Ehsan Forootan

• Professor
• Professor at Aalborg University

Simulating hydrological processes within the (semi-)arid region of the Murray-Darling Basin (MDB), Australia, is very challenging specially during droughts. In this study, we investigate whether integrating remotely sensed terrestrial water storage changes (TWSC) from the Gravity Recovery And Climate Experiment (GRACE) mission into a global water r...

Observing global terrestrial water storage changes (TWSCs) from (inter-)seasonal to (multi-)decade timescales is very important to understand the Earth as a system under natural and anthropogenic climate change. The primary goal of the Gravity Recovery And Climate Experiment (GRACE) satellite mission (2002-2017) and its follow-on mission (GRACE-FO,...

Climate variability and change along with anthropogenic water use have affected the (re)distribution of water storage and fluxes across the Contiguous United States (CONUS). Available hydrological models, however, do not represent recent changes in the water cycle. Therefore, in this study, a novel Bayesian Markov Chain Monte Carlo-based Data Assim...

Global estimation of thermospheric neutral density (TND) on various altitudes is important for geodetic and space weather applications. This is typically provided by models, however, the quality of these models is limited due to their imperfect structure and the sensitivity of their parameters to the calibration period. Here, we present an ensemble...

Improving thermospheric neutral density (TND) estimates is important for computing drag forces acting on low-Earth-orbit (LEO) satellites and debris. Empirical thermospheric models are often used to compute TNDs for the precise orbit determination experiments. However, it is known that simulating TNDs are of limited accuracy due to simplification o...

Plain Language Summary Monitoring Earth's water resources from space is essential for understanding droughts and managing water supplies. The Gravity Recovery and Climate Experiment (GRACE) satellite and its Follow‐On (GRACE/FO) track changes in terrestrial water storage by detecting tiny variations in Earth's gravity, providing crucial information...

Modeling sub-daily mass changes, dominated by the atmosphere and the oceans, is not only essential for understanding weather and climate change but also serves as a fundamental requirement for nearly all existing terrestrial or space-borne geodetic observations to perform signal separation. Removing these high-frequency mass changes, through the us...

Atmospheric de-aliasing is one of the most important background models for recovering Earth’s temporal gravity field from gravity satellite missions. To meet the needs of China’s gravimetric satellite platform, an independent atmospheric dealiasing model that relies on Chinese meteorological data needs to be developed. The release of CRA-40, as the...

One of the important challenges in the coastal altimetry is the contamination of the waveforms by non-oceanic effects. These effects, which have complex behavior, appear in different forms on the waveforms. Therefore, omitting or mitigating their effects is one of the important issues in preparing the precise time series of Sea Surface Height (SSH)...

The estimation of the wet refractivity indices is crucial for applications like weather predictions or improving the accuracy of real‐time positioning techniques. Traditionally, solving the inverse tomography problem to estimate these atmospheric parameters has been challenging due to its ill‐posed nature and high computational demands, necessitati...

Understanding mass (re‐)distribution within the Earth system, and addressing global challenges such as the impact of climate change on water resources requires global time‐variable terrestrial water storage (TWS) estimates along with reasonable uncertainty fields. The Gravity Recovery and Climate Experiment (GRACE) and GRACE‐FO satellite missions p...

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions have enabled consistent production of monthly gravity field solutions by international institutes, contributing to the International Centre for Global Earth Models. Each institute employs distinct processing strategies, yielding varied estimates of terrestri...

Data Assimilation (DA) of time-variable satellite gravity observations, e.g., from the Gravity Recovery and Climate Experiment (GRACE), GRACE-Follow On (GRACE-FO) and future gravity missions, can be applied to constrain the vertical sum of water storage simulations of Global Hydrological Models (GHMs). However, the state-of-the-art DA of these meas...

The existing water vapor present in the lower regions of the atmosphere plays a pivotal role in both weather forecasting and the propagation of signals in satellite-based observations. This parameter introduces a delay in GNSS observations, known as tropospheric wet delay. Accurately predicting the spatial distribution of this parameter can signifi...

Climate change triggers a wide range of hydrometeorological, glaciological, and geophysical processes that span across vast spatiotemporal scales. With the advances in technology and analytics, a multitude of remote sensing (RS), geodetic, and in situ instruments have been developed to effectively monitor and help comprehend Earth’s system, includi...

