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Absorption index of super- cooled liquid water in the microwave range 0 – 300 GHz. Absorption coefficient in Np /km for different models. The ‚ straight way ‘:. The total optical thickness can be derived via the well- known T_mr approach : while
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The total opticalthicknesscanbederived via the well-knownT_mrapproach:
Whenthewatervaporand dry componentcanbeestimatedwith a RT model, andif an INDEPENT informationabout LWP andcloudtemperatureisavailable, the liquid opticaldepthcanbeusedtoderivetheapsorptioncoefficient via:
PROBLEM: Difficulttodisentanglethe different errorsources, atmosphericconditionsneedtobeknownveryprecisely.
Idea: Fast opacitychangesaremainly due to liquid waterfluctuations. Ifonetakessmall time (e.g. 10 min) periodsthevariationsΔτareonly due to
The ratiosof such changesat different frequenciesareindependentofΔLWP andthusidenticaltotheratioofabsorptioncoefficients:
Ratioscanbederivedbyplottingopticalthicknessesat f1 against f2 (linear in contrastto TBs!).
Ratiosare also very robust (e.g. offsetsdon‘tchangetheratio a lot).
Idea: Find a frequencywhereyou ‚trust‘ a model (e.g. Stogrynat 90 GHz). Havingthisspecificandratioscontainingthisspecificfrequencyyoucandirectlyderivetheabsorptioncoefficientattheotherfrequencyby:
In thefollowingslidesthishasbeendone, assumingthatStogryniscorrectat 90 GHz so that e.g. αat 150 GHz canbedirectlyderived (orateveryother 90/f combination!
Alternative: Use fast LWP changesfromHatproand fast opacitychangesfrom DPR toderiveabsorptioncoeff. At 90 or 150 directly:
Idea: Again, theassumptionis, thatthe fast changes in opt. thicknessareonly due to liquid water. Thus, wecanwrite:
Iftherearecorrelated IWV variationsonehastoestimatetheirinfluence but thatispossiblewithourdata, I think. The benefithereisagain, thatthediefferencesarelesspronetoretrieval/calibrationoffsets. On the Zugspitze therearesometimes strong and fast LWP changes due toturbulenceoverthemountaincrestwhichcanbenicelyusedtogothisway…
I triedtheapproachwiththe AMF/UFS data…