Peter Bechtold and Christian Jakob

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Parametrization of diabatic processesMoist
convection

• Peter Bechtold and Christian Jakob
• Original ECMWF lecture has been adjusted to fit
  into todays schedule
• Roel Neggers, KNMI

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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/
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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

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Convection Parameterization 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
  lAtmosphère, 1975 Institut Royal météorologique
  de Belgique
• Anbaum, 2010 Thermal Physics of the atmosphere.
  J Wiley Publishers

APAcademic Press OUPOxford University Press
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What does it look like ?
In the lab
From above
From the ground
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Convective modes Shallow cumulus
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Convective modes Deep cumulus
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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
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Congestus
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Reality often a mixture of multiple modes
Gulf of Mexico
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Moist convection Global occurrence
IR GOES METEOSAT 7/04/2003
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Convection dynamics in the (sub)tropics
Low-level trade-wind flow
Stratocumulus
Shallow cu
Deep cu
IR GOES, tropical Eastern Pacific
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Convective cloud types Global
distributionproxy distribution of deep and
shallow convective clouds as obtained from IFS
Cy33r1 (spring 2008)
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Convection and tropical circulations ITCZ and
the Hadley meridional circulation the role of
trade-wind cumuli and deep tropical towers
Deep Cu
Shallow Cu
Strato Cu
Surface rain
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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
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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
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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)
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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 concepts will be
  introduced
• Buoyancy
• CAPE
• Tephi diagrams
• Impact of convection on the larger-scale flow
• Quasi-Equilibrium

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Concept I Buoyancy - physics of Archimedes
(1)
Body in a fluid
Forces
Proportional to density difference!
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Buoyancy (2)
The impact of a density difference on the
vertical momentum
Neglect second order terms
Buoyancy
Pressure perturbation
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Buoyancy (3)Contributions
Buoyancy acceleration
Dry air
Assume
Buoyancy is temperature driven
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Buoyancy (4) Contributions
Cloudy air
effects of humidity and condensate need to be
taken into account
where Tv is the virtual temperature
Liquid water loading (gravity acting on
condensate)
Impact of different specific gas constant for
moist air
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
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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

dz
g
env
cld
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.
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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
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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
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Concept III Impact of convection on the
large-scales
Thermodynamic equation (dry static energy)
Energy from phase changes
s CpTgz Conserved for adiabatic motions
Radiation
Gives
large-scale observable terms
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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
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Impact of convection on large-scalesTropical
deep convection
Net effect heating (Q1gt0) and drying (Q2gt0)
throughout the troposphere
Tropical Pacific
Tropical Atlantic
Yanai et al., 1973, JAS
Yanai and Johnson, 1993
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Impact of convection on large-scalesSubtropical
shallow convection
Net effect deepening and moistening the
boundary layer! (Q2lt0)
Subtropical Atlantic (Caribbean)
Boundary layer top
Nitta and Esbensen, 1974, MWR
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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)
Q1gt0 heating
Q1lt0 cooling
Q1gt0
Q2gt0 drying
Q2gt0
Q2lt0 moistening
Q1gt0
Q2gt0
Q2lt0
Q2lt0
Q2lt0
Surface rain
E
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Zonal average convective Q1 in IFS
P (hPa)
Latitude
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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)
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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.
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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..

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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
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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