Mesoscale Convective Systems Associated with African Easterly Waves

500 km

Adapted from Fink and Reiner (2003)

Squall Line Generation

12.5°-20°N

5°-12.5°N

Mid-level Vortex

What do we know about the AEW-MCS Relationship?

Longitude

Lati

tud

e

NW

NE

• Fink and Reiner (2003) found that squall line generation occurred NW of the mid-level AEW vortex and south of the northern AEW low-level vortex.

• Dissipation occurred in the ridge.

Based on Berry and Thorncroft (2005)

What is responsible for this relationship?

• The NW squall line initiation maximum is related to a westerly low-level jet (LLJ) with high equivalent potential temperature (θe) (Berry and Thorncroft, 2005).• The westerly LLJ is associated with the northern low-level vortex.

SHL – Saharan heat lowLLJ – Low-level jet

ITD – Intertropical discontinuityAEJ – Africa easterly jet (at ~600 hPa)

X – Vorticity maximum or trough

What is still not understood?

a) The Properties of the Precipitation Features…– The size, intensity, and vertical development of the

precipitation features (PFs).– The vertical structure of convective and stratiform

rain.– These properties are crucial to diagnosing heating

rates and the upscale effect of convection on the AEWs.

b) How AEW Structure Controls this Relationship…– The vertical distribution of moisture and temperature.– This is important for forecasting intense events.

Convection as Viewed by the TRMM Platform

• TRMM 3B42 is a continuous gridded rainrate dataset (Huffman et al., 2007).– 3-hrly observations at 0.25° grid-spacing.– Utilizes passive microwave, precipitation radar (PR),

geostationary infrared (IR), and rain gauge data.– Wavenumber-frequency filtered TRMM 3B42 is used to

determine AEW phase and amplitude. • The NCEP Climate Forecast System Reanalysis (CFSR) (Saha et al.,

2010) is one of the latest-generation reanalyses.– Analyses have a horizontal grid-spacing of 0.5° and 29 vertical

levels between 1000-50 hPa.– Analyses are composited by AEW phase to diagnose the

thermodynamic and kinematic AEW structure.

Data Used to Determine AEW Phase, Amplitude, and Structure

Climatological Rain Rate

The plot shows the rainrate from averaging all the TRMM PR passes. The unconditional rainrate includes zeros. The PR

samples each point about every 3 days.

Climatological Rain Rate Intensity

In this plot the averaging is conditional, it excludes 0 mm hr-1 rain rates. This indicates the rain rate intensity.

Sahelian Belt

Congo

Identification of Precip. Features (PFs)• Precipitation features (PFs) are defined as

contiguous regions of rain rate > 0 mm hr-1 from the TRMM PR (no size criteria are applied).

• Using the rain rate and reflectivity profiles for each PF, a variety of criteria can be calculated:– Rain rate: PF size, volumetric rain,

convective/stratiform ratios.– Reflectivity: Echo tops, the area > 20 dBZ above 10 km.– TRMM Microwave Imager (TMI): Minimum 85 gHZ

polarization corrected temperature.• Since small PFs are unimportant, we focus on the

total rainfall from PFs with characteristics.

Volumetric Rain Contributions from PFs

50% 50%

• We can determine the relative importance of certain PF characteristics by calculating their contribution to the volumetric rain fall in each region.

• To summarize the distribution of PF contributions, the 50% divide is calculated (see Liu 2011).

50% of the rainfall comes from PFs larger than 21,500 km2. The percent number of PFs larger

than this is much smaller.

10,000 km2 marks the approximate separation of “MCS” from “Sub-MCS” rainfall.

50% of the rainfall comes from PFs larger than 21,500 km2. The percent number of PFs larger

than this is much smaller.

10,000 km2 marks the approximate separation of “MCS” from “Sub-MCS” rainfall.

Sub-MCS MCS

Volumetric Rain Contribution: System Size

The “50/50 split” of the volumetric rain bins at each 1.0°x1.0° grid box.

The regions with high rain rates are dominated by contributions from large PFs

while the cold tongue and Egypt are dominated by very small systems.

10000

Volumetric Rain Contribution: Echo Tops

• There is an axis of intense convection stretching from Sudan westwards toward Senegal.

• A second region of intense convection is found on the slopes where the Great Escarpment meets the Congo Basin.

• Add 85 gHZ PCT

Climatological CAPE and Shear

AEW Phase Partitioning using Wavenumber-Frequency Filtered Rain

MAXMIN

INC

DEC

NOTUSED

1.5σ

TRMM PR Binning Schematic

AEW Activity Measured by Filtered 3B42

AEW Composite Domain

5-15°N, 10°W-20°E is used to construct the AEW composites.

This region has convection ahead of the trough and a similar amplitude (e.g. Kiladis et al. 2006).

Partitioning AEWs into Phases Using TD Filtered TRMM 3B42

Kelvin and TD signalscontoured every 1σ.

Phases masked foramplitude >1.5σ

Max.

Min.

Dec.

Inc.

Variation of Rainfall and Stratiform Ratios by Wave Phase

The TD waves show a clear transition from convective to stratiform rainfall between the leading and trailing edges of the convective envelope.

Variation in stratiform fractions are due to changes in spatial coverage AND conditional intensity.

