Efficient single-scattering lookup table for lidar and polarimeter phytoplankton studies

Coupled atmosphere and ocean remote sensing retrievals of aerosol, cloud, and oceanic phytoplankton microphysical properties, such as those carried out by the NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, involve single-scattering calculations that are time consuming. Lookup tables (LUTs) exist to speed up these calculations for aerosol and water droplets in the atmosphere. In our new Lorenz-Mie lookup table, we tabulate single scattering by an ensemble of coated isotropic spheres representing oceanic phytoplankton at wavelengths from 0.355 µm. The lookup table covers phytoplankton particles with radii in the range of 0.15-100 µm at an increase of up to 104 in computational speed compared to single-scattering calculations. The allowed complex refractive indices range from 1.05 to 1.24 for the shell's real part, from 10-7 to 0.3 for the shell's imaginary part, from 0 to 0.001 for the core's imaginary part, and equal to 1.02 for the core's real part. We show that we precisely compute inherent optical properties for the phytoplankton size distributions ranging up to 5 µm for the effective radius and up to 0.6 for the effective variance. We test wavelengths from 0.355 to 1.065 µm and find that all the inherent optical properties of interest agree with the single-scattering calculations to within 1% for 99.9% of cases. We also provide an example of using the lookup table to reproduce the phytoplankton optical datasets listed in the PANGAEA database for synthetic hyperspectral algorithm development. The table together with C++, Fortran, MATLAB, and Python codes to apply different complex refractive indices and phytoplankton size distributions is freely available online.

Efficient single-scattering look-up table for lidar and polarimeter water cloud studies

Combined lidar and polarimeter retrievals of aerosol, cloud, and ocean microphysical properties involve single-scattering cloud calculations that are time consuming. We create a look-up table to speed up these calculations for water droplets in the atmosphere. In our new Lorenz-Mie look-up table we tabulate the light scattering by an ensemble of homogeneous isotropic spheres at wavelengths starting from 0.35 µm. The look-up table covers liquid water cloud particles with radii in the range of 0.001-500 µm while gaining an increase of up to 10⁴ in computational speed. The covered complex refractive indices range from 1.25 to 1.36 for the real part and from 0 to 0.001 for the imaginary part. We show that we can precisely compute inherent optical properties for the particle size distributions ranging up to 100 µm for the effective radius and up to 0.6 for the effective variance. We test wavelengths from 0.35 to 2.3 µm and find that the elements of the normalized scattering matrix as well as the asymmetry parameter, the absorption, backscatter, extinction, and scattering coefficients are precise to within 1% for 96.7%-100% of cases depending on the inherent optical property. We also provide an example of using the look-up table with in situ measurements to determine agreement with remote sensing. The table together with C++, Fortran, MATLAB, and Python codes to interpolate the complex refractive index and apply different particle size distributions are freely available online.

Active remote sensing of atmospheric dust using relationships between their depolarization ratios and reflectivity

The backscattered light from agglomerated debris particles shows that an approximate linear correlation exists between the logarithm of the geometric albedo $ \log(A )$ of polydispersions of agglomerated debris particles and their lidar linear or circular depolarization ratios, $ \unicode{x00B5}_L$ and $ \unicode{x00B5}_C$. The nature of the relationship depends on the complex refractive index of the particles in the distribution. This extension of the Umov law can be used for lidar and radar characterizations by placing constraints on the reflectivity of the particles. It suggests that an approximate inverse relationship exists between the lidar ratio and the lidar depolarization ratios whose scaling parameter depends on the refractive index of the aerosol population.

