EMS 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

EMS Sep Reading UK Motivation (1) GABLS3 SCM case 01-July hours moderate geowind (7 m/s) Emphasis on stable boundary layer and the transition periods Intercomparison and evaluation of 19 Models

EMS Sep Reading UK Motivation (2) Long wave downward radiation at 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!

EMS Sep Reading UK Strategy Start with Brunt (1932) L ↓ as function of 2-m temperature ( T 2 ) and water vapour pressure ( e 2 ) 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)

EMS 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 clouds Cloud radar – cirrus clouds L ↓ from Cabauw BSRN station: Accuracy 2-3 W m -2 1 year of data (May 2008 – Apr 2009)

EMS Sep Reading UK Results (Brunt, 1932) Optimized parameters a = 0.660± b = ± Brunt deviates at day and night because upper air is warm at night and cool at day time

EMS Sep Reading UK Results (Extensions of Brunt, 1932) Separate Day and Night Substitute part of surface radiation by radiation emitted at temperature of 200 m Use e 200 instead of e 2

EMS Sep Reading UK Results (Optimizations) 10 minute mean clear sky observations N = 2450 all N = 1480 night N = 600 day Brunt (1932) flavours Sig (y) W m -2 Original7.57 Add offset All6.32 Day5.43 Night5.91 Add T 200 All4.62 Day4.38 Night4.46 Change e 2 -> e 200 All4.14 Day3.94 Night3.96 Add offset : improved but significant difference between night and day Add T 200 : Improved and much less difference between night and day Change e 2 -> e 200 : Improved further

EMS Sep Reading UK Results (Improved and more general fit) Brunt Extended deviations almost gone because upper air temperature is taken into account Brunt deviates at day and night because upper air is warm at night and cool at day time

EMS 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

EMS 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 overview See for information and data of CESARwww.cesar-observatory.nl