Ocean Albedo Research Articles

Influence of Phytoplankton on Ocean Albedo

Using numerical methods for solving the radiation transfer equation, ocean albedo values were calculated for a set of bio-op tical characteristics corresponding to situations with different chlorophyll concentrations (1 μg/L and 10 μg/L) and the case of intense coccolithophore bloom (8–12 million cells/L). Calculations were carried out in the spectral range of 280–2800 nm for cases of cloudless sky at various wind speeds and atmospheric transmission. It has been shown that for Case 1 waters, a change in chlorophyll concentration from 1 to 10 μg/L does not lead to changes in albedo. In the case of intense coccolithophore blooms, the ocean albedo can increase more than threefold. Calculation of average monthly albedo values for selected points in the Barents and Black seas showed that the presence of intense coccolithophore blooms significantly increases average monthly albedo values. The calculation of the values of radiation absorbed in the seawater column depending on the time of day, carried out for these points, demonstrated that the presence of blooms significantly reduces the values of absorbed radiation. It is shown that the contribution to the albedo of radiation emerging from water used in the state-of-the-art NEMO numerical ocean model, amounting to 0.005 ± ± 0.0005, corresponds only to Case 1 waters. Intense coccolithophore blooms can increase this contribution by more than 14 times. A simple formula is proposed for correcting albedo values taking into account the influence of bio-optical characteristics.

An effective parameterization of broadband ocean surface albedo applicable to all skies

The ocean surface albedo (OSA) is an important parameter in ocean and climate models for air-sea heat flux calculations. Current OSA schemes are either too simple, making them only suitable for clear sky conditions, or too complex, because they depend on parameters that are not often measured in conventional ocean observations. Using radiation observations at a fixed offshore platform, we propose a simple but effective parameterization scheme of OSA, in which the broadband OSA is an analytical function of both the solar zenith angle and atmospheric transparency. It depends only on the downward shortwave radiation measured at the ocean surface and applies to all sky conditions. During our 15-month radiation observations, the correlation coefficient between the calculated OSA and the observations reached 0.90 for all skies, and the root mean square deviation was 0.0130. Three other OSA observation datasets are also introduced to verify this scheme.

Sensitivity of Arctic Surface Temperature to Including a Comprehensive Ocean Interior Reflectance to the Ocean Surface Albedo Within the Fully Coupled CESM2

AbstractAlmost all current climate models simplify the ocean surface albedo (OSA) by assuming the reflected solar energy without the ocean interior contribution. In this study, an improved ocean surface albedo scheme is incorporated into the Community Earth System Model version 2 (CESM2) to assess the sensitivity of Arctic surface temperature to including ocean interior reflectance to the OSA. Fully coupled CESM2 simulations with and without ocean interior reflectance are subsequently performed, we focus on the analysis of Arctic surface temperature responses. Incorporating ocean interior reflectance increases absorbed solar radiation and warms the ocean, enhancing seasonal heat storage and release across the Arctic Ocean, and increasing sea ice reduction and positive climate feedbacks that elevates Arctic surface temperature. Seasonal variations in air‐surface temperature differences induce changes in turbulent heat flux patterns, concurrently modifying dynamic advection and moisture processes that affect boundary layer humidity and low clouds, especially in winter. Based on partitioning physical processes in the thermodynamic energy equation, surface air warming is induced primarily through positive heating anomalies of vertical advection, latent heat release, and longwave radiative forcing. Through an examination of the surface energy budget, skin temperature warming is driven predominantly by increased downward longwave radiation, positive surface albedo feedback in summer, and increased conductive heat transport from the ocean particularly in winter. Significant effects of ocean interior reflectance on the Arctic Ocean, including sea surface warming and sea ice reduction, justify the importance of ocean interior reflectance in climate models for better understanding of ongoing Arctic climate changes.

