a direct measure of entrainment david m. romps workshop on large-scale circulations
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A direct measure of entrainment David M. Romps Workshop on Large-Scale Circulations in Moist Convecting Atmospheres October 17, 2009. Conclusion. The bulk -plume equations , when used to diagnose large-eddy simulations, underestimate convective entrainment. Implication. - PowerPoint PPT PresentationTRANSCRIPT
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A direct measure of entrainment
David M. Romps
Workshop on Large-Scale Circulationsin Moist Convecting Atmospheres
October 17, 2009
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The bulk-plume equations, when used to diagnose large-eddy simulations, underestimate convective entrainment.
Conclusion
Implication
Way forward
Design an independent way to diagnose entrainmentand use that rate in convective parameterizations.
Too little entrainment Too little sensitivity to mid-tropospheric moisture Muted convectively coupled waves
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We need an algorithm for classifying the atmosphere into two categories:
active & inactive
Define the “activity operator”, , such that
Example:
We can now define entrainment and detrainment in terms of :
An air parcel entrains when it flips from inactive to active
An air parcel detrains when it flips from active to inactive
Entrainment into what?
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Standard bulk-plume diagnosis of entrainment
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Bulk-plume approximation:
Standard bulk-plume diagnosis of entrainment
Bulk
-plu
me
equa
tions
Consider the case of = total water.....
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The bulk-plume equations:
1. Underestimate entrainment
Problems with the bulk-plume diagnosis of entrainment
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LES of shallow convection
25.6
km
25.6 km
25.6 km
200-metergrid cells
Lin-Lord-Krueger6-class microphysics
RRTM
300-K SST
3 km
12.8 km
12.8 km50-metergrid cells
Lin-Lord-Kruegerw/o precip
SST = 300.4 K
DAM(Romps, “The dry-entropy budget
of a moist atmosphere”, JAS, 2008)
LES of deep convectionModel setup
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The bulk-plume equations:
1. Underestimate entrainment
2. Produce tracer-dependent estimates of entrainment
3. Produce unphysical values of entrainment
Problems with the bulk-plume diagnosis of entrainment
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Direct measurement of entrainment
Entrainment is the local source of active fluid
Detrainment is the local sink of active fluid
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Direct measurement of entrainment
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Entr
ainm
ent
Detr
ainm
ent
Activ
e ai
r
40 minutes 60 minutes 80 minutes
Direct measurement of entrainment
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Bulk
-plu
me
equa
tions
Dire
ct m
easu
rem
ent
Direct measurement vs. Bulk-plume equations
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Direct measurement vs. Bulk-plume equations
Bulk-plume equations underestimate entrainment by roughly a factor of 2
Fractional entrainment does not go like 1/z.
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Entrainment-buoyancy relationships
Some relationship:
No relationship:
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Boom and bust cycle at the melting line
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Other conclusions
Standard method produces tracer-dependent and
unphysical estimates of entrainment
Direct measurement offers new diagnostic possibilities
Standard method underestimates entrainment
by roughly a factor of 2
There is a linear relationship between buoyancy and detrainment
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(Romps and Kuang, “Nature versus nurture in shallow convection,” JAS, in review)
From LES of BOMEX, we learn that the variability of total water in shallow,
non-precipitating clouds is large.
This source of this variability is not cloud-base variability, but, instead, is a source of variability intrinsic to all convection: turbulent entrainment.
Must understand cloud variance to understand sensitivity to humidity
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Entrainment can be measured directly.
Other conclusion
Implication
Caution
Be wary of bulk-plume parameterizations!
We can learn what sets the entrainment rate Use this to inform our convective parameterizations Improve simple models of convectively coupled waves
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Extend the bulk-plume analysis to deep convection using the purity tracer
In shallow, non-precipitating convection, we can use total water for .
In deep convection, there is no such quantity that is strictly conserved (not even MSE).
Specify “purity” tracer with sources away from convection that maintain:
(Romps and Kuang, “Do undiluted convective plumes exist in the upper tropical troposphere?”, JAS, in press)
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Direct measurement of entrainment
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Bulk-plume analysis with artificial tracers
In shallow, non-precipitating convection, we can use = total water.
In deep convection, there is no quantity that is strictly conserved (not even MSE).
Define “purity” tracer with sources away from convection that maintain:
Purity tracer
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The bulk-plume equations:
1. Underestimate entrainment
2. Produce unphysical values of entrainment
Problems with the bulk-plume diagnosis of entrainment
![Page 23: A direct measure of entrainment David M. Romps Workshop on Large-Scale Circulations](https://reader036.vdocuments.net/reader036/viewer/2022062520/568164ba550346895dd6c778/html5/thumbnails/23.jpg)
Bulk-plume analysis with artificial tracers
In shallow, non-precipitating convection, we can use = total water.
In deep convection, there is no quantity that is strictly conserved (not even MSE).
Define “purity” tracer with sources away from convection that maintain:
Define “exponential” tracer with sources away from convection that maintain:
Purity tracer
Exponential tracer