What to believe? Models or GRACE data?

From the UNIVERSITY OF TEXAS AT AUSTIN and the “not so amazing GRACE” department.

Discrepancies between satellite and global model estimates of land water storage

Research led by The University of Texas at Austin has found that calculations of water storage in many river basins from commonly used global computer models differ markedly from independent storage estimates from GRACE satellites.

The findings, published in the Proceedings of the National Academy of Sciences on Jan. 22, raise questions about global models that have been used in recent years to help assess water resources and potentially influence management decisions.

The study used measurements from GRACE satellites from 2002 to 2014 to determine water storage changes in 186 river basins around the world and compared the results with simulations made by seven commonly used models.

These are mouths of the Amazon River in Brazil. CREDIT NASA

The GRACE satellites, operated by NASA and the German Aerospace Center, measure changes in the force of gravity across the Earth, a value influenced by changes in water storage in an area. The computer models used by government agencies and universities were developed to assess historical and/or scenario-based fluxes in the hydrological cycle, such as stream flow, evapotranspiration and storage changes, including soil moisture and groundwater.

“People are depending more and more on global models to determine projections of the impacts of human water use and climate on water resources,” said lead author Bridget Scanlon, a senior research scientist at the university’s Bureau of Economic Geology. “We are now able to evaluate water storage changes from models with GRACE data, which suggests that the models may underestimate large water storage changes, both large declining and rising storage trends.”

For example, in the Amazon River basin, GRACE data indicate that water storage increased by 41 to 43 cubic kilometers during the study period — the largest increase in water storage of any basin in the world. But most of the models projected huge declines in water storage, with one simulating a loss of 70 cubic kilometers. The model that most closely matched the GRACE data calculated an increase of only 11 cubic kilometers.

In the Ganges River basin, GRACE showed a loss of 12 to 17 cubic kilometers of water per year over the 12-year period — the biggest decrease in water storage measured by the study. The models range between a loss of 7 cubic kilometers and an increase of 7 cubic kilometers.

Overall, the model results calculated a decline in global water storage during the study period, while GRACE data indicate it was on the rise. However, the study notes that while the climate increased water storage globally, humans caused significant declines in certain regions. The study area covered about 63 percent of global land area and excluded Greenland and Antarctica because most of the water in those areas is trapped in glaciers or ice sheets.

The study shows that there are regions where global models need to be improved, said co-author Hannes Müller Schmied, a senior research scientist at Goethe University Frankfurt. The global perspective of the research can help scientists understand why models are performing better in some areas than others, and zero-in on the areas where they’re not matching up with the data.

“Based on the comprehensive assessment of water storage changes, we have important additional information and can now focus on challenging regions and analyze more in-depth which processes need to be included, for example floodplain dynamics and backwater effects in the Amazon, or to be improved upon, like human water extractions in the Ganges River basin,” Müller Schmied said.

The study also notes that scientists should work on improving regional assessments.

“GRACE is great because it highlights the global picture of what’s happening with global water storage, and at a coarse grid-scale it’s really nice to see what’s happening,” Scanlon said. “But ensuring water availability for human consumption and agriculture is in many cases a regional to local issue, and we should put increased emphasis on analyses at this scale by, for example, integrating local data. The specific situation could be much better investigated than with global-scale studies only.”



102 thoughts on “What to believe? Models or GRACE data?

  1. Well I’m sure the emerging “Water Crisis” industry approves/believes the GIGO models over any real data.

    • Generally, magma movements are slow and usually quite localized. Annual volcanic eruptions total 4 cubic km, 3 of which are from the Mid Atlantic Ridge. Also, what comes up must go down. My daughter (Post Doc at Jep Propulsion Lab at the time) and colleague have a publication in “Science” a few years ago of surface uplift in the mid Andes Altoplano measured in 3D by paired Euro satellites (~10mm a year) surrounded by a ring of subsidence. The cute title of the paper gives an image of what is happening:


      Moreover, magmas from crustal melting are lower density than the source rocks but greater volume. This subdues gravitational variation. Of course, large basalt trap extrusions redistribute matter broadly.

