Characteristics of the Global Radio Frequency Interference in the Protected Portion of L-Band

The National Aeronautics and Space Administration’s (NASA’s) Soil Moisture Active–Passive (SMAP) radiometer has been providing geolocated power moments measured within a 24 MHz band in the protected portion of L-band, i.e., 1400–1424 MHz, with 1.2 ms and 1.5 MHz time and frequency resolutions, as its Level 1A data. This paper presents important spectral and temporal properties of the radio frequency interference (RFI) in the protected portion of L-band using SMAP Level 1A data. Maximum and average bandwidth and duration of RFI signals, average RFI-free spectrum availability, and variations in such properties between ascending and descending satellite orbits have been reported across the world. The average bandwidth and duration of individual RFI sources have been found to be usually less than 4.5 MHz and 4.8 ms; and the average RFI-free spectrum is larger than 20 MHz in most regions with exceptions over the Middle East and Central and Eastern Asia. It has also been shown that, the bandwidth and duration of RFI signals can vary as much as 10 MHz and 10 ms, respectively, between ascending and descending orbits over certain locations. Furthermore, to identify frequencies susceptible to RFI contamination in the protected portion of L-band, observed RFI signals have been assigned to individual 1.5 MHz SMAP channels according to their frequencies. It has been demonstrated that, contrary to common perception, the center of the protected portion can be as RFI contaminated as its edges. Finally, there have been no significant correlations noted among different RFI properties such as amplitude, bandwidth, and duration within the 1400–1424 MHz ban

Relationship between vegetation microwave optical depth and cross-polarized backscatter from multiyear Aquarius observations

Soil moisture retrieval algorithms based on passive microwave remote sensing observations need to account for vegetation attenuation and emission, which is generally parameterized as vegetation optical depth (VOD). This multisensor study tests a new method to retrieve VOD from cross-polarized radar backscattering coefficients. Three years of Aquarius/SAC-D data were used to establish a relationship between the cross-polarized backscattering coefficient σHV and VOD derived from a multitemporal passive dual-channel algorithm (VODMT). The dependence of the correspondence is analyzed for different land use classes. There are no systematic differences in the slope for woody versus nonwoody vegetation, resulting in a strong correlation (80% explained-variance) and a global linear relationship when all classes are combined. The relationship is stable over the years of observations. The comparison of the Aquarius-derived VODMT to Soil Moisture and Ocean Salinity's multi-angular VOD estimates shows similar spatial patterns and temporal behavior, evident in high correlations. However, VODMT has considerably higher mean values, but lower dynamic range globally. Most of the differences can be attributed to differences in instrument sampling. The main result of this study, a relationship between backscatter and VOD, will permit high-resolution mapping of VOD with synthetic aperture radar measurements. These maps allow future studies of scaling and heterogeneity effects of vegetation on soil moisture retrieval at the coarser scales of land microwave radiometry. The study shows that VOD based on passive measurements and predicted by active measurements are comparable globally and that the breakdown by land cover classification does not affect the relationship appreciably

Analyse des cycles gel/dégel des régions nordiques par télédétection micro-ondes passives en bande L