Transboundary effects from idealized regional geoengineering

Regional geoengineering, by reflecting sunlight over a very limited spatial domain, might be considered as a means to target specific regional impacts of climate change. One of the obvious concerns raised by such approaches is the extent to which the resulting effects would be detectable well beyond the targeted region (e.g. in neighbouring countries). A few studies have explored this question for targeted regions that are still comparatively large. We consider idealized simulations with increased ocean albedo over relatively small domains; the Gulf of Mexico (0.23% of Earth's surface) and over the Australian Great Barrier Reef (0.07%), both with negligible global radiative forcing. Applied over these very small domains, the only statistically significant non-local changes we find are some limited reduction on summer precipitation in Florida in the Gulf of Mexico case (adjacent to the targeted region). The lack of transboundary effects suggests that governance needs for such targeted interventions are quite distinct from those for more global sunlight reflection.

Broadband radiometric measurements from GPS satellites reveal summertime Arctic Ocean Albedo decreases more rapidly than sea ice recedes

New measurements from the Arctic ± 40 days around the summer solstice show reflected sunlight from north of 80°N decreases 20–35%. Arctic sea ice coverage decreases 7–9% over this same time period (as reported by the NSIDC) implying Arctic sea ice albedo decreases in addition to the sea ice receding. Similar Antarctic measurements provide a baseline to which Arctic measurements are compared. The Antarctic reflected sunlight south of 80°S is up to 30% larger than the Arctic reflectance and is symmetric around the solstice implying constant Antarctic reflectivity. Arctic reflected sunlight 20 days after solstice is > 100W/m2 less than Antarctic reflected sunlight. For perspective, this is enough heat to melt > 1 mm/hour of ice. This finding should be compared with climate models and in reanalysis data sets to further quantify sea ice albedo’s role in Arctic Amplification. The measurements were made with previously unpublished pixelated radiometers on Global Positioning System satellites from 2014 to 2019. The GPS orbits give each radiometer instantaneous and continuous views of 37% of the Earth, two daily full views of the Arctic and Antarctic. Furthermore, the GPS constellation gives full-time full-Earth coverage that may provide data that complements existing limited field of view instruments that provide a less synoptic Earth view.

Scheme to estimate water-leaving albedo from remotely sensed chlorophyll-a concentration.

Water-leaving albedo (αw(λ)) is an important component of the ocean surface albedo and is conventionally estimated based on remotely sensed chlorophyll-a concentration (Chl) (termed Chl-αw). We show that estimated αw(λ) by Chl-αw could be significantly biased in global oceans, because there is no guarantee of closure between the modeled remote sensing reflectance (Rrs(λ)) from Chl-inferred inherent optical properties (IOPs) and the input Rrs(λ) that is used to derive Chl. We thus propose a simple and improved scheme, termed Chl-αw_new, and show that the step to infer IOPs from Chl is not necessary, where αw(λ) can be accurately estimated from satellite-measured Rrs(λ) and a Chl-based look-up-table (LUT) for the bidirectional relationships of angular Rrs(λ). Evaluations with both HydroLight simulations and satellite measurements show that Chl-αw_new is equivalent to the recently developed αw scheme based on IOPs (IOPs-αw, [Remote Sens. Environ. 269, 112807]), where both schemes could significantly improve the estimation of αw(λ) compared to Chl-αw. Comparisons among Chl-αw, Chl-αw_new, and IOPs-αw highlight that optical closure of Rrs(λ) is essential for accurate remote sensing of αw(λ), while the model for Rrs(λ) bidirectionality has rather minor impacts. The impact of improved αw(λ) estimations on the solar flux exchanges at the air-sea interface is preliminarily evaluated in this effort, where the use of Chl-αw_new could increase the estimation of reflected solar radiation by over 68.7% in turbid waters compared to that using Chl-αw, highlighting the necessity of incorporating accurate αw schemes into the coupled ocean-atmosphere models, especially for regional models in coastal oceans.

Direct measurement system of water-leaving albedo in the field by the skylight-blocked approach: Monte Carlo simulations.

