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Tangent Height Effects in SCIAMACHY Solar Occultation Measurements

Tangent Height Effects in SCIAMACHY Solar Occultation Measurements. S. Noël IFE/IUP, University of Bremen, Germany. Questions. What is the tangent height of a solar occultation measurement? What is the vertical resolution of a solar occultation measurement?.

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Tangent Height Effects in SCIAMACHY Solar Occultation Measurements

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  1. Tangent Height Effects in SCIAMACHY Solar Occultation Measurements S. Noël IFE/IUP, University of Bremen, Germany

  2. Questions • What is the tangent height of a solar occultation measurement? • What is the vertical resolution of a solar occultation measurement?

  3. SCIAMACHY Occultation Measurements • SCIAMACHY occultation measurements are performed once per orbit in Northern latitudes • Vertical scans over the sun during sunrise (local sunset) • Scan centred around rising sun • Different scan strategies above ~100 km

  4. Scan Effects • Inst. Field of View (IFOV) vertical size = 0.045° ~ 2.6 km • Scan speed: vscan = 0.33°/s • Typical integration time (solar occ.): tint = 0.125 s • vertical range covered by scan during tint: 0.04125° ~ 2.4 km • Approx. solar elevation rate: vsun = 0.05 °/s • vertical range covered by rising sun during tint: 0.00625° ~ 0.4 km • Total (Δz): 2.6 km (IFOV) + 2.4 km (Scan) ± 0.4 km (Sunrise) • Δz = 5.4 km (upscan) Δz = 4.6 km (downscan) Δz <z>

  5. Density Gradient • Density varies in vertical direction • Assumption: Barometric distribution in Δz: n(z) = n0 exp(-z/H) • H = scale height = H(T(z)) • Average density: <n>= 1/Δz Δz n(z) dz =: n(<z>) <z> = average altitude • Assuming a constant scale height within Δz leads to: <z> = zgeo – H ln(-H/Δz(exp(-Δz/H) – 1)) – Δz/2 Δz <z>

  6. Density Gradient • Effect depends via scale height on temperature and therefore altitude • Effect depends on scan mode and direction (Δz)

  7. Geopotential Height • Zgeopot = z+r0r0 1/r2 dr r0 = Earth radius; z = geometr. height • Derived from Zgeopot = 1/g(r0)z+r0r0 g(r) dr with g(r) = Gm/r2 G=grav. constant, m=mass • ECMWF data contain geopotential heightasverticalcoordinate • Different fromgeometricalheightgiven in SCIAMACHY data • To be considered when e.g. combining p, T from ECMWF with density from SCIAMACHY to get VMRs • Effect ~ 400 m at 50 km altitude

  8. Detector Readouts • SCIAMACHY detectors 1-5 are sequentially read out • Readout time: tread = 28.771 ms (for 1024 pixel) • Offset depends on scan direction and pixel no. • Shift = tread/1024*(vscan ± vsun)

  9. Other Problems • Mismatch between upscan and downscan tangent heights of ~400 m derived from PMD measurements • Tangent height drifts?

  10. Questions • What is the tangent height of a solar occultation measurement? • What is the vertical resolution of a solar occultation measurement?

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