the structural evolution of african easterly waves

The Structural Evolution of African Easterly Waves

Matthew A. Janiga and Chris ThorncroftDEPARTMENT OF ATMOSPHERIC AND

ENVIRONMENTAL SCIENCESUniversity at Albany, State University of New York

Northeast Tropical Conference5/18/2011

Supported by NSF Grant: ATM0507976

Much is known about the mean kinematic and thermodynamic structure of African easterly waves (AEWs) (e.g. Reed et al., 1977). However, comparatively little is known about there mean structural evolution.

Relationships between AEWs and organized convection have been observed (Fink and Reiner, 2003). However, the role of the 3D flow and sub-synoptic scale features associated with the AEW in this relationship is poorly understood.

Lastly, while the importance of the upscale impact of convection on AEWs has been demonstrated in case studies (e.g. Berry and Thorncroft, 2005; Schwendike and Jones, 2010), the representativeness of these studies is not known.

Background and Motivation

The Composite Evolution of African Easterly Waves

Methodology

Tracks of AEWs were determined by tracking long-lived synoptic-scale vorticity maxima at 700 hPa during JAS 1998-2009 (see Hodges et al., 1999).

The composite structural evolution of AEWs was determined by compositing analyses and forecasts from the NCEP Climate Forecast System Reanalysis (CFSR) and TRMM 3B42 rainrate estimates.

SV

AEJ

NV

2000-3000 km Wavelength~8 ms-1surface

jet-level

Carlson (1969)

Composite Location for Developing Phase

GATE Array

MIT Radar

20°E

Composite Location for Mature Baroclinic Phase

GATE Array

MIT Radar

5°W

Composite Location for Coastal Transition Phase

GATE Array

MIT Radar

15°W

Composite Location for Oceanic Phase

GATE Array

MIT Radar

30°W

Mid-Level Kinematic Structure: Developing Phase

700 hPa PV (0.1 PVU, shaded) and Streamfunction (x106m2s-1, contours)

1.0° CFSR Reanalysis

Mid-Level Kinematic Structure: Mature Baroclinic Phase



Mid-Level Kinematic Structure: Coastal Transition Phase



Mid-Level Kinematic Structure: Oceanic Phase



Low-Level Kinematic Structure: Developing Phase

925 hPa Vorticity (x10-5 s-1, shaded), Wind (ms-1, vectors), and Streamfunction (x106 m2s-1, contours)


NV

Low-Level Kinematic Structure: Mature Baroclinic Phase



NV

Low-Level Kinematic Structure: Coastal Transition Phase



NV

Low-Level Kinematic Structure: Oceanic Phase



Low-Level Thermodynamic Structure: Developing Phase

925 hPa 2-10 day Filtered θ (K, shaded), θ (K, contours), and 2-10 day Filtered Wind (ms-1, vectors)


NV

Low-Level Thermodynamic Structure: Mature Baroclinic Phase



NV

Low-Level Thermodynamic Structure: Coastal Transition Phase



NV

+θ΄ no longer

ahead of SV

Low-Level Thermodynamic Structure: Oceanic Phase



Northerly flow out of phase with

+θ΄

Sub-Synoptic-Scale Structures in AEWs

Helene AEW (2006)

N = 925 hPa NV. M = 700 hPa SV. L = 925 hPa SV.Dashed lines denote trough axes defined at 700 hPa.

Except for being stronger than most AEWs the evolution of the AEW associated with Hurricane Helene (2006) was somewhat typical of the composite evolution.

The 0.5° AMMA reanalysis is used to highlight sub-synoptic scale features which are “washed out” in the composites.

