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Evaluating modelled surface long wave downward radiation with Cabauw observations: The GABLS3 SCM case. Fred Bosveld (KNMI). Content: Motivation An improved formula for Long Wave Downward radiation High quality validation data Optimization of Improved formula Application in GABLS3 SCM - PowerPoint PPT Presentation
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EMS2013 09 - 13 Sep Reading UK
Evaluating modelled surface long wave downward radiation with Cabauw observations: The GABLS3 SCM
case.
Fred Bosveld (KNMI)
Content:
• Motivation
• An improved formula for Long Wave Downward radiation
• High quality validation data
• Optimization of Improved formula
• Application in GABLS3 SCM
• Conclusions
EMS2013 09 - 13 Sep Reading UK
Motivation (1)
GABLS3 SCM case01-July-200624 hoursmoderate geowind (7 m/s)
Emphasis on stable boundary layer and the transition periods
Intercomparison and evaluation of 19 Models
EMS2013 09 - 13 Sep Reading UK
Motivation (2)
Long wave downward radiationat the surface:
Models spread and underestimate
Models differ in:
- Thermodynamic profiles- Profiles of GHG- Radiation parameterization
How to evaluate?
- Get rid of the influence of Thermodynamic profiles!
EMS2013 09 - 13 Sep Reading UK
Strategy
Start with Brunt (1932) L↓ as function of 2-m temperature (T2) and water
vapour pressure (e2)
Extend simple formula of Brunt (1932) with profile information
Optimize against high quality observations.
Half of L↓ at the surface originates from lowest 200 m (Ohmura, 2001)
Temperature gradient is important for L↓ (Dürr and Philipona, 2004)
422 )( TebaL
EMS2013 09 - 13 Sep Reading UK
Observations from CESAR:Cabauw Experimental Site for Atmospheric Research
Observations of Cabauw (NL) (Boers et al., 2011)
Clear Sky detection very strict :
Nubiscope – low and middle cloudsCloud radar – cirrus clouds
L↓ from Cabauw BSRN station:
Accuracy 2-3 W m-2
1 year of data (May 2008 – Apr 2009)
EMS2013 09 - 13 Sep Reading UK
Results (Brunt, 1932)
422 )( TebaL
Optimized parameters
a = 0.660± 0.002
b = 0.0419± 0.0007
Brunt deviates at day and nightbecause upper air is warm at
nightand cool at day time
EMS2013 09 - 13 Sep Reading UK
Results (Extensions of Brunt, 1932)
fTebaL 422 )(
Separate Day and Night
fTTedcTebaL )()()( 42
4200200
422
422 )( TebaL
Substitute part of surface radiation by radiation emitted at temperature of 200 m
fTTedcTebaLref )()()( 42
4200200
42200
Use e200 instead of e2
EMS2013 09 - 13 Sep Reading UK
Results (Optimizations)
10 minute mean clear sky observations
N = 2450 allN = 1480 nightN = 600 day
Brunt (1932) flavours
Sig (y)W m-2
Original 7.57
Add offset
All 6.32
Day 5.43
Night 5.91
Add T200
All 4.62
Day 4.38
Night 4.46
Change e2 -> e200
All 4.14
Day 3.94
Night 3.96
Add offset : improved but significant difference between night and day
Add T200 : Improved and much less difference between night and day
Change e2 -> e200 : Improved further
EMS2013 09 - 13 Sep Reading UK
Results (Improved and more general fit)
Brunt Extended deviations
almost gone because upper
air temperature is takeninto account
Brunt deviates at day and night
because upper air is warm at night
and cool at day time
EMS2013 09 - 13 Sep Reading UK
Coming back to GABLS3 SCM
Under estimation almost disappears after correcting for the effect of
thermodynamic profiles on L↓.
ΔL↓= L↓ - L↓ref
0400 UTC
EMS2013 09 - 13 Sep Reading UK
Conclusions
T
An extension of the Brunt (1932) LWD formula is found:• More general in time than the original simple formula• Residuals in the fit approaches the high accuracy of the
observation• The effect of e and T profile above 200 m is only statistically
taken into account.• Thus the coefficients are most likely location dependent.
Application in GABLS3 SCM case:• LWD bias in most Models explained• Thermal coupling to the surface dominates T-profile. (Bosveld
et al., 2013, BLM (In review)).
See Holtslag et al. (2013) (BAMS) for GABLS overviewSee www.cesar-observatory.nl for information and data of CESAR