Water-leaving albedo (αw(λ)) is an important component of the ocean surface albedo. Direct measurement of αw(λ) in the field is not yet available due to difficulties in removing the contribution of surface-reflected solar radiation. Following the concept of the skylight-blocked approach (SBA), a novel system, termed αwSBA, is proposed in this study to directly measure Ew(λ), where a wide-angle black cone is used to block the surface-reflected radiance. The shading errors associated with the cone and the measuring system are examined via Monte-Carlo (MC) simulations for a wide range of water inherent optical properties (IOPs), solar zenith angle, and different configurations of the αwSBA system (i.e., half cone angle, and the length of supporting arm). Based on sensitive analysis using MC simulations, an optimal configuration of αwSBA is recommended. We further propose a mathematical expression to parameterize the shading error (ɛ), along with an error correction scheme (αwOPT). It is found that, with the optimal configuration and αwOPT, the uncertainties of obtained αw(λ) by αwSBA are generally less than 7% for a wide range of waters with different IOPs and particulate scattering phase functions.

Top-of-Atmosphere Clear-Sky Albedo Estimation Over Ocean: Preliminary Framework for MODIS

Top-of-atmosphere (TOA) albedo is a significant factor of earth energy budget, climate change, and environmental change. As tremendous regional and global changes are happening over ocean, more details are needed to monitor the ocean environment. However, there were still no high-spatial resolution TOA albedo products over ocean. In this study, a new algorithm for clear-sky TOA albedo estimation over ocean was proposed, based on Moderate Resolution Imaging Spectroradiometer (MODIS) data. Instead of building angular distribution models, direct retrieval models between TOA reflectance and TOA albedo were developed based on extensive radiative transfer (RT) simulations, covering thousands of ocean and atmosphere types. Three-component ocean water albedo model was involved to take account for the ocean surface anisotropy at different wind speed, wind direction, and chlorophyll concentration, while Modtran 5 was utilized to simulate different atmospheric conditions. Our results showed good agreement with the Clouds and the Earth’s Radiant Energy System (CERES) based on a global comparison on August 4, 2011, with RMSE = 0.015 and bias = 0.002. And our MODIS-based products provide more spatial details due to higher spatial resolution (1 km), which will be a good data source for regional environmental and climatic research and will also enhance the understanding of Earth’s radiation budget.

Estimating the water-leaving albedo from ocean color

Water-leaving albedo (αw), defined as the ratio of water-leaving irradiance to downwelling irradiance just above the surface, is a major component of ocean surface albedo (α) but has long been ignored or underrepresented. A semi-analytical scheme based on inherent optical properties (IOPs), termed IOPs-αw, is proposed in this study to estimate spectral αw(λ) from ocean color measurements. Evaluations with numerical simulations of radiative transfer show that IOPs-αw outperforms the conventional scheme based on chlorophyll-a (Chl) concentration. The median absolute percentage difference (MAPD) of derived αw(λ) from IOPs-αw is generally less than 3% in the blue-green spectral domain, in comparison to MAPD of over 40% for estimated αw(λ) from the Chl-based scheme. IOPs-αw is later implemented to monthly composite data of the Visible Infrared Imaging Radiometer Suite (VIIRS), where reasonable spatial distributions and seasonal patterns of αw(λ) are obtained. In particular, broadband αw in the visible domain, termed αw_VIS, obtained via IOPs-αw is over 50% higher than the previous estimation by the Chl-based scheme in most oceanic waters. Furthermore, this study concludes that αw_VIS could contribute up to 20% to α in oceanic waters under low solar-zenith angles. Thus, we suggest that neither the spatial variability of αw_VIS nor the contribution of αw_VIS to α shall be neglected, and it is necessary to incorporate IOPs-αw into current parameterizations of α in coupled ocean-atmosphere and climate models.

The rôles of plankton and neuston microbial organic matter in climate regulation

Abstract Plankton and neuston microbes produce organic matter (OM), which accumulates in the sea surface microlayer (SML). Fluxes of heat and momentum exchange across the sea-air interface, as do fluxes of matter, including greenhouse gases, aerosols, microbes (algae, bacteria sensu lato and viruses) and other substances. At least at calm to moderate windspeeds, microbial OM (MOM) in the SML reduces these fluxes. Another MOM fraction, foam, covers a part of the ocean surface. Ocean foam increases mean ocean albedo because it reflects solar radiation, thus cooling the ocean and the Earth. The rheological properties of MOM and the reduction of sea-air fluxes depend on microbial abundance and taxonomic composition, as do the formation and persistence of foam. Genomic regulation of MOM secretion may thus be helping to regulate air-sea fluxes and climate. Unpredictable changes in abundance and taxonomic composition of these microbial communities may be adding uncertainty to global and more local climate. Some of this uncertainty could be mitigated by studying the ecology and genomics of the surface microbial community together with chemical and rheological properties of their secreted MOM and its effects on sea-air fluxes and foam coverage, to incorporate into climate models.

