Uncertainty of simulated brightness temperature due to sensitivity to atmospheric gas spectroscopic parameters

Abstract. Atmospheric radiative transfer models are extensively used in Earth observation to simulate radiative processes occurring in the atmosphere and to provide both upwelling and downwelling synthetic brightness temperatures for ground-based, airborne, and satellite radiometric sensors. For a meaningful comparison between simulated and observed radiances, it is crucial to characterise the uncertainty of such models. The purpose of this work is to quantify the uncertainty in radiative transfer models due to uncertainty in the associated spectroscopic parameters, and to compute simulated brightness temperature uncertainties for millimeter- and submillimeter-wave channels of downward-looking satellite radiometric sensors (MWI, ICI, MWS and ATMS) as well as upward looking airborne radiometers (ISMAR and MARSS). The approach adopted here is firstly to study the sensitivity of brightness temperature calculations to each spectroscopic parameter separately, then to identify the dominant parameters and investigate their uncertainty covariance, and finally to compute the total brightness temperature uncertainty due to the full uncertainty covariance matrix for the identified set of relevant spectroscopic parameters. The approach is applied to a recent version of the Millimiter-Wave propagation model, taking into account water vapor, oxygen, and ozone spectroscopic parameters, though it is general and can be applied to any radiative transfer code. A set of 135 spectroscopic parameters were identified as dominant for the uncertainty of simulated brightness temperatures (26 for water vapor, 109 for oxygen, none for ozone). The uncertainty of simulated brightness temperatures is computed for six climatology conditions (ranging from sub-Arctic winter to Tropical) and all instrument channels. Uncertainty is found to be up to few kelvin K in the millimeter-wave range, whereas it is considerably lower in the submillimeter-wave range (less than 1 K).