      • This particular study covers a period of 12 years.
        Magma wouldn’t have moved much, but then it doesn’t need to in order to make a measurable difference.

    • Agreed: when that magma shifts as the Earth’s crust slowly rises or sinks, due to changes in the loads upon it, if affects the Earth’s gravity field just like ice accumulating or diminishing does.. GRACE can’t tell the difference.

      For instance, it is commonly claimed by climate alarmism propagandists, like JPL’s “vital signs” site by their “Earth Science Communications Tream,” that, based on GRACE measurements, it has been determined that Antarctica is losing ice (albeit at a glacially slow rate, though of course they don’t describe it that way). However, based on ICESat and ERS Zwally et al found that Antarctica is gaining ice (also albeit at a glacially show rate).

      I think that in both cases the model-derived Glacial Isostatic Adjustment (GIA) figures are a critically important part of the “measurements”[sic]. In fact, I’ve read somewhere (maybe the late, great Michael Crichton?) that ALL of the Antarctic ice loss in the GRACE studies is actually due to subtracting off GIA/PGR estimates. (Can anyone here confirm or refute that?)

      • Oops, that’s what happens when I start to write a comment, then go away, and come back later to finish it: I end up asking questions that someone else has already answered: in this case, tty and Jim Steele. I should have read the rest of the comments before clicking “Post Comment.”

        Sorry for the dumb comment, and thank you for the info, tty & Jim.

  2. GRACE measurements are quite precise though geographically “smeared out”, and in these cases interpretation is straightforward (in contrast to ice-caps) so it is pretty clear that the models are faulty.

  3. So, um, the models give a rough idea of what Mr Hyde looks like while GRACE gives a photo of Dr, Jekyl(?)

  4. Neither. I will believe what I see/experience in the REAL WORLD. Liars lie, liars payed by governments spew the lies they are ordered to spew.

  5. I don’t believe a single data-point that comes out of Grace.

    All of it needs to be processed and adjusted to arrive at whatever signal the operators want it to show. There is a huge amount of money being spent for this kind of result. In other words, every agency needs to stop wasting so much taxpayers money on useless algorithms.

    • Do you believe the models better?

      The more different sources of data we have, the harder it is for the corrupt to get away with cooking the data. It’s a corollary to:

      You can fool all the people some of the time and some of the people all the time, but you cannot fool all the people all the time. link

      • Not believing GRACE is not evidence that you believe the models. It’s equally likely that both are wrong.

      • As I understand GRACE, you are at one end of the auditorium holding the end of a string, and a guy way across the room holds the other end. Then a guy in the middle reaches up and pushes the string sideways an inch or two, and you’re supposed to figure out how much he pushed it sideways by how much your end of the string moved. Count me skeptical.

      • Mike McMillan January 26, 2018 at 10:52 pm

        … and you’re supposed to figure out how much he pushed it sideways by how much your end of the string moved.

        If it’s just guys holding a string, you’re right. If we take one end as fixed, substitute the string with wire whose characteristics are well known, and pull on the other end with a known force (ie. a spring), we have an experiment that could be successfully done in a high school Physics class.

        For a long time, prospectors and geologists and archaeologists have been measuring gravity to find structures underground. link link The technique is well known and reliable.

        The quibble should not be about measuring gravity. The quibble should be about the reliability of gravity measurements in assessing how much water is stored in an area.

    • I would not go that far, but the fact is that Grace measures gravity in a very indirect way. The processing of results is extremely complicated and therefore error-prone.

      • The gravity measurements in themselves aren’t particularly “indirect”. They are based on measuring the difference between the two satellites by interferometry which is an extremely precise method.

        It however does not give any information why the gravity has changed. However it this particular case most of the confounding factors mentioned above do not apply. Wind does not affect groundwater. River basins with deep sedimentary layers aren’t volcanic and there is no magma close to the surface.

        That there are no major problems with the processing algorithms is shown by the close match between GRACE results and ground-based gravity measurements.

