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Parametrization of diabatic processes Moist convection

Parametrization of diabatic processes Moist convection

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Parametrization of diabatic processes Moist convection

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  1. Parametrization of diabatic processesMoist convection Peter Bechtold and Christian Jakob Original ECMWF lecture has been adjusted to fit into today’s schedule Roel Neggers, KNMI

  2. Convection • Lecture: • Overview of the phenomenon • Concepts • Parameterization of convection • ECMWF convection scheme 1st hour 2nd hour IFS Documentation, Part IV: Physical processes, Chapter V: Convection Available at

  3. Outline of first hour Overview of moist convection • Appearance, modes, global occurrence • Budgets Useful concepts • Buoyancy • Convective Available Potential Energy • Soundings and thermodynamic diagrams • Impact of convection on larger-scales • Convective quasi-equilibrium

  4. Convection Parameterization and DynamicsText Books • Emanuel, 1994: Atmospheric convection, OUP • Houze R., 1993: Coud dynamics, AP • Holton, 2004: An introduction to Dynamic Meteorology, AP • Bluestein, 1993: Synoptic-Dynamic meteorology in midlatitudes, Vol II. OUP • Peixoto and Ort, 1992: The physics of climate. American Institute of Physics • Emanuel and Raymond, 1993: The representation of cumulus convection in numerical models. AMS Meteor. Monogr. • Smith, 1997: The physics and parametrization of moist atmospheric convection. Kluwer • Dufour et v. Mieghem: Thermodynamique de l’Atmosphère, 1975: Institut Royal météorologique de Belgique • Anbaum, 2010: Thermal Physics of the atmosphere. J Wiley Publishers AP=Academic Press; OUP=Oxford University Press

  5. What does it look like ? In the lab: From above: From the ground:

  6. Convective modes: Shallow cumulus

  7. Overshoot Anvil cloud Updraft Convective modes: Deep cumulus

  8. Convective modes: Congestus Recent studies indicate, that there is a third important mode of convection (besides deep and shallow) in the tropics consisting of mainly cumulus congestus clouds terminating near the melting level at around 5 km. Johnson et al., 1999, JCL

  9. Congestus 9

  10. Reality: often a mixture of multiple modes Gulf of Mexico 10

  11. Intense deep Deep and shallow convection ITCZ at 10ºN Sc convection African Squall lines Moist convection : Global occurrence IR GOES METEOSAT 7/04/2003

  12. Convection & dynamics in the (sub)tropics Low-level trade-wind flow Stratocumulus Shallow cu Deep cu IR GOES, tropical Eastern Pacific

  13. Convective cloud types – Global distributionproxy distribution of deep and shallow convective clouds as obtained from IFS Cy33r1 (spring 2008)

  14. Convection and tropical circulationsITCZ and the Hadley meridional circulation: the role of trade-wind cumuli and deep tropical towers Deep Cu Shallow Cu Strato Cu Surface rain

  15. Convection and precipitation2000/2001 rainfall rate as simulated by IFS Cy36r4 (autumn 2010) and compared to GPCP version 2.1 dataset about 3 mm/day is falling globally, but most i.e. 5-7 mm/day in the Tropics

  16. The role of tropical convection - Budgets The driving force for atmospheric dynamics and convection is the radiative cooling Above the boundary layer, there is an equilibrium Radiation-Clouds-Dynamics-Convection for Temperature, whereas for moisture there is roughly an equilibrium between dynamical transport (moistening) and convective drying. - Global Budgets are very similar

  17. What we will not talk about (much): • Storm dynamics: Squall lines, Mesoscale convective systems, Tropical superclusters, Tornados, etc. Highly complex (three-dimensional) systems Bulk parameterizations in GCMs typically have a much simpler form (for computational efficiency)

  18. What we will talk about (a lot): Parameterizations: Simple conceptual models for the net effect of certain convective elements (e.g. updrafts, downdrafts, rain) Before we go in, some important conceptswill be introduced: Buoyancy CAPE Tephi diagrams Impact of convection on the larger-scale flow Quasi-Equilibrium 18

  19. Assume fluid to be in hydrostatic equlibrium Top Bottom Gravity Net Force: Acceleration: Concept I:Buoyancy - physics of Archimedes (1) Body in a fluid Forces: Proportional to density difference!

