On Effective Spectral Wideband Models for Clear Sky Atmospheric Emissivity and Transmissivity

Abstract

AbstractClear sky emittance models provide critical information for the determination of downwelling longwave irradiance at the Earth's surface. This study updates existing calculations which relate clear sky longwave emissivity with the main (and most variable) greenhouse gas in the atmosphere, water vapor. Impacts of station elevation and data quality control are quantified. Empirical results are used to validate highly resolved spectral models, and the resultant simplified calculates provide accurate estimations of clear sky emissivity without the need for extensive computation. Results show that correlation coefficients are mostly robust to nuanced data processing choices when regressed from sufficiently large data sets (≥104 samples) with the exceptions of altitude adjustment and measurement bias corrections. The empirical results from this study are compared to results from other leading empirical, physics‐based, and hybridized phenomenological models. Correlations for effective clear sky emissivity, transmissivity and optical depth are provided, based on parameterized line‐by‐line (LBL) model results, for the broadband 0–2,500 cm−1 and for seven wavenumber wideband of interest. Results for the (b3) wideband 580–750 cm−1 are particularly relevant because of its disaggregated and combined carbon dioxide‐water vapor contributions. The broadband effective optical depth (δ) of water vapor is found to be , where pw is the dimensionless partial pressure of water vapor at the surface. Equivalently, the broadband effective optical depth of carbon dioxide in the presence of water vapor is found to be . Processed training data sets are provided as supplementary content for comparative studies.