      • Best thing Grace did…was find sea floor volcanoes…..but the idiots running it said that was a constant hump of sea level

      • One problem is that gravity doesn’t go just straight up. It propagates in all directions simultaneously.
        You have to puzzle out gravity changes directly below the satellite vs gravity changes miles away from that point.
        Beyond that, there are a number of things that influence gravity. Wind driven movement of water, magma moving around in the mantle. Floods moving soil downstream and depositing it at river mouths. Man made earthworks, etc.
        I am not convinced that those who run GRACE understand all these other factors, much less have accurately accounted for them.

      • The area in question doesn’t have to be directly on the coast, mass changes hundreds of miles away can impact the altitude of a satellite.
        The area doesn’t have to be volcanic, since magma in the mantle is known to be in motion.

      • “One problem is that gravity doesn’t go just straight up. It propagates in all directions simultaneously.”

        Which is true for all gravity measurements.

        “The area doesn’t have to be volcanic, since magma in the mantle is known to be in motion.”

        Certainly. And how fast does it move? A centimeter or two per year.

      • It is true for all gravity measurements, but the closer to the ground you are the easier it is to ignore changes that occur miles away.

        A few centimeters a year, and we are talking a dozen years and a claimed increase or decrease of a few centimeters of water over that period.

    • I believe lots of data and models. Generally, where people do not have skin in the game and are providing it because their agency is seeking factual truth.

      Let’s say something like the initial jobless unemployment claims from the Department of Labor. They have been doing it since 1967 every week and have proven credibility over time. They have processes designed to ensure integrity and the nerds producing the data just provide the factual data and don’t care what the data says.

      Climate research, not so much. They get fired if they don’t produce the right results. The data nerds at the Department of Labor get promoted if they keep producing the nerdy factual data. Completely opposite from climate science and Grace scientists.

      That is where people need to draw the line in our new fake news world.

      • Bill, are you referring to U1, U2, U3, U4, U5 or U6. Which one is most representative of the state of the economy?

      • I have not read up on this recently, but as I recall, there have been all kinds of changes to the way they count the unemployed over the years and the politicians of both parties have tweaked it to make it look better so they don’t get hammered in elections. As others have pointed out, it is seasonally adjusted and has other adjustments as well and there are multiple definitions of unemployment with the various U-numbers. And once someone has stopped looking for a certain period of time, they no longer count them as unemployed, even though they are capable and willing to work and under 65.

    • commieBob, the geologists etc are measuring gravity changes from just a few feet above the ground.
      Way different from trying the same measurements from orbit.
      The geologists etc. are taking a single snapshot of gravity variations over a small area over a couple of hours. None of the complications that myself and others have brought up apply is such a situation.

      • Way different from trying the same measurements from orbit.

        And yet satellite gravity data is useful to some folks. link link link

        It boggles my mind that some folks are resistant to the idea that the technique could even work at all. It works folks, along with other remote sensing techniques, satellite gravity measurement has become a standard tool in mineral exploration. case study

    • If Grace were a total bust, the people who conceived it would still save face by putting out confident products. The CO2 satellite was so good, they rarely want to put out information because it confounds theory. Similarly, the shock discovery of 14% increased forest cover they let slip out to much consternation by the Gang Green who began putting out how CO2 was going to destroy forests and crops. Heard much from Nasa lately on this…er..no.

      Re Grace, I can see calculations that must be made to sensibly get something from it, though. When ice accumulates, it also pushes the substrate down so the gravitational changes will continue to occur over thousands of years. Your earliest measurement for accumulated ice and snow would be the best. With water, you have more data to work with. Annual rainfall, evapotranspiration data, river discharge and their differences annually and how they effect Grace measurements should give you decent estimates to calculate volume and Grace will give the distribution of these volumes. To Bill Illis’s point, there was a big concern expressed about Grace’s performance re ice but I haven’t (naturally) heard anything since. Science these days is more prone to restrict information that they consider “problematical” to the plan.

  6. I am SHOCKED by this information. Please be aware that I was given no prior warning whatsoever that this research was going to be presented in this manner and before I was able to move to a safe space away from this blog where I could be shielded from the apparent failure of these models. Surely the GRACE data should have been no-platformed by the owner of this blog. I have serious mental and emotional trauma. Please send money.

