Atmosphere Ocean Science Friday Seminar

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Earth’s infrared background

Speaker: Ofer Shamir

Location: Warren Weaver Hall 1314

Date: Monday, April 22, 2024, 4 p.m.

Synopsis:

Is one person’s noise another person’s signal?A long-standing, practical, challenge when using OLR observations for studying physical processes is the identification of the background, which is just a (not so) fancy word for noise. The question is: How much of the observed variability, say at a given point in physical/spectral space, should be attributed to noise? Part of the problem, as I see it, is the lack of agreed upon definition of the background, with members in “the community” having different, often vague, notions. In this work, we suggest an objective definition of the background, based on its physical origins and desired mathematical properties. Our notion of the background is the planetary equivalent of Johnson noise in electric circuits. Much like a resistor exhibits non-zero instantaneous currents due to thermal agitation, a planetary atmosphere exhibits spatiotemporal OLR fluctuations associated with the absorption and re-emission of the infrared radiation on its way of the atmosphere.A quantitative description of the background is obtained by employing a minimal stochastic model, which is capable of explaining the natural variability in OLR observations using only 3 parameters. The model places a lower bound, and perhaps the best available estimate, on the contribution of natural variability. We find that about 20% of the global mean OLR variance and about 40% of the total variance ought to be attributed to natural variability. The remaining two parameters characterize the intrinsic spatiotemporal correlations of the atmosphere on a global scale. Finally, results from a CMIP6 model indicate that the background depends on the climate state, and, as such, may be useful for constraining/improving climate models.