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WEATHER FORECASTINGWEATHER FORECASTING IN MID-LATITUDE REGIONS IN MID-LATITUDE REGIONS
Prepared in close collaboration with the “Working Group on Convection” in the frame of the Plan de Prepared in close collaboration with the “Working Group on Convection” in the frame of the Plan de Formation des Prévisionnistes program of Météo-France. This group, headed by J-Ch Rivrain Formation des Prévisionnistes program of Météo-France. This group, headed by J-Ch Rivrain and with the and with the support of the scientific expertise provided by J-Ph Laforesupport of the scientific expertise provided by J-Ph Lafore, is composed of Mrs Canonici, Mercier, Mithieux , is composed of Mrs Canonici, Mercier, Mithieux and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, references by Jean Paul Billerot. references by Jean Paul Billerot.
CONVECTI ON
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DENSITY CURRENT(cold pool)
1. Definition
2. Example of a density current (DC):
– Radar animation of a squall line
– Signature at the surface
3. Structure of a DC:
– Without shear
– With shear
– Spatial extension
– Propagation
4. Combination of DCs: Merging
5. Conclusion
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Precipitation
Formation: Updraft
Droplets growth
Loading by hydrometeors
Condensation
Downdraft
Dry air Evaporation Cooling
DRYAIR
DRYAIR
Definition: Air mass of higher density spreading at the surface
DENSITY CURRENT
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DC
Amplification of downdrafts
Allowing the feeding of the DC
ReplayCell during its dissipation stage
Without wind shear
DENSITY CURRENT
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Signature of a DC at the surface
• Rotation and intensification of the wind
gusts up to 25m/s
• Temperature drop
2 to 10°C
• Pressure jump
1 to 2 hPa
• Drop of the water vapor mixing ratio, but the relative humidity increases
• ’w drop
• Fast evolution of the above parameters at the storm passage Sharp discontinuity (a few km to less than a km)
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SQUALL LINE PASSAGE RADAR ANIMATION
10 dec. 2000 -- 1230 to 1530 UTC
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SQUALL LINE PASSAGE
OVER THE AISNE DEPARTEMENT
ST-QUENTIN
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SAINT-QUENTIN10 Dec. 2000
0
5
10
15
20
25
30
35
40
45
50
0
1
2
3
4
5
6
7
8
9
10
FMAX
Pstation
Tempé
Wind Bursts
>33m/s
P = 1,6 hPa
ms-1
T = 3, 3 ° C
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STRUCTURE AND IMPACTOF A DENSITY CURRENT
– Without wind shear
NO LOCAL
CONVERGENCE
SYMMETRICAL
SOLUTION
Spreading of the Density Current
H
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STRUCTURE WITH WIND SHEAR
DISSYMMETRIC
STRONG AND LOCALIZED
CONVERGENCE
GUST FRONT
CONVECTION
DRY AIR
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Rain shafts: Evidence of Evaporation
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STRUCTURE OF ADENSITY CURRENT
•The gust front can precede the storm cell of a few tens of km (20 to 40 km).
•Rotor circulation in the DC head
•DC depth 1 km.
•Often thinner over ocean (200 à 300 m)
•Often deeper over continent (up to 2 km) and plateau
•(dry conditions)
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PROPAGATION SPEED OF A DENSITY CURRENT
• The propagation of a density current is given by a Bernoulli equation:
• h: depth of the density current
v: mean difference of potential virtual temperature between the DC and the environment
• ql+qs: loading term by liquid and solid hydrometeors
• Numerical example:
v = -3°C at the surface. We assume a linear vertical profile of v
h = 1 kmC*=10m/s
hqqv
vgC sl
o
2*
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COMBINATION OF DENSITY COURANTS (MERGING)
BRIDGE
Combinaison of DCs + Triggering of new cell
Gravity Waves
CD1 CD2
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CONCLUSION
The density current is an air mass of higher density spreading at the surface. It is fed by the downdrafts of the storm.
