1 PVU) of qualified AEWs.  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 west coast AEWs. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary 30W 15W 0 15E 30E 45E 25N 20N 15N 10N 5N 0 25N 20N 15N 10N 5N 0 30W 15W 0 15E 30E 45E

Production of MCS Tracks 30min, 4km resolution IR data from JJAS was used to objectively track mesoscale convective systems (MCSs). MCSs are defined as IR clusters cooler than 233K with a minimum area of 5000km K 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. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary NASA Merged 10.8 μm IR 2006/8/30 6 UTC Clusters <233 K 2006/8/30 6 UTC 20N 10N 0 20N 10N 0 20W 10W 0 10E 20E 30E 40E

AEW Relative Long-Lived (>6 h) MCS Genesis  Location of LL MCS genesis relative to the AEW mid-level vortex (MLV).  There is increased genesis ahead of the trough and decreased genesis in the ridge and southerlies.  Based on the composite structure of an AEW why do we see this? Number of Genesis Events Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Relative Longitude Relative Latitude MLV

Streamfunction and V Plan Views and Cross-Sections  All reanalysis composites show use 1-20 day band-pass filtered fields.  The low-levels are characterized by a low-level vortex (LLV) on the baroclinic zone and the bottom of the mid-level vortex (MLV).  The bowed wind perturbations suggest barotropic energy conversions.  The trough tilts eastward with height below the AEJ and westward above the AEJ suggesting baroclinic energy conversions. LLV MLV Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary LLV MLV MLV 900 Streamfunction and Wind Streamfunction and Wind (0 Lat.) Streamfunction and Wind (0 Lon.) 650 Streamfunction and Wind Rel. Longitude Rel. Latitude MLV Pressure E W S N Stream Func. (10 4 m 2 s -1 ), Wind (m s -1 )

Q-Vector Convergence and Vertical Velocity Plan Views  Q-vector convergence of the non-divergent wind (e.g. Kiladis et al., 2006).  The structure of the MLV and LLV ascent/descent couplets in the two fields compare well. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary 800 Q-Vectors and Q-Vector Conv.800 Pressure Vertical Velocity Rel. Longitude Rel. Latitude Rel. Longitude Rel. Latitude QVC ( Pa -1 s -3 ), Q ( m Pa -1 s -3 )ω ( hPa -1 hr -1 )

Q-Vector Convergence and Vertical Velocity Cross-Sections  Adiabatic forcing for ascent associated with the MLV is greatest at 800 hPa and weak above the AEJ.  While the forcing for ascent is only at low-levels moist convection results in deep ascent. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Rel. Longitude Pressure Q-Vector Convergence (0 Lat.)Pressure Vertical Velocity (0 Lat.) QVC ( Pa -1 s -3 )Omega ( hPa -1 hr -1 ) E WE W

Specific Humidity Plan-Views and Cross-Sections  Specific humidity perturbations associated with the MLV include moist convection. Those associated with the LLV are dominated by advection.  Gradients matter! The low-level moisture anomalies are where the boundary between the Saharan air layer and monsoon layer is located. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Rel. Longitude 900 Specific Humidity 500 Specific Humidity LLV Specific Humidity (+6 Lat.) Rel. Longitude MLV Axis Specific Humidity (0 Lat.) Rel. Longitude Rel. Latitude Pressure Specific Hum (g kg -1 ) E W E W

Parcel Buoyancy Plan Views  Buoyancy is defined as… Tv parcel – Tv environment for a parcel in psuedo- adiabatic ascent.  The southerly moisture flux enhances low-level buoyancy (reduces negative area).  Advection of dry SAL air reduces low-level buoyancy.  Moist southerlies undercutting the SAL enhances upper-level buoyancy (increaces positive area); the warm anomaly above the MLV reduces it. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary PV cool anomaly & moisture Less CIN PV warm anomaly Less CAPE Moisture More CAPE Rel. Longitude Dry air More CIN Rel. Latitude Rel. Longitude 800 Parcel Buoyancy 500 Parcel Buoyancy Buoyancy (K)

AMMA Campaign 2006 Radiosondes  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  Is the thermodynamic modification by the AEW more important to the north than to the south? Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary LFC Parcel Buoyancy (K) JJAS N 10N 15N 20N 25N 30N Parakou Niamey

Niamey and Parakou Radiosonde Composites  Negative area in the southerlies is also about half that of the northerlies.  Moisture advection and changes in negative area are more important the farther north one goes. Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Niamey Negative Area Parakou Negative Area Parakou Parcel Buoyancy (K) Niamey Parcel Buoyancy (K) Lag (Days) Pressure Lag (Days)

Summary Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary Moist Air Dry Air Low-level adiabatic forcing.  Low-level adiabatic forcing ahead of the MLV agrees well with the location of MCS genesis.  Thermodynamics are important for AEWs that track north or for locations near the MLV.  The LLV moisture advection played a role in 17 of the 30 convective events impacting Niamey during AMMA SOP2 (Poster 9A-1).

Future Work Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary  How do these composites vary with: geography, season, and through the lifetime of the AEW? Curvature Vorticity and TRMM Rain