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