IR (shaded) and 700 hPa Streamfunction (x106 m2s-1, contours)

700 hPa PV (0.1 PVU, shaded), Streamfunction (x106 m2s-1, contours),

and Winds (ms-1, vectors)

Sep. 5, 0000ZDeveloping




Sep. 9, 0000ZMature Baroclinic




Sep. 11, 0000ZCoastal Transition

IR (shaded) and 925hPa Streamfunction

(x106 m2s-1, contours)

925 hPa θv (K, shaded), Streamfunction (x106 m2s-1, contours), Winds (ms-1, vectors), and

Vorticity (> 2.5x10-5 s-1, pattern)





Sep. 8, 0000ZDeveloping Modified

Monsoon Dry

Moist





SV

NV





SVNV

Upscale Impact of Moist Convection on AEWs

Climatological Latent Heating and PV Generation in CFSR

TRMM 3B42 Rainrate (mm day-1, shaded)

CFSR Rainrate Bias (mm day-1, shaded)

Relative to 3B42

[mm day-1]

[mm day-1]

Climatological Latent Heating and PV Generation in CFSR

Resolved heating (K day-1, shaded) and ω (hPa hr-1,

contours)

PV Tendency due to resolved latent heating (PVU day-1, shaded)

Cross-Sections 5-15°N JAS 1998-2009

Total PV tendency (PVU day-1, shaded)

Heating over Land: Comparison with Radar Observations

Pre

ssur

e (h

Pa)

Regression between rain rate (derived from ZR relationship, Russell et al., 2010) and divergence estimated from the radial wind (Mapes and Lin, 2005).

Divergence (x10-5 s-1 per mm hr-1)

JAS 2006-2007

MIT C-Band radar operated in Niamey, Niger during JAS 2006-2007.

Radar observations suggest a peak heating rate ~300-500 hPa.

Low-level divergence was much stronger than other tropical sites examined in Mapes and Lin, (2005).

Approx. Peak Heating

Heating over East Atlantic: Comparison with GATE

Apparent heat source (Q1) derived from Global Atmospheric Research Program Atlantic Tropical Experiment

During Aug. 30 Sep. 18, 1974.

The level of peak heating over the East Atlantic is also qualitatively similar to the results from GATE.

Peak heating near ~600 hPa.

Rainrate in CFSR and TRMM: Developing Phase

TRMM 3B42 CFSR F06 h

Total Rainrate (mm day-1, shaded), 2-10 day Filtered Rainrate (contours at 0.5, 2.5, 5, 10 mm day-1)

Rainrate in CFSR and TRMM: Mature Baroclinic Phase



Rainrate in CFSR and TRMM: Coastal Transition Phase



Rainrate in CFSR and TRMM: Oceanic Phase



PV Production Sources: Developing Phase

Cumulus + Diff DiabaticFriction + Mom. FluxResolved LHRadiation

Underground

~600-750

700 hPa Diabatic PV Tendency (PVU day-1) and 700 hPa Streamfunction

(x106 m2s-1, contours),

Circle defines averaging domainof profile (3° radius from SV).

PV Production Sources: Mature Baroclinic Phase





PV Production Sources : Coastal Transition Phase


~850




PV Production Sources: Oceanic Phase


~950




Summary and Conclusions

Recent observations show a much richer structure in AEWs. These results highlight sub-synoptic scale structures within AEWs.

A qualitative picture of the upscale impact of moist convection on AEWs is beginning to emerge. There are strong contrasts between convection over interior Africa and the East Atlantic. These effect the levels that PV production occurs at.

Future work will focus on quantitative estimates of the upscale impact of convection on AEWs and exploring the variability of AEW-convection relationships.

Total

Radiation

Heat Diffusion

CumulusMomentum

Friction

Advection

Cumulus

Microphysics

Time Tendency DiabaticResidualAdvection

PV Budget: Developing Phase


Underground Underground

~600-750


PV Budget: Mature Baroclinic Phase


~750-1000




~850




~950

the structural evolution of african easterly waves

Documents

streamfunction x106m2s

oceanic phase700 hpa

wind ms

streamfunction x106

subsynoptic scale features

organized convection

case studies

upscale impact of convection