Space-time Green functions for diffusive radiation transport, in application to active and passive cloud probing

Abstract

Clouds are a feast for the eye but, when contemplating their fluid beauty, it is important — at least for scientists — to bear in mind that they are also key elements of the Earth’s climate system. They are indeed the first-order regulators of the intake in solar energy: What portion goes back to space? What reaches the surface (then warms the ground, drives photosynthesis, etc.)? Clouds also contribute strongly to the vertical distribution of solar heating and, from there, the thermal balance of the atmosphere. These are well-known and relatively well-understood/modeled climate roles of clouds, as can be expected for such naturally occurring components of the atmosphere. We note that these roles involve radiative transfer across the electromagnetic spectrum. What is far less understood about clouds is how they interact microphysically, chemically and thermo-hydrodynamically, with other natural and anthropogenic constituents, especially aerosols. These are known as cloud feedback mechanisms in the parlance of climate science, and have been identified as the single most resilient roadblock in the way of reducing uncertainty in future climate prediction [1], an enterprise that relies heavily on models to explore various scenarios in global greenhouse gas emissions.