    Ps: /sarc off

  7. My models model better than your models, and I’ve got models that (after adjustments) show it, so keep funding ME.

      • Ummmm…my model’s big red button is bigger and redder than yours…and it gets bigger, er…badder…results…
        Is that what you were going to say?
        I can’t stop laughing. :-)
        Childish, I know…but people in far more public spots get away amusing themselves much the same, so humor us.

  8. Am I to understand Grace can interpret the relative wetness or dryness of underground water basins.

    • Ryan GRACE’s assessments are theoretically sound. A full aquifer contains more water thus more mass which exerts a greater gravitational pull, compared to a dry aquifer. GRACE does detect seasonal changes in the water cycle.

      However because the earth’s mantle is also flowing, GRACE can not distinguish between an increased gravitational pull due to a fluid mantle or due to changes in water storage.

      The weight of the glaciers displaced the mantle analogous to squeezing a water balloon and causing the water to bulge out at either end, except where you squeeze. When you stop squeezing the ballon returns to is original shape. However to what degree this Glacial Isostatic Adjustments (GIA) are happening at the earth’s surface depends on 1) modeled weight and areal estimates of the earth’s ice history and 2) models of the earth’s viscoelasticity which is estimated largely by modeling how the earth’s surface has been rebounding, so there can be a bit of circular reasoning. Unfortunately those models have been ever changing.

      Furthermore GRACE’s resolution is poor, so various algorithms and masking schemes are developed to determine where exactly any given anomalous change is taking place.

  9. GRACE has great potential but the problem is the Glacial Isostatic Adjustments. Between 2003 and 2008 the processed data suggested no increases in ocean mass. So different sea level researchers added 1 to 2 mm/year worth of adjustments to argue for increasing water mass, with each researchers choosing something different to better “close the budget”!

    • “So different sea level researchers added 1 to 2 mm/year worth of adjustments”
      This is not a sea level study. It is studying the mass of water in land aquifers. There is no level involved, so I can’t see the role of a GIA, or even how it could be used.

      I couldn’t find a copy of this paper, but I did find a similar study by the same authors here. There is no mention of a GIA.

      • Jim is talking about GRACE methodologies and the interesting relationship to sea level calculation. No-one is claiming that this paper is about sea level measurement.

        I was struck by the comment “….The global perspective of the research can help scientists understand why models are performing better in some areas than others..”

        The default answer to that would be ‘co-incidence’

      • Nick, Such an odd comment. If you doubt my word simply ask for a citation and I’d gladly give it to you. To say you did a search and found no mention of GIA is rather ridiculously meaningless.

        From Cazenave (2008) Sea level budget over 2003–2008: A reevaluation from GRACE space gravimetry,
        satellite altimetry and Argo

        Here is their Figure 1

        “The raw GRACE-based ocean mass time series is dominated by an annual cycle caused by the annual exchange of water between land and oceans (Cazenave et al., 2000). Aswe are interested here in the interannual fluctuations,we remove the annual cycle. The resulting time series, shown in Fig.1, has a slightly negative slope of∼−0.12+/−0.06mm/yrover the time span January 2003–December 2007 (we consider this time span – called 2003–2008 – to work with an integer number of years). However, a GIA correction has to be applied to this raw ocean mass time series. In effect, GIA causes a secular change in the mean oceanic geoid that needs to be removed from the GRACE-based raw ocean mass time series to obtain the real water mass change of the oceans. This linear correction is quite large and available from GIA modelling only. It
        varies from ∼1 mm/yr to 2 mm/yr (in ESL unit), depending on modelling assumptions”

        “Willis et al. (2008) used a value closer to 1 mm/yr. Recently Peltier (submitted for publication) reevaluated, under various modelling assumptions, the GIA corrections that need to be applied to satellite data (satellite altimetry and GRACE) when determining global mean sea level rise and ocean mass change. He shows that Earth rotation effects have strong influence on the ocean mass GIA correction and recommends to use an ocean mass GIA correction of ∼2 mm/yr that accounts for the rotational effects. Here we use this value.”

