Satellite Remote-sensing Observations Research Articles

Extreme melting at Greenland's largest floating ice tongue

Abstract. The 79° North Glacier (Nioghalvfjerdsbrae, 79NG) is one of three remaining glaciers with a floating tongue in Greenland. Although the glacier has been considered exceptionally stable in the past, earlier studies have shown that the ice tongue has thinned in recent decades. By conducting high-resolution ground-based and airborne radar measurements in conjunction with satellite remote-sensing observations, we find significant changes in the geometry of 79NG. In the vicinity of the grounding line, a 500 m high subglacial channel has grown since ∼ 2010 and has caused surface lowering of up to 7.6 m a−1. Our results show extreme basal melt rates exceeding 150 m a−1 over a period of 17 d within a distance of 5 km from the grounding line, where the ice has thinned by 32 % since 1998. We find a heterogeneous distribution of melt rates, likely due to variability in water column thickness and channelization of the ice base. Time series of melt rates show a decrease in basal melting since 2018, indicating an inflow of colder water into the cavity below 79NG. We discuss the processes that have led to the changes in geometry and conclude that the inflow of warm ocean currents has led to the extensive thinning of 79NG's floating ice tongue near the grounding line over the last 2 decades. In contrast, we hypothesize that the growth of the channel results from increased subglacial discharge due to a considerably enlarged area of summer surface melt due to the warming of the atmosphere.

Spatio-Temporal Trends of Surface Energy Budget in Tibet from Satellite Remote Sensing Observations and Reanalysis Data

Being the highest and largest land mass of the earth, the Tibetan Plateau has a strong impact on the Asian climate especially on the Asian monsoon. With high downward solar radiation, the Tibetan Plateau is a climate sensitive region and the main water source for many rivers in South and East Asia. Although many studies have analyzed energy fluxes in the Tibetan Plateau, a long-term detailed spatio-temporal variability of all energy budget parameters is not clear for understanding the dynamics of the regional climate change. In this paper, satellite remote sensing and reanalysis data are used to quantify spatio-temporal trends of energy budget parameters, net radiation, latent heat flux, and sensible heat flux over the Tibetan Plateau from 2001 to 2019. The validity of both data sources is analyzed from in situ ground measurements of the FluxNet micrometeorological tower network, which verifies that both datasets are valid and reliable. It is found that the trend of net radiation shows a slight increase. The latent heat flux increases continuously, while the sensible heat flux decreases continuously throughout the study period over the Tibetan Plateau. Varying energy fluxes in the Tibetan plateau will affect the regional hydrological cycle. Satellite LE product observation is limited to certain land covers. Thus, for larger spatial areas, reanalysis data is a more appropriate choice. Normalized difference vegetation index proves a useful indicator to explain the latent heat flux trend. Despite the reduction of sensible heat, the atmospheric temperature increases continuously resulting in the warming of the Tibetan Plateau. The opposite trend of sensible heat flux and air temperature is an interesting and explainable phenomenon. It is also concluded that the surface evaporative cooling is not the indicator of atmospheric cooling/warming. In the future, more work shall be done to explain the mechanism which involves the complete heat cycle in the Tibetan Plateau.

Accurate Evaluation of Sea Surface Temperature Cooling Induced by Typhoons Based on Satellite Remote Sensing Observations

We introduce a novel method to accurately evaluate the satellite-observed sea surface temperature (SST) cooling induced by typhoons with complex tracks, which is widely used but only roughly calculated in previous studies. This method first records the typhoon forcing period and the SST response grid by grid, then evaluates the SST cooling in each grid by choosing the maximum decrease in SST within this time period. This grid-based flexible forcing date method can accurately evaluate typhoon-induced SST cooling and its corresponding date in each grid, as indicated by applying the method to the irregular track of Typhoon Lupit (2009) and three sequential typhoons in 2016 (Malakas, Megi, and Chaba). The method was used to accurately calculate the impact of Typhoon Megi by removing the influence of the other two typhoons. The SST cooling events induced by all typhoons in the northwest Pacific from 2004 to 2018 were extracted well using this method. Our findings provide new insights for accurately calculating the response of the ocean using multi-satellite remote sensing and simulation data, including the sea surface salinity, sea surface height, mixed layer depth, and the heat content of the upper levels of the ocean.

