The Structure of AEWs in the CFSR and their Relationship with Convection.

The Structure of AEWs in the CFSR and their Relationship with Convection

Motivation

Long-lived synoptic-scale disturbances known as African Easterly Waves (AEWs) play an important role in modulating rainfall during the West African Monsoon (WAM).

The diabatic heating associated with deep moist convection is important to determining both the structure of synoptic systems and their upscale impact.

Recent observations such as those from the MIT radar (e.g. Barthe et al., 2010) and the latest generation of reanalyses (CFSR and MERRA) finally allow for a comparison.

Tracking Methodology

• Tracking field: 2 day low-pass filtered 700 hPa circulation (3° radius).

• Tracked vortices: Iterative solving of a cost function of speed change and direction change (e.g. Hodges, 1999) for maxima exceeding 2x10-5s-1 over 1998-2009.

• Composited vortices with >10° westward propagation lasting > 2 days in 5° wide longitudinal bins every 5° from 30°W to 30°E excluding > TD.

Intensity Change of AEWs

700 hPa Streamfunctionand Rainrate (TRMM 3b42 0.25° 3-hrly)

20°E, 15°E , 10°E , 5°E , 0°E , 5°W , 10°W , 15°W , 20°W , 25°W , 30°W

TRMM 3B42 3hrly Average

925 hPa θ and θ’

20°E, 15°E , 10°E , 5°E , 0°E , 5°W , 10°W , 15°W , 20°W , 25°W , 30°W

CFSR Analysis

925 Relative Vorticity and Wind

20°E, 15°E , 10°E , 5°E , 0°E , 5°W , 10°W , 15°W , 20°W , 25°W , 30°W

CFSR Analysis

850 hPa RH

20°E, 15°E , 10°E , 5°E , 0°E , 5°W , 10°W , 15°W , 20°W , 25°W , 30°W

CFSR Analysis

850 hPa Pressure Vertical Velocity

20°E, 15°E , 10°E , 5°E , 0°E , 5°W , 10°W , 15°W , 20°W , 25°W , 30°W

CFSR Analysis

Total Diabatic Heating 15°W vs. 15°E

Are these heating profiles realistic?

c/s = 0.98

JAS

10-5 s-1 per mm/hr

Sep 11 – Oct. 2 2001 EPIC (East Pacific)

JAS 2006-2007 AMMA (Niamey, Niger)

The large area of increasing convergence with height is suggests a confirmation of the deep area of downdrafts and cooling in the CFSR composites of inland AEWs.

Mapes and Lin (2005)

Open Questions

1. Why do AEWs intensify so rapidly near the coast?– Increased convection over Guinea Highlands.– Lack of friction.– Shift of convection toward trough.– Changing nature of convection (increased low-

level θe and RH > less downdrafts increased ω > low-level vorticity stretching).

Open Questions

2. How well is convection parameterized in AEWs?– Basic structure of heating, momentum transport,

etc. appear consistent with the limited observations.

Open Questions

3. What is the spread?– Very little is known about the range of these

relationships both in analyses and observations.– Different modes of convective organization.– How does intraseasonal variability impact these

relationships.

The Structure of AEWs in the CFSR and their Relationship with Convection.

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