2. 1 FORMATION DES PREVISIONNISTES CONVECTION WEATHER 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 Formation des Prévisionnistes program of Météo-France. This group, headed by J-Ch Rivrain and with the support of the scientific expertise provided by J-Ph Lafore, is composed of Mrs Canonici, Mercier, Mithieux and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, references by Jean Paul Billerot.

3. 2 FORMATION DES PREVISIONNISTES CONVECTION 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

4. 3 FORMATION DES PREVISIONNISTES CONVECTION Precipitation Formation: Updraft Droplets growth Loading by hydrometeors Condensation Downdraft Dry air  Evaporation  Cooling DRY AIR Definition: Air mass of higher density spreading at the surface DENSITY CURRENT

5. 4 FORMATION DES PREVISIONNISTES CONVECTION DC Amplification of downdrafts Allowing the feeding of the DC Replay Cell during its dissipation stage Without wind shear DENSITY CURRENT

6. 5 FORMATION DES PREVISIONNISTES CONVECTION 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)

7. 6 FORMATION DES PREVISIONNISTES CONVECTION SQUALL LINE PASSAGE RADAR ANIMATION 10 dec. 2000 -- 1230 to 1530 UTC

8. 7 FORMATION DES PREVISIONNISTES CONVECTION SQUALL LINE PASSAGE OVER THE AISNE DEPARTEMENT ST-QUENTIN

9. 8 FORMATION DES PREVISIONNISTES CONVECTION SAINT-QUENTIN 10 Dec. 2000 Wind Bursts >33m/s  P = 1,6 hPa ms -1  T = 3, 3 ° C

10. 9 FORMATION DES PREVISIONNISTES CONVECTION STRUCTURE AND IMPACT OF A DENSITY CURRENT –Without wind shear NO LOCAL CONVERGENCE SYMMETRICAL SOLUTION Spreading of the Density Current H

11. 10 FORMATION DES PREVISIONNISTES CONVECTION STRUCTURE WITH WIND SHEAR DISSYMMETRIC STRONG AND LOCALIZED CONVERGENCE GUST FRONT CONVECTION DRY AIR

12. 11 FORMATION DES PREVISIONNISTES CONVECTION Rain shafts: Evidence of Evaporation

13. 12 FORMATION DES PREVISIONNISTES CONVECTION STRUCTURE OF A DENSITY 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)

14. 13 FORMATION DES PREVISIONNISTES CONVECTION 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 q l +q s : 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 km C*=10m/s

15. 14 FORMATION DES PREVISIONNISTES CONVECTION COMBINATION OF DENSITY COURANTS (MERGING) BRIDGE Combinaison of DCs +  Triggering of new cell Gravity Waves CD 1 CD 2

16. 15 FORMATION DES PREVISIONNISTES CONVECTION 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.

17. 16 FORMATION DES PREVISIONNISTES CONVECTION DOWNWARD MOTIONS: SUBSIDENCES 1.Definition 2Different types of subsidence: –Subsidence at Large Scales –Subsidence at Small Scales 3.Intensity of downdrafts The DCAPE parameter 4. Conclusion

18. 17 FORMATION DES PREVISIONNISTES CONVECTION 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.

19. 18 FORMATION DES PREVISIONNISTES CONVECTION 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

20. 19 FORMATION DES PREVISIONNISTES CONVECTION SUBSIDENCE AT 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 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)

21. 20 FORMATION DES PREVISIONNISTES CONVECTION 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.

22. 21 FORMATION DES PREVISIONNISTES CONVECTION DCAPE Between 2 theoretical maxima –Dry adiabatic –Wet adiabatic ? Reality? Depends on: –Subsidence –Precipitation –Humidity

23. 22 FORMATION DES PREVISIONNISTES CONVECTION TD n°3 Subsidences. 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?

24. 23 FORMATION DES PREVISIONNISTES CONVECTION TD n°3 Subsidences. DCAPE

25. 24 FORMATION DES PREVISIONNISTES CONVECTION 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

26. 25 FORMATION DES PREVISIONNISTES CONVECTION 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)