Le réchauffement climatique dans les régions nordiques, fort important depuis le milieu du siècle dernier, a de multiples impacts sur la dynamique des écosystèmes, notamment sur les cycles gel/dégel de surface qui influencent les flux de carbone, l'activité biogéochimique des sols, l'hydrologie et le pergélisol aux hautes latitudes. La télédétection satellitaire du gel/dégel par micro-ondes passives est un outil très prometteur permettant un suivi continu et global, mais comporte des difficultés souvent reliées à l’effet d’hétérogénéité spatiale intra-pixel relié aux résolutions grossières des capteurs micro-ondes passives à basse fréquence. L’objectif principal du projet est d’évaluer l’utilisation de la télédétection micro-onde passive en bande L (1.4 GHz) pour le suivi de l’état de gel/dégel de la surface en forêt boréale. Un premier objectif spécifique est d’évaluer un nouveau produit des cycles de gel/dégel de surface estimée à partir des radiomètres bande L satellitaires Aquarius. Cette base de données de 3.5 années a été mise en ligne au National Snow and Ice Data Center (NSIDC). Le deuxième objectif spécifique est d’analyser l’effet de la variabilité spatiale intrapixel de l’état de gel du sol et de son impact sur les températures de brillance (TB) mesurées par le radiomètre de la mission Soil Moisture Active Passive (SMAP) en période de transition afin de quantifier la fraction de sol gelé. Les résultats pour le premier objectif montrent que la nouvelle base de données possède une bonne capacité à estimer l’état de gel/dégel de la surface sur l’ensemble de l’Hémisphère Nord (> 50°N). Cette recherche offre également une rare intercomparaison entre produits de gel/dégel satellitaires en comparant le produit Aquarius au Freeze/Thaw-Earth System Data Record (FT-ESDR) développé avec les données à plus hautes fréquences du capteur SSM/I. Pour le deuxième objectif, des capteurs de température distribués le long de transects de plusieurs kilomètres sur deux différents sites de taïga montrent que la variabilité spatiale du gel à l’automne peut être de 7.5 à 9.5 semaines. Il est également démontré que les mesures de SMAP sont sensibles à cette variabilité et un algorithme développé permet d’estimer le pourcentage intrapixel de sol gelé avec des coefficients de détermination (R2) entre 0.63 et 0.88 lorsque comparé aux mesures in situ. Ces résultats offrent de nouveaux outils pour mieux comprendre et quantifier les cycles de gel/dégel de l’environnement boréal et leurs impacts sur les processus biogéophysiques, hydrologiques et sur le pergélisol.Abstract: Climate change in nordic regions, which has been of growing significance over the past century has multiple impacts on the dynamic of ecosystems, notably on the surface freeze/thaw cycles, which influences carbon flux, soil biogeochemical activity, hydrology and permafrost at high latitudes. Satellite remote sensing of freeze/thaw with passive microwaves is a promising tool to offer continuous and global monitoring, but can also entail some difficulties due to intra-pixel spatial variability effects coming from the low resolution of low-frequency passive microwave sensors. The primary objective of the project is to evaluate the use of passive microwave remote sensing in L-band (1.4 GHz) for monitoring of the surface freeze/thaw in the boreal forest. A first specific objective is to evaluate a new surface freeze/thaw product estimated by the Aquarius satellite L-band radiometers. This 3.5 year-old database has been put online at the National Snow and Ice Data Center (NSIDC) website. The second specific objective is to analyse the effect of intra-pixel spatial variability of freeze/thaw and its impact on brightness temperatures (TB) measured by the Soil Moisture Active Passive (SMAP) radiometer during transition periods in order to quantify the frozen soil fraction. Results for the first objective show that the new database possesses a good capacity to estimate the surface freeze/thaw state for the entirety of the Northern Hemisphere (>50°N). This research also offers a rare intercomparison between freeze/thaw satellite products by comparing the Aquarius product to the Freeze/Thaw-Earth System Data Record (FT-ESDR) product developed with higher frequencies data of the SSM/I sensor. For the second objective, temperature sensors distributed along transects of several kilometers on two different taiga sites show that the spatial variability of autumn soil freeze onset can be between 7.5 and 9.5 weeks. It demonstrates that SMAP measurements are sensitive to this variability and a developed algorithm offers estimations of the intrapixel soil frozen fraction with coefficients of determination (R2) between 0.63 and 0.88 when compared to in situ measurements. These results offer new tools for a better understanding and quantification of freeze/thaw cycles in boreal environments and their impacts on biogeochemical and hydrologic processes and on permafrost

Statistical analysis and combination of active and passive microwave remote sensing methods for soil moisture retrieval

