NASA Glory Mission

by Kirk Knobelspiesse

Global climate change has been has attracted considerable interest lately. Much of the attention is about the role of greenhouse gases, about which there is a high degree of scientific certainty. However, the global climate is a very complicated system, and some components are much less well understood. Once such component is particulate matter in the atmosphere, also known as aerosols1.

Current global climate models are unable to fully account for all aerosol radiative effects.  This inability, in turn, is due to the lack of comprehensive measurements of aerosols in the atmosphere. Unlike greenhouse gases, which persist in the atmosphere for decades or centuries, aerosols only remain aloft for days. They also have a variety of both natural and anthropogenic (human) sources, such as desert dust, forest fire smoke, and oxidation of industrial sulfur dioxide emissions. This means their global distribution is highly heterogeneous, requiring measurements from orbit in order to fully comprehend the potential climate effects on a global scale.

Remote sensing technology has advanced dramatically in the past three decades, but the current set of orbital instruments are not capable of measuring all the aerosol descriptive parameters that are required for climate modeling2. This is partly because the radiative effects of aerosols are quite complicated, involving both ‘direct’ radiative forcing (simple reflection or absorption of radiation by aerosols) and ‘indirect’ forcing (the potentially large influence on the lifetime and optical nature of clouds). For this reason, climate models require not only the quantity of aerosols, but also information about their size distribution, index of refraction and shape. Over the years, NASA and other organizations have launched a variety of remote sensing instruments. Aerosol property retrieval generally involves optimizing atmospheric radiative transfer models to match satellite observations. However, this process is often underdetermined, and requires assumptions about components of the radiative transfer system.

Therefore, increasing the quantity of observations about a parcel of the atmosphere can lead to retrieval of more accurate climate relevant parameters.

The NASA Glory mission, due to be launched in December, intends to reduce the uncertainty associated with aerosols with its Aerosol Polarimetry Sensor (APS). The APS observes the earth at a variety of view angles in nine spectral channels, and is also sensitive to polarization. This combination offers the potential to reduce aerosol uncertainties by reducing assumptions. For example, land surface reflectance, which is often a large source uncertainty with other aerosol remote sensing instruments, can be determined directly from APS measurements. In addition to aerosol retrievals, the APS sensor will measure cloud properties. Another instrument on Glory, the Total Irradiance Monitor (TIM), will measure the total solar irradiance reaching the earth, whose variability is another source of climate change uncertainty3.

The Department of Applied Physics and Applied Mathematics at Columbia is involved with the NASA Glory mission in its collaboration with the NASA Goddard Institute of Space Studies (GISS). Several research scientists have joint appointments with APAM and GISS, and the department occasionally sends graduate students to do their research at NASA.

As one of those graduate students, I am developing surface radiative transfer models that will be used for the retrieval of aerosol properties. These models are based upon numerical solutions of Maxwell’s equations for scattering from arbitrarily shaped objects (such as the wax crystals on the surface of leaves). Long wavelength observations, where aerosol effects are minimal, will the used to identify the appropriate surface model for a scene. This information will be used to constrain the surface reflectance at shorter wavelengths during the atmospheric model optimization used for aerosol retrieval.

As part of this model development effort, I have had the opportunity to participate in several field campaigns with an airborne APS prototype, called the Research Scanning Polarimeter (RSP). Field campaigns often have many research objects, but I was able to make observations of reflectance over a variety of surface types, which will prove invaluable when validating surface reflectance models. I have traveled to the plains of central Oklahoma, the gulf coast of Mexico, and will depart again later this summer for the Canadian Arctic. So, my experience as a graduate student has certainly been in synch with APAM’s interdisciplinary nature. I have been combining radiative transfer and computational mathematics for the purposes of understanding atmospheric aerosols, and ultimately, climate change. All while I get to ride around in small airplanes!

[1] IPCC. Climate Change 2007 - The Physical Science Basis: Contribution of the Working Group I to the Fourth Assessment Report of the IPCC. Cambridge University Press, New York, NY, 2007.

[2] M. Mishchenko, I. Geogdzhayev, B. Cairns, B. Carlson, J. Chowdhary, A. Lacis, L. Liu, W. Rossow, and L. Travis. Past, present, and future of global aerosol climatologies derived from satellite observations: A perspective. J. Quant. Spectrosc. Radiat. Transfer, 106:325–347, 2007.

[3] M. Mishchenko, B. Cairns, G. Kopp, C. Schueler, B. Fafaul, J. Hansen, R. Hooker, T. Itchkawich, H. Maring, and L. Travis. Accurate monitoring of terrestrial aerosols and total solar irradiance: introducing the Glory mission. Bull. Amer. Meteorol. Soc, 88, 2007.

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