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**ECMWF Training Course Peter Bechtold and Christian Jakob**

02 May 2000 Numerical Weather Prediction Parametrization of diabatic processes Convection I An overview Peter Bechtold and Christian Jakob Moist Processes

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**Lectures: Exercises Convection**

ECMWF Training Course 02 May 2000 Convection Lectures: An overview (only about 5 simple principles to remember) Parametrisation of convection The ECMWF mass-flux parametrisation and Tracer transport Forecasting of Convection Cellular automaton (in preparation) Exercises The big secret !!! Moist Processes

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**Offline convection Code:**

ECMWF Training Course 02 May 2000 Convection Aim of Lectures: The aim of the lecture is only to give a rough overview of convective phenomena and parameterisation concepts in numerical models. The student is not expected to be able to directly write a new convection code- the development and full validation of a new convection scheme takes time (years). There are many details in a parameterisation, and the best exercise is to start with an existing code, run some offline examples on Soundings and dig in line by line ….. there is already a trend toward explicit representation of convection in limited area NWP (no need for parameterization) but for global we are not there yet, and still will need parameterizations for the next decade Offline convection Code: Can be obtained from Moist Processes

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**Convection Parametrisation and Dynamics - Text Books**

ECMWF Training Course 02 May 2000 Convection Parametrisation and Dynamics - Text 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 Moist Processes

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How does it look like ? Pre-frontal deep convection July 2010 near Baden-Baden Germany Rayleigh-Benard cellular convection Cumulus under Stratocumulus, South UK, July 2008 Classic plume experiment

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**Moist convection : Global**

Deep and shallow convection Intense deep ITCZ at 10ºN Sc convection African Squall lines SPCZ IR GOES METEOSAT 7/04/2003

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**Convection and role of water vapor**

Interaction of Tropics and midlatitudes: Dry air intrusions modulate convection (Rossby wave breaking)

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**Convection and upper-level Divergence (determine divergence from variation of cold cloud top areas)**

Mc is the convective mass flux (see later)

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**Outline General: Convection and tropical circulations**

ECMWF Training Course 02 May 2000 Outline General: Convection and tropical circulations Midlatitude Convection Shallow Convection Useful concepts and tools: Buoyancy Convective Available Potential Energy Soundings and thermodynamic diagrams Convective quasi-equilibrium Large-scale observational budgets Moist Processes

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Convection and tropical circulations (1) It’s raining again… 2000/2001 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

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**Model Tendencies – Tropical Equilibria**

ECMWF Training Course Model Tendencies – Tropical Equilibria 02 May 2000 Nevertheless, the driving force for atmospheric dynamics and convection is the radiation 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 Moist Processes

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**Distribution of convective clouds**

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Global: Convective cloud types (2) proxy distribution of deep and shallow convective clouds as obtained from IFS Cy33r1 (spring 2008)

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**A third convective mode**

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

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Comparison Cloudsat precip (left) from low, middle and high clouds (space radar) and IFS Cy31r1 (right) Angela Benedetti, Graeme Stephens

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Convection and tropical circulations (3) ITCZ and the Hadley meridional circulation: the role of trade-wind cumuli and deep tropical towers ECMWF Training Course 02 May 2000 Moist Processes

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**Convection and tropical circulations (4) The Walker zonal Circulation**

ECMWF Training Course 02 May 2000 Convection and tropical circulations (4) The Walker zonal Circulation From Salby (1996) Moist Processes

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Convection and tropical circulations (5) Tropical waves: Rossby, Kelvin, Gravity, African easterly waves a Squall line Analytical: solve shallow water equations

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**Convection and tropical circulations (7)**

The KELVIN wave V=0 50/50 rotational/divergent 50/50 KE/PE Strong zonal wind along the Equator Symmetric around the Equator Eastward moving ~18 m/s

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**Convection and tropical circulations (8) The Kelvin wave, OLR composite**

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**Convection and tropical circulations (9)**

The (n=1) Equatorial Rossby wave Symmetric KE>PE KE max at Equator,PE max off the Equator Westward moving ~ 5 m/s

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**Convection and tropical circulations (10) The Equatorial Rossby wave, OLR composite**

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**African easterly waves**

ECMWF Training Course 02 May 2000 African easterly waves African easterly waves have periods of 2-6 days, typical wavelengths of about 2500 km and propagation speeds around -8 m/s. These waves are thought to originate from barotropic and baroclinic instability, but the effects of diabatic (Cumulus) heating, the diurnal cycle and orography also modulate the waves. For instability to exist, the quasi-potential vorticity gradient must change sign in the domain, i.e for tropical North Africa it must become negative. Although the shear instability associated with the jet is present throughout the rainy season, the waves appear to contribute to the development of rainfall systems only during late summer as only then the can access the necessary moisture in the low-level monsoon flow. The exit region of the Tropcial easterly Jet – which is the consequence of the outflow=divergence due to the Asian Monsoon - might also have an effect on convection over tropical North Africa The waves are generally confined to a latitudinal zone close and south of the Jet Further reading: Diedhiou et al. (1998, GRL), Nicholson and Grist (2003,J. Clim), Hsieh and Cook (2004, MWR), Grist (2002, MWR) Moist Processes

