The impact of water vapour continuum on absorption of

solar radiation – a preliminary analysis of the impact of the

new RAL measurements

Julie Chenery, Igor Ptashnik, Keith Shine Department of Meteorology, University of Reading

September 2010

(More than) a hint that (MT)-CKD is incorrect by an order of magnitude in near-IR windows

Measurements in near-IR band wings

Paynter et al. (JGR 2009) plus other evidence

Latest results support earlier preliminary measurements

2000 3000 4000 5000 6000 7000 8000

10-3

10-2

10-1

100

/ 430K, 1.8 atm H2O, Res=0.2cm-1 /

Uncertainty in baseline about 0.3% or 0.003 OD

Lines & MTCKD: 430K, 20m, 1.8 atm H2O, Res=0.2cm-1

cm-1

Op

tica

l dep

th

HITRAN-2008 lines MTCKD-1.3 MTCKD-2.5 RAL continuum: RAL - PS_lines RAL continuum: RAL - HITRAN-2008

Measurements at RAL (Smith, McPheat,Williams)

Analysis by Igor Ptashnik

430 K measurements and comparison with MT-CKD

Method

• Apply 430 K ratio at all temperatures

• Clear sky, no scattering, near-IR only, includes CO2, CH4 etc

• From previous work (Ptashnik, Paynter, …) we do not expect a huge effect

Vertical optical depth in near-IR for mid-lat summer

atmosphere

Impact on surface irradiance (overhead sun) mainly seen

at edges of windows

Dependence on solar zenith angle

0o 85o

Dependence on zenith angle

• Marked difference in dependence between the two continua

Dependence on atmosphere

NewOld

0o

85o

July latitudinal dependence Absolute

(0.6%)

(0.4 Wm-2)

Next steps

• Update continuum with more recent analyses including refined temperature dependence

• Include band centres• Extend to cloudy skies etc.• Possibly extend to examining

impact on water vapour feedbacks in response to climate change