Retrieval of Surface Spectral Emissivity in Polar Regions Based on the Optimal Estimation Method

Abstract

Surface emissivity is defined as the ratio of actual surface thermal emission to the radiation of a blackbody with the same temperature as the surface. It is a spectrally dependent property of surface composition that directly affects surface upward thermal emission and reflection of atmospheric downward thermal radiation at the surface, thus playing an important role in the radiation budget of the climate system. In addition, surface emissivity is also a key quantity to observe in remote sensing, for its inseparable role in atmospheric sounding (

Joint Use of Far‐Infrared and Mid‐Infrared Observation for Sounding Retrievals: Learning From the Past for Upcoming Far‐Infrared Missions

“…The retrieval algorithm used here largely follows the OE method described in Rodgers (2000) and Xie, Huang, Chen, L’Ecuyer, Drouin, et al. (2022). According to the Bayes' theorem, $$$\begin{array}{c}P(\mathbf{x}\vert \mathbf{y})=\frac{P(\mathbf{y}\vert \mathbf{x})\cdot P(\mathbf{x})}{P(\mathbf{y})}\end{array}$$$ where $$\mathbf{y}$$ denotes the radiances observed at TOA and $$\mathbf{x}$$ denotes the state vector.…”

“…In an effort to fill the gap of far‐IR observations, two future satellite missions are selected and scheduled to be launched in this decade: PREFIRE (Polar Radiant Energy in the far‐IR Experiment; L’Ecuyer et al., 2021) by NASA and FORUM by ESA (far‐IR Outgoing Radiation Understanding and Monitoring; Palchetti et al., 2020) will open up a new era for far‐IR surface emissivity retrievals in polar regions. Previous work has shown promising results with surface spectral emissivity retrieved in mid‐IR and far‐IR from simulated PREFIRE measurements in the Arctic (Xie, Huang, Chen, L’Ecuyer, Drouin, et al., 2022) and simulated FORUM observations at different latitudes including polar regions (Ridolfi et al., 2020; Sgheri et al., 2022). It is therefore necessary and interesting to carry out a retrieval effort with real far‐IR satellite observations.…”

“…A PCRTM‐based simulator has been developed by Chen et al. (2013) and successfully applied in multiple studies (Bantges et al., 2016; Chen et al., 2018; Huang et al., 2014; Xie, Huang, Chen, L’Ecuyer, Drouin, et al., 2022). It is applied in our study as the forward model to calculate spectral radiances and Jacobians in clear‐sky conditions.…”

“…First, surface emissivity retrievals in the far‐IR are subject to the strong water vapor absorption. Mid‐IR window channels can stay nearly transparent (clear‐sky transmittances >0.8) even for extremely humid scenarios (Xie, Huang, Chen, L’Ecuyer, Drouin, et al., 2022). However, the far‐IR spectrum will become almost opaque with the clear‐sky transmittances less than 0.05 when the total column water vapor (TCWV) exceeds 1 cm as indicated by Feldman et al.…”

Sensitivity of Arctic Surface Temperature to Including a Comprehensive Ocean Interior Reflectance to the Ocean Surface Albedo Within the Fully Coupled CESM2

“…where SKT represents the CTR climatological SKT; ε s is calculated by ε s = LWUP∕−ε bb σ(SKT) 4 (the reader is referred to Xie et al, 2022 for the definition of ε s ), LWUP is the model output CTR climatological LWUP at the surface, and ε bb is the blackbody emissivity (unity). Term G is vanishingly small as z is a very thin air-surface interface (Lee et al, 2017;Lesins et al, 2012).…”