Bubble effects on upper ocean acoustics under wind-driven seas

Dense clouds of bubbles—ranging in scale from 10s of micrometers to cm—are generated by breaking waves on wind-driven seas. Wave-induced bubbles increase ocean albedo and play an important role in air-sea exchange processes by enhancing the transfer of gases and generating sea spray aerosol. Because bubbles both scatter sound efficiently and generate pulses of sound on formation, they create important acoustical effects, including the generation of ambient sound, the attenuation and scattering of sound forward scattered from the sea surface, and enhanced surface backscattering at low grazing angles. Examples of these effects will be illustrated along with the treatment of a bubble cloud acoustical scattering regime characterized by low absorption but high phase distortion relevant to high-frequency, coherent acoustic communications. [Work supported by ONR Ocean Acoustics.]

Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt

Ensuring that global warming remains <2 °C requires rapid CO2 emissions reduction. Additionally, 100–900 gigatons CO2 must be removed from the atmosphere by 2100 using a portfolio of CO2 removal (CDR) methods. Ocean afforestation, CDR through basin-scale seaweed farming in the open ocean, is seen as a key component of the marine portfolio. Here, we analyse the CDR potential of recent re-occurring trans-basin belts of the floating seaweed Sargassum in the (sub)tropical North Atlantic as a natural analogue for ocean afforestation. We show that two biogeochemical feedbacks, nutrient reallocation and calcification by encrusting marine life, reduce the CDR efficacy of Sargassum by 20–100%. Atmospheric CO2 influx into the surface seawater, after CO2-fixation by Sargassum, takes 2.5–18 times longer than the CO2-deficient seawater remains in contact with the atmosphere, potentially hindering CDR verification. Furthermore, we estimate that increased ocean albedo, due to floating Sargassum, could influence climate radiative forcing more than Sargassum-CDR. Our analysis shows that multifaceted Earth-system feedbacks determine the efficacy of ocean afforestation.

The Tuning Strategy of IPSL‐CM6A‐LR

AbstractThe assessment of current and future risks for natural and human systems associated with climate change largely relies on numerical simulations performed with state‐of‐the‐art climate models. Various steps are involved in the development of such models, from development of individual components of the climate system up to free parameter calibration of the fully coupled model. Here, we describe the final tuning phase for the IPSL‐CM6A‐LR climate model. This phase alone lasted more than 3 years and relied on several pillars: (i) the tuning against present‐day conditions given a small adjustment of the ocean surface albedo to compensate for the current oceanic heat uptake, (ii) the release of successive versions after adjustments of the individual components, implying a systematic and recurrent adjustment of the atmospheric energetics, and (iii) the use of a few metrics based on large scale variables such as near‐global mean temperature, summer Arctic sea‐ice extent, as targets for the tuning. Successes, lessons and prospects of this tuning strategy are discussed.

Ocean surface energy balance allows a constraint on the sensitivity of precipitation to global warming

Climate models generally predict higher precipitation in a future warmer climate. Whether the precipitation intensification occurred in response to historical warming continues to be a subject of debate. Here, using observations of the ocean surface energy balance as a hydrological constraint, we find that historical warming intensified precipitation at a rate of 0.68 ± 0.51% K−1, which is slightly higher than the multi-model mean calculation for the historical climate (0.38 ± 1.18% K−1). The reduction in ocean surface albedo associated with melting of sea ice is a positive contributor to the precipitation temperature sensitivity. On the other hand, the observed increase in ocean heat storage weakens the historical precipitation. In this surface energy balance framework, the incident shortwave radiation at the ocean surface and the ocean heat storage exert a dominant control on the precipitation temperature sensitivity, explaining 91% of the inter-model spread and the spread across climate scenarios in the Intergovernmental Panel on Climate Change Fifth Assessment Report.