  20. Buoyancy (2) The impact of a density difference on the vertical momentum: Neglect second order terms Pressure perturbation Buoyancy

  21. ¢ T ¢ ¢ p T » B g << T p T Buoyancy (3)Contributions Buoyancy acceleration: Dry air: Assume Buoyancy is temperature driven

  22. Buoyancy (4)Contributions Cloudy air: effects of humidity and condensate need to be taken into account where Tv is thevirtual temperature: Impact of different specific gas constant for moist air Liquid water loading (gravity acting on condensate) In general all 3 terms are important. 1 K perturbation in T is equivalent to 5 g/kg perturbation in water vapor or 3 g/kg in condensate

  23. Example: Much larger than observed - what’s going on ? Concept II :Convective Available Potential Energy (CAPE) Using vertically integrated buoyancy to predict convective activity (“triggering”) and intensity (“closure”) Definition: top ò Bdz = CAPE base top - T T ò » cld env g dz T env base CAPE represents the amount of potential energy of a parcel lifted to its level of neutral buoyancy. This energy can potentially be released as kinetic energy in convection.

  24. LNB CAPE CIN LFC LCL Convection in thermodynamic diagrams (1)using Tephigrams T Tdew dry adiabat isobar Tephigrams: A complex (but instructive) way of plotting vertical profiles CAPE appears as an enclosed area on this diagram isotherm moist adiabat humidity

  25. No dilution Moderate dilution Heavy dilution Effect of mixing on parcel ascent Mixing partially explains inefficient conversion of potential energy into kinetic energy Mixing: “Entrainment” of dry air into the rising cloud

  26. Apply Reynolds’ averaging (see boundary-layer lecture): and In convective regions these terms will be dominated by convection “sub-grid” terms Concept III:Impact of convection on the large-scales Thermodynamic equation (dry static energy) : Energy from phase changes s =CpT+gz Conserved for adiabatic motions Radiation Gives “large-scale observable” terms

  27. Impact of convection on larger-scalesQ1 and Q2 Apparent heat source Define: Apparent moisture sink Note: h: Moist static energy (conserved for moist processes and precipitation) with These Q-quantities: * describe the influence of the “sub-grid” processes on the atmosphere. * can indirectly be derived from observations by estimating the “large-scale” terms on the l.h.s. of the area-averaged equations

  28. Impact of convection on large-scalesTropical deep convection Net effect: heating (Q1>0) and drying (Q2>0) throughout the troposphere Tropical Pacific Tropical Atlantic Yanai et al., 1973, JAS Yanai and Johnson, 1993

  29. 500 600 700 P(hPa) Q2 800 Q1 900 QR 1000 -14 -10 -6 -2 0 2 (K/day) Impact of convection on large-scalesSubtropical shallow convection Net effect: deepening and moistening the boundary layer! (Q2<0) Subtropical Atlantic (Caribbean) Boundary layer top Nitta and Esbensen, 1974, MWR

  30. Q1 and Q2 in the Hadley circulation Subtropical boundary-layer convection driven by surface evaporation moistens the lower atmosphere, which in turn acts as fuel for the deep convective heating in the ITCZ (surface rain) Q1>0 : heating Q1<0 : cooling Q1>0 Q2>0 : drying Q2>0 Q2<0 : moistening Q1>0 Q2>0 Q2<0 Q2<0 Q2<0 Surface rain E 30

  31. Zonal average convective Q1 in IFS P (hPa) Latitude

  32. Vertical integrals of Q1 and Q2 Surface sensible Heat flux (H) Surface Precipitation flux Pr E H The net effect of convection on the column is equal to the sum of the surface turbulent flux +/- the precipitation flux Surface Precipitation flux Surface latent Heat flux (E)

  33. Concept IV:Quasi-equilibrium Large-scale Forcing that slowly builds up instability (generates CAPE) The fast convective process that stabilizes environment (removes CAPE) Quasi-equilibrium: A near-balance is maintained, even when F is varying with time, i.e. cloud ensemble follows the Forcing.

  34. Concept IV:Quasi-equilibrium: Earthly analogue Free after Dave Randall: • Think of CAPE as the length of the grass • Forcing as an irrigation system • Convective clouds as sheep • Quasi-equilibrium: Sheep eat grass no matter how quickly it grows, so the grass is allways short. • Precipitation………..

  35. LS ascent: w<0 Concept IV:Quasi-equilibrium: Observational evidence GARP Atlantic Tropical Experiment (GATE, 1974) Precipitating convection is observed to react to instability caused by large-scale convergence: Thompson et al., JAS, 1979 -w (700 hPa) Precipitation

  36. Summary • Convection affects the atmosphere through condensation / evaporation and eddy transports • On large horizontal scales convection is in quasi-equilibrium with the large-scale forcing • Q1 and Q2 are quantities that reflect the time and space average effect of convection (“unresolved scale”) on the larger-scales • An important parameter for the strength of convection is CAPE • Shallow convection is present over very large (oceanic) areas, it determines the redistribution of the surface fluxes and the transport of vapor and momentum from the subtropics to the ITCZ