An Overview of Atmospheric Features Over the Western North Atlantic Ocean and North American East Coast - Part 1: Analysis of Aerosols, Gases, and Wet Deposition Chemistry

The Western North Atlantic Ocean (WNAO) and adjoining East Coast of North America are of great importance for atmospheric research and have been extensively studied for several decades. This broad region exhibits complex meteorological features and a wide range of conditions associated with gas and particulate species from many sources regionally and other continents. As Part 1 of a 2-part paper series, this work characterizes quantities associated with atmospheric chemistry, including gases, aerosols, and wet deposition, by analyzing available satellite observations, ground-based data, model simulations, and reanalysis products. Part 2 provides insight into the atmospheric circulation, boundary layer variability, three-dimensional cloud structure, properties, and precipitation over the WNAO domain. Key results include spatial and seasonal differences in composition along the North American East Coast and over the WNAO associated with varying sources of smoke and dust and meteorological drivers such as temperature, moisture, and precipitation. Spatial and seasonal variations of tropospheric carbon monoxide and ozone highlight different pathways toward the accumulation of these species in the troposphere. Spatial distributions of speciated aerosol optical depth and vertical profiles of aerosol mass mixing ratios show a clear seasonal cycle highlighting the influence of different sources in addition to the impact of intercontinental transport. Analysis of long-term climate model simulations of aerosol species and satellite observations of carbon monoxide confirm that there has been a significant decline in recent decades among anthropogenic constituents owing to regulatory activities.

New treatment of strongly anisotropic scattering phase functions: The Delta-M+ method

The treatment of strongly anisotropic scattering phase functions is still a challenge for accurate radiance computations. The new Delta-M+ method resolves this problem by introducing a reliable, fast, accurate, and easy-to-use Legendre expansion of the scattering phase function with modified moments. Delta-M+ is an upgrade of the widely-used Delta-M method that truncates the forward scattering peak with a Dirac delta function, where the '+' symbol indicates that it essentially matches moments beyond the first M terms. Compared with the original Delta-M method, Delta-M+ has the same computational efficiency, but for radiance computations the accuracy and stability have been increased dramatically.

Inferring inherent optical properties and water constituent profiles from apparent optical properties

The BP09 experiment conducted by the Centre for Maritime Research and Experimentation in the Ligurian Sea in March 2009 provided paired vertical profiles of nadir-viewing radiances L(u)(z) and downward irradiances E(d)(z) and inherent optical properties (IOPs, absorption, scattering and backscattering coefficients). An inversion algorithm was implemented to retrieve IOPs from apparent optical properties (AOPs, radiance reflectance R(L), irradiance reflectance R(E) and diffuse attenuation coefficient K(d)) derived from the radiometric measurements. Then another inversion algorithm was developed to infer vertical profiles of water constituent concentrations, including chlorophyll-a concentration, non-algal particle concentration, and colored dissolved organic matter from the retrieved IOPs based on a bio-optical model. The algorithm was tested on a synthetic dataset and found to give reliable results with an accuracy better than 1%. When the algorithm was applied to the BP09 dataset it was found that good retrievals of IOPs could be obtained for sufficiently deep waters, i.e. for L(u)(z) and E(d)(z) measurements conducted to depths of 50 m or more. This requirement needs to be satisfied in order to obtain a good estimation of the backscattering coefficient. For such radiometric measurements a correlation of 0.88, 0.96 and 0.93 was found between retrieved and measured absorption, scattering and backscattering coefficients, respectively. A comparison between water constituent values derived from the measured IOPs and in-situ measured values, yielded a correlation of 0.80, 0.78, and 0.73 for chlorophyll-a concentration, non-algal particle concentration, and absorption coefficient of colored dissolved organic matter at 443 nm, respectively. This comparison indicates that adjustments to the bio-optical model are needed in order to obtain a better match between inferred and measured water constituent values in the Ligurian Sea using the methodology developed in this paper.

Long-term comparisons of UV index values derived from a NILU-UV instrument, NWS, and OMI in the New York area

A comparison is presented of UV index (UVI) values obtained under different cloud conditions from a Norwegian Institute for Air Research UV (NILU-UV) instrument, the ozone monitoring instrument (OMI) onboard the Aura satellite, and the National Weather Service (NWS) model for the time period of 2010-2014. The NILU-UV irradiance meter is a ground-based, multi-channel, moderate bandwidth filter instrument. UVI values derived from measurements by a NILU-UV instrument deployed in the New York area (40.74°N, -74.03°E) to monitor the erythemal UV radiation from 2010 to present is compared to UVI values derived from OMI measurements and predicted by the NWS model. OMI overestimated the UVI values by 13.06% for all cloud conditions compared with the UVI values derived from measurements by the NILU-UV instrument. The heavier the cloud cover, the higher the overestimation. The mean relative difference between the UVI derived from the NWS model and from NILU-UV measurements was 11.54%. The UVI prediction by NWS was also overestimated under cloudy conditions. Under overcast conditions the NWS predictions of UVI values differ significantly from those derived from NILU-UV measurements, yielding a correlation of only 0.8025 and a mean relative difference of 28.25%.