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**West-African meteorology – easterly waves**

Mid-level dry “Harmattan” Low-levelMonsoon flow Upper-level easterlies Monsoon flow ,Easterly waves, and midlatitude-tropical mixing Hovmoeller plots as an easy way to plot wave (propagation)

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Reminder : useful relations for the Streamfunction expressing the rotational (non divergent part) of the horizontal wind field Streamfunction like vorticity is a scalar function that allows to describe the flow (i.e. both u and v) Streamfunction and vorticity: Geostrophic Streamfunction and Geopotential: Nota: Streamfunction ≠ Streamline which is defined by

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**Hovmoeller plots as an easy way to plot wave (propagation)**

Analysis Comparison Analysis – Forecast for African 850 hPa winds (“easterly waves”). No filtering required from a series of 2-day Forecasts

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**MJO 11 Nov. 2011 Sat+ECMWF analysis**

ECMWF Training Course 02 May 2000 MJO 11 Nov Sat+ECMWF analysis U850 U200 @ECMWF Moist Processes

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**Wavenumber frequency Diagrams of OLR**

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Wavenumber frequency Diagrams of OLR same as previous but latest cycle and background spectrum substracted Cy38r1 (2012) NOAA Satellite

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**Convection and tropical circulations Summary of tropical motions and scales**

There are still uncertainties concerning our knowledge about the interaction between convective and synoptic scales in the Tropics. Horizontal temperature fluctuations in the Tropics are small <1K/1000 km; and in the absence of precipitation the vertical motions(subsidence) tend to balance the cooling through IR radiation loss: w dθ/dz = dθ/dt_rad = -1-2 K/day => w ~ -.5 cm/s In the absence of condensation heating, tropical motions must be barotropic and cannot convert PE in KE. Therefore they must be driven by precipitating disturbances or lateral coupling with midlatitude systems. When precipitation takes place, heating rates are strong; e.g. 100 mm/day precip ~ energy flux of 2900 W/m2 or an average 30 K/day heating of the atmospheric column => w ~ 8.6 cm/s. However, this positive mean motion is composed of strong ascent of order w ~ 1 m/s in the Cumulus updrafts and slow descending motion around (“compensating subsidence”) when analysing the vorticity equation it appears that in precipitating disturbances the vertical transport of vorticity (momentum) through Cumulus is important to balance the divergence term

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Midlatitude Convection (1) Convection associated to synoptic forcing, orographic uplift, and/or strong surface fluxes A Supercell over Central US, Mai 1998, flight level m

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**Midlatitude Convection (2) It’s raining again… Europe climatology (Frei and Schär, 1998)**

In Europe most intense precipitation is associated with orography, especially around the Mediterranean, associated with strong large-scale forcing and mesoscale convective systems

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Midlatitude Convection (3) European MCSs (Morel and Sénési, 2001) Density Map of Triggering ….. over Orography

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Midlatitude Convection (4) European MCSs (Morel and Sénési, 2001) Time of Trigger and mean propagation European (midlatitude) MCSs essentially form over orography (convective inhibition –see later- offset by uplift) and then propagate with the midtropospheric flow (from SW to NE)

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Midlatitude Convection (5) along the main cold frontal band and in the cold core of the main depression – 17/02/97 during FASTEX A Supercell over Central US, Mai 1998, flight level m

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Midlatitude Convection (6) Forcing of ageostrophic circulations/convection in the right entrance and left exit side of upper-level Jet Acceleration/deceleration of Jet Thermally indirect circulation Total energy is conserved: e.g. at the exit region where the Jet decelerates kinetic energy is converted in potential energy Thermally direct circulation

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Midlatitude Convection (7) Conceptual model of a Squall line system with a trailing stratiform area (from Houze et al. 1989) Evaporation of precipitation creates negatively buoyant air parcels. This can lead to the generation of convective-scale penetrative downdraughts. In the stratiform part there is heating/cooling couple with an upper-level mesoscale ascent, and a lower-level mesoscale downdraught, due to the inflow of dry environmental air and the evaporation of stratiform rain.

