<title>Sensitivity of broadband and spectral measurements of outgoing radiance to changes in water vapor content</title>

Response to reviewer #2

“…Such a value is consistent with the accuracy provided by the manufacturer and the absolute uncertainty of 2 W m −2 suggested by Marty (2003) for such sensors. This is very satisfactory for a sensor sensitive only up to 42 µm while a significant part of the energy lies beyond, and considering that the calibration was done above 2 • C. This agreement gives high confidence in the atmospheric profile measurements, as well as in the aerosols modelled in MODTRAN, because errors in aerosol profiles could result in discrepancies of several W m −2 (Sauvage et al, 1999). Regarding the upper extrapolated part of the atmosphere, comparisons of measured and simulated downwelling LW fluxes (not shown) are also in reasonably good agreement, which gives confidence in the ERA-Interim fields.…”

The Far‐infrared Earth

“…In fact, a very interesting partially transparent window in the 500–600 cm −1 region opens up, raising the intriguing question of enhanced surface to space radiative cooling, as reported by Shaw et al [1999] on the basis of low water content measurements from a high mountain. Rizzi et al [2002] also demonstrated that the FIR was particularly sensitive to water vapor changes in the upper troposphere and suggested that spectrally resolved measurements were more sensitive to humidity changes at all levels in the atmosphere than broadband (spectrally integrated) measurements.…”

“…As suggested by section 2.4 the FIR spectrum at the top of the atmosphere is highly sensitive to changes in atmospheric water vapor [ Rizzi et al , 2002]. Figure 6 shows the difference in outgoing longwave radiance between three simulations: (1) case A (tropical standard atmosphere with water vapor concentration scaled by 0.9 and skin temperature decreased by 1 K); (2) case B (tropical standard atmosphere with only water vapor concentration scaled by 0.9 and no other change); and (3) case C, the control (tropical standard atmosphere with no scaling).…”

“…Recently, Rizzi et al [2002] analyzed the potential water vapor retrieval performance attainable from spectral measurements with a resolution of 0.5 cm −1 in the FIR, from 200 to 1000 cm −1 . A technique known as the σ ‐Infrared Atmospheric Sounding Interferometer ( σ ‐IASI) methodology [ Amato et al , 2002] was applied, and it was found that the sensitivity of the rotational band to water vapor perturbations, as measured by the matrix of the derivative of spectral radiance with respect to water vapor amount (the Jacobian), may reach values 6–7 times greater than that exhibited using the ν 2 band; this implies greater sensitivity of the radiance to changes in water vapor amount in the FIR than in the MIR.…”

Can IASI be used to simulate the total spectrum of outgoing longwave radiation?

“…Currently retrievals of upper tropospheric water vapour (UTWV) by space-borne instruments exclusively focuses on the vibrational-bending mode (ν 2 ) 15 which is centred at 1595 cm −1 (6.3 µm). However, research has shown that the radiance from the rotational mode may be up to 6-7 times more sensitive to water vapour changes (Rizzi et al, 2002;Huang et al, 2007). Harries et al (2008) estimate the accuracy of of the retrieval performance of the FIR to be comparable to, and sometimes slightly better than, an equivalent mid infrared sounder.…”