Using Multi-Angle, Multispectral Photo-Polarimetry from the Aerosol Polarimetry Sensor to Constrain Optical Properties of Aerosols And Clouds: Demonstration of Capabilities Using Results from Similar Measurements in Four Field Experiments

Tropospheric aerosols play a crucial role in climate and can cause a climate forcing directly by absorbing and reflecting sunlight, thereby cooling or heating the atmosphere, and indirectly by modifying cloud properties. The indirect aerosol effect may include increased cloud brightness, as aerosols lead to a larger number of smaller cloud droplets (the so-called Twomey effect), and increased cloud cover, as smaller droplets inhibit rainfall and increase cloud lifetime. Both forcings are poorly understood and may represent the largest source of uncertainty about future climate change. In this poster we present results from various field experiments demonstrating the contribution that the multi-angle multi-spectral photopolarimetric remote sensing measurements of the Aerosol Polarimetry sensor will make to the determination of the direct and indirect radiative effects of aerosols.

Remote sensing of aerosols from satellites is plagued by the need to make prior assumptions about the composition and size of the aerosols that are present, whether this is to calculate the phase functions of the aerosols for passive remote sensing, or the extinction to backscatter ratio for elastic backscatter lidar measurements. Measurements made by the Research Scanning Polarimeter (RSP) have demonstrated that many of these assumptions can be eliminated using polarimetric remote sensing and that it is possible to retrieve the optical depth, single scattering albedo, refractive index and the location and width of a bimodal size distribution. Moreover, polarimetric remote sensing provides this capability over both land and water surfaces. Measurements from the CLAMS and IHOP field experiments and over smoke from fires in Southern California are used to demonstrate these capabilities.

In passive remote sensing of clouds using the intensity of reflected solar radiation a common problem has been that the retrieved droplet effective radii are biased high compared with in situ measurements with the implication that there may be “anomalous absorption”. It is also the case that for water clouds the effective variance of the droplet size distribution and in the case of ice clouds the particle shape distribution must be assumed globally constant. We find that our particle size retrievals in water clouds using polarimetric measurements agree with in situ measurements to within the uncertainty caused by spatial variability. These measurements also allow the effective variance at cloud top to be accurately determined in the case of water clouds and allow a reasonable particle shape distribution to be estimated in the case of ice clouds. In the case of water clouds polarimetric measurements also allow a plausible estimate of the vertical profile of droplet size because of the significantly different vertical weighting profiles of the polarization and intensity measurements. Furthermore, polarimetric measurements allows the cloud top and cloud base pressure to be determined which allows the physical thickness of a cloud to be estimated. This in turn allows us to estimate the number concentration of droplets, or ice particles, in clouds which is the crucial parameter needed in evaluating the indirect effect of aerosols on clouds. These capabilities are demonstrated and validated using measurements taken during the CSTRIPE and CRYSTAL-FACE field experiments.