EASTERLY WAVE STRUCTURAL EVOLUTION OVER WEST AFRICA AND THE EAST ATLANTIC Matthew A. Janiga Department of Atmospheric and Environmental Sciences, University at Albany Albany, NY Supported by NSF grant ATM th 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 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 s -1 ] thick (thin) = composited (excluded) track solid (dashed) = composited (excluded) portion Frances Ivan hPa Relative Vorticity Karl Lisa

Track Zonal Velocity 64 tracks identified for compositing over the period JAS 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 10 6 m 2 s -1, solid contours] 20°W 5°E 700 hPa Potential Vorticity and Stream Function

θ [K, contours], θ’ [K, shaded] 5°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 H H W 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 L W C 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 H H L W 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 H L W 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 H H W 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 20°W H W C No more baroclinic growth

Convection and and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 5°E hPa Q Convergence and Q Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

Convection and and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 0° hPa Q Convergence and Q Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

Convection and and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 5°W hPa Q Convergence and Q Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

Convection and and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 10°W hPa Q Convergence and Q 10°W Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

Convection and and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 15°W hPa Q Convergence and Q Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

Convection and Q-Vectors SF [x 10 6 m 2 s -1, contours], IR [% exceedance, shaded] 700 hPa Stream function and % times IR cooler than 220K 20°W hPa Q Convergence and Q 20°W Q Conv [x Pa -1 s -3, shaded], Q [x mPa -1 s -3, vectors]

10°W 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” Jkg -1.

θ +θ+θ -θ-θ θ +θ+θ -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 hPa mean-layer Q conv. (x Pa -1 s -3, shaded), Q vector (x Pa -1 s -3, vectors) average composite position (star)

Forcing for Vertical Motion z x θ θ z y 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