The global estimation of Thermospheric Neutral Density (TND) and electron density (Ne) on various altitudes are provided by upper atmosphere models, however, the quality of their forecasts needs to be improved. In this study, we present the impact of assimilating space‐based TNDs, measured along Low Earth Orbit (LEO) mission, into the NCAR Thermosp...

Satellite altimetry has enhanced the understanding of ocean dynamics through high-rate sampling and global coverage. However, there are some issues related to coastal areas due to land contamination and bad reflection effects in the altimeter and radiometer footprints. This study presents a geometrical method for retracking Jason-2 and Jason-3 alti...

Estimating global and multi-level Thermosphere Neutral Density (TND) is important for studying coupling processes within the upper atmosphere, and for applications like orbit prediction. Models are applied for predicting TND changes, however, their performance can be improved by accounting for the simplicity of model structure and the sampling limi...

The Gravity Recovery and Climate Experiment (GRACE, 2003‐2017) and its Follow‐On mission GRACE‐FO (2018‐now) provide global estimates of the vertically integrated Terrestrial Water Storage Changes (TWSC). Since 2015, the Soil Moisture Active Passive (SMAP) radiometer observes global L‐band brightness temperatures, which are sensitive to near‐surfac...

Estimating global and multi-level Thermosphere Neutral Density (TND) is important for studying coupling processes within the upper atmosphere, and for applications like orbit prediction. Available models fall short in predicting realistic TND changes due to the simplicity of model structure or sampling limitations. In this study, a simultaneous Cal...

An accurate estimation of ionospheric variables such as the total electron content (TEC) is important for many space weather, communication, and satellite geodetic applications. Empirical and physics-based models are often used to determine TEC in these applications. However, it is known that these models cannot reproduce all ionospheric variabilit...

A new special collection invites papers focusing on the processing, modeling, and analysis of all types of big datasets in the Earth and space sciences, including the influence of solar forcing on Earth's climate.

Constructing accurate models that provide information about water vapor content in the troposphere improves the reliability of numerical weather forecasts and the position accuracy of low-cost Global Navigation Satellite System (GNSS) receivers. However, developing models with high spatial-temporal resolution demands compact observational datasets...

In this report, the possibility of using the space-based along-track Thermospheric Neutral Density (TND) estimates for generating the European Space Agency (ESA)'s Level 3 (L3) global multi-level TND data products is assessed. For this, the TND estimates along the CHAMP, GRACE, and Swarm satellites are used as observation within the sequential Cali...

California's Central Valley, one of the most agriculturally productive regions, is also one of the most stressed aquifers in the world due to anthropogenic groundwater over-extraction primarily for irrigation. Groundwater depletion is further exacerbated by climate-driven droughts. Gravity Recovery and Climate Experiment (GRACE) satellite gravimetr...

Citation: Chen, W.; Shum, C.K.; Forootan, E.; Feng, W.; Zhong, M.; Jia, Y.; Li, W.; Guo, J.; Wang, C.; Li, Q.; et al. Understanding Water Level Changes in the Great Lakes by an ICA-Based Merging of Multi-Mission Altimetry Measurements. Remote Sens. 2022, 14, 5194. https://doi.

Hydrological droughts are events of prolonged water scarcity and cause many devastating impacts. It is, therefore, extremely crucial to understand their spatiotemporal evolution to guide prevention and mitigation policies. The Gravity Recovery and Climate Experiment (GRACE, April 2002-June 2017) and GRACE Follow-On (GRACE-FO, June 2018-present) mis...

An accurate estimation of the Thermospheric Neutral Density (TND) is important for predicting the orbit of satellites and objects, for example, those with the altitude of less than 1000 km. Models are often used to simulate TNDs but their accuracy is limited due to modelling restrictions and sensitivity to the calibration period. Satellite missions...

Under review at Science of Total Environment

This GIF illustrates the testing GRACE(-FO) TWSA fields, BCNN's reconstructions and predictive uncertainties (std)

Dynamical changes in the ionosphere and thermosphere during geomagnetic storm times can have a significant impact on our communication and navigation applications, as well as satellite orbit determination and prediction activities. Because of the complex electrodynamics coupling processes during storms, which cannot be fully described with the spar...

