1 FORMATION DES PREVISIONNISTES CONVECTION WEATHER FORECASTING IN MID-LATITUDE REGIONS IN MID-LATITUDE REGIONS Prepared in close collaboration with the

1 FORMATION DES PREVISIONNISTES CONVECTION

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


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


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


DC

Amplification of downdrafts

Allowing the feeding of the DC

ReplayCell during its dissipation stage

Without wind shear

DENSITY CURRENT


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)


SQUALL LINE PASSAGE RADAR ANIMATION

10 dec. 2000 -- 1230 to 1530 UTC


SQUALL LINE PASSAGE

OVER THE AISNE DEPARTEMENT

ST-QUENTIN


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


STRUCTURE AND IMPACTOF A DENSITY CURRENT

– Without wind shear

NO LOCAL

CONVERGENCE

SYMMETRICAL

SOLUTION

Spreading of the Density Current

H


STRUCTURE WITH WIND SHEAR

DISSYMMETRIC

STRONG AND LOCALIZED

CONVERGENCE

GUST FRONT

CONVECTION

DRY AIR


Rain shafts: Evidence of Evaporation


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)


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*


COMBINATION OF DENSITY COURANTS (MERGING)

BRIDGE

Combinaison of DCs + Triggering of new cell

Gravity Waves

CD1 CD2


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.


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


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.


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


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)


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.


DCAPE

• Between 2 theoretical maxima– Dry adiabatic– Wet adiabatic

?

• Reality? Depends on:– Subsidence– Precipitation– Humidity


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?


TD n°3 Subsidences. DCAPE


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


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)

Documents

1 FORMATION DES PREVISIONNISTES CONVECTION WEATHER FORECASTING IN MID-LATITUDE REGIONS IN MID-LATITUDE REGIONS Prepared in close collaboration with the