Climate More Responsive to Marine Cloud Brightening Than Ocean Albedo Modification: A Model Study

AbstractSolar radiation modification has been suggested as a backup option to reduce anthropogenic warming. Marine cloud brightening (MCB) and ocean albedo modification (OAM) are two proposed approaches to intentionally reflect sunlight back to space over oceanic regions. Using the NCAR Community Earth System Model, we compare climate response to MCB and OAM under the framework of fast adjustment and slow feedback. We implement MCB and OAM uniformly over the global ocean to offset CO2‐induced warming. We find that to offset 3.3 K global mean warming from a doubling of CO2, diagnosed effective radiative forcing is −4.8 and −3.6 W m−2 for OAM and MCB, respectively. Correspondingly, radiative forcing efficacy of OAM is about 70% of MCB. Fast climate adjustment differs in response to MCB and OAM forcing. MCB cools the lower atmosphere by reflecting sunlight from cloud, causing a reduction in sunlight absorption in the atmosphere. In contrast, OAM, by reflecting more sunlight from surface, increases shortwave heating of the lower atmosphere, leading to a decrease in low marine clouds and hence a positive cloudy‐sky shortwave forcing that partly compensates the negative clear‐sky shortwave forcing. The slow climate response and pattern of equilibrium climate change are similar between MCB and OAM. As for hydrological cycle, relative to the climate under a doubling of CO2, both MCB and OAM produce an increase in precipitation and runoff over tropical land.

Ensemble Kalman Filter Parameter Estimation of Ocean Optical Properties for Reduced Biases in a Coupled General Circulation Model

AbstractCoupled general circulation models (GCM), and their atmospheric, oceanic, land, and sea‐ice components have many parameters. Some parameters determine the numerics of the dynamical core, while others are based on our current understanding of the physical processes being simulated. Many of these parameters are poorly known, often globally defined, and are subject to pragmatic choices arising from a complex interplay between grid resolution and inherent model biases. To address this problem, we use an ensemble transform Kalman filter, to estimate spatiotemporally varying maps of ocean albedo and shortwave radiation e‐folding length scale in a coupled climate GCM. These parameters are designed to minimize the error between short term (3–28 days) forecasts of the climate model and a network of real world atmospheric, oceanic, and sea‐ice observations. The data assimilation system has an improved fit to observations when estimating ocean albedo and shortwave e‐folding length scale either individually or simultaneously. However, only individually estimated maps of shortwave e‐folding length scale are also shown to systematically reduce bias in longer multiyear climate forecasts during an out‐of‐sample period. The bias of the multiyear forecasts is reduced for parameter maps determined from longer DA cycle lengths.

Assessing Uncertainties and Approximations in Solar Heating of the Climate System

AbstractIn calculating solar radiation, climate models make many simplifications, in part to reduce computational cost and enable climate modeling, and in part from lack of understanding of critical atmospheric information. Whether known errors or unknown errors, the community's concern is how these could impact the modeled climate. The simplifications are well known and most have published studies evaluating them, but with individual studies it is difficult to compare. Here, we collect a wide range of such simplifications in either radiative transfer modeling or atmospheric conditions and assess potential errors within a consistent framework on climate‐relevant scales. We build benchmarking capability around a solar heating code (Solar‐J) that doubles as a photolysis code for chemistry and can be readily adapted to consider other errors and uncertainties. The broad classes here include: use of broad wavelength bands to integrate over spectral features; scattering approximations that alter phase function and optical depths for clouds and gases; uncertainty in ice‐cloud optics; treatment of fractional cloud cover including overlap; and variability of ocean surface albedo. We geographically map the errors in W m−2 using a full climate re‐creation for January 2015 from a weather forecasting model. For many approximations assessed here, mean errors are ∼2 W m−2 with greater latitudinal biases and are likely to affect a model's ability to match the current climate state. Combining this work with previous studies, we make priority recommendations for fixing these simplifications based on both the magnitude of error and the ease or computational cost of the fix.

