NERC-NSFGEO: Constraining Longwave Energy Flows in Cold Climates (CLEFCC)

Over the last century the Arctic has been warming three times faster than the Earth as a whole. Inferences from ice core records imply that the warming rate in Antarctica is also faster than the global average. These rapid changes are already having major a socio-economic impact locally within the high latitude regions (e.g. food and water security) and through their remote influences (e.g. extreme weather). These impacts are predicted to intensify with time. However, current climate models fail to capture the observed rate of polar warming while predictions of future high latitude climate show significant model to model variation, severely limiting confidence in our ability to plan for and mitigate future change. The amount of warming primarily depends on the overall balance between the incoming solar (shortwave) and outgoing thermal (longwave) energy. Although there is conjecture as to the dominant physical mechanisms at play, accelerated high-latitude warming is strongly influenced by processes involving longwave energy flows. Ice and mixed-phase clouds are particularly prevalent at these high latitudes and exert a strong control on these energy transfers. Cold surface and cloud emitting temperatures mean that more than 60 % of this longwave energy is located at wavelengths longer than 15 microns in the so-called ‘far-infrared’. Despite its energetic importance, observations of the far-infrared energy spectrum are severely lacking. For example, we have only a handful of measurements of the surface thermal emission at these wavelengths while observations of the outgoing far-infrared energy spectrum from space are only just being realised. Such measurements, in combination with in-situ observations characterizing the surface and atmospheric state, are critical for testing and improving key components of the large-scale models used to predict our future climate. CLEFCC will exploit a unique set of observations from two field campaigns, designed to deliver the measurements we so urgently need. Coupling these with ground-breaking satellite observations of the Earth’s outgoing longwave spectrum, we will focus on three critical questions: (1) Do current representations of surface properties capture the longwave emission spectrum of snow and ice surfaces correctly? (2) Is a new light-scattering model able to reconcile ice cloud microphysics (ice crystal sizes, shapes) with energetic (radiative) impact across the longwave spectrum? (3) Can our radiative transfer models successfully match simultaneous observations of the longwave energy spectrum at the surface, within the atmosphere and at the top of the atmosphere under a variety of different atmospheric and surface conditions? In each case, should we obtain a negative answer to the question posed we will use the observations to inform us how to refine the underlying assumptions in our models to achieve closer agreement. The project brings together world-leading experts in Earth Observation, climate modelling, and light scattering and radiative transfer from the UK and US. It will deliver new, observationally evaluated tools for immediate use by the wider climate modelling and remote sensing communities. For example, our thoroughly tested ice-cloud optical property model will be well placed to become the globally leading tool for those working to derive ice cloud microphysics from satellite observations. Further, we anticipate that our results will motivate improvements in the treatment of surface-atmosphere energetic coupling and the representation of ice-clouds across the climate modelling community, both critical to enhancing confidence in our ability to predict polar, and therefore global, climate.