Mesoscale Convective Systems Associated with African Easterly Waves

30th AMS Conference on Hurricanes and Tropical Meteorology

Matthew A. Janiga and Chris D. ThorncroftUniversity at Albany, SUNY

NiameyBamako

500 km

Adapted from Fink and Reiner (2003)

Squall Line Generation

12.5°-20°N

5°-12.5°NMid-level Vortex

What do we know about the AEW-MCS Relationship?

Longitude

Lati

tude

NW

NE

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

• Dissipation occurred in the ridge.

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

Based on Berry and Thorncroft (2005) and others

What is still not understood?

a) How AEW Structure Supports Convective Development…– The vertical distribution of moisture and temperature.– This is important for forecasting convective systems

associated with AEWs.

b) Properties of Precipitation Features (PFs) in the AEW envelope…

– The size, intensity, and vertical development of the PFs.– The vertical structure of convective and stratiform regions.– The vertical structure of convection affects the heating

profiles and the upscale impact of convection on the AEWs.

• TRMM 3B42 is a continuous gridded rain rate dataset (Huffman et al., 2007) utilizing 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 are composited by AEW phase to diagnose the

thermodynamic and kinematic AEW structure.

• TRMM PR and TMI orbital swaths contain information on the vertical structure of convection and PF properties.– Orbital data is binned by AEW phase using 3B42 rain rate

filtered for synoptic westward propagating convection.

Data

Convection as Viewed by the TRMM Platform

C

S

Determining Maximum and Minimum Phases of TD Filtered 3B42

3B42 (shaded, mm hr-1) and TD filtered 3B42 RR (every 1.5σ).

Composite Domain

Maximum

MinimumStandard Deviation of TD filtered 3B42 RR

TD FilterPeriod: 2.5-10 days

Wavenumbers: -20 to -7 (2000-5600 km)

Domain: 5-12.5°N, 10°W-20°E

Determining Increasing and Decreasing Phases of TD Filtered 3B42

3B42 (shaded, mm hr-1) and TD filtered 3B42 dRR/dt (every 1.5σ).

Increasing

Decreasing

Composite Domain

Standard Deviation of TD filtered 3B42 dRR/dt

TD FilterPeriod: 2.5-10 days

Wavenumbers: -20 to -7 (2000-5600 km)

Domain: 5-12.5°N, 10°W-20°E

AEW Phase Partitioning using Wavenumber-Frequency Filtered 3B42

MAXMIN

INC

DEC

TOOWEAK

1.5σ

TRMM Orbital Binning Schematic

MAX

MIN

DEC

INC

AEW Phases for > 1.5σ Amplitude (shaded) and TD Filtered 3B42 RR (every 1.5σ).

8 AEW phases are identified using the 3B42 TD filtered RR and dRR/dt. The AEW phase and amplitude is used to partition orbital data binned to the same 1.0°x1.0° grid.

3B42 TD RR (standard deviations)

3B42

TD

dRR/

dt (s

tand

ard

devi

ation

s)

Maximum

Minimum

AEW Structure

Phase 1 is located ahead of the trough.

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.

Trough Ridge

Moisture and Thermodynamics

There is increased moisture ahead of the and within the AEW trough. Within and ahead of the ridge is dry.

High low-level θe (and CAPE) is found ahead of the trough with lower low-level θe in the southerlies.

Moist Dry

HighCAPE

LowCAPE

ShearThe AEJ strengthens ahead of the trough and weakens behind it.

Surface westerlies are enhanced ahead of the trough.

Both of these act to increase the shear ahead of the trough and weaken it behind the trough.

Enhanced Shear Reduced Shear

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.

Does the structure of the convective and stratiform precipitation vary too?

Black solid = Total rain rateBlue solid = Stratiform rain rateRed solid = Convective rain rate

Black dashed = Stratiform fraction

RIDGE N TROUGH S RIDGE

Wave Phase (Precipitation – 3B42)

Wave Phase (Dynamics - CFSR)

Wave-Phase CFADs (Convective)

• Two convective modes are apparent over this region.– A deep mode where

convection reaches the tropopause.

– A shallow mode where most of what is observed is below the melting level.

The % of reflectivity values falling in each bin at each level. Composite of all 8 phases for convective precipitation.

Domain: 5-12.5°N, 10°W-20°E.

Deep ModeShallow

Mode

Wave-Phase CFADs (Convective)

7:INC 8

1: MAX

5: MIN

3: DEC2

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)

• The stratiform regions have lower reflectivity values than the convective regions. The reflectivity also doesn’t extend as high.

• A kink toward high reflectivity values is found between 4-5 km associated with the bright band / melting level.

The % of reflectivity values falling in each bin at each level. Composite of all 8 phases for stratiform precipitation.

Domain: 5-12.5°N, 10°W-20°E.

Bright Band / Melting Level

Wave-Phase CFADs (Stratiform)

7:INC 8

1: MAX

5: MIN

3: DEC2

4 6

Comp.

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

The enhanced phases have a deeper column of observable reflectivity above the melting level.

Modification of the Diurnal Cycle

Enhanced(Phase 1)

Suppressed(Phase 5)

The enhanced phase has the “flattest” diurnal cycle with the largest fractional contributions from early morning (0000-0600 LT) rainfall.

In the suppressed phase, the greatest percentage occurs at 1800 LT. There is very little contribution from the diurnal minimum (0600-1200 LT).

Climatology

The % of the diurnal cycle’s rainfall coming from each 3-hr block (local time) for Phase 1, Phase 5,

and the domain climatology.

Domain: 5-12.5°N, 10°W-20°E

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).

• 3 PF characteristics are defined using the rain rate, reflectivity profiles, and collocated TMI data.– PF Size– 18 dBZ echo top– Minimum 85 gHZ temperature

• We focus on the total rainfall from PFs at falling in each PF characteristic value bin.

The % of the rainfall coming from a range of PF characteristic bins for Phase 1,

Phase 5, and the domain climatology.

Domain: 5-12.5°N, 10°W-20°E

Enhanced(Phase 1)

Suppressed(Phase 5)

Climatology

Object Size

85 GHz PCT

Compared to the suppressed phase, the enhanced phase has a larger contribution of its rainfall from PFs with • higher echo tops • larger PFs • colder minimum 85 GHz temps.

Summary of the AEW-Convection Relationship

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

• The enhanced (suppressed) phases particularly enhance (suppress) rainfall early in the day (0000-0900 LT).

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

Questions?

Percent of Rainfall From Various PF Characteristic Values

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 PFswhile 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

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)

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)

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)

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)

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)

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).

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

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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)

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)

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)

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)

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 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