Seminar

Abstract

Climate and air-quality models initialize aerosol sources with an injection height and a source strength. From space-based remote sensing, we are able to constrain both these quantities, especially when the source produces a distinct plume that can be imaged at multiple angles. Multi-spectral, multi-angle snapshots from low-Earth orbit can also provide qualitative information about particle microphysical properties, making it possible to map plume particle evolution on regional scales downwind of sources.

In case studies of wildfire and volcanic plumes, we observe injection within as well as above the planetary boundary layer (PBL) is space-based stereo imagery; when used to initialize model simulations, the details of injection height can significantly affect the simulation of downwind dispersion. Model plume dispersion differences are greatest when injection height is initialized within vs. above the PBL. Depending on the wind-shear profile, dispersion forecasts can also be significantly different when initialization is at different elevations in the free troposphere, as might be expected.

Inverse modeling has been attempted to deduce smoke plume source strength from aerosol optical depth (AOD) regional snapshots. One limitation of this approach is that the problem is highly underdetermined. Source location is among the few external constraints that can be applied to this problem, but is difficult to effect in the inverse-modeling framework. Using a forward modeling approach, we compare model simulations, initialized with different source-strength values and sampled at the time of satellite overpass, with regional AOD observations from space. This approach works best where the AOD contribution from the plume dominates background values, which occurs most commonly for boreal and tropical forest fires. When the background AOD is high, such as in polluted regions of northern India and eastern China, or when the fire plume is small or optically very thin, which is common in agricultural burning situations, the derived source-strength constraints from forward modeling are less certain.

Top-of-atmosphere radiance measurements from the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR) also contain qualitative information about particle size, shape, and light-absorption properties under favorable retrieval conditions. When mid-visible AOD exceeds about 0.15, about three-to-five bins in particle size, two-to-four bins in single-scattering albedo, and spherical vs. non-spherical shape can be derived. For well-formed volcanic plumes, we often observe downwind decreases in AOD, effective particle size, and non-spherical AOD fraction, consistent with expected preferential settling of larger, non-spherical ash particles relative to sulfates, water droplets, and background aerosol. We sometimes also observe increases in single-scattering albedo downwind, consistent with particle hydration and/or oxidation as the particles evolve. We have used these plume observations to deduce aspects of Kamchatka volcano behavior, and are currently applying the same approach to wildfire plumes. This presentation will review our work in these areas to date, and will summarize what we might glean in ongoing work from the 18-year, global MISR and MODIS aerosol plume data records.

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Ralph Kahn is a Senior Research Scientist at NASA’s Goddard Space Flight Center. He received his PhD in applied physics from Harvard University, and spent 20 years as a Research Scientist and Senior Research Scientist at the Jet Propulsion Laboratory, where he studied climate change on Earth and Mars. Kahn is Aerosol Scientist for the NASA Earth Observing System's Multi-angle Imaging SpectroRadiometer (MISR) instrument. He focuses on using MISR’s unique observations, combined with other data and numerical models, to learn about wildfire smoke, desert dust, volcano and air pollution particles, and to apply the results to regional and global climate-change questions.