A simple parameterization for detrainment in shallow cumulus

Hirlam results for RICO

Wim de Rooy & Pier SiebesmaRoyal Netherlands Meteorological Institute (KNMI)

Hirlam 1D

Hirlam but with:• Statistical cloud scheme• Tiedtke mass flux convection scheme with

updates:– Mass flux closure at cloud base (Neggers et al.

2002)– Triggering (Jacob & Siebesma 2003)– versions with a conventional or a new lateral

mixing concept

Mass flux concept

MM M

•Fixed and :No dependence on environmental

humidity conditions

•Buoyancy sorting conceptComplex, fundamental problems

Fraction of environmental air

z

MM

c

c

1Courtesy: Stephan de Roode

Conventional fixed =z-1 and =0.00275 m-1 okay for BOMEX but for RICO?

What’s going on?

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

4 6 8 10 12 14 16

RICO (LES and SCM, No Rain)

qt [gr/kg]

z [m

] LES no rain (24:00 UTC) fixed and (24:00 UTC) inital profile (0:00 UTC)

LES results for BOMEX, ARM and RICO show:

• Not much variation in . For a correct simulation of the Mass flux profile, =z-1 is good enough.

• Much more variation in . The value of mainly depends on:- Cloud Layer Height- Environmental conditions

(e.g. BOMEX)

Cloud layer depth=1000m

Mass flux profiles with =z-1 and =0.00275

Cloud layer depth=200m

Cloud layer depth=2000me.g. RICO

Cloud ensembles

z

M

M

M

z

zbot

ztop

ztop

ztop

zbot

zbot

Cloud Layer Height dependence

LES Non-dimensionalized mass flux profiles

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0ARM LES

(z

-zbo

t)/(z

top-

z bot)

M/Mb

17:30 UTC 18:30 UTC 19:30 UTC 20:30 UTC 21:30 UTC 22:30 UTC 23:30 UTC 24:30 UTCz*

ARM case LES

Eliminate cloud height dependence by looking at a non-dimensionalized mass flux profile

Dependence on environmental conditions

Suppose we would know the non-dimensionless mass flux m* halfway the cloud layer at height z*

Mz

Mcz

mM

M(z :zz atbot

**

)(

)

1

*

)(

)ln(

*

*

*

bot

bot

zzmz

z

Z*

Zbot

Ztop

0.00 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27 0.30 0.330.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

m*

mean c

m* RICO m* Bomex m* ARM

From LES, the non-dimensionalized mass flux half way the cloud layer as a function of c

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

4 6 8 10 12 14 16

RICO (LES and SCM, No Rain)

qt [gr/kg]

z [m

]

LES (24:00 UTC) fixed and (24:00 UTC) new (24:00 UTC) initial profile (0:00 UTC)

Good results with the new parameterization for ARM, BOMEX and:

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

2 4 6 8 10 12 14 16 18

RICO (LES with and without rain, SCM with Rain)

qt [gr/kg]

z [m

]

LES with Rain (24:00 UTC) new (24:00 UTC) fixed and (24:00 UTC) LES no rain (24:00 UTC)

ConclusionsThe proposed detrainment parameterization is simple but includes two important dependencies:

•Cloud layer height dependenceCurrent mass flux schemes ignore this dependence which evidently can lead to large discrepancies with observed mass flux profiles.

•Environmental conditionsWith the c dependence the new scheme can be seen as an alternative for more complex buoyancy sorting schemes (without some of the disadvantages)

Conclusions• Good results for a wide range of shallow convection cases

(BOMEX, ARM, RICO)

• Easy to incorporate in existing mass flux schemes (and will be incorporated in an EDMF dual mass flux

environment)

45 50 55 60 65 70 75 80 850.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

m*

Relative humidity [%]

m* RICO m* Bomex m* ARM

LES: The non-dimensionalized mass flux half way the cloud layer as a function of RH

LES Non-dimensionalized

mass flux profilesNon-dimensionalized mass

flux profiles with fixed and

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0ARM LES

(z-z

bot)/

(zto

p-z

bot)

M/Mb

17:30 UTC 18:30 UTC 19:30 UTC 20:30 UTC 21:30 UTC 22:30 UTC 23:30 UTC 24:30 UTC

ARM case

Eliminate cloud height dependence by looking at a non-dimensionalized mass flux profile

Dependence on environmental conditions