Dear Jonathan:
I asked our remote sensing algorithm scientist to answer your question. This is what he came up with. Note that in addition to the error estimate value, which is the example I have provided, there is also a wind vector quality indicator, which is a unitless value that can range from 0 to 100, that is also discussed in his response immediately below. The error estimate and quality indcator can be used to identify a subset of high-quality DMW depending on the sensitivity of the user application (e.g., assimilation into numerical weather prediction models). Public reference: http://www.star.nesdis.noaa.gov/goesr/product_winds_dmw.php "Quality control of the retrieved DMWs is performed in two ways. The first is through the application of target selection, feature tracking, and height assignment error checks as described in the previous sections. The second way involves the calculation of two quality indicators for each of the DMWs using two different, but related, algorithms: the Quality Indicator (QI) (Holmlund, 1998; Holmlund et al., 2001) and the Expected Error (EE) (LeMarshall et al., 2004; Berger et al. 2008) " "The statistically-based quality indicator (QI) developed at EUMETSAT estimates the reliability of each Derived Motion Winds (DMW) based on several quality control tests (Holmlund, 1998, Holmlund et. al 2001). These tests not only analyze the consistency in space and time of each of the intermediate DMW vector components, but also the height and temperature of the tracers used in the vector determination, the symmetry of vector pairs achieved from tracking tracers between consecutive images, differences with surrounding vectors, and differences from a forecast field (optional). " "The Expected Error (EE) algorithm, originally developed at the Australian Bureau of Meteorology (LeMarshall et al, 2004) is an extension of the QI algorithm described in the previous section. It is designed to express quality in terms of a physical vector error metric (meters/second, m/s), rather than a normalized score such as the QI." "The outputted EE and QI quality indicators associated with each DMW estimate can be used synergistically in order to optimize the quality and geographic coverage of the final DMW dataset passed onto the user community. The synergistic use of these quality indicators is designed to take advantage of the strengths of each. The EE is superior at identifying the quality of relatively slow DMWs, whereas the QI is better at identifying the quality of relatively fast DMWs. A study conducted under the GOES-R Risk Reduction (Berger et al. 2008) seeked to identify thresholds for each parameter that could serve as a potential starting point for users to use, if so desired, in any process they may have established to select a subset of the highest quality DMWs. " very respectfully, randy ---------------------------------------- From: "Jonathan Gregory" <[email protected]> Sent: Monday, July 08, 2013 3:13 PM To: [email protected] Subject: Re: [CF-metadata] how to represent a non-standard error Dear Randy My question is perhaps the same as Jim's. The question is, how is this number interpreted, by those who use it? In what way does it quantify the error? Best wishes Jonathan ----- Forwarded message from "[email protected]" <[email protected]> ----- > Date: Fri, 5 Jul 2013 13:43:25 -0400 > From: "[email protected]" <[email protected]> > To: Jim Biard <[email protected]>, > "[email protected] List" <[email protected]> > Subject: Re: [CF-metadata] how to represent a non-standard error > > > > Jim: > > > > An expected error value is individually calculated for EACH wind vector in > the product file based on the factors I identified a few emails back. > > > > This expected error allows the users of the GOES-R derived motion wind > products to know what the error is for each wind vector in the product > file. I think there are weather prediction models that make use of these > error values. There may be other uses, but I don't know for sure. > > > > > > very respectfully, > > > > randy > > ---------------------------------------- > From: "Jim Biard" <[email protected]> > Sent: Friday, July 05, 2013 1:22 PM > To: "[email protected] List" <[email protected]> > Cc: "[email protected] Horne" <[email protected]> > Subject: Re: [CF-metadata] how to represent a non-standard error > > Randy, > Thanks. I guess I'm still wondering how you can validly assign a label > such as "error" to a number that doesn't somehow tell you the likelihood of > the variable value being within some sort of interval. Without getting too > hung up on statistical formalisms, can you explain what use this number is > to you? > Grace and peace, > Jim > Jim Biard > Research Scholar > Cooperative Institute for Climate and Satellites > Remote Sensing and Applications Division > National Climatic Data Center > 151 Patton Ave, Asheville, NC 28801-5001 > > [email protected] > 828-271-4900 > Follow us on Facebook! > On Jul 5, 2013, at 1:13 PM, [email protected] wrote: > > Jim: > > The magnitude of the Expected Error is a function of the calculated wind > speed. > > In perusing the Expected Error algorithm documentation to compose the last > email, it appears the algorithm does not assume any type of an error > distribution (normal or otherwise). The error estimate is absolute and is > not associated with a confidence level. > > The use of this algorithm is based on the results it has achieved on > predecessor weather satellite programs (empirical data has been used to > determine its effectiveness.) I can provide you additional information on > the Expected Error algorithm if you are interested, > > The point I am trying to make is that this is a specific error estimation > approach that is unrelated to a sampling distribution. I would think > there are others. > > very respectfully, > > randy > > From: "Jim Biard" <[email protected]> > Sent: Friday, July 05, 2013 11:22 AM > To: "[email protected] List" <[email protected]> > Cc: "[email protected] Horne" <[email protected]> > Subject: Re: [CF-metadata] Fwd: how to represent a non-standard error > > Randy, > Could you help me understand a touch more about this? You say it is an > error that comes from a custom algorithm, but what defines what magnitude > it has? How do you relate it to anything? Does it represent some sort of > confidence interval? > Grace and peace, > Jim > Jim Biard > Research Scholar > Cooperative Institute for Climate and Satellites > Remote Sensing and Applications Division > National Climatic Data Center > 151 Patton Ave, Asheville, NC 28801-5001 > > [email protected] > 828-271-4900 > <CicsLogoTiny.png>Follow us on Facebook! > On Jul 5, 2013, at 9:28 AM, "[email protected]" > <[email protected]> wrote: > > Dear Jonathan: > > In the case of the GOES-R derived motion winds product, the error estimate > (i.e. more formally referred to as Expected Error) is based on a custom > algorithm. > > This expected error algorithm is specific to atmospheric wind vectors > derived from satellte data. The overarching concept of the wind algorithms > generated from satellite data is doing pattern matching of phenomena (like > clouds) across multiple images of the same region separated by some period > of time > > The GOES-R incarnation of this Expected Error approach makes use of a set > of error predictors including (1) NWP model data (wind shear, temperature > gradient), (2) wind speed, direction, and consistency quality indicators > output from the winds algorithm proper, and (3) a wavelength dependent > constants (GOES-R generates sets of wind vectors from a visible and several > emissive bands) > > I also found an article on the web that discusses it: > > https://www.eumetsat.int/cs/idcplg?IdcService=GET_FILE&dDocName=pdf_conf_p42 > _s2_le_marshall&allowInterrupt=1&noSaveAs=1&RevisionSelectionMethod=LatestRe > leased > > very respectfully, > > randy > > Dear all > > OK, I agree that if it's useful to compare them, then they should be > described > in a standardised way. > > Why is this *not* a standard error? I suppose that to be described as a > standard error it should be a number you could regard as the standard > deviation > of the true value around the stated value. If it's not that, are there > other > ways you would use such a number? > > Best wishes > > Jonathan > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > <CicsLogoTiny.png> > > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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