Gravity Recovery and Climate Experiment (GRACE) and its Follow-On mission (GRACE-FO) have become an indispensable tool in monitoring global mass variations. However, separating GRACE(-FO) signals into its individual Terrestrial Water Storage Changes (TWSC) and surface deformation contributors, i.e. Post-Glacial Rebound (PGR), is desirable for many...

A P-band SAR moisture estimation method is introduced for complex soil permittivityand penetration depth estimation using fully polarimetric P-band SAR signals. This methodcombines eigen- and model-based decomposition techniques for separation of the totalbackscattering signal into three scattering components (soil, dihedral, and volume). Theincorp...

During geomagnetic storms, rapid changes in the upper atmosphere can dramatically affect communication and navigation instruments. Therefore, it is necessary to monitor these events through a good representation of the thermosphere-ionosphere (TI) system. This can be achieved by assimilating data into physical models by means of an ensemble Kalman...

During geomagnetic storms, communication and navigation instruments can be dramatically affected by the rapid changes that occur in the upper atmosphere. The assimilation of data in physics-based models such as the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model through and ensemble Kalman filter, can improve the represen...

Ionospheric models are applied for computing the Total Electron Content (TEC) in ionosphere to reduce its effects on the Global Navigation Satellite System (GNSS)-based Standard Point Positioning (SPP) applications. However, the accuracy of these models is limited due to the simplified model structures and their dependency on the calibration period...

Complex dynamical changes in the ionosphere and thermosphere during geomagnetic storm times could have a significant impact on our communication and navigation applications, as well as satellite orbit determination and prediction activities. Because of the complex electrodynamics coupling processes during storms, which cannot be fully described wit...

Accelerometer-derived neutral mass density (NMD) is an important measurement of the variability in upper atmosphere and one of the widely used measurements to calibrate and validate models used for satellite orbit determination and prediction. Providing precise estimates of the true uncertainty of these NMD products is a challenging task but essent...

The monthly terrestrial water storage anomaly (TWSA) observations during the gap period between the Gravity Recovery and Climate Experiment (GRACE) satellite and its Follow-On (GRACE-FO) are missing, leading to discontinuity in the time series, and thus, impeding full utilization and analysis of the data. Despite previous efforts undertaken to tack...

The current state‐of‐the‐art of satellite gravity data processing makes use of de‐aliasing products to reduce high‐frequency mass anomalies. For example, the most recent official Atmosphere and Ocean De‐aliasing products (AOD1B‐RL06) are applied for the Gravity Recovery and Climate Experiment (GRACE) and GRACE‐Follow On (GRACE‐FO) missions. The tem...

The GNSS tomography provides the opportunity to estimate atmospheric wet refractivity, which is important for precise positioning and navigation, as well as for constraining weather and climate models. The GNSS-derived water vapor is often estimated by implementing voxel-based inversions, but this technique is numerically unstable due to the high n...

The response of the Ionosphere - Thermosphere (IT) system to severe storm conditions is of great importance to fully understand its coupling mechanisms. The challenge to represent the governing processes of the upper atmosphere depends, to a large extent, on an accurate representation of the true state of the IT system, that we obtain by assimilati...

The development of space-geodetic observation techniques has brought out a wide range of applications such as positioning and navigation, where the Global Navigation Satellite System (GNSS) is the main tools to provide surveying measurements in these applications. Though GNSS signals enable the calculation of receiver's position, some errors restri...

In this study, we present a data processing framework to apply measurements of the Global Navigation Satellite System (GNSS) technique for analyzing and predicting the movements of civil structures such as bridges. The proposed approach reduces the noise level of GNSS measurements using the Kalman Filter (KF) approach and enables the estimation of...

A new approach is presented to improve the spatial and temporal resolution of the Vertical Total Electron Content (VTEC) estimates for regional positioning applications. The proposed technique utilises a priori information from the Global Ionosphere Maps (GIMs) of the Center for Orbit Determination in Europe (CODE), provided in terms of Spherical H...

The Gravity Recovery and Climate Experiment (GRACE) mission ended its operation in October 2017 and the GRACE Follow-On mission was launched only in May 2018, leading to approximately one year of data gap. Given that GRACE-type observations are exclusively providing direct estimates of Total Water Storage Change (TWSC), it would be very important t...

We demonstrate a new approach to recover water mass changes from GRACE satellite data at a daily temporal resolution. Such a product can be beneficial in monitoring extreme weather events that last a few days and are missing by conventional monthly GRACE data. The determination of the distribution of these water mass sources over networks of juxtap...

