| 17 Oct 2025
Past, Present, and Future Arctic Radiative States Simulated by Polar-WRF
Abstract. Two recurring radiative states (“transmissive” and “opaque”) strongly modulate the Arctic surface energy balance through their control on downwelling longwave radiation (DLR). Because these states are primarily governed by cloud processes, many coarse-resolution models fail to capture their behavior. This study evaluates how well the Polar-optimized Weather Research and Forecasting model (PWRF) simulates present-day DLR distributions associated with these states and examines projected changes into the future. While most physics parameterizations mirror those of the widely used Arctic System Reanalysis (ASR), we test several advanced microphysics schemes and assess the impact of model resolution. Both the P3 and Morrison two-moment schemes (candidates for the next ASR version) overproduce the opaque mode, whereas the Goddard scheme used in ASR overproduces the transmissive mode. The opaque bias in P3 and Morrison arises mainly from excessive low-level, optically thick clouds over sea ice. Among all schemes, P3 best preserves the distinctiveness of the two radiative modes. Using this scheme, PWRF forced with end-of-century CESM1 output projects a shift toward more frequent opaque conditions, consistent with long-term observations at the North Slope of Alaska. While PWRF shows promise as a tool for dynamically downscaling climate model output, persistent cloud-related biases, especially over ice, warrant caution in future projections. Continued improvements in cloud representation are essential to obtain more quantitative insight into Arctic radiative regime changes.
Summary of the main contribution of the paper:
Bertossa et al. study the evolution of Arctic radiative states (Transmissive Vs Opaque) using a high-resolution climate model designed for polar regions (Polar-WRF). They combine observations from satellites (CALIPSO-CloudSat) and ground station (ARM-NSA) to evaluate the representation of Downwelling Longwave Radiation (DLR) from climate models and reanalyses. They found that the bimodality of the DLR distribution is misrepresented for almost all seasons (which is critical as we advance in simulating future Arctic trajectories). Using the PWRF (with some specific microphysical schemes) allows us to get a DLR distribution close to the observed one, with two distinct radiative states: Transmissive and Opaque, although some biases remain (overproducing opaque clouds or too frequent intermediate conditions). A sensitivity study showed that the biases come mainly from over sea ice and need further improvements. However, PWRF does a good job in allowing us to see the broader picture of DLR and CRE-LW evolution as the climate warms. Specifically, the authors show that going from past-present to future climate, the DLR will shift toward larger values with a higher occurrence. Decomposing into contributions, the DLR-clear will shift toward larger values, and the CRE-LW will increase the occurrence of large values (higher than 70 W/m2), mostly due to the low cloud cover increase.
General comments:
The study is well presented and nicely constructed. The findings are interesting. I suggest some minor/specific comments.
Specific comments:
Figure comments: