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## Parametrization of diabatic processes Moist convection

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**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**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 http://www.ecmwf.int/research/ifsdocs/**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**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**What does it look like ?**In the lab: From above: From the ground:**Overshoot**Anvil cloud Updraft Convective modes: Deep cumulus**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**Reality: often a mixture of multiple modes**Gulf of Mexico 10**Intense deep**Deep and shallow convection ITCZ at 10ºN Sc convection African Squall lines Moist convection : Global occurrence IR GOES METEOSAT 7/04/2003**Convection & dynamics in the (sub)tropics**Low-level trade-wind flow Stratocumulus Shallow cu Deep cu IR GOES, tropical Eastern Pacific**Convective cloud types – Global distributionproxy**distribution of deep and shallow convective clouds as obtained from IFS Cy33r1 (spring 2008)**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**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**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**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)**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**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!**Buoyancy (2)**The impact of a density difference on the vertical momentum: Neglect second order terms Pressure perturbation Buoyancy**¢**T ¢ ¢ p T » B g << T p T Buoyancy (3)Contributions Buoyancy acceleration: Dry air: Assume Buoyancy is temperature driven**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**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.**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**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**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**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**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**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**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**Zonal average convective Q1 in IFS**P (hPa) Latitude**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)**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.**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………..**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**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