Satellite remote-sensing observations for definitions of areas for marine conservation: Case study of the Scotian Slope, Eastern Canada

Characterizing offshore areas for marine conservation faces impediment from logistics and costs of in situ data collection. This study leveraged the high spatial and temporal coverage provided by remote-sensing ocean colour products, and the concept of biogeographical classification to delineate areas in the pelagic environment with application to marine conservation. Satellite-derived and modelled biogeophysical data were used to delineate an area of biological and physical homogeneity spatially consistent with the static boundaries of the Scotian Slope Ecologically and Biologically Significant Area (EBSA), off eastern Canada. We used iterative cluster analysis applied to an archive (2004–2014) that consisted of remotely-sensed Chlorophyll a, sea-surface temperature, and primary production derived from the Moderate resolution Imaging Spectroradiometer (MODIS), and simulated mixed layer depth to define “dynamic” boundaries of the EBSA at bi-weekly, seasonal and annual resolutions. The final cluster extended further east than the static EBSA in summer and fall, indicating that characteristics of the static EBSA environment persist beyond the current boundaries in these seasons. In winter and spring, the final areas derived by our analysis was smaller than the static EBSA, but again showed extension beyond the current eastern boundary. Both ice and cloud cover affecting remotely-sensed data and the extent of water column mixing were important in determining the size of the final cluster. The dynamic cluster east of the original static boundary incorporated an area of lower Chlorophyll a and water column primary production in the spring, but higher values in the autumn relative to the static area. Overall physical and biological characteristics of the static and dynamic EBSA considered in this research were similar within and across years. This methodology incorporates ocean-colour data and modelled estimates of multiple biological and physical characteristics to objectively refine areas of ecological interest at a spatial scale relevant for the management of marine conservation areas.

Progress in satellite remote sensing for studying physical processes at the ocean surface and its borders with the atmosphere and sea ice

Physical oceanography is the study of physical conditions, processes and variables within the ocean, including temperature–salinity distributions, mixing of the water column, waves, tides, currents and air–sea interaction processes. Here we provide a critical review of how satellite sensors are being used to study physical oceanography processes at the ocean surface and its borders with the atmosphere and sea ice. The paper begins by describing the main sensor types that are used to observe the oceans (visible, thermal infrared and microwave) and the specific observations that each of these sensor types can provide. We then present a critical review of how these sensors and observations are being used to study: (i) ocean surface currents, (ii) storm surges, (iii) sea ice, (iv) atmosphere–ocean gas exchange and (v) surface heat fluxes via phytoplankton. Exciting advances include the use of multiple sensors in synergy to observe temporally varying Arctic sea ice volume, atmosphere–ocean gas fluxes, and the potential for four-dimensional water circulation observations. For each of these applications we explain their relevance to society, review recent advances and capability, and provide a forward look at future prospects and opportunities. We then more generally discuss future opportunities for oceanography-focused remote sensing, which includes the unique European Union Copernicus programme, the potential of the International Space Station and commercial miniature satellites. The increasing availability of global satellite remote-sensing observations means that we are now entering an exciting period for oceanography. The easy access to these high quality data and the continued development of novel platforms is likely to drive further advances in remote sensing of the ocean and atmospheric systems.

Review: advances in in situ and satellite phenological observations in Japan.