Knowledge about soil moisture and its spatio-temporal dynamics is essential for the improvement of climate and hydrological modeling, including drought and flood monitoring and forecasting, as well as weather forecasting models. In recent years, several soil moisture products from active and passive microwave remote sensing have become available with high temporal resolution and global coverage. Thus, the validation and evaluation of spatial and temporal soil moisture patterns are of great interest, for improving soil moisture products as well as for their proper use in models or other applications. This thesis analyzes the different accuracy levels of global soil moisture products and identifies the major influencing factors on this accuracy based on a small catchment example. Furthermore, on global scale, structural differences betweenthe soil moisture products were investigated. This includes in particular the representation of spatial and temporal patterns, as well as a general scaling law of soil moisture variability with extent scale. The results of the catchment scale as well as the global scale analyses identified vegetation to have a high impact on the accuracy of remotely sensed soil moisture products. Therefore, an improved method to consider vegetation characteristics in pasive soil moisture retrieval from active radar satellite data was developed and tested. The knowledge gained by this thesis will contribute to improve soil moisture retrieval of current and future microwave remote sensors (e.g. SMOS or SMAP)

Variability and uncertainty of satellite sea surface salinity in the subpolar North Atlantic (2010-2019)

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yu, L. Variability and uncertainty of satellite sea surface salinity in the subpolar North Atlantic (2010-2019). Remote Sensing, 12(13), (2020): 2092, doi:10.3390/rs12132092.Satellite remote sensing of sea surface salinity (SSS) in the recent decade (2010–2019) has proven the capability of L-band (1.4 GHz) measurements to resolve SSS spatiotemporal variability in the tropical and subtropical oceans. However, the fidelity of SSS retrievals in cold waters at mid-high latitudes has yet to be established. Here, four SSS products derived from two satellite missions were evaluated in the subpolar North Atlantic Ocean in reference to two in situ gridded products. Harmonic analysis of annual and semiannual cycles in in situ products revealed that seasonal variations of SSS are dominated by an annual cycle, with a maximum in March and a minimum in September. The annual amplitudes are larger (>0.3 practical salinity scale (pss)) in the western basin where surface waters are colder and fresher, and weaker (~0.06 pss) in the eastern basin where surface waters are warmer and saltier. Satellite SSS products have difficulty producing the right annual cycle, particularly in the Labrador/Irminger seas where the SSS seasonality is dictated by the influx of Arctic low-salinity waters along the boundary currents. The study also found that there are basin-scale, time-varying drifts in the decade-long SMOS data records, which need to be corrected before the datasets can be used for studying climate variability of SSSThis research was funded by NASA Ocean Salinity Science Team (OSST) activities through Grant 80NSSC18K1335

Northern Hemisphere surface freeze–thaw product from Aquarius L-band radiometers

In the Northern Hemisphere, seasonal changes in surface freeze–thaw (FT) cycles are an important component of surface energy, hydrological and eco-biogeochemical processes that must be accurately monitored. This paper presents the weekly polar-gridded Aquarius passive L-band surface freeze–thaw product (FT-AP) distributed on the Equal-Area Scalable Earth Grid version 2.0, above the parallel 50∘&thinsp;N, with a spatial resolution of 36&thinsp;km&thinsp;×&thinsp;36&thinsp;km. The FT-AP classification algorithm is based on a seasonal threshold approach using the normalized polarization ratio, references for frozen and thawed conditions and optimized thresholds. To evaluate the uncertainties of the product, we compared it with another satellite FT product also derived from passive microwave observations but at higher frequency: the resampled 37&thinsp;GHz FT Earth Science Data Record (FT-ESDR). The assessment was carried out during the overlapping period between 2011 and 2014. Results show that 77.1&thinsp;% of their common grid cells have an agreement better than 80&thinsp;%. Their differences vary with land cover type (tundra, forest and open land) and freezing and thawing periods. The best agreement is obtained during the thawing transition and over forest areas, with differences between product mean freeze or thaw onsets of under 0.4 weeks. Over tundra, FT-AP tends to detect freeze onset 2–5 weeks earlier than FT-ESDR, likely due to FT sensitivity to the different frequencies used. Analysis with mean surface air temperature time series from six in situ meteorological stations shows that the main discrepancies between FT-AP and FT-ESDR are related to false frozen retrievals in summer for some regions with FT-AP. The Aquarius product is distributed by the U.S. National Snow and Ice Data Center (NSIDC) at https://nsidc.org/data/aq3_ft/versions/5 with the DOI https://doi.org/10.5067/OV4R18NL3BQR.</p

