Parametrization of diabatic processes moist convection
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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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Parametrization of diabatic processes Moist convection

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Parametrization of diabatic processes moist convection

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

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

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 dynamics text books

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

What does it look like ?

In the lab:

From above:

From the ground:


Convective modes shallow cumulus

Convective modes: Shallow cumulus


Convective modes deep cumulus

Overshoot

Anvil cloud

Updraft

Convective modes: Deep cumulus


Convective modes congestus

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


Congestus

Congestus

9


Reality often a mixture of multiple modes

Reality: often a mixture of multiple modes

Gulf of Mexico

10


Moist convection global occurrence

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

Convection & dynamics in the (sub)tropics

Low-level trade-wind flow

Stratocumulus

Shallow cu

Deep cu

IR GOES, tropical Eastern Pacific


Parametrization of diabatic processes moist convection

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


Parametrization of diabatic processes moist convection

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


Parametrization of diabatic processes moist convection

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 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

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

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


Concept i buoyancy physics of archimedes 1

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

Buoyancy (2)

The impact of a density difference on the vertical momentum:

Neglect second order terms

Pressure perturbation

Buoyancy


Buoyancy 3 contributions

¢

T

¢

¢

p

T

»

B

g

<<

T

p

T

Buoyancy (3)Contributions

Buoyancy acceleration:

Dry air:

Assume

Buoyancy is temperature driven


Buoyancy 4 contributions

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


Concept ii convective available potential energy cape

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.


Convection in thermodynamic diagrams 1 using tephigrams

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


Effect of mixing on parcel ascent

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


Concept iii impact of convection on the large scales

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 scales q 1 and q 2

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 scales tropical deep convection

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


Impact of convection on large scales subtropical shallow convection

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


Parametrization of diabatic processes moist convection

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

Zonal average convective Q1 in IFS

P (hPa)

Latitude


Vertical integrals of q 1 and q 2

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)


Parametrization of diabatic processes moist convection

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.


Parametrization of diabatic processes moist convection

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………..


Concept iv quasi equilibrium observational evidence

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

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


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