      • Furthermore Nick you need to get up to speed, There are always GIA adjustments to all GRACE data simply because GRACE can not differentiate the causes of mass change in any given vertical profile whether over land, over sea or over ice. The GIA adjustments are model dependent

        From Xiao (2015) Monitoring Groundwater Variations from Satellite Gravimetry and Hydrological Models: A Comparison with in-situ Measurements in the Mid-Atlantic Region of the United States

        “we use the results calculated by Swenson et al. [35,36] which had been proven to improve estimates of mass variability from GRACE. Glacial isostatic adjustment (GIA) is corrected based on the model of Paulson et al. [37].”

      • “Nick, Such an odd comment.”
        Jim, I don’t think you read it. I’ll say it again
        “This is not a sea level study. It is studying the mass of water in land aquifers. There is no level involved”
        So you cite Cazenave on sea level budget? GIA is for relating a sea level to volume, when the boundaries of the volume may change. This is a study about mass of water in an aquifer. The title is
        “Discrepancies between satellite and global model estimates of land water storage”

        Look at the image posted. There hasn’t been any glaciation around the Amazon. But even if there had, there still isn’t any level being observed that would need correction.

    • GIA is not a big problem in these cases. It is very slight and changes very slowly far from the former ice-covered areas. On the other hand the uncertainty due to GIA means that GRACE data can’t really tell whether the amount of ice in Antarctica is increasing or decreasing. That it is decreasing (very slightly) in Greenland seems secure.

      • As a real “Climate Scientist” Nick Stokes doesn’t see any necessity to actually read a post to authoritatively rubbish it.

  10. GRACE needs GIA corrections. Therein lies an Antarctic problem. But where those are not necessary, GRACE puts the lie to much. See essay PseudoPrecision in ebook Blowing Smoke dor a couple of hilarious examples.

  11. “The global perspective of the research can help scientists understand why models are performing better in some areas than others, and zero-in on the areas where they’re not matching up with the data.”

    The first theory which should be put forth is random chance.

  12. A USGS colleague of mine who conducted dozens of gravity surveys across the western US gave me the best advice about interpreting gravity data/anomalies.

    The measured data (strength of gravitational attraction) are insensitive mass properties, subject to multiple interpretations based on distribution of density with depth and spatial distribution. The interpretation of the data is an exercise in zero-sum arithmetic – many models of depth/density distributions will calculate to the same result of “measured gravitational attraction”. There is no unique solution to the interpretation of the data.

    GRACE is the perfect device for measuring “data” that can be modeled/interpreted in any way desired.

    Perfect black box.

    Perfect scam generator.

    GRACE is like using a photographer’s light-meter to locate “bright spot” sources of illumination in a wide field of view. B.S.

    • Your colleague is right for static interpretations, but it is a reasonable assumption that “the distribution of density with depth and spatial distribution” doesn’t change much over time, so for water catchment changes a GRACE assessment may be quite appropriate.

  13. If you have not done so already , read Useless Arithmetic, in which Roger Pielke, Sr and his co-author look at the problems associated with Government-approved models.

  14. The world’s oceans cover some 70% of Earth’s surface and have an average depth of something like 4-5 km.. That’s a helluva lot of water: salty to be sure but humans are already using desalination technology. Any perceived problems stem from a chronic lack of imagination and an asinine desire to mandate the exclusive use of only the most expensive technologies for generating electricity.

  15. “…The study shows that there are regions where global models need to be improved, said co-author Hannes Müller Schmied…”

    Well EVERY study shows that there are regions where global models need to be improved.

    So much stock is put into models that can’t really even get global temps right. On a regional scale, they are a complete and utter failure.

  16. Well .. that is the question I am asking too. Why are temperatures not lower with clouds (but rather higher), if clouds would have a net cooling effect (which is essential to the GHE theory)? Empiric data suggest the opposite.

    (based on about 100 mio records 2015-2017 US)

    The data are yet unadjusted for rain chill and other co-factors, which would make clouds look even warmer.


    I am curious .. has never anyone done that before? I mean looking at the cloud/temperature relation?