Historically, hydrological models have been developed to represent land-atmosphere interactions by simulating water storage and water fluxes. These models, however, have their own unique characteristics (strength and weakness) in capturing different aspects of the water cycle, and their results are typically compared to or calibrated against in-sit...

Extracting the main cyclic fluctuations from sea level changes of the Persian Gulf and Oman Sea is vital for understanding the behavior of tides, as well as for isolating non-tidal impacts such those related to climate and changes in the ocean-sea circulations. This study compares two spectral analysis methods: Least Squares Spectral Analysis and L...

Global Terrestrial Water Storage from 2003 to 2018, on 1° grid and smoothed by a Gaussian 400km half-width filter.

Defining Essential Geodetic Variables (EGVs), as it is also done e.g. in ocean- and climate- sciences, can be extremely useful in representing the complex characteristics and processes of the Earth system and advancement of scientific research, sustainable stewardship and policy. So far, various geodetic variables are defined describing (changes in...

Quantifying spatial and temporal changes in thermospheric neutral density is important for various applications such as precise orbit determination, estimating mission lifetime and re-entry prediction of Earth orbiting objects. It is also crucial for analysis of possible collisions between active satellite missions and space debris. Empirical model...

https://meetingorganizer.copernicus.org/EGU2019/EGU2019-4385-2.pdf The interpretation of spatial and temporal changes in the Ionospheric Electron Density (IED) and the Thermospheric Neutral Density (TND) is important for various applications such as global communication, precise orbit determination (POD), estimating mission lifetimes or re-entry p...

Satellite Altimetry technique and Complex Independent Component Analysis (CICA, Forootan et al., Surveys in Geophysics 2018, https://link.springer.com/article/10.1007/s10712-017-9451-1) techniques are applied to monitor Great lakes in the North USA. The animation shows 14 days 0.05° maps, for which CICA is used to extend (model) the data spatially...

Droughts often evolve gradually and cover large areas, and therefore, affect many people and activities. This motivates developing techniques to integrate different satellite observations, to cover large areas, and understand spatial and temporal variability of droughts. In this study, we apply probabilistic techniques to generate satellite derived...

In order to reduce high frequency non-tidal mass changes, while inverting for the Earth’s time-variable gravity fields from the Gravity Recovery And Climate Experiment (GRACE) measurements, it is usual to apply the Atmospheric and Oceanic De-aliasing (AOD1B) products. However, limitations in these products count as a potential threat to the accurac...

With a growing number of available datasets especially from satellite remote sensing, there is a great opportunity to improve our knowledge of the state of the hydrological processes via data assimilation. Observations can be assimilated into numerical models using dynamics and data-driven approaches. The present study aims to assess these assimila...

GRACE temporal gravity field solutions have been widely used for hydrologic related research. Generally, two forms of mathematical representations of GRACE solutions, the spherical harmonic solution (Level 2 or L2 data products), and the constrained mascon solutions are more popular and their respective data products are publicly available. Spheric...

GRACE temporal gravity field solutions have been widely used for hydrologic related research, e.g. terrestrial water storage variation, ground water depletion, drought and flood. Generally, two forms of GRACE solutions, unconstrained spherical harmonic solution (L2) and constrained mascon solution, published by three official institutions JPL, CSR...

With the construction of the largest dam in Africa, the Grand Ethiopian Renaissance Dam (GERD) along the Blue Nile, the Nile is back in the news. This, combined with Bujagali Dam on the White Nile are expected to bring ramification to the downstream countries. A comprehensive analysis of the Nile's waters (surface, soil moisture and groundwater) is...

The standard ensemble data assimilation schemes often violate the dynamical balances of hydrological models, in particular, the fundamental water balance equation, which relates water storage and water flux changes. The present study aims at extending the recently introduced Weak Constrained Ensemble Kalman Filter (WCEnKF) to a more general framewo...

Data assimilation, which relies on explicit knowledge of dynamical models, is a well-known approach that addresses models' limitations due to various reasons, such as errors in input and forcing data sets. This approach, however, requires intensive computational efforts, especially for high-dimensional systems such as distributed hydrological model...

Hydrological models are necessary tools for simulating the water cycle and for understanding changes in water resources. To achieve realistic model simulation results, real-world observations are used to determine model parameters within a “calibration” procedure. Optimization techniques are usually applied in the model calibration step, which assu...

