Developing Optical Algorithms to Advance Airborne Measurements of Aerosol and Meteorological Properties

Abstract

The marine atmospheric boundary layer (MABL), the layer between the ocean and free troposphere, hosts a suite of important atmospheric processes such as heat and temperature flux, gas exchange of carbon dioxide and water vapor, cloud evolution, and aerosol particle transport. To measure these complex processes and provide a complete picture of the MABL, organizations such as the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the Office of Naval Research (ONR) conduct airborne field campaigns that use a multitude of in-situ and remote sensing platforms. This dissertation introduces two studies that aim to improve airborne measurements of 1) ocean surface wind speeds and 2) atmospheric aerosol particles. Both of these studies focus on the in-situ and remote sensing instruments used in NASA’s Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) field campaign that took place from 2020 – 2022. The first study of this dissertation introduces a new 10 m ocean surface wind speed product from the High Spectral Resolution Lidar – generation 2 (HSRL-2) developed at the NASA Langley Research Center (LaRC) and evaluates it using coincident dropsonde surface wind speed data collected during the NASA ACTIVATE field campaign. The HSRL-2 directly retrieves vertically resolved aerosol backscatter and extinction profiles without relying on an assumed lidar ratio or other external aerosol constraints, enabling accurate estimates of the attenuation of the atmosphere and direct retrieval of surface wind speed through probing the variance of ocean wave slopes (i.e., wave-slope variance). The important findings from this study are 1) HSRL-2 surface wind speed retrieval accuracy is 0.15 m s-1 ± 1.80 m s-1, 2) dropsonde surface wind speed measurements most closely match with the Hu et al. (2008) wind speed-wave-slope variance model for surface wind speeds below 7 m s-1, showing that this model is best to use for HSRL-2 retrievals, 3) the fine horizontal spatial resolution of the HSRL-2 (0.5 s or ~75 m along track) provides near-continuous profiles of surface wind speed over time, allowing for the instrument to probe MABL processes such as sea surface temperature (SST) dynamics and cloud evolution, and 4) the HSRL-2 can detect the ocean surface in broken cloud scenes, showing that the retrievals are not limited to aerosol-free conditions, thus enabling substantial retrievals in scenes with high cloud fraction over the northwest Atlantic. The second study focuses on improving airborne measurements of atmospheric aerosol particles through evaluation of the following microphysical and optical property data: aerosol number concentration (N_a), aerosol effective radius (r_eff), aerosol extinction at 532 nm (ε_532nm), and single scattering albedo (SSA) at 555 nm. A rigorous comparison analysis between ACTIVATE’s in-situ and remote sensing instruments (i.e., external closure) is conducted to see if measurements of the aforementioned aerosol data agree with one another. It is difficult to perform closure between these two instrument platforms because in-situ instruments provide dry (~ 20% relative humidity (RH)) aerosol measurements while remote sensors retrieve these data at ambient RH conditions. Also, in-situ instruments can only sample fine-mode particles due to the sampling inlet of the aircraft only allowing particles with diameters < 5 µm to pass through; this is problematic for intercomparisons with remote sensors that retrieve information about particles extending into coarser sizes. To overcome these limitations, the In Situ Aerosol Retrieval Algorithm (ISARA) is introduced, a forward optical algorithm that adjusts dry in-situ aerosol data into ambient data for both fine- and coarse-mode particles. This study demonstrates that for marine environments, appropriate a priori assumptions for coarse-mode aerosol allow for consistent closure between in-situ measurements and lidar and polarimetric retrievals of total (fine- + coarse-mode) aerosol properties. The second main finding is that it is possible to systematically close in-situ and polarimeter aerosol data, which has not been shown in the literature to date. Overall, it is hoped that optical technologies and algorithms can continue to advance our knowledge of the atmosphere by providing state-of-the-art measurements of critical MABL parameters.