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Midlatitude Convection (8a) Conceptual model of a rotating mesoscale convective system – tornadic thunderstorm (from Lemon and Doswell, 1979) Forward Flank downdraft induced by evaporation of precipitation Rear Flank Downdraft induced by dynamic pressure perturbation: Interaction of updraft with shear vector of environment: The linear part of the dynamic pressure perturbation is proportional to the horizontal gradient of the vertical velocity perturbation (updraft) times the environmental shear vector

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**Midlatitude Convection (8b) Origin and mechanism of generation of vertical vorticity**

Conversion of horizontal vorticity at surface frontal boundary in vertical vorticity by tilting in updraft A useful quantity in estimating the storm intensity is the “bulk” Richardson number R=CAPE/S2, where CAPE is the convective available energy (see later) and S is the difference between the mean wind vector at 500 and 925 hPa

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**Summary: What is convection doing, where does it occur**

ECMWF Training Course 02 May 2000 Summary: What is convection doing, where does it occur Convection transports heat, water vapor, momentum … and chemical constituents upwards …. Water vapor then condenses and falls out -> net convective heating/drying Deep Convection (precipitating convection) stabilizes the environment, an approximate not necessarily complete picture is to consider it as reacting to the large-scale environment (e.g. tropical waves, mid-latitude frontal systems) =“quasi-equilibrium”; shallow convection redistributes the surface fluxes The tropical atmosphere is in radiative(cooling) / convective(heating) equilibrium 2K/day cooling in lowest 15 km corresponds to about 5 mm/day precipitation. The effect of convection (local heat source) is fundamentally different in the midlatitudes and the Tropics. In the Tropics the Rossby radius of deformation R=N H/f (N=Brunt Vaisala Freq, f=Coriolis parameter, H=tropopause height) is infinite, and therefore the effects are not locally bounded, but spread globally via gravity waves – “throwing a stone in a lake” Moist Processes

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**What we have not talked about**

Organization of convection: Squall lines, Mesoscale convective systems, tropical superclusters, and the influence of vertical wind shear The diurnal cycle of convection over land (see lecture Notes and last lecture) Follow some Tools and Concepts !

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**Buoyancy - physics of Archimedes (1)**

ECMWF Training Course 02 May 2000 Buoyancy - physics of Archimedes (1) Body in a fluid Assume fluid to be in hydrostatic equlibrium Forces: Top Bottom Gravity Net Force: Acceleration: Emanuel, 1994 Moist Processes

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**Buoyancy (2) Vertical momentum equation: Neglect second order terms**

ECMWF Training Course 02 May 2000 Buoyancy (2) Vertical momentum equation: Neglect second order terms Moist Processes

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**B - buoyancy acceleration**

ECMWF Training Course 02 May 2000 Buoyancy (3) = = = B - buoyancy acceleration Moist Processes

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**Buoyancy (4) Contributions**

ECMWF Training Course 02 May 2000 Buoyancy (4) Contributions Buoyancy acceleration: Dry air: Often (but not always): Then Hence Moist Processes

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**Buoyancy (5) Contributions**

ECMWF Training Course 02 May 2000 Buoyancy (5) Contributions Cloudy air: effects of humidity and condensate need to be taken into account 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 Moist Processes

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**Non-hydrostat. Pressure gradient effects**

ECMWF Training Course 02 May 2000 Non-hydrostat. Pressure gradient effects 15 10 5 -0.02 0.02 0.04 -0.04 P Z (km) (ms-2) B Physics: Vector field of the buoyancy pressure-gradient force for a uniformly buoyant parcel of finite dimensions in the x-z-plane. (Houze, 1993, Textbook) CRM analysis of the terms Guichard and Gregory Moist Processes

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**Convective Available Potential Energy (CAPE)**

ECMWF Training Course 02 May 2000 Definition: Example: Much larger than observed - what’s going on ? 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. Moist Processes

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**Convection in thermodynamic diagrams (1) using Tephigram/Emagram**

ECMWF Training Course 02 May 2000 LNB CAPE Idealised Profile CIN LFC LCL Moist Processes

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**Θe is conserved during moist adiabatic ascent**

ECMWF Training Course 02 May 2000 Convection in thermodynamic diagrams (2) using equivalent Potential Temperature and saturated equivalent Potential Temperature GATE Sounding θ Θe is conserved during moist adiabatic ascent CAPE Θesat(T) Θe(T,q) Note that no CAPE is available for parcels ascending above 900 hPa and that the tropical atmosphere is stable above 600 hPa (θe increases) – downdrafts often originate at the minimum level of θe in the mid-troposphere. Moist Processes

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**Importance of choice of moist adiabat in CAPE calculations**

ECMWF Training Course 02 May 2000 Importance of choice of moist adiabat in CAPE calculations Reversible moist adiabat: Condensate remains in parcel at all time. Consequences: Water loading (gravity acting on condensate) Condensate needs to be heated - different heat capacity than dry air Phase transition from water to ice leads to extra heating Irreversible moist adiabat (Pseudo-adiabat): Condensate is removed from parcel instantly Moist Processes

