Abstract
The Arctic radiation balance is strongly affected by clouds and surface albedo. Prior work has identified Arctic cloud liquid water path (LWP) and surface radiative flux biases in the Community Atmosphere Model, version 5 (CAM5), and reductions to these biases with improved mixed-phase ice nucleation schemes. Here, CAM5 net top-of-atmosphere (TOA) Arctic radiative flux biases are quantified along with the contributions of clouds, surface albedos, and new mixed-phase ice nucleation schemes to these biases. CAM5 net TOA all-sky shortwave (SW) and outgoing longwave radiation (OLR) fluxes are generally within 10 W m−2 of Clouds and the Earth’s Radiant Energy System Energy Balanced and Filled (CERES-EBAF) observations. However, CAM5 has compensating SW errors: Surface albedos over snow are too high while cloud amount and LWP are too low. Use of a new CAM5 Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar simulator that corrects an error in the treatment of snow crystal size confirms insufficient cloud amount in CAM5 year-round. CAM5 OLR is too low because of low surface temperature in winter, excessive atmospheric water vapor in summer, and excessive cloud heights year-round. Simulations with two new mixed-phase ice nucleation schemes—one based on an empirical fit to ice nuclei observations and one based on classical nucleation theory with prognostic ice nuclei—improve surface climate in winter by increasing cloud amount and LWP. However, net TOA and surface radiation biases remain because of increases in midlevel clouds and a persistent deficit in cloud LWP. These findings highlight challenges with evaluating and modeling Arctic cloud, radiation, and climate processes.
Highlights
Arctic near-surface air temperatures have warmed faster than the global average over the late twentieth and early twenty-first centuries
With an assessment of cloud amount using the new CAM5 lidar simulator with a correction for an error in the treatment of snow crystal size complete, we evaluate net TOA radiative fluxes in the model
We specify spherical shape, which is the default and what is assumed in the CAM5 cloud optics code and the cloud microphysics code
Summary
Arctic near-surface air temperatures have warmed faster than the global average over the late twentieth and early twenty-first centuries. This ‘‘Arctic amplification’’ has been attributed to numerous possible processes and feedbacks, sea ice loss (Serreze et al 2009). Modeling this observed Arctic climate change and projecting future Arctic climate change with general circulation models (GCMs) is challenging because of numerous complex processes and feedbacks. Climate models continue to disagree with representation of cloud cover in the current Arctic climate (Cesana and Chepfer 2012) and projection of surface warming and sea ice loss in the future (Karlsson and Svensson 2013; Liu et al 2013)
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