To accurately evaluate the responses of spatial and temporal variation of ecosystem functioning (evapotranspiration and photosynthesis) and services (regulating and cultural services) to the rapid changes caused by global warming, we depend on long-term, continuous, near-surface, and satellite remote sensing of phenology over wide areas. Here, we review such phenological studies in Japan and discuss our current knowledge, problems, and future developments. In contrast with North America and Europe, Japan has been able to evaluate plant phenology along vertical and horizontal gradients within a narrow area because of the country’s high topographic relief. Phenological observation networks that support scientific studies and outreach activities have used near-surface tools such as digital cameras and spectral radiometers. Differences in phenology among ecosystems and tree species have been detected by analyzing the seasonal variation of red, green, and blue digital numbers (RGB values) extracted from phenological images, as well as spectral reflectance and vegetation indices. The relationships between seasonal variations in RGB-derived indices or spectral characteristics and the ecological and CO2 flux measurement data have been well validated. In contrast, insufficient satellite remote-sensing observations have been conducted because of the coarse spatial resolution of previous datasets, which could not detect the heterogeneous plant phenology that results from Japan’s complex topography and vegetation. To improve Japanese phenological observations, multidisciplinary analysis and evaluation will be needed to link traditional phenological observations with “index trees,” near-surface and satellite remote-sensing observations, “citizen science” (observations by citizens), and results published on the Internet.

Efficient Parallel GPU Design on WRF Five-Layer Thermal Diffusion Scheme

Satellite remote-sensing observations and ground-based radar can detect the weather conditions from a distance and are widely used to monitor the weather all around the globe. The assimilated satellite/radar data are passed through the weather models for weather forecasting. The five-layer thermal diffusion scheme is one of the weather models, handling with an energy budget made up of sensible, latent, and radiative heat fluxes. The model feature of no interactions among horizontal grid points makes this scheme very favorable for parallel processing. This study demonstrates implementation of this scheme using graphics processing unit (GPU) massively parallel architecture. By employing one NVIDIA Tesla K40 GPU, our GPU optimization effort on this scheme achieves a speedup of $311 \times$ with respect to its CPU counterpart Fortran code running on one CPU core of Intel Xeon E5-2603, whereas the speedup for one CPU socket (four cores) with respect to one CPU core is only $3.1 \times$ . We can even boost the speedup of this scheme to $398 \times$ with respect to one CPU core when two NVIDIA Tesla K40 GPUs are applied.

Wet and dry deposition of mineral dust particles in Japan: factors related to temporal variation and spatial distribution

Abstract. Recent ground networks and satellite remote-sensing observations have provided useful data related to spatial and vertical distributions of mineral dust particles in the atmosphere. However, measurements of temporal variations and spatial distributions of mineral dust deposition fluxes are limited in terms of their duration, location, and processes of deposition. To ascertain temporal variations and spatial distributions of mineral dust deposition using wet and dry processes, weekly deposition samples were obtained at Sapporo, Toyama, Nagoya, Tottori, Fukuoka, and Cape Hedo (Okinawa) in Japan during October 2008–December 2010 using automatic wet and dry separating samplers. Mineral dust weights in water-insoluble residue were estimated from Fe contents measured using an X-ray fluorescence analyser. Wet and dry deposition fluxes of mineral dusts were both high in spring and low in summer, showing similar seasonal variations to frequency of aeolian dust events (Kosa) in Japan. For wet deposition, highest and lowest annual dust fluxes were found at Toyama (9.6 g m−2 yr−1) and at Cape Hedo (1.7 g m−2 yr−1) as average values in 2009 and 2010. Higher wet deposition fluxes were observed at Toyama and Tottori, where frequent precipitation (> 60% days per month) was observed during dusty seasons. For dry deposition among Toyama, Tottori, Fukuoka, and Cape Hedo, the highest and lowest annual dust fluxes were found respectively at Fukuoka (5.2 g m−2 yr−1) and at Cape Hedo (2.0 g m−2 yr−1) as average values in 2009 and 2010. The average ratio of wet and dry deposition fluxes was the highest at Toyama (3.3) and the lowest at Hedo (0.82), showing a larger contribution of the dry process at western sites, probably because of the distance from desert source regions and because of the effectiveness of the wet process in the dusty season. Size distributions of refractory dust particles were obtained using four-stage filtration: > 20, > 10, > 5, and > 1 μm diameter. Weight fractions of the sum of > 20 μm and 10–20 μm (giant fraction) were higher than 50% for most of the event samples. Irrespective of the deposition type, the giant dust fractions generally decreased with increasing distance from the source area, suggesting the selective depletion of larger giant particles during atmospheric transport. Based on temporal variations of PMc (2.5 < D < 10 μm), ground-based lidar, backward air trajectories, and vertical profiles of potential temperatures, transport processes of dust particles are discussed for events with high-deposition and low-deposition flux with high PMc. Low dry dust depositions with high PMc concentrations were observed under stronger (5 K km−1) stratification of potential temperature with thinner and lower (< 2 km) dust distributions because the PMc fraction of dust particles only survived after depletion of giant dust particles by rapid gravitational settling at the time they reach Japan. In contrast, transport through a thicker (> 2 km) dust layer with weak vertical gradient of potential temperature carry more giant dust particles to Japan. Because giant dust particles are an important mass fraction of dust accumulation, especially in the North Pacific, which is known as a high-nutrient, low-chlorophyll (HNLC) region, the transport height and fraction of giant dust particles are important factors for studying dust budgets in the atmosphere and their role in biogeochemical cycles.

