Boundary Layer Verification

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Boundary Layer Verification

• ECMWF training course
• May 2010
• Maike Ahlgrimm

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What does the BL parameterization do?
Attempts to integrate effects of small scale
turbulent motion on prognostic variables at grid
resolution.
Turbulence transports temperature, moisture and
momentum (tracers).
Ultimate goal correct model output
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Which aspect of the BL can we evaluate?

• 2m temperature/humidity
• Depth of BL
• Diurnal variability of BL height
• Structure of BL (temperature, moisture, velocity
  profiles)
• Turbulent transport within BL
• Boundaries entrainment, surface fluxes, clouds
  etc.

large scale
small scale
Chandra et al., sub. to J. Climate
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Part 1

• Depth of the boundary layer

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BL depth from radiosondes

• Problem Define the top of the BL!
• Concept At he top of the BL, the air motion
  transitions from turbulent to laminar flow.
• For an equitable comparison, apply the same
  criteria for identification of this transition to
  model profiles and radiosonde profiles.
• Alternative for convectively driven boundary
  layers turbulent mixing leads to T and q
  gradients at the BL top. Identify these gradients
  in the profile.

normalized BL height
Figure Martin Köhler
DSE/cp
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Richardson number-based approach

• Richardson number defined as
• flow is turbulent if Ri is negative
• flow is laminar if Ri above critical value
• calculate Ri for model/radiosonde profile and
  define BL height as level where Ri exceeds
  critical number

buoyancy production/consumption shear production
(usually negative)
Ri
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Difficulties with this approach

• discrete model layers -gt bulk Ri number
• where is the top and bottom of the bulk layer?
• how much do surface fluxes increase buoyancy? not
  most reliable model field
• for sonde profiles, surface fluxes usually
  unavailable
• noise in sonde profiles can introduce
  uncertainties

diagnostic BLH in IFS is currently tuned to
best agree with paramete- rization based BL height
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How-to

• Need T, u,v,q,z and some constants
• Define conserved variable, e.g. virtual dry
  static energy
• Apply smoothing in the vertical if necessary
• Starting at lowest model level, calculate Ri
  number, adding an excess to the dse to make up
  for missing surface fluxes
• Iterate, until Ri exceeds critical level (e.g.
  0.25)
• Assign height of nearest layer as BL top height

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Example dry convective boundary layer NW Africa
2K excess
1K excess
Figures Martin Köhler
Theta K profiles shifted
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Example Inversion-topped BL

• Inversion capped BLs dominate in the subtropical
  oceanic regions
• Identify height of jump across inversion

EPIC, October 2001 southeast Pacific
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Limitations of sonde measurements

• Sonde measurements are limited to populated areas
• Depend on someone to launch them (cost)
• Model grid box averages are compared to point
  measurements (representativity error)

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Took many years to compile this map
Neiburger et al. 1961
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Calipso tracks
CALIPSO tracks
Arabic peninsula - daytime
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BL from lidar how-to

• Easiest use level 2 product (GLAS)
• Algorithm searches from the ground up for
  significant drop in backscatter signal
• Align model observations in time and space with
  satellite track and compare directly, or compare
  statistics

molecular backscatter
backscatter from BL aerosol
surface return
Figure GLAS ATBD
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Example Lidar-derived BL depth from GLAS
Only 50 days of data yield a much more
comprehensive picture than Neiburgers map.
Ahlgrimm Randall, 2006
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Limitations to this method

• Definition of BL top is tied to aerosol
  concentration - will pick residual layer
• Does not work well for cloudy conditions
  (excluding BL clouds), or when elevated aerosol
  layers are present
• Overpasses only twice daily, same local time
• Difficult to monitor given location

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The case of marine stratocumulus

• Well mixed convective layer underneath strong
  inversion
• Are clouds part of the BL?
• As Sc transition to trade cumulus, where is the
  BL top?

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Stratocumulus cloud top height
Model underestimates Sc top height
SEP
EPIC
Hannay et al. 2009
Köhler Ahlgrimm, sub.
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Part 2

• Diurnal cycle of boundary layer height

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Diurnal cycle of convective BL from radiosonde

• Example stratocumulus-topped marine BL in the
  south-east Pacific East Pacific Investigation of
  Climate (EPIC), 2001
• Clear diurnal cycle of 200m with minimum in
  early afternoon, maximum during early morning.

Bretherton et al. 2004, BAMS
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Diurnal cycle from CALIPSO
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Part 3

• Turbulent transport

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Flux towers

• Example Cabauw, 213m mast
• obtain measurements of roughness length, drag
  coefficients etc.

KNMI webpage
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Bomex trade cumulus regime
Stevens et al. 2001
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Bomex - DualM

• Dual Mass Flux parameterization - example of
  statistical scheme mixing K-diffusion and mass
  flux approach
• Updraft and environmental properties are
  described by PDFs, based on LES
• Need to evaluate PDFs!

Neggers et al. 2009
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Turbulent characteristics humidity

• Raman lidar provides high resolution (in time and
  space) water vapor observations

Plot Franz Berger (DWD)
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Turbulent characteristics vertical motion

• Observations from mm-wavelength cloud radar at
  ARM SGP, using insects as scatterers.

reflectivity
doppler velocity
reflectivity
Chandra et al., sub. to J. Climate
local time
red dots ceilometer cloud base
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Turbulent characteristics vertical motion

• Variance and skewness statistics in the
  convective BL (cloud free) from four summer
  seasons at ARM SGP

Chandra et al., sub. to J. Climate
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Part 4

• Boundaries

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Forcing

• BL turbulence driven through surface fluxes, or
  radiative cooling at cloud top.
• Check albedo, soil moisture, roughness length,
  clouds
• BL top entrainment rate important but elusive
  quantity

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Entrainment rate - DYCOMS II

• Example DYCOMS II - estimate entrainment velocity

mixed layer concept
Stevens et al. 2003
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Summary Considerations

• What parameter do you want to verify?
• What observations are most suitable?
• Define parameter in model and observations in as
  equitable and objective a manner as possible.
• Compare!
• Are your results representative?
• How do model errors relate to parameterization?

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References (in no particular order)

• Neiburger et al.,1961 The Inversion Over the
  Eastern North Pacific Ocean
• Bretherton et al., 2004 The EPIC Stratocumulus
  Study, BAMS
• Stevens et al., 2001 Simulations of trade wind
  cumuli under a strong inversion, J. Atmos. Sci.
• Stevens et al., 2003 Dynamics and Chemistry of
  Marine Stratocumulus - DYCOMS II, BAMS
• Chandra, A., P. Kollias, S. Giangrande, and S.
  Klein Long-term Observations of the Convective
  Boundary Layer Using Insect Radar Returns at the
  SGP ARM Climate Research Facility, submitted to
  J. Climate
• Hannay et al., 2009 Evaluation of forecasted
  southeast Pacific stratocumulus in the NCAR,
  GFDL, and ECMWF models. J. Climate
• Köhler et al. Stratocumulus in the ECMWF model.
  submitted to QJRMS
• Ahlgrimm Randall, 2006 Diagnosing monthly mean
  boundary layer properties from reanalysis data
  using a bulk boundary layer model. JAS
• Neggers, 2009 A dual mass flux framework for
  boundary layer convection. Part II Clouds. JAS