matthew a. janiga and chris d. thorncroft university at albany 3 rd amma conference 7/24/2009

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)


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


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


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.


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.


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


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


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.


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.


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.


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.


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?


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.


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


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


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

matthew a. janiga and chris d. thorncroft university at albany 3 rd amma conference 7/24/2009

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