Remotely sensed retrieval of midday air temperature considering atmospheric and surface moisture conditions

Near-surface air temperature (T a) is an important variable for various scientific communities, and many previous studies have attempted to estimate T a with high spatial resolution using remotely sensed data. The present study proposes a new, practical method for estimating T a near the land surface based on moisture conditions of the atmosphere and surface. First, match-up data sets of ground observations from 76 meteorological stations and satellite remote-sensing observations were established over the Republic of Korea during 2006. Four cases of atmospheric and surface moisture conditions were then determined using Moderate Resolution Imaging Spectroradiometer brightness temperatures (at 11 and 12 μm) and the normalized difference water index for all match-ups. Using stepwise multiple regression analysis for each case, land surface temperature and the normalized difference vegetation index were statistically selected as significant independent variables for estimating T a using the development data set (75% of the match-ups for each case). The T a values estimated from the multiple regression algorithm with different constants and coefficients for each case showed reasonably good performance (nearly zero mean bias and approximately 3°C root mean square error) compared with the measured T a values used as the validation data set (25% of the match-ups for each case). Thus, the proposed T a estimation approach based on moisture conditions of the atmosphere and surface is not confined to a specific season or land type and can be applied to all areas and seasons in the Republic of Korea.

Combined remote-sensing, model, and in situ measurements of sea surface temperature as an aid to recreational navigation: crossing the Gulf Stream

Combined in situ, model, and satellite remote-sensing observations are used to determine the location of the Gulf Stream as an aid to safe navigation for small recreational vessels. A field study was executed from Hamilton, Bermuda, to Virginia Beach, USA, over a period of 5 days, from 30 June 2010 to 4 July 2010 to test the feasibility of using remote-sensing products as an aid to cross the Gulf Stream from the point of view of a small, slow-moving (˜6 knots, 3 m s−1) sailboat. The in situ data collected were compared to NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) remote-sensing data, to the Global High Resolution Sea Surface Temperature (GHRSST) microwave and infrared blended data set, to the National Oceanic and Atmospheric Administration Real-Time Ocean Forecast System (NOAARTOFS) ocean model, and to selected NOAA buoy and ship measurements. A spatio-temporal analysis was performed by comparing the in situ measurements with observations retrieved at the same time and location in each of the data sets. The least error (correlation coefficient r = 0.94) was obtained using MODIS data, and the largest error (r = 0.78) was obtained using the RTOFS model data. Overall, most observations agree with the general spatio-temporal trend of the in situ data, with 95% of the errors within ±1°C and 98% of the errors within ±2°C. The study shows that MODIS data are particularly suited to identification of the location of the Gulf Stream, which can be used by small vessels to optimize the crossing route and to minimize the risks associated with the passage.

