Abstract
The Research Scanning Polarimeter (RSP) is an airborne along-track scanner measuring the polarized and total reflectances in 9 spectral channels. The RSP was a prototype for the Aerosol Polarimetery Sensor (APS) launched on-board the NASA Glory satellite. Currently the retrieval algorithms developed for the RSP are being adopted for the measurements of the space-borne polarimeters on the upcoming NASA’s Plankton, Aerosol, Cloud Ocean Ecosystem (PACE) satellite mission. The RSP’s uniquely high angular resolution coupled with the high frequency of measurements allows for characterization of liquid water cloud droplet sizes using the polarized rainbow structure. It also provides geometric constraints on the cumulus cloud’s 2D cross section yielding the cloud’s geometric shape estimates. In this study we further build on the latter technique to develop a new tomographic approach to retrieval of cloud internal structure from remote sensing measurements. While tomography in the strict definition is a technique based on active measurements yielding a tomogram (directional optical thickness as a function of angle and offset of the view ray), we developed a “semi-tomographic” approach in which tomogram of the cloud is estimated from passive observations instead of being measured directly. This tomogram is then converted into 2D spatial distribution of the extinction coefficient using inverse Radon transform (filtered backprojection) which is the standard tomographic procedure used e.g., in medical CT scans. This algorithm is computationally inexpensive compared to techniques relying on highly-multi-dimensional least-square fitting; it does not require iterative 3D RT simulations. The resulting extinction distribution is defined up to an unknown constant factor, so we discuss the ways to calibrate it using additional independent measurements. In the next step we use the profile of the droplet size distribution parameters from the cloud’s side (derived by fitting the polarized rainbows) to convert the 2D extinction distribution into that of the droplet number concentration. We illustrate and validate the proposed technique using 3D-RT-simulated RSP observations of a LES-generated Cu cloud. Quantitative comparisons between the retrieved and the original optical and microphysical parameters are presented.
Highlights
Tomography is a retrieval technique inverting a 2D spatial density from a dataset of measured “slices”, which are the integrals of this density over transect lines
We presented a new remote sensing tomographic technique allowing for retrieval of cloud internal structure from external measurements made by the Research Scanning Polarimeter
While tomography is an active technique incompatible with the geometry of atmospheric remote sensing, we developed a “semi-tomographic” approach in which the tomogram of the cloud is estimated from passive measurements instead of being measured directly
Summary
Tomography is a retrieval technique inverting a 2D spatial density from a dataset of measured “slices” (τoμoς, tomos means “slice” in Greek), which are the integrals of this density over transect lines (chords). It is ideologically closer than LSF approaches to “tomography” in the traditional sense, while relying on passive optical measurements Such measurements obviously cannot directly provide a proper tomogram (directional optical thickness as a function of angle and offset of the viewing ray), but allow for estimation of it using a nested family of “cloud shapes” corresponding to an array of thresholds in the measured total reflectance. Once such tomogram is obtained, we proceed following the standard tomographic procedure and apply inverse Radon transform to it deriving a 2D field of the extinction coefficients (which requires calibration using independent measurements). We will report analyses of the actual RSP data from CAMP2Ex in subsequent publications following this paper
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