Revisiting the global patterns of seasonal cycle in sea surface salinity

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(4), (2021): e2020JC016789, https://doi.org/10.1029/2020JC016789.Argo profiling floats and L-band passive microwave remote sensing have significantly improved the global sampling of sea surface salinity (SSS) in the past 15 years, allowing the study of the range of SSS seasonal variability using concurrent satellite and in situ platforms. Here, harmonic analysis was applied to four 0.25° satellite products and two 1° in situ products between 2016 and 2018 to determine seasonal harmonic patterns. The 0.25° World Ocean Atlas (WOA) version 2018 was referenced to help assess the harmonic patterns from a long-term perspective based on the 3-year period. The results show that annual harmonic is the most characteristic signal of the seasonal cycle, and semiannual harmonic is important in regions influenced by monsoon and major rivers. The percentage of the observed variance that can be explained by harmonic modes varies with products, with values ranging between 50% and 72% for annual harmonic and between 15% and 19% for semiannual harmonic. The large spread in the explained variance by the annual harmonic reflects the large disparity in nonseasonal variance (or noise) in the different products. Satellite products are capable of capturing sharp SSS features on meso- and frontal scales and the patterns agree well with the WOA 2018. These products are, however, subject to the impacts of radiometric noises and are algorithm dependent. The coarser-resolution in situ products may underrepresent the full range of high-frequency small scale SSS variability when data record is short, which may have enlarged the explained SSS variance by the annual harmonic.L. Yu was funded by NASA Ocean Salinity Science Team (OSST) activities through Grant 80NSSC18K1335. FMB was funded by the NASA OSST through Grant 80NSSC18K1322. E. P. Dinnat was funded by NASA through Grant 80NSSC18K1443. This research is carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.2021-09-1

Monitoring Water and Energy Cycles at Climate Scale in the Third Pole Environment (CLIMATE-TPE)

A better understanding of the water and energy cycles at climate scale in the Third Pole Environment is essential for assessing and understanding the causes of changes in the cryosphere and hydrosphere in relation to changes of plateau atmosphere in the Asian monsoon system and for predicting the possible changes in water resources in South and East Asia. This paper reports the following results: (1) A platform of in situ observation stations is briefly described for quantifying the interactions in hydrosphere-pedosphere-atmosphere-cryosphere-biosphere over the Tibetan Plateau. (2) A multiyear in situ L-Band microwave radiometry of land surface processes is used to develop a new microwave radiative transfer modeling system. This new system improves the modeling of brightness temperature in both horizontal and vertical polarization. (3) A multiyear (2001–2018) monthly terrestrial actual evapotranspiration and its spatial distribution on the Tibetan Plateau is generated using the surface energy balance system (SEBS) forced by a combination of meteorological and satellite data. (4) A comparison of four large scale soil moisture products to in situ measurements is presented. (5) The trajectory of water vapor transport in the canyon area of Southeast Tibet in different seasons is analyzed, and (6) the vertical water vapor exchange between the upper troposphere and the lower stratosphere in different seasons is presented

A roadmap for high-resolution satellite soil moisture applications – confronting product characteristics with user requirements

Soil moisture observations are of broad scientific interest and practical value for a wide range of applications. The scientific community has made significant progress in estimating soil moisture from satellite-based Earth observation data, particularly in operationalizing coarse-resolution (25-50 km) soil moisture products. This review summarizes existing applications of satellite-derived soil moisture products and identifies gaps between the characteristics of currently available soil moisture products and the application requirements from various disciplines. We discuss the efforts devoted to the generation of high-resolution soil moisture products from satellite Synthetic Aperture Radar (SAR) data such as Sentinel-1 C-band backscatter observations and/or through downscaling of existing coarse-resolution microwave soil moisture products. Open issues and future opportunities of satellite-derived soil moisture are discussed, providing guidance for further development of operational soil moisture products and bridging the gap between the soil moisture user and supplier communities