Derivation of pan-Arctic soil decomposition rate constant, heterotrophic respiration and NEE using AMSR-E and MODIS data

An approach was developed for regional assessment and monitoring of land-atmosphere carbon dioxide (CO2) exchange, soil heterotrophic respiration (R h), and vegetation productivity of Arctic tundra using global satellite remote sensing at optical and microwave wavelengths. C- and X-band brightness temperatures were used from the Advanced Microwave Scanning Radiometer Earth Observing System (AMSR-E) to extract surface wetness and temperature, and MODerate Resolution Imaging Spectroradiometer (MODIS) data were used to derive land cover, Leaf Area Index (LAI), and Net Primary Production (NPP) information. Calibration and validation activities involve comparisons between satellite remote sensing and tundra CO2 eddy flux towers, and hydroecological process model simulations. Analyses of spatial and temporal anomalies and environmental drivers of land-atmosphere net CO2 exchange at weekly and annual time steps were conducted. Surface soil moisture and temperature, as detected from satellite remote-sensing observations, were found to be major drivers for spatial and temporal patterns of tundra net ecosystem CO2 exchange and photosynthetic and respiration processes. Satellite microwave measurements are capable of capturing seasonal variations and regional patterns in tundra soil heterotrophic respiration and CO2 exchange, while the ability to extract spatial patterns at the scale of surface heterogeneity is limited by the coarse spatial scale of the satellite remote-sensing footprint. The microwave-derived surface temperature and soil moisture were used to estimate net ecosystem carbon exchange (NEE) at the boreal-Arctic region. These were validated using flux tower sites data. Existing satellite-based measurements of vegetation structure (i.e. LAI) and productivity (i.e. Gross Primary Production (GPP) and NPP) from the Aqua/Terra MODIS with the AMSR-E-derived land-surface temperature and soil moisture were used and integrated. Spatially explicit estimates of NEE for the pan-Arctic region at daily, weekly and annual intervals were derived. Comparative analysis of satellite data-derived NEE with measurements from CO2 eddy flux tower sites and the BIOME-BGC model were carried out and good agreement was found. The comparative analysis is statistically significant with high regression (i.e. R 2 = 0.965), especially in the R h calculation and the overall NEE regression is 0.478. The results also indicate that the carbon cycle response to climate change is nonlinear and is strongly coupled to Arctic surface hydrology.

Assessment and inter-comparison of five high-resolution sea surface temperature products in the shelf and coastal seas around China

Recently the global ocean data assimilation experiment (GODAE) High-Resolution SST Pilot Project (GHRSST-PP) has delivered several sea surface temperature (SST) products based on satellite remote-sensing observations. These cloud-free, high spatial (up to 2–5 km) and temporal resolution (daily or 6-hourly) SST products provide important data sources for operational ocean forecast of both global and shelf/coastal seas. Before applying the GHRSST products to initialization or data assimilation for ocean forecasting of shelf and coastal seas, it is necessary to make inter-comparison and assessment of these products. In this paper, five products were investigated by means of inter-comparison and comparison to in situ observations at the shelf and coastal sea around China and in the northwest Pacific over 1-year period (from October 2005 to September 2006). The inter-comparison shows that in the coastal and the shelf sea around China, there are some significant differences among these products. Some have warm bias with reference to the five-member mean, while some have cold biases. In shelf seas around China, the spread of the five products is between 0.4 and 0.6 °C. In the coastal sea (shallower than 40 m), the spread increases dramatically to 0.95 °C. Further, the comparison of daily GHRSST products with in situ night-time SST observations from surface drifters and ship reports indicated that most of these SST products are excellent in deep water region. The root mean square differences (RMSDs) of some products with reference to in situ observations are less than 0.4 °C. But near the coastal, relatively large bias and RMSD exist in all five products. In order to reduce the bias and RMSD in near coast, a simple, depth-dependent bias correction scheme is applied to two of the five products. After correction their biases can be reduced, and their RMSDs could be also decreased about 6–14%. This study suggests that the GHRSST products can be used for operational ocean forecasting with proper error estimation for shelf seas and the open ocean. For the coastal seas where the depths are shallower than 40 m, large uncertainties exist in these products. Improvements are needed for future application in the coastal seas.