New neural-network-based method to infer total ozone column amounts and cloud effects from multi-channel, moderate bandwidth filter instruments

A new method is presented based on a radial basis function neural network (RBF-NN) to analyze data obtained by ultraviolet (UV) irradiance instruments. Application of the RBF-NN method to about three years of data obtained by a NILU-UV device, which is a multi-channel, moderate bandwidth filter instrument, revealed that compared to the traditional Look-up table (LUT) method, the RBF-NN method yielded better agreement with a 1% decrease in relative difference and an increase of 0.03 in correlation with total ozone column (TOC) values obtained from the Ozone Monitoring Instrument (OMI). Furthermore, the RBF-NN method retrieved more valid results (daily average values within a meaningful range (200-500 DU)) than the LUT method. Compared with RBF-NN retrievals, TOC values obtained from the OMI are underestimated under cloudy conditions. This finding agrees with conclusions reached by Anton and Loyola (2011).

Comparisons of three NILU-UV instruments deployed at the same site in the New York area

The Norwegian Institute for Air Research ultraviolet (NILU-UV) irradiance meter is a ground-based, multichannel, moderate bandwidth filter instrument that measures irradiances at ultraviolet (UV) and visible wavelengths with five channels in the UV (302, 312, 320, 340, and 380 nm) and one channel in the visible (400-700 nm) part of the spectrum. Minute-by-minute irradiances recorded in these channels are used to infer the total ozone column (TOC) amount, and a radiation modification factor (RMF) designed to have a value close to 100 under cloud-free conditions. The performance of three NILU-UV instruments deployed side-by-side in the New York area (40.74°N, -74.03°E) is assessed, and derived TOC values are compared with those derived from the ozone monitoring instrument (OMI) deployed on NASA's AURA satellite. Based on about three years of data, it was found that the three instruments yielded similar TOC values that were in close agreement with those derived from the OMI. The relative difference in TOC values derived from the three NILU-UV instruments was generally less than 2.5%. Cloud cover affects the accuracy of the inferred TOC, but reliable values can be obtained in the presence of clouds, although the accuracy deteriorates under heavy overcast conditions with RMF values smaller than 65 (low cloud transmittance).

Comparison of discrete ordinate and Monte Carlo simulations of polarized radiative transfer in two coupled slabs with different refractive indices

A comparison is presented of two different methods for polarized radiative transfer in coupled media consisting of two adjacent slabs with different refractive indices, each slab being a stratified medium with no change in optical properties except in the direction of stratification. One of the methods is based on solving the integro-differential radiative transfer equation for the two coupled slabs using the discrete ordinate approximation. The other method is based on probabilistic and statistical concepts and simulates the propagation of polarized light using the Monte Carlo approach. The emphasis is on non-Rayleigh scattering for particles in the Mie regime. Comparisons with benchmark results available for a slab with constant refractive index show that both methods reproduce these benchmark results when the refractive index is set to be the same in the two slabs. Computed results for test cases with coupling (different refractive indices in the two slabs) show that the two methods produce essentially identical results for identical input in terms of absorption and scattering coefficients and scattering phase matrices.

Stokes scattering matrix for human skin

We use a layered model of normal human skin based on size distributions of polydisperse spherical particles and their complex refractive indices to compute the Stokes scattering matrix at wavelengths in the visible spectral band. The elements of the Stokes scattering matrix are required in a polarized radiative transfer code for a coupled air-tissue system to compute the polarized reflectance and examine how it is dependent on the vertical structure of the inherent optical properties of skin, including the phase matrix. Thus, the elements of the Stokes scattering matrix can be useful for investigating polarization-dependent light propagation in turbid optical media, such as human skin tissue.