Arctic sea ice and snow cover albedo variability and trends during the last three decades

The aim of the present study is to assess the full effect on the albedo of both sea ice extent decrease and snowline retreat in the Arctic during the last three decades. Averaged over the globe, the overall warming effect due to Arctic land and ocean albedo change corresponds to adding about 44% to the direct effect of human CO2 emissions during the same period. In fact, the area and thickness of Arctic sea ice have both been declining in this time frame. This has caused feedbacks affecting the whole global climate system. One such is albedo feedback of sea ice shrinking which was previously estimated (Pistone et al., 2014) to add about 25% to the direct warming effect of anthropogenic CO2 emissions. In this study, we demonstrate that the role of snowline retreat in albedo decrease is comparable to that of sea ice shrinking. To this aim, we estimate the radiative forcing (W/m2) due to snow and ice decrease during 34 years (1982–2015) from the analysis of changes of observed albedo based on the Clouds and the Earth's Radiant Energy System Energy Balanced And Filled (CERES EBAF) dataset, paired with sea ice and snow cover data from the US National Snow & Ice Data Center (NSIDC).

Daily spectral ocean surface albedo (OSA) parameterization in case of clearness index (Kt) and phytoplankton variability in Malacca Strait

In this study, we examined the albedo's dependence on clearness index (Kt) based on measured global radiation data and phytoplankton variability influence represented by chlorophyll (a) concentration at the upper layers over the Malacca Strait. Half-hourly global solar radiation (5°26′53″ N, 100°11′36″ E) measurement over the period from January to October 2017 was analyzed to investigate the correlation between clearness index and ocean surface albedo (OSA) in case of phytoplankton influence in Malacca Strait coastal waters. Three days (January 29, February 26 and March 22, 2017) with high irradiation values were selected for daily OSA variability study. The parameterization includes bio-optical analytical modelling based on apparent optical properties and photosynthetically active range (PAR) estimation. Daily timescale analysis of spectral albedo in Malacca Strait enables detail examination for the chlorophyll maximum absorption wavelength (λ = 440 nm) parameterization. OSA diurnal variability was estimated using Jin et al. scheme, which is based on the separation of albedo components (direct and diffuse). Hourly changes in radiation (including Kt) and solar position properties provide valuable information to investigate the pathways of daily spectral albedo in case of small concentrations of chlorophyll (a) and clearness index parameterization.

An improved air mass factor calculation for nitrogen dioxide measurements from the Global Ozone Monitoring Experiment-2 (GOME-2)

Abstract. An improved tropospheric nitrogen dioxide (NO2) retrieval algorithm from the Global Ozone Monitoring Experiment-2 (GOME-2) instrument based on air mass factor (AMF) calculations performed with more realistic model parameters is presented. The viewing angle dependency of surface albedo is taken into account by improving the GOME-2 Lambertian-equivalent reflectivity (LER) climatology with a directionally dependent LER (DLER) dataset over land and an ocean surface albedo parameterisation over water. A priori NO2 profiles with higher spatial and temporal resolutions are obtained from the IFS (CB05BASCOE) chemistry transport model based on recent emission inventories. A more realistic cloud treatment is provided by a clouds-as-layers (CAL) approach, which treats the clouds as uniform layers of water droplets, instead of the current clouds-as-reflecting-boundaries (CRB) model, which assumes that the clouds are Lambertian reflectors. On average, improvements in the AMF calculation affect the tropospheric NO2 columns by ±15 % in winter and ±5 % in summer over largely polluted regions. In addition, the impact of aerosols on our tropospheric NO2 retrieval is investigated by comparing the concurrent retrievals based on ground-based aerosol measurements (explicit aerosol correction) and the aerosol-induced cloud parameters (implicit aerosol correction). Compared with the implicit aerosol correction utilising the CRB cloud parameters, the use of the CAL approach reduces the AMF errors by more than 10 %. Finally, to evaluate the improved GOME-2 tropospheric NO2 columns, a validation is performed using ground-based multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements at different BIRA-IASB stations. At the suburban Xianghe station, the improved tropospheric NO2 dataset shows better agreement with coincident ground-based measurements with a correlation coefficient of 0.94.