EASTERLY WAVE STRUCTURAL EVOLUTION OVER WEST AFRICA

AND THE EAST ATLANTIC

Matthew A. JanigaDepartment of Atmospheric and Environmental Sciences,

University at AlbanyAlbany, NY

Supported by NSF grant ATM0507976

29th AMS Conference on Hurricanes and Tropical Meteorology

May 14, 2010 Tucson, AZ

5D.6

1. Introduction and motivation.2. Composite methodology.3. Evolution of kinematic structure.4. Synoptic forcing for moist

convection.5. Conclusions and Open Questions.

Outline

θ [K, shaded], Wind [ms-1, vectors]

PV [PVU, shaded], U [ms-1, contours]

700 hPa Potential Vorticity and Zonal Wind

925 hPa Potential Temp. and Wind

• Reversal of mid-level PV strip allows for barotropic growth.

• Strong low-level temperature gradient allows for baroclinic growth.

• Large change in basic state at West Coast.

Motivation and Scientific Issues

• Composite study evaluation of intense AEW case studies.

• How does the AEW respond to changes in the zonal basic state?

• How does the AEW influence the development of moist convection using a convective ingredients approach?

• 2 day low-pass vorticity averaged over a 4.5° radius at each grid point.

• Maxima exceeding 1 x 10-5 s-1 are tracked by finding maxima closest to extrapolated positions.

• Keep those tracks that maintain westward velocity > 2 ms-1 between at least 7.5°E-22.5°E.

Relative Vorticity [shaded, x 10-5 s-1]thick (thin) = composited (excluded) track

solid (dashed) = composited (excluded) portion

Frances

Ivan

2004

700 hPa Relative Vorticity

Karl

Lisa

Track Zonal Velocity

• 64 tracks identified for compositing over the period JAS 1989-2005.

• Composites produced for 5° width bins centered every 5° from 5°E to 20°W.

• ECMWF Interim Reanalysis grids and Cloud User Archive Service (CLAUS) brightness temperature composited relative to the 700 hPa vortices.

Track Age

PV [PVU, shaded], Stream function [x 106 m2s-1, solid contours]

20°W 5°E

700 hPa Potential Vorticity and Stream Function

θ [K, contours], θ’ [K, shaded]

5°E5°E

925 hPa θ and 2-10 day filtered θ

Evolution of the Northern Vortex

925 hPa 2-10 day filtered height and wind

Height’ [m, shaded], Wind’ [ms-1, vectors]

L HHW

C

0°

925 hPa θ and 2-10 day filtered θ 925 hPa 2-10 day filtered height

and wind

θ [K, contours], θ’ [K, shaded] Height’ [m, shaded], Wind’ [ms-1, vectors]

Evolution of the Northern Vortex

0°

H LWC H

5°W

925 hPa θ and 2-10 day filtered θ 925 hPa 2-10 day filtered height

and wind

Evolution of the Northern Vortex

θ [K, contours], θ’ [K, shaded] Height’ [m, shaded], Wind’ [ms-1, vectors]

5°W

HH

LW

C

925 hPa θ and 2-10 day filtered θ 925 hPa 2-10 day filtered height

and wind

Evolution of the Northern Vortex

10°W

Height’ [m, shaded], Wind’ [ms-1, vectors] θ [K, contours], θ’ [K, shaded]

10°W

H HLW

C

Height’ [m, shaded], Wind’ [ms-1, vectors] θ [K, contours], θ’ [K, shaded]

925 hPa θ and 2-10 day filtered θ 925 hPa 2-10 day filtered height

and wind

Evolution of the Northern Vortex

15°W 15°W

HH

WC

Height’ [m, shaded], Wind’ [ms-1, vectors] θ [K, contours], θ’ [K, shaded]

925 hPa θ and 2-10 day filtered θ 925 hPa 2-10 day filtered height

and wind

Evolution of the Northern Vortex

20°W 20°W

HW

C

No more baroclinic growth

Convection and and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

5°E 5°E

900-700 hPa Q Convergence and Q

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

Convection and and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

0° 0°

900-700 hPa Q Convergence and Q

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

Convection and and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

5°W 5°W

900-700 hPa Q Convergence and Q

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

Convection and and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

10°W 10°W

900-700 hPa Q Convergence and Q

10°W

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

Convection and and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

15°W 15°W

900-700 hPa Q Convergence and Q

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

Convection and Q-Vectors

SF [x 106 m2s-1, contours], IR [% exceedance, shaded]

700 hPa Stream function and % times IR cooler than 220K

20°W20°W

900-700 hPa Q Convergence and Q

20°W

QConv [x 10-19 Pa-1s-3, shaded], Q [x 10-14 mPa-1s-3, vectors]

10°W

CAPE and CIN

CAPE and CIN

CAPE [J kg-1, shaded], CIN [J kg-1, contours]

• CAPE is enhanced by mid-level northerlies and reduced in the southerlies.

• CIN is reduced by low-level southerlies and enhanced in the northerlies.

• In the area of greatest convection (northerlies) CIN is enhanced but still “low enough” 25-75 Jkg-1.

θ+θ

-θ

θ+θ

-θ +θ΄-θ΄

+θ΄ -θ΄θ

Developing (East of 5°E)

Baroclinic Growth (0°-10°W)

West Coast Transition (10-20°W)

• Start with a disturbance on the PV strip.

• Strong baroclinic and barotropic tilts.

• Convection ahead of trough.

• Intensification of baroclinic wave.

• Enhancement (suppression) of convection in northerlies (southerlies) increases.

• Cessation of baroclinic growth.• Mid-level vortex becomes more

important.• Convection moves into the

trough.

• Mesoscale Issues– How is the low-level flow modified by complex

terrain such as the Air Mountains and Guinea Highlands?

– What is the mesoscale structure of the northern vortex and perturbations to the ITD?

– More complete diagnosis of balanced vertical motion.

• What is the spectrum of AEW disturbances: structure, origins, impacts?

Open Questions / Future Work

Thanks you for your time!

925 hPa θ (K, contours), 2-10 day filtered θ (K, shaded), vector wind (kts, barbs)average composite position (star)

Frontal Wave

925 hPa height (m, contours) 2-10 day filtered height (m, shaded)

Perturbation Low-level Flow

Q-Vectors & Q-Vector Convergence

925-700 hPa mean-layer Q conv. (x 10-19 Pa-1 s-3, shaded), Q vector (x 10-13 Pa-1 s-3, vectors)average composite position (star)

Forcing for Vertical Motion

zx

θ

θ

zy

Adapted from Raymond and Jiang (1990)

• If the potential vorticity is steady the isentropic upglide represents the adiabatic vertical motion from a Lagrangian perspective.

• In reality the shear deforms the PV lifting isentropic surfaces.

PV in West African Shear

With Baroclinicity

Easterly Jet

Monsoon

Unfiltered and 2-10 day filtered parcel buoyancy

Pb’ [K, shaded], Pb [K, contours]

LFC