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Mesoscale Convective Systems Associated with African Easterly Waves. Niamey. Bamako. 30 th AMS Conference on Hurricanes and Tropical Meteorology. Matthew A. Janiga and Chris D. Thorncroft University at Albany, SUNY. 500 km. What do we know about the AEW-MCS Relationship?. 12.5°-20°N. - PowerPoint PPT Presentation
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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