Ultra-sensitive space-borne accelerometers on board of low Earth orbit (LEO) satellites are used to measure non-gravitational forces acting on the surface of these satellites. These forces consist of the Earth radiation pressure, the solar radiation pressure and the atmospheric drag, where the first two are caused by the radiation emitted from the...

During the last few decades, decomposition techniques have found increasing application for dimension reduction as well as exploring information from geophysical time series. Traditionally, the principal component analysis (PCA)/ empirical orthogonal function (EOF) method, and more recently independent component analysis (ICA) has been applied to e...

Africa, a continent endowed with huge water resources that sustain its agricultural activities is increasingly coming under threat from impacts of climate extremes (droughts and floods), which puts the very precious water resource into jeopardy. Understanding the relationship between climate variability and water storage over the continent, therefo...

The computerized ionospheric tomography is a method for imaging the Earth’s ionosphere using a sounding technique and computing the slant total electron content (STEC) values from data of the global positioning system (GPS). The most common approach for ionospheric tomography is the voxel-based model, in which (1) the ionosphere is divided into vox...

Climate change can significantly influence terrestrial water changes around the world particularly in places that have been proven to be more vulnerable such as Bangladesh. In the past few decades, climate impacts, together with those of excessive human water use have changed the country's water availability structure. In this study, we use multi-m...

The arid Middle East is experiencing water scarcity due to shortages in rainfall, ongoing drought, overexploitation of surface water and groundwater, and economic development. Evaluating long-term trends of water resources and the interaction with climate forcing and human intervention is crucial for future water resource management. In this contex...

We propose the globalization of the continent-scale Kalman Filtering (KF) previously developed by Ramillien et al. (2015) to produce time series of daily maps of surface mass variations by progressive integration of daily geopoten-tial variations measured by orbiting satellites. These geopotential variations can be determined from very accurate int...

Traditionally, data assimilation is a well-known Bayesian approach for updating model simulations with new incoming observations. This integration is necessary due to an uncertainty in model outputs, which is the result of several drawbacks, e.g., limitations in accounting for all complex water cycle processes, uncertainties of (unknown) empirical...

Identity gravity change signals caused by 2008 South China snow/ice storm and 2010 North America snow storm from GRACE solutions.

For Brazil, a country frequented by droughts and whose rural inhabitants largely depend on groundwater, reliance on isotope for its monitoring, though accurate, is expensive and limited in spatial coverage. We exploit total water storage (TWS) derived from Gravity Recovery and Climate Experiment (GRACE) satellites to analyse spatial-temporal ground...

The Gravity Recovery And Climate Experiment (GRACE) satellite mission provides time-variable gravity fields that are commonly used to study regional and global terrestrial total water storage (TWS) changes. These estimates are superimposed by different error sources such as the north–south stripes in the spatial domain and spectral/spatial leakage...

Assimilating Gravity Recovery And Climate Experiment (GRACE) data into land hydrological models provides a valuable opportunity to improve the models’ forecasts and increases our knowledge of terrestrial water storages (TWS). The assimilation, however, may harm the consistency between hydrological water fluxes, namely precipitation, evaporation, di...

Assimilation of terrestrial water storage (TWS) information from the Gravity Recovery And Climate Experiment (GRACE) satellite mission can provide significant improvements in hydrological modeling. However, the rather coarse spatial resolution of GRACE TWS and its spatially correlated errors pose considerable challenges for achieving realistic assi...

The time-variable terrestrial water storage (TWS) products from the Gravity Recovery And Climate Experiment (GRACE) have been increasingly used in recent years to improve the simulation of hydrological models by applying data assimilation techniques. In this study, for the first time, we assess the performance of the most popular data assimilation...

Monitoring surface soil moisture (SSM) variability is essential for understanding hydrological processes, vegetation growth, and interactions between land and atmosphere. Due to sparse distribution of in-situ soil moisture networks, over the last two decades, several active and passive radar satellite missions have been launched to provide informat...

We present a state-of-the-art approach of passive-ocean Modified Radial Basis Functions (MRBFs) that improves the recovery of time-variable gravity fields from GRACE. As is well known, spherical harmonics (SHs), which are commonly used to recover gravity fields, are orthogonal basis functions with global coverage. However, the chosen SH truncation...