an atmospheric modeling tool for site characterization and telescope calibration using ... · 2017....
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An Atmospheric Modeling Tool for Site Characterization and Telescope Calibration using MERRA2
Andrew Wang, Denis Barkats, Scott Paine, Simon Radford, John Kovac
Abstract
UsingNASA’sMERRA2atmosphericreanalysisandtheamradia7vetransfermodel,wehavedevelopedanewtoolthatpredictstheskybrightnesstemperatureandopacityanywhereonEarthforanydatefrom1970tothepresent.WevalidateMERRA2bycomparingourpredic7onswithlong-baselineopacitymeasurementcampaignsandquan7fytheaccuracyofourpredic7ontool.Finally,wehavecreatedacommandlinetoolandwebinterfaceforresearcherstogeneratepredic7onsindependently.
Introduction
Method
Results
1 M. M. Rienecker, et al. 2011 “MERRA, NASA's Modern-Era Retrospective Analysis for Research and Applications.” J. Climate 24:3624. , https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/
2 Paine, Scott. (2017, March 29). The am atmospheric model (v. 9.2). Zenodo. http://doi.org/10.5281/zenodo.438726
3 Radford & Peterson, 2016, arXiv:1602.08795 4 Matsushita et al., 2017, PASP, 129, 025001 5 C-L. Kuo, arXiv: 1707.08400 6 NOAA/NCEP Global Forecast System (GFS) Atmospheric Model
Relevance to CMB-S4
• SiteCharacteriza1on:Ourtoolcanprovidepredic7onsofanypoten7alsite’sopacityorbrightnesstemperaturetodetermineviabilityasafutureobservingloca7on.• Recentworkhasbeendonetocompareexis7ngCMB
observingloca7onswithnewsites,includingSummitSta7on4inGreenland,Ali5ObservatoryinTibet,andDomeAinAntarc7ca.
• TelescopeCalibra1on:Zenithbrightnesstemperature7mestreamcanbeusedtocalibrateores7matereal-7mesensi7vityatcurrentCMBobservatories(Figure5).
References
HowaccuratearetheMERRA2-basedpredic1ons?• WecompareourMERRA2skybrightnesstemperaturepredic7onsto850GHz7ppermeasurements
takenfrom1998-2017atSouthPole(Fig2),ChajnantorPlateau,CerroChajnantor,andMaunaKea.
• Themedian7pperuncertaintyis7-10K(5-6%)atallsites,andremainsbelow10%over93%ofthe7meatMaunaKeaandSouthPoleandover88%ofthe7meattheChajnantorsites.
• WeassigntheMERRA2predic7onanuncertaintyrela7vetothe7pper’sknownuncertainty.Modelingtheresidual’svarianceasthesumofthe7pperσt2andtheMERRA2σm2variances,calcula7ngchi-squaredvaluesoftheresidualsallowsustosolvefortheunknownσm(Fig3):χ2=(7ppermeasurement-MERRA2modelpredic7on)2/(σt2+σm2)
• IfMERRA2wereaperfectmodel,wewouldexpectχ2=1(i.e.σm=0).Forallsites,wefindχ2=2–6whichsuggeststheMERRA2modelhasbetween1and2.37mesthe7pperuncertainty(Fig4).
• AtSouthPoleandMaunaKea,theMERRA2predic7onsareremarkablyclosetothemeasuredbrightnesstemperature.Predic7onsattheChajnantorsitesareslightlylessaccurate,buts7llwithin10-15%ofthemeasuredvalue.ThisisexpectedgiventhatthecomplexlocalweatherpajerninthemountainousChajnantorregionishardertospa7allyresolveaccuratelyonMERRA2’s50x50kmgrid.
• Ground-basedCMBtelescopesobservethroughtheatmosphere,whichabsorbsasignificantfrac7onofthemm/sub-mmcosmicsignalandaddsthermalnoise.
• Atmosphericturbulencealsoaddsnoisefluctua7onstoincomingradia7on.
• WedeterminetheatmosphericstateusingMERRA2.MERRA2’sfocusontheatmosphericwatervaporisidealforthisapplica7on.
• WevalidateMERRA2’spredic7onsbyquan7fyingitsuncertain7esagainstlong-baselinehistorical850GHzand225GHzopacity7pper3measurements.
• Interpolate3-hourMERRA21dataproducts(e.g.ver7calprofilesofpressure,temperature,watermixingra7o,andotherrelevantquan77es)toanyposi7ononEarth.
• Usethe2DMERRA2productstoobtaingroundvaluesoftheatmosphericprofiles.
• Runtheamradia7vetransfermodel2ontheseprofilestogeneratefrequency-dependentatmospherictransmissionspectra,alongwithPWVvalue.
• Integratethesespectraoveraninstrumentbandpasstopredicttheskybrightnesstemperatureforthatinstrument.
Publicly Available Tool
• WebInterface(seebelow)andcommandlinetoolcurrentlyindevelopment;betaversionath$ps://goo.gl/czB24x
• Userinputsaloca7on,daterange,andbandpass,andreceivesanoutputfileandplotswiththebrightnesstemperatureandPWVpredic7onsat3-hourintervals.
• FutureWork:Weareaddingthecapabilitytomakepredic7onsforfuturedatesusingGFS6forecasts,touploadcustombandpassfiles.Wearealsoworkingonapaperdescribingthedetailedmethodsandvalida7onofthistool.
Figure1:Transmissionspectraoftheatmospherewithvaryinglevelsofprecipitablewatervapor(PWV).TransmissiondropssignificantlyasatmosphericPWVincreases,whichiswhyweneedtoiden7fylow-PWVsites.
Figure5:Brightnesstemperaturepredic7onsatSouthPolein2015fortheBICEP/KeckArray100/150/220/270GHzbands.
Figure2Le#:Time-seriescomparisonofMERRA2brightnesstemperaturepredic7onstoreal7pperdataevery3hoursfromFebruarytoApril2015atSouthPole.Right:One-to-onecomparisonandbest-fitlineforafullyear.
Figure3:ResidualsforFig2-lenatSouthPolefromFebruarytoApril2015,withaχ2of3.
Figure4(below):χ2valuesfortheaboveresidualsateachofthesites.Amedianχ2of2.9(asinSPandMK)meansthestandarddevia7onoftheMERRA2predic7onis1.47mesthatofthe7pper.Aχ2of6(asinChajnantor)impliesapredic7onuncertainty2.37mesthe7pper’suncertainty.
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