Simulating PACE Global Ocean Radiances

Abstract

The NASA PACE mission is a hyper-spectral radiometer planned for launch in the next decade. It is intended to provide new information on ocean biogeochemical constituents by parsing the details of high resolution spectral absorption and scattering. It is the first of its kind for global applications and as such, poses challenges for design and operation. To support pre-launch mission development and assess on-orbit capabilities, the NASA Global Modeling and Assimilation Office has developed a dynamic simulation of global water-leaving radiances, using an ocean model containing multiple ocean phytoplankton groups, particulate detritus, particulate inorganic carbon (PIC), and chromophoric dissolved organic carbon (CDOC) along with optical absorption and scattering processes at 1 nm spectral resolution. The purpose here is to assess the skill of the dynamic model and derived global radiances. Global bias, uncertainty, and correlation are derived using available modern satellite radiances at moderate spectral resolution. Total chlorophyll, PIC, and the absorption coefficient of CDOC (a_CDOC), are simultaneously assimilated to improve the fidelity of the optical constituent fields. A 5-year simulation showed statistically significant (P <0.05) comparisons of chlorophyll (r = 0.869), PIC (r = 0.868), and a_CDOC (r = 0.890) with satellite data. Additionally, diatoms (r = 0.890), cyanobacteria (r = 0.732), and coccolithophores (r = 0.716) were significantly correlated with in situ data. Global assimilated distributions of optical constituents were coupled with a radiative transfer model (Ocean-Atmosphere Spectral Irradiance Model, OASIM) to estimate normalized water-leaving radiances at 1 nm for the spectral range 250-800 nm. These unassimilated radiances were within -0.074 mW cm^-2 μm¹ sr^-1 of MODIS-Aqua radiances at 412, 443, 488, 531, 547, and 667 nm. This difference represented a bias of -10.4% (model low). A mean correlation of 0.706 (P < 0.05) was found with global distributions of MODIS radiances. These results suggest skill in the global assimilated model and resulting radiances. The reported error characterization suggests that the global dynamical simulation can support some aspects of mission design and analysis. For example, the high spectral resolution of the simulation supports investigations of band selection. The global nature of the radiance representations supports investigations of satellite observing scenarios. Global radiances at bands not available in current and past missions support investigations of mission capability.

Keywords: PACE; biogeochemical model; ocean color; radiative transfer model; water-leaving radiances.

FIGURE 1. irradiance pathways in OASIM

E_d is direct downwelling irradiance, E_s is diffuse downwelling. ρ surface reflectance, E_u is diffuse upwelling irradiance, and L_wN is normalized water-leaving radiance. All irradiances and radiances are spectrally resolved at 25 nm for E_d, E_s, and E_u. and 1 nm for L_wN.

FIGURE 2

Spectral absorption and scattering coefficients of water, phytoplankton, detritus, PIC, and CDOC in OASIM.

FIGURE 3. OASIM spectral upwelling radiance and dependencies in the ocean

Shown are the visible bands. The spectral resolution for upwelling radiance is 1 nm. Inherent optical properties are derived from spectral characteristics of water, phytoplankton groups, detritus. PIC, and CDOC.

FIGURE 4

Model assimilated total chlorophyll for June and December 2007 compared to MODIS-Aqua chlorophyll.

FIGURE 5

Model assimilated PIC for June and December 2007 compared to MODIS-Aqua PIC.

FIGURE 6

Model assimilated a_CDOC 443 nm for June and December 2007 compared to MODIS-Aqua a_CDM 443 nm.

FIGURE 7. Global statistics on model normalized water-leaving radiances L_wN(λ) compared to MODIS-Aqua data for 2003–2011

Mean radiance and difference is mW cm⁻² μm⁻¹ sr⁻¹. Correlation is r-value. All correlations are significant (P < 0.05: N > 3.7 × 10⁶. Error bars represent semi-interquartile range.

FIGURE 8

Model normalized water-leaving radiances L_wN(λ) for 412 and 443 nm compared to MODIS-Aqua radiances.

FIGURE 9

Model normalized water-leaving radiances L_wN(λ) for 488 and 531 nm compared to MODIS-Aqua radiances.

FIGURE 10

Model normalized water-leaving radiances L_wN(λ) for 547 and 667 nm compared to MODIS-Aqua radiances.

FIGURE 11

Model normalized water-leaving radiances for selected wavelengths in the ultraviolet and visible region.

FIGURE 12. Model normalized water-leaving radiances for selected wavelengths in the ultraviolet, long visible, and near-infrared region

Note scale change.

FIGURE 13

Normalized water-leaving radiances from two locations in the Pacific Ocean: a gyre location (low chlorophyll) and a high latitude location (high chlorophyll).