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WEATHER FORECASTING IN MID-LATITUDE REGIONS

WEATHER FORECASTING IN MID-LATITUDE REGIONS.

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WEATHER FORECASTING IN MID-LATITUDE REGIONS

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  1. WEATHER FORECASTING 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.

  2. 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

  3. Downdraft Precipitation DRYAIR DRYAIR DENSITY CURRENT • Definition: Air mass of higher density spreading at the surface Formation: Updraft • Condensation Droplets growth Loading by hydrometeors Dry air  Evaporation  Cooling

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

  5. 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 • q’w drop • Fast evolution of the above parameters at the storm passage  Sharp discontinuity (a few km to less than a km)

  6. SQUALL LINE PASSAGE RADAR ANIMATION10 dec. 2000 -- 1230 to 1530 UTC

  7. SQUALL LINE PASSAGE OVER THE AISNE DEPARTEMENT ST-QUENTIN

  8. P = 1,6 hPa  T = 3, 3 ° C Wind Bursts >33m/s SAINT-QUENTIN10 Dec. 2000 ms-1

  9. H SYMMETRICAL SOLUTION NO LOCAL CONVERGENCE Spreading of the Density Current STRUCTURE AND IMPACTOF A DENSITY CURRENT • Without wind shear

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

  11. Rain shafts: Evidence of Evaporation

  12. 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)

  13. PROPAGATION SPEED OF A DENSITY CURRENT • The propagation of a density current is given by a Bernoulli equation: • h: depth of the density current • Dqv: mean difference of potential virtual temperature between the DC and the environment • ql+qs: loading term by liquid and solid hydrometeors • Numerical example: Dqv = -3°C at the surface. We assume a linear vertical profile of qv h = 1 km C*=10m/s

  14. BRIDGE COMBINATION OF DENSITY COURANTS (MERGING) Combinaison of DCs +  Triggering of new cell Gravity Waves CD1 CD2

  15. 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.

  16. DOWNWARD MOTIONS:SUBSIDENCES • Definition • Different types of subsidence: • Subsidence at Large Scales • Subsidence at Small Scales • Intensity of downdrafts The DCAPE parameter 4. Conclusion

  17. 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 q’w is minimum  need to check the q’w profile and its minimum value NB: It should be recalled that q’w corresponds to the minimum temperature that a parcel may reach in a downdraft when evaporation is involved.

  18. 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

  19. The LS signature is weak (no low levels convergence) SUBSIDENCEAT SMALL SCALES The compensation occurs in the vicinity or within the convective area The LS signature is weak (no convergence at low levels) • Different types of subsidence: • micro-subsidence (+ microbursts): scale < 1 km but very intense: > 15m/s • subsidence at convective scale: a few km, intense: 1 to 10 m/s • subsidence at mesoscale (stratiform parts): 10 to 100 km, less intense: ~10 cm/s

  20. 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 ConvectiveAvailablePotentialEnergy 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.

  21. DCAPE • Reality?Depends on: • Subsidence • Precipitation • Humidity • Between 2 theoretical maxima • Dry adiabatic • Wet adiabatic ?

  22. 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?

  23. TD n°3 Subsidences. DCAPE

  24. 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 q’w • “onion shape” soundings • Minimum of q’w

  25. SITUATION for 10 Dec 2000 • Identification of a dry air area • Water vapor imagery (darker areas) • Minimum of q’w (vertical sounding) • Vertical cross-section(ARPEGE 12H)

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