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The impact of African easterly The impact of African easterly waves on the environment and waves on the environment and characteristics of convection characteristics of convection
over West Africaover West Africa
Matthew A. Janiga and Chris D. ThorncroftMatthew A. Janiga and Chris D. Thorncroft
University at AlbanyUniversity at Albany
33rdrd AMMA Conference 7/24/2009 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
ζζ’ > 0’ > 0AEJ
Thorncroft and Hoskins (1990)
Trough
Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary
DeepConvection
ECMWF Interim Reanalysis 650 hPa PV during June-September 2004-2008 was degraded and maxima were objectively tracked.
Disturbances lasting 2 days or longer, moving at least 10° westward, and moving through the domain (7.5-17.5N, 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
0.5 1 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
30N
20N
10N
0
2006/8/30 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-1 2.5 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
030W 15W 0 15E 30E 45E
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.25 1.75 2.25 2.75 3.25 3.75 4.25
Production of MCS Tracks
• 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.
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
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
2 10 18 26 34 42 50
16
8
0
-8
-16
Relative Longitude
Re
lati
ve
La
titu
de
-16 -8 0 8 16
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.
LLVLLV
MLVMLV
Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary
LLVLLV
MLVMLVMLVMLV
900 Streamfunction and Wind
Streamfunction and Wind (0 Lat.) Streamfunction and Wind (0 Lon.)
650 Streamfunction and Wind
Rel. Longitude
Rel. Longitude
Rel. Longitude
Rel. Latitude
Rel
. Lat
itu
de
Rel
. Lat
itu
de
MLVMLV
Pre
ssu
re
Pre
ssu
re
EW S N
Stream Func. (104 m2 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
. Lat
itu
de
Rel. Longitude
Rel
. Lat
itu
de
QVC (10-20 Pa-1 s-3), Q (10-14 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 Rel. Longitude
Pre
ssu
re
Pre
ssu
re
Q-Vector Convergence (0 Lat.) Pressure Vertical Velocity (0 Lat.)
QVC (10-20 Pa-1 s-3) Omega ( hPa-1 hr-1)
EW EW
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
MLV Axis
Specific Humidity (0 Lat.)
Rel. LongitudeRel. Longitude
Rel
. Lat
itu
de
Rel
. Lat
itu
de
Pre
ssu
re
Pre
ssu
re
Specific Hum (g kg-1)
EW EW
Parcel Buoyancy Plan Views
Buoyancy is defined as…
Tvparcel – Tvenvironment
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
. Lat
itu
de
Rel
. Lat
itu
de
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
LFCLFC
Parcel Buoyancy (K) JJAS 04-08
0 5N 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)
6543210
-1-2-3-4
6543210
-1-2-3-4
Lag (Days)
Lag (Days)
Pre
ssu
reP
ress
ure
Lag (Days)
Lag (Days)
Summary
Introduction | Methods | Convection | Reanalysis | Radiosonde | Summary
Moist AirDry 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
http://www.atmos.albany.edu/student/janiga/weather_maps.htm