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The environment and characteristics of convective events over Niamey, Niger:

AMMA SOP2 observations and climatological context

Matthew A. Janiga1, Chris D. Thorncroft1, and Earle Williams2

University at Albany1, MIT Lincoln Laboratory2

Introduction

AEW Context

Monsoon Context SOP2 Convection

1 2Aїr Mountains

ConvectionAїr Mountains

Convection

3 4Local

Convection

The Role of AEWs

July 26 Event

August 3 Event

The hour (UTC) of maximum <233 K pixel frequency in 10.8 μm IR imagery during JAS 2004-2008 with elevation greater than 500 m overlaid in blue.

The fraction of time pixels are cooler than 233 K in 10.8 μm IR imagery during JAS 2004-2008 at each 30 min interval (UTC).

• 16 events associated with LLV moisture transport

• 11 with MLV adiabatic forcing for ascent

• 10 with gust fronts from existing convection

• 4 with extratropical PV streamers interacting with the baroclinic zone

Aїr Mountains

• In a composite AEW adiabatic forcing for ascent and rainfall were found ahead of the mid-level vortex (MLV). Moisture advection was found southeast of the low-level vortex (LLV).

• Niamey has a brief climatological window of favorable instability and shear (mid-July to mid-September) for the development of organized convection.

• Propagating convection dominates up to 10 degrees downstream of the Aїr Mountains. Niamey has one of the strongest nocturnal convection peaks in West Africa.

• The formation of most Niamey SOP2 convective events occurred in distinct clusters over the Jos Plateau and Aїr Mountains.

• The formation (passage) of these convective events occurred in the late afternoon (early morning), except in the case of gust front developments.

• Moisture advection associated with the LLV or adiabatic forcing associated with the MLV played a role in the formation of 22 of the 30 SOP2 convective events.

Diurnal Cycle Context

To determine the composite structure of AEWs passing through the Niamey region during JAS 2004-2008, fields from the ERA Interim Reanalysis and CMORPH were first 1-20 day band-pass filtered to highlight the perturbation AEW structures.

LLV

MLV

The seasonal cycle of positive area, shear, and rainfall from 0-10°E.

Gust Front

* Cumulus Field Jos Plateau

Aїr Mountains

0 3 6 9 12 15 18 21

The genesis hour (UTC, colors) of convective events impacting Niamey during JAS 2006 overlaid on elevation (m, grayscale).

The hour (UTC) of the genesis and passage time of the above events.

Relative Longitude

Rel

ativ

e L

atit

ud

e

The relative latitude and longitude of AEW mid-level (MLVs, O) and low-level vortices (LLVs, X) to convection genesis. Red LLVs (MLVs) are associated with enhanced moisture transport (850 hPa q-vector convergence).

Aїr Mountains

Jos PlateauNiamey

4 123

The AEW circulation tilts southeast with height below the AEJ. Adiabatic forcing is found ahead of the MLV associated with the low to mid-level shear. Enhancement of the low-level monsoon flow and moisture is found east of the LLV. Terrain and mesoscale boundaries also aid in the development of convection.

Mid-levelVortex(MLV)

Baroclinic Zone

Mid-levelPV Strip

Low-levelVortex(LLV)

AEJ

3/00 900 Q, Rel. Vor.

1500

1250

1000

750

500

250

0

3/00 600 PV, 850 Q-Vector Conv.

25/18 900 Q, Rel. Vor.

Diabatic PV Maxima

Low-level Flow

25/18 600 PV, 850 Q-Vector Conv.

3/00 3/03

3/06 3/14

1500

1250

1000

750

500

250

0

1500

1250

1000

750

500

250

0

N

25/14 25/16

26/0225/23

9

7

5

3

1

9

7

5

3

1

18

16

14

12

10

8

6

4

2

18

16

14

12

10

8

6

4

2

J F M A M J J A S O N D

GPCP (0-10E) Daily Avg. Precip. (mm/day) 1997-2007

ERA Int. (0-10E) Daily Avg. 600-900 Shear (m/s) 2004-2008

J F M A M J J A S O N D

ERA Int. (0-10E) 12 UTC Pos. Area (J/kg) 2004-2008

Aїr Mountains Convection

J F M A M J J A S O N D

Summary

The genesis of convection that passed over Niamey during AMMA SOP2 was analyzed to understand whether terrain, gust fronts, or African easterly waves (AEWs) played a role in the development. These findings were placed within the context of the mean AEW structures, monsoon, and diurnal cycle from 2004-2008.

The relationship between adiabatic forcing for ascent and moisture transport with convection impacting Niamey was examined. Two events where each process dominated were examined.

Track Density (10-2 passages day-1) of MLVs JAS 04-08

PDF of poster available at http://www.atmos.albany.edu/student/janiga/cv.htm