    • Different latitudes have different temperature and different more persistent cloud types so you “empiric data” averages aren’t showing you anything. A weather station with a thermometer and a pyranometer will show a strong relationship. Everyone recognizes the temperature drop when a cloud goes over.

      • “Everyone recognizes the temperature drop when a cloud goes over.” Great logic!

        I have news for you: next to the day we have something called night..

      • Your Problem is that you are measuring Air Temperature, most of the Solar Radiation goes in to the Oceans, (ie 70% of the Surface) when the Clouds block the much more energetic Solar Radiation it is preventing the heat from being stored. Whereas at night the clouds slow down the loss of heat from less energetic LWIR.
        The overall affect of H2O in the Atmosphere is to control the swings in Diurnal Temperatures.
        Try your analysis in Dry Desert Areas as well for comparison.

        As to has anyone ever done it before, yes they have.

      • @tty & Osborn If had read the paper you would know better. If have also analyzed a small sample of 26 stations all located in the tropics. Also all of them a have a maritime climate. And what the sample shows is, that clouds have even more of a warming effect there. Sure I would love to have more raw data on this, suggestions will be welcome.

        Condition Temp. C
        CLR 24.79
        FEW 25.51
        SCT 25.93
        BKN 25.97
        OVC 24.96
        (tropic subset, unadjusted for diurnal cloud trends)

        But please stop arguing like “on the one hand, but on the other hand..”. I f**** know there is day AND night. Yet that is not an insurmountable problem, in fact it is no issue at all. Long term averages are long term averages.
        Btw. I have looked into the specific patterns of night time cooling before, which is interesting enough.

  17. LOL….GRACE data is good when it suits the purpose of the person citing it (dissing models.) However whenn GRACE is used to infer Greenland and Antarctica ice mass, these same people claim the data is bad.

      • Mike Borgelt: I suppose that you are aware that the “raw” GRACE data for Antarctic ice mass change are not significantly different from zero?

        Which means that the much-touted “decrease” is 100% due to GIA adjustments. And none of those GIA models comes even close to matching actual measurements. Take a look at figure 5 in this paper:


      • If you have read my responses above, you would already know the answer to that.
        That you have not read them is not surprising. That’s what trolls do.

    • GRACE measures gravity fields..

      Both Greenland and the Antarctic are over known active volcanic regions.

      I think even you can figure it out !!

    • MB, please go read steve McIntyre’s detailed critique of Grace ice mass in Antarctica. Modeled GIA then after 2013 diff GPS measured GIA. Then get back.

      • No. IceSat measured actual altitude changes of the ice-surface which is much less affected by GIA. The result there was that the ice in Antarctica was growing. There are uncertainties there too however, for example the density of snow probably varies slightly depending on average wind.

        The fact is that the changes in Antarctica are so small and slow that it is virtually impossble to measure them.

  18. Stupid question. Wouldn’t an increase in biomass look exactly like an increase in water retention to a satellite measuring gravitational variances?

    • Yes. But the more precise GRACE measurements showed – surprise – that existing tidal models had significant errors in shallow waters.

  19. As somebody once said, an unvalidated model is nothing more than an illustration of someone’s hypothesis.

    • Good memory, Orson. That’s one of the (many) advantages of tide gauges over satellite altimetry for measuring sea-level: tide gauges are referenced to geodetic reference markers, and satellites aren’t.

      That’s obviously because there are no geodetic reference markers in space. To address that problem, in 2011 NASA proposed (and re-proposed in 2014 / 2015) a new satellite mission called the Geodetic Reference Antenna in SPace (GRASP).

      In addition to the WUWT article you mentioned, it was discussed on WUWT here.

      The Europeans also proposed a similar mission (E-GRASP).

      But neither mission has flown, so getting the precise satellite positioning necessary for accurate satellite altimetry is problematic.

      BTW, I don’t understand how the GRASP & E-GRASP missions would work. Does someone here know: would they really solve the problem, and enable us to know the precise positions of other satellites?

  20. Modern mass transfers in the mantle are not accounted for by GRACE!

    Until such can be done, any suggestions about “terrestrial water storage change” can be regarded as bullshit.