Black solid = Total rainrateBlue solid = Stratiform rainrateRed solid = Convective rainrate

Black dashed = % stratiform rainrate

Trough RidgeSNRidge

Relative Importance of Intensity and Fractional Areal Coverage

Conditional Rain Rates

Fractional Area Coverage

Blue solid = Stratiform rainrateRed solid = Convective rainrate

Blue solid = Stratiform coverageRed solid = Convective

coverage

Wave-Phase CFADs

(Convective)

7:INC 8

1: MAX

5: MAX

3: MAX2

4 6

Comp.

Shows a composite CFAD for all phases and the differences between each phase. CFADs are normalized for each phase.

The main difference is a tendency towards increased shallow-warm rain in the suppressed phase and deep convection in the enhanced phase.

Wave-Phase CFADs

(Stratiform)

7:INC 8

1: MAX

5: MAX

3: MAX2

4 6

Comp.

The main difference is a tendency towards increased reflectivity in the enhanced phase and reduced reflectivity in the suppressed phase.

Wave-Phase Rainfall

ContributionA 2D histogram of the rainfall contributions as a function of PF size / echo top and wave-phase.

Most of the rainfall from large/deep PFs occurs in the convective envelope.

The distributions are slightly shifted toward small/less-deep systems in the suppressed phases.

Modification of the Diurnal Cycle

Phase 1Enhanced

Phase 5Suppressed

In the enhanced phase, rain rate is somewhat constant except for a reduction at 1200 LT.

In the suppressed phase, rain rate peaks at 1800 LT and quickly drops to 25% this amount between 0600-1200 LT.

Vortex Structure

The cold-core AEWs have troughs and ridges which peak between 600-700 hPa.

There is a quadrapole of temperature anomalies. This is because the temperature gradient reverses above 700 hPa.

Reduced Stability

IncreasedStability

Trough Ridge

U and V Structure

The cold-core AEWs have troughs and ridges which peak between 600-700 hPa.

The AEJ strengthens ahead of the trough and weakens behind it (could be inflow to stratiform precip.).

Surface westerlies are enhanced ahead of the trough.

Enhanced Shear Reduced Shear

Vertical Motion

Structure

The vertical motion and divergence profiles agree with TRMM showing a transition from shallow to deep convective and then to stratiform precip.

Shallow - Deep - Stratiform

Instability and

Moisture

Mid-level moisture is greater in the northerlies.

This is closely related to the profiles of θe. Temperature matters in the low-levels.

Moist Dry

HighCAPE

LowCAPE

Summary Illustration of the AEW-Convection Relationship

• AEWs increase the vertical extent, size, and rainfall intensity of convective systems.

• They also support convection through the unfavorable portions of the diurnal cycle.

• Shear, instability, lift, and moisture are all increased in the northerlies.

Illustration

Future Work

• Kinematic properties of convection using velocity data from Bamako and Niamey radars.

• The vertical profiles of convective & stratiform heating rates using TRMM estimated Q1-QR.

• How does the convection-AEW phase relationship (as determined by TRMM) vary by region and season?

• How do these results compare to other waves like Kelvin waves?

Questions?

MCSs associated with the “Helene AEW” of 2006

Triggering occurs near the Jos Plateau ahead of the AEW trough.

Life-Cycle of the Niamey MCS

Sep. 7, 1200Z2006

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

In the late afternoon convection expands in coverage.

Life-Cycle of the Niamey MCS

Sep. 7, 1800Z2006

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

Most of the other convection weakens but the Niamey MCS strengthens as the nocturnal LLJ develops.

Life-Cycle of the Niamey MCS

Sep. 8, 0000Z2006

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

More intensification occurs through 0600Z when the LLJ reaches its peak intensity.

Life-Cycle of the Niamey MCS

Sep. 8, 0600Z2006

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

As the LLJ weakens at 1200Z the MCS begins to dissipate.

Life-Cycle of the Niamey MCS

Sep. 8, 1200Z2006

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

925 hPa θe (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

925 hPa θe (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).MCS

NV

NV

MCS

925 hPa θv (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

925 hPa θv (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

Sep. 8, 0600Z2006

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Niamey Radar

150 km

Sep. 9, 1800Z2006

Triggering occurs in the late afternoon.

Life-Cycle of the Bamako MCS

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

Sep. 10, 0000Z2006

Intensification occurs as the LLJ begins to develop.

Life-Cycle of the Bamako MCS

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

Sep. 10, 0600Z2006

Continued intensification occurs up through 0600Z when the LLJ reaches its peak intensity.

Life-Cycle of the Bamako MCS

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

Sep. 10, 1200Z2006

The MCS begins to weaken at 1200Z as the LLJ begins to weaken.

Life-Cycle of the Bamako MCS

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

IR (shaded) and 700 hPa Streamfunction

(x106 m2s-1, contours)

NV

NV

MCS

MCS

925 hPa θe (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

925 hPa θe (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

925 hPa θv (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

925 hPa θv (K, shaded), streamfunction (x106 m2s-1,

contours), winds (ms-1, vectors), and vorticity (> 2.5x10-5 s-1,

pattern).

Sep. 10, 0600Z2006

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km

Bamako Radar

150 km