Convective Parameterization in NWP Models

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Convective Parameterization in NWP Models

• Jack Kain
• And
• Mike Baldwin

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What is convective parameterization?

• A technique used in NWP to predict the collective
  effects of (many) convective clouds that may
  exist within a single grid elementAs a function
  of larger-scale processes and/or conditions.

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Why do NWP models need to worry about it?

• Direct Concern To Predict convective
  precipitation
• Feedback to larger Scales Deep convection
  overturns the atmosphere, strongly affecting
  mesoscale dynamics
• - Changes vertical stability
• - generates and redistributes heat
• - removes and redistributes moisture
• - makes clouds, strongly affecting surface
  heating and atmospheric radiation

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A convective parameterization must decide 3
things

• Activation? ? Trigger function

• Intensity? ? Closure Assumptions

• Vertical Distribution? ? Cloud model or
  specified profile

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How is the parameterized information fed back to
the model?

• Consider the Temperature-Tendency Equation in a
  model

Where the convective term is simply
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Consider two very different approaches

• BMJ Scheme (convective adjustment)
• KF scheme (mass flux scheme)

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Procedure followed by BMJ scheme
1) Find the most unstable air in lowest 200 mb
2) Draw a moist adiabat for this air
3) Compute a first-guess temperature-adjustment
profile (Tref)
4) Compute a first-guess dewpoint-adjustment
profile (qref)
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The Next Step is an Enthalpy Adjustment
With Parameterized Convection, each grid-point
column is treated in isolation. Total column
latent heating must be directly proportional to
total column drying, or dH 0.
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Enthalpy is not conserved for first-guess
profiles for this sounding! Must shift Tref and
qvref to the left
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Imposing Enthalpy Adjustment
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Adjusted Enthalpy Profiles
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Suppose the cloud layer was drierreduce RH by
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Enthalpy is conserved, but the net temperature
change is negative, and the net moisture change
is positive Negative Precipitation!
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If we systematically change cloud-layer RH in
this sounding, it can be shown that precipitation
rate generated by the scheme is very sensitive to
deep-layer moisture
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If the environment is too dry or CAPE layer is
less than 200 mb deep, the scheme attempts to
initiate shallow (non-precipitating) convection
1) Set cloud-top height as the level within 200
mb of LCL where RH falls off most rapidly with
height.
2) Find LCL of cloud-top air line connecting
LCLs of subcloud and cloud-top air is a mixing
line.
3) Assume Tref has same slope as mixing line
first-guess Tref is anchored on ambient
temperature curve.
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With Shallow Convection, there is no net
temperature or moisture change
and
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Consider the impact of parameterized BMJ shallow
convection in a normal diurnal cycle
Model Initial Condition Raob BMX 12 Z 11 May 2000
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Convective Adjustment Profiles
Initial time
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Convective Adjustment Profiles
6 h forecast BMJ convection inactive because
convective entropy change would be negative.
Sounding characteristics that lead to negative
entropy change are not easily identified.
3 h forecast
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Other constraints that cause BMJ shallow
convection to abort
- Net entropy change in cloud layer would be
negative
- Tref is super-adiabatic
- Tref is isothermal
- qref gives a negative q at some level
- qref gives an increase in q with height
- qref gives super-saturated q at some level
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Back to the convective adjustment profiles
9 h forecast 2100 UTC
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Compare with raob at 00 Z 12 h forecast
Model forecast Raob BMX 00Z 12 May 2000
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Consider a transition from shallow to deep
convection
FWD 00Z 20 April 2001
Model Initial Condition Raob
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Convective Adjustment Profiles
1h Forecast
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Compare with raob at 12 Z 12 h forecast
Model forecast Raob FWD 12Z 20 April 2001
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More Convective Adjustment Profiles
16 h forecast
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Continuing to work on the sounding
18 h forecast
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Compare with raob at 00 Z 24 h forecast
Raob Model Forecast FWD 12Z 20 April 2001
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BMJ Deep convection activated only briefly at
FWD, but 100 miles to the north (ADM), BMJ deep
convection was more persistent and strongly
modified soundings
EtaKF Model Forecast Model Forecast ADM 22 Z 20
April 2001
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OK, consider the KF scheme, a Mass-flux
parameterization
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Basic procedures
1) Starting at the surface, mix 50 mb deep
layers, lift to LCL
2) Give parcel a boost based on low-level
convergence. Can it reach the LFC?
3) If parcel makes it to LFC, allow it to rise
and overshoot equilibrium level.
4) Form downdraft from air within 200 mb of
cloud base
5) Overturn mass in updraft, downdraft, and
surrounding environment until stabilization is
achieved.
If cloud depth ? 3 km, parameterize shallow
convection
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KF adjustment profiles
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Focus on deep convectionwhat is the Updraft Mass
Flux (UMF)?
The mass of air that goes through cloud base
divided by the initial mass in the 50 mb
updraft source layer UMF Mu/Musl
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How is UMF determined?
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What is UMF sensitive to?

• qe of downdraft air
• Lapse rates in cloud layer

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Increasing humidity in the 900 550 mb layer
increases downdraft qe. This makes stabilization
of the boundary layer less efficient and UMF
increases.
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Summary

• Parameterized shallow convection can distort
  sounding structures, significantly affecting CIN
  and CAPE more problematic with BMJ than with KF
• BMJ deep convection very sensitive to cloud-layer
  RH
• KF mass flux particularly sensitive to lapse
  rates in lower half of cloud layer.