Occurrence of dry air in the mid troposphere favors downdrafts.
Rain evaporation in this dry air feeds the DC and intensifies it.
Without vertical wind shear, the DC spreading at surface is isotropic.
convection is not well organized and weak
With vertical wind shear, the DC spreads downward the shear
convection is well structured and intense
new cells appear downward the shear along a gust front.
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DOWNWARD MOTIONS:SUBSIDENCES
1. Definition
2 Different types of subsidence:
– Subsidence at Large Scales
– Subsidence at Small Scales
3. Intensity of downdrafts
The DCAPE parameter
4. Conclusion
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DOWNWARD MOTIONS: SUBSIDENCES
These play two important roles:
1) The compensation of upward motions
To maintain the mass conservation
2) The feeding of DCs
To help organize convection
The air feeding the DCs can originate from mid-troposphere where ’w is minimum
need to check the ’w profile and its minimum value
NB: It should be recalled that ’w corresponds to the minimum temperature that a parcel may reach in a downdraft when evaporation is involved.
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SUBSIDENCE AT LARGE SCALE
The compensation can occur far from the convective area (at large scale)
Driven by radiative cooling (Example: the Hadley cell)
Convergence at lower levels
and Divergence at upper levels
SUBSIDENCE in dry air
Weak downward motion: a few cm/s
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SUBSIDENCEAT SMALL SCALES
The compensation occurs in the vicinity or within the convective area
The LS signature is weak (no low levels convergence)
Different types of subsidence:
1. micro-subsidence (+ microbursts): scale < 1 km but very intense: > 15m/s
2. subsidence at convective scale: a few km, intense: 1 to 10 m/s
3. subsidence at mesoscale (stratiform parts): 10 to 100 km, less intense: ~10 cm/s
The LS signature is weak
(no convergence at low levels)
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INTENSITY OF DOWNDRAFTS
Difference between oceans and continents
Stronger intensity over continents: Why?
CAPE is designed to analyze the convective updrafts, but cannot explain the above difference
Similarly, DCAPE is defined to analyze downdrafts. It corresponds to the
Downdraft Convective Available Potential Energy
Contrary to updrafts, there is a high degree of uncertainty to forecast the downdraft intensity, that strongly depends on complex diabatic processes: evaporation, microphysics, mixing, pressure field…
DCAPE only provides a theoretical maximum intensity which can not be physically reached.
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DCAPE
• Between 2 theoretical maxima– Dry adiabatic– Wet adiabatic
?
• Reality? Depends on:– Subsidence– Precipitation– Humidity
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TD n°3Subsidences. DCAPE
QUESTION 5: What is the lowest temperature the Density Current may reach?
QUESTION 6: Similarly to what is involved in the definition of CAPE, what area on the graphic represents the work of the buoyancy forces applied to the subsiding parcel?
QUESTION 7: For a parcel with initial state in A, undergoing a theoretical transformation without évaporation – that is, along a dry adiabat -:a) What sign is its buoyancy at level 700 hPa? et quel est le gain de température ?b) If some forcing (e.g. fœhn effect), keeps this parcel subsiding, what will be its temperature when reaching the ground?c) On the graphic,what represents the energy to be provided to this parcel to make it reach the ground (forcing)?
QUESTION 8: Do you think these two theoretical trajectories we simulated are plausible?
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TD n°3 Subsidences. DCAPE
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CONCLUSION
We showed in this chapter the importance of downdrafts, which help structure convection and strengthen it
DCAPE allows to estimate the potential of a given atmosphere to develop downdrafts if sufficient rain precipitation occurs
Rain evaporation, and thus the existence of dry air is crucial for the generation of intense downdraft and DCs
Special attention must be given to analyze the observed and forecast profiles of temperature and of ’w
- “onion shape” soundings
- Minimum of ’w
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SITUATION for 10 Dec 2000
Identification of a dry air area• Water vapor imagery (darker areas)• Minimum of ’w (vertical sounding)• Vertical cross-section(ARPEGE 12H)