    A major “design” flaw of GRACE, remember that “E” in GRACE stands for Experiment, is that the orbits of A/B are forced to Earth center of figure! Earth, and Moon, orbit the barycenter that is located a few thousands of km in the mantle of Earth! Ah! Plate Tectonics!

  21. As of 2016 instrumentation errors were unresolved. As the instruments and batteries aged instrumentation errors increased forcing reliance on models to reduce the errors.

    “Although the accuracy of gravity field solutions has increased considerably during the last years, there still remains an offset in the order of one magnitude between the error level of present gravity field solutions and the GRACE baseline accuracy, i.e. the predicted accuracy from pre-launch simulations (Kim and Tapley, 2002). Therefore, even after more than 14 years of mission operation, efforts are ongoing to identify the remaining error sources. The accuracy of gravity field solutions is not yet only limited by aliasing effects (due to under-sampling in space and time) and imperfections of the background models, but also by errors within the instrument data. This implies that improvements within the data processing and sensor calibration modeling can contribute to the overall accuracy of gravity field solutions. Although many investigations have been carried out (Bandikova and Flury, 2014; Peterseim et al., 2012; Harvey, 2016), not all error sources and perturbations acting upon satellite instruments and sensors are resolved yet.

    Both satellites are equipped with space accelerometers measuring the non-gravitational accelerations acting on the spacecraft (Touboul et al., 1999). These forces are due to atmospheric drag, as well as solar and Earth radiation pressure. The main observables for the gravity field recovery by GRACE are the inter-satellite ranging measurements, carried out by the onboard K-Band microwave ranging system (KBR) with micrometer precision (Kim, 2000). For the purpose of gravity field recovery, the effects induced by the gravitational and non-gravitational forces have to be separated. For this reason, the accelerometer measurements are essential, as they provide information about the non-gravitational forces acting on the satellites. However, the accelerometer measurements cannot be used directly and have to be calibrated, as they are affected by an instrument bias and scale.

    With increasing lifetime, GRACE is facing additional challenges due to the degraded battery capacity that limits the availability of power-supply in certain orbital configurations (Herman et al., 2012). To extend the batteries’ lifetime the onboard instruments are shut down for approximately 40–50 days during each 161-day
    cycle (since April 2011), i.e. data are not collected during these time periods (Tapley et al., 2015). The angle represents the angle between the orbital plane and the Earth-Sun line, hence it varies between . Every 161 days, at crossings, the Sun is in the orbital plane of the satellites. During this period, the spacecraft spend nearly half their revolution time in the Earth’s shadow, and thus rely on their batteries for power. As a consequence, the GRACE instruments are shut down for small angles. In between, when the satellite is in full-sun orbit (

    ), the Sun is visible to the satellite at all time and primarily illuminates the side panels (Herman et al., 2012).

    As a result of reduced battery capacity, active thermal control was switched off in April 2011 (Tapley et al., 2015). Since then, temperature variations directly affect the onboard instruments. To avoid a degradation of the monthly gravity field solutions, the effects of thermal variations on the accelerometer measurements have to be modeled during gravity field recovery.

    As the results of the gravity field recovery also depend on the quality of the accelerometer data, the recovered solutions might reveal the quality of accelerometer data calibration to a certain extent. The calibration of the accelerometer data is a rather difficult task, as effects from the onboard environment on the sensor have to be considered.

    Over the years, several approaches for accelerometer calibration using derived accelerations from Precise Orbit Determination (POD) have been implemented (e.g. Bezdek, 2010). During gravity field recovery, however, different parameterizations for bias and scale parameters and/or empirical parameters are used by the various analysis centers (e.g. Bettadpur, 2012; Dahle et al., 2013). The accelerometer calibration parameters can vary a lot depending on the calibration method and the context of data usage.”


    The GRACE Follow On mission scheduled for an 18 March launch date includes the $300 million dollar satellites. The original GRACE satellite program was $127 million.


    • Rather odd isn’t it that these (relatively) simple gravity measurements have trouble interpreting data and calibrating instruments while the vastly more difficult problem of measuring sea-level changes to 0.1 mm can apparently be solved. Particularly as GRACE among other things has shown that tidal models are off by up to 20 centimeters on continental shelves.

