Close Correlation Between Vertically Integrated Tropospheric Water Vapor and the Downward, Broadband Thermal-Infrared Irradiance at the Ground: Observations in the Central Arctic During MOSAiC

The impact of the vertical distribution of tropospheric water vapor on the cloud-free downward, broadband thermal-infrared irradiance $\left(F_{\text{TIR}}\right)$ was quantified using observations in the Central Arctic, north of 85°N, collected during the Arctic winter. The water vapor profiles were measured with a temporal resolution of $30s$ using a Raman lidar. The observations revealed maximum values of integrated water vapor (IWV) contents of $3.6kg{m}^{-2}$. Seven measurement cases of several-hour durations of slowly changing air masses were examined. Furthermore, 53 rather short-term (10 min) measurement cases were studied. The temporal evolution of the slowly changing air masses revealed a linear relationship between $F_{\text{TIR}}$ and IWV with slopes between 7.17 and $12.95Wk{g}^{-1}$ and a coefficient of determination larger than 0.95 for most of the selected cases. The slopes and the ordinate intercepts showed a dependence on the water vapor-weighted mean temperature (representative temperature of the water vapor distribution). The temperature determined with the Stefan-Boltzmann law from $F_{\text{TIR}}$ correlated with the representative temperature with a coefficient of determination of 0.92. The analysis of 53 independent short-term observations of different air masses confirmed the linear relationship between $F_{\text{TIR}}$ and IWV at wintertime cloud-free conditions in the Arctic (coefficient of determination of 0.75, slope of $19.95Wk{g}^{-1}$, and ordinate intercept of $107.22W{m}^{-2}$).