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**Importance of choice of moist adiabat in CAPE calculations**

ECMWF Training Course 02 May 2000 Importance of choice of moist adiabat in CAPE calculations CAPE - reversible adiabat without freezing vs. irreversible adiabat Reversible CAPE much smaller, typically by a factor of 2 with respect to irreversible Emanuel, 1994 Moist Processes

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**Non-dimensional dynamical equations**

no distinction has been made between a horizontal and vertical length scale L If we now express the geopotential as a hydrostatic Φ0 and non-hydrostatic contribution, then the non-hydrostatic vertical acceleration can be expressed through a perturbation term where

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**Non-dimensional dynamical equations**

Dividing by UU/L one obtains the final non-dimensional form where the Richardson number, the Rossby number and the Reynolds number become apparent. In atmospheric motions the Reynolds number is very large, so the molecular diffusion term is neglected. Also, for large scales the Rossby number is of O(1) or smaller, only for small scales it becomes large and the Coriolis term can be neglected.

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**Mixing and 3D flow subcloud and cloud-layer Circulations**

From high-resolution LES simulation (dx=dy=50 m) Vaillancourt, You, Grabowski, JAS 1997

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**Mixing models undiluted entraining plume cloud top entrainment**

ECMWF Training Course 02 May 2000 Mixing models undiluted entraining plume cloud top entrainment stochastic mixing after Raymond,1993 Moist Processes

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**Effect of mixing on parcel ascent**

ECMWF Training Course 02 May 2000 Effect of mixing on parcel ascent No dilution Moderate dilution Heavy dilution Moist Processes

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**Large-scale effects of convection (1) Q1 and Q2**

ECMWF Training Course 02 May 2000 Large-scale effects of convection (1) Q1 and Q2 Thermodynamic equation (dry static energy) : why use s and not T s =CpT+gz ds/dz= CpdT/dz+g If dT/dz=-g/Cp (dry adiabatic lapse rate), then ds=0 Define averaging operator over area A such that: and Apply to thermodynamic equation, neglect horizontal second order terms, use averaged continuity equation: In convective regions these terms will be dominated by convection “sub-grid” terms “large-scale observable” terms Moist Processes

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**Large-scale effects of convection (2) Q1 and Q2**

ECMWF Training Course 02 May 2000 Large-scale effects of convection (2) Q1 and Q2 Apparent heat source Define: Apparent moisture sink Analogous: Apparent momentum source This quantity can be derived from observations of the “large-scale” terms on the l.h.s. of the area-averaged equations and describe the influence of the “sub-grid” processes on the atmosphere. Note that: with Moist static energy Moist Processes

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**Large-scale effects of convection (3) vertical integrals of Q1 and Q2**

ECMWF Training Course 02 May 2000 Large-scale effects of convection (3) vertical integrals of Q1 and Q2 Surface Precipitation flux Surface sensible Heat flux Surface Precipitation Surface latent Heat flux Moist Processes

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**Large-scale effects of convection (3) Deep convection**

ECMWF Training Course 02 May 2000 Large-scale effects of convection (3) Deep convection Tropical Atlantic Yanai and Johnson, 1993 Tropical Pacific Yanai et al., 1973, JAS Note the typical tropical maximum of Q1 at 500 hPa, Q2 maximum is lower and typically at 800 hPa Moist Processes

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**Large-scale effects of convection (5) Shallow convection**

ECMWF Training Course 02 May 2000 Large-scale effects of convection (5) Shallow convection 500 600 700 800 900 1000 -2 2 -10 -6 -14 Q2 Q1 QR (K/day) P(hPa) q-difference of simulation without and with shallow convection. Without shallow boundary-layer is too moist and upper-troposphere too dry ! -2 Nitta and Esbensen, 1974, MWR Moist Processes

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**Effects of mesoscale organization The two modes of convective heating**

ECMWF Training Course 02 May 2000 Effects of mesoscale organization The two modes of convective heating Structure Effects on heating 700 -2 (K/day) convective mesoscale total 1000 500 200 100 2 4 6 P(hPa) 300 Moist Processes

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**Zonal average convective Q1 in IFS**

P (hPa) Latitude

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**Convective quasi-equilibrium (1)**

ECMWF Training Course 02 May 2000 Convective quasi-equilibrium (1) Arakawa and Schubert (1974) postulated that the level of activity of convection is such that their stabilizing effect balances the destabilization by large-scale processes. v (700 hPa) -w (700 hPa) Precipitation Observational evidence: GARP Atlantic Tropical Experiment (1974) Thompson et al., JAS, 1979 Moist Processes

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ECMWF Training Course 02 May 2000 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, Q2 and Q3 are quantities that reflect the time and space average effect of convection (“unresolved scale”) and stratiform heating/drying (“resolved scale”) 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 Moist Processes

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