  22. computer models predict what the model builders subconsciously believe to be true. otherwise the builders would judge the models to be in error and change them.

    in any system where there is more than one possible outcome mother nature is free to chose and no computer yet built can predict the actual outcome.

    even a simple system like a coin toss is beyond the ability of the most powerful computer to reliably predict the next toss.

  23. Same theme again,the measurement method is fuzzy,imprecise, we NEED BETTER instruments.Some wonderful speculation can be inferred, but honestly cannot say much using tools we have.Must build better instruments.
    Being the conclusion of the scientists and engineers involved.

    Political decision,money best spent elsewhere.
    Same politicians insist Climate Change TM is priority one, direct threat to civilization.
    Frustrated government scientists are pressured to draw policy conclusions based on insufficient data.
    Policy based evidence manufacturing results.

  24. Wow! I just cannot fathom this piece of dribble! Comparing a satellite reporting system, with 7 models ( and we know how accurate climate models are!), with no actual comparison to real data? What? Are these people stupid or something? Unless you can back up your findings with falsifiable data, you are just pi$$ing in the wind! Unless I’m missing something about the supposed accuracy of GRACE…………………………!

  25. The ability of GRACE to detect a modest gravity anomaly such as is posited for water storage may be checked by at least two ways. Nether is expensive or requires any additional data collection.

    As an afterthought a third option is suggested. It involves fielding a gravimeter, which can be expensive unless slave labor willingly supplied by starving grad students is used.

    First, examine GRACE passes over the Bay of Fundy at low tide and high tide and compare the profiles. Because of different orbital passes they are probably not exact duplicates, so one is forced to comparing profiles extracted from fields. DamnifIknow what a GRACE data ‘point’ looks like. Hopefully somewhere in there is some data that might be used to see if GRACE can detect a the change in a body of water that is more or less comparable with the gravity anomaly expected from the stored water in an aquifer. If a difference between the low tide and high tide observations can be seen then there might be hope. If one could take other observations and successfully detect other high and low tide times then there are grounds for even more hope.

    The second way is simple. It can be done by anyone with access to modest PC based 2.5 D gravity modeling software. Determine the gravitational effect of a prism of water 100 meters thick, 5 km long and 2 km wide. Add that to a regional gravity profile as observed by GRACE somewhere along the Gulf Coast away from any river draining into the GOM. Calculate profiles of the ‘observed’ field with and without the water prism at increasing heights above the model. Intuition suggests the observed differences will fade into the noise level well below GRACE’s orbital height. However, I am quite willing to be surprised at the answers.both

    As I am tapping away here it occurs to me that the UT Geophysics department could unearth one of their Worden gravimeters and head to the field with some fancy new GPS positioning equipment over the aquifer. Very precise location information, especially elevation, is needed to make the corrections necessary to determine the actual gravity value at each station’s location. A series of profiles or better yet a grid of data should be collected during the dry season and the wet season. If one really wanted to tip the budget a series of holes could be drilled and logged with a bore hole gravimeter. After the observations are reduced one could see if any changes are observable in a uncharged vs fully charged aquifer. Intuition and some experience with gravity data collection, reduction, and modelling suggest the effect, if observable at all, will be quite small.

    But doing that would mean leaving the comfort of the air condition offices and doing real boots in the mud, feeding the bugs and being very mindful of the cotton mouths field work. I love watching Ph.Ds ankle deep in swampy waters reading instruction manuals while swatting mosquitos.

    Not quite sure those Teasips are up for that.

    Mosquito bites to itch, real data to reformat and reduce…. Meaningful measurements of something we think we might be able to see. That’s how science moves forward.

    Naw, lets just stay in the office and model the number trail GRACE gives us and call it good. Oh, and write up another grant proposal real quick now.

    Finally, I remember something called the Eotvos effect as being a real bear to deal with while reducing gravity data collected from a moving object, like a ship or airplane. Would someone who knows more about these things please chime in?

    Thanks, Tom Bakewell, sorta-retired geophysicist.

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