Parametrization of diabatic processes Moist 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
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Peter Bechtold and Christian Jakob
Original ECMWF lecture has been adjusted to fit into today’s schedule
Roel Neggers, KNMI
1st hour
2nd hour
IFS Documentation, Part IV: Physical processes, Chapter V: Convection
Available at http://www.ecmwf.int/research/ifsdocs/
Overview of moist convection
Useful concepts
AP=Academic Press; OUP=Oxford University Press
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
Deep and shallow convection
ITCZ at 10ºN
Sc convection
African Squall lines
Moist convection : Global occurrenceIR GOES METEOSAT 7/04/2003
Lowlevel tradewind 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)
Deep Cu
Shallow Cu
Strato Cu
Surface rain
about 3 mm/day is falling globally, but most i.e. 57 mm/day in the Tropics
The driving force for atmospheric dynamics and convection is the radiative cooling
Above the boundary layer, there is an equilibrium RadiationCloudsDynamicsConvection for Temperature, whereas for moisture there is roughly an equilibrium between dynamical transport (moistening) and convective drying.  Global Budgets are very similar
Highly complex (threedimensional) systems
Bulk parameterizations in GCMs typically have a much simpler form (for computational efficiency)
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 largerscale flow
QuasiEquilibrium
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!
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)ContributionsBuoyancy acceleration:
Dry air:
Assume
Buoyancy is temperature driven
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
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.
CAPE
CIN
LFC
LCL
Convection in thermodynamic diagrams (1)using TephigramsT
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
Moderate dilution
Heavy dilution
Effect of mixing on parcel ascentMixing partially explains inefficient conversion of potential energy into kinetic energy
Mixing: “Entrainment” of dry air into the rising cloud
Apply Reynolds’ averaging (see boundarylayer lecture):
and
In convective regions these terms will be dominated by convection
“subgrid” terms
Concept III:Impact of convection on the largescalesThermodynamic equation (dry static energy) :
Energy from phase changes
s =CpT+gz Conserved for adiabatic motions
Radiation
Gives
“largescale observable” terms
Apparent heat source
Define:
Apparent moisture sink
Note:
h: Moist static energy (conserved for moist processes and precipitation)
with
These Qquantities:
* describe the influence of the “subgrid” processes on the atmosphere.
* can indirectly be derived from observations by estimating the “largescale” terms on the l.h.s. of the areaaveraged equations
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
600
700
P(hPa)
Q2
800
Q1
900
QR
1000
14
10
6
2
0
2
(K/day)
Impact of convection on largescalesSubtropical shallow convectionNet 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 boundarylayer 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
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:Quasiequilibrium
Largescale Forcing that slowly builds up instability (generates CAPE)
The fast convective process that stabilizes environment (removes CAPE)
Quasiequilibrium:
A nearbalance is maintained, even when F is varying with time, i.e. cloud ensemble follows the Forcing.
Concept IV:Quasiequilibrium: Earthly analogue
Free after Dave Randall:
LS ascent: w<0
Concept IV:Quasiequilibrium: Observational evidenceGARP Atlantic Tropical Experiment (GATE, 1974)
Precipitating convection is observed to react to instability caused by largescale convergence:
Thompson et al., JAS, 1979
w (700 hPa)
Precipitation