Tangent height verification algorithm Chris Sioris, Kelly Chance, and Thomas Kurosu Smithsonian Astrophysical Observatory

Algorithm described in Sioris et al., JGR, in press. - find the longest median wavelength for the set of pixel that has the same knee TH on the measured TH grid such that the median is < 305 nm -compare this knee TH (e.g. ~45 km) to one produced by a limb radiative transfer model at the median wavelength for the same conditions (SZA, d , T[z], p[z], O 3 [z]) -algorithm takes 3 sec per limb scan (at 800 MHz) with McLinden et al. model. -this approach has been compared to one where a quadratic in is fitted to observed and simulated knee THs (and the difference between the ‘obs’ and ‘sim’ is the TH offset)

 Median used for 305 nm technique

Results Orbit 3422: nm gives wrong TH offset due to wrong mesospheric O 3 in the model

# of limb scans=325, # of orbits = 17, mostly from Aug. and Sep. ’02

Analysis of five consecutive SCIAMACHY orbits

Paired t-test: every 2nd orbit has same latitude sampling scheme, Applied to NH extratropics where algorithm is less sensitive to assumed atm orbit pairs lat range (°) p (that drift is insignificant) drift/orbit (km) 2995 vs % vs % vs % -0.24

Offset statistics (km, obs - model TH) GLOBAL Average: 0.06 stdev: 1.42 maximum: 6.03 minimum: -4.38

Expected error budget for spectral knee at ~305 nm 1. theoretical limit to due spectral resolution and sampling and shot noise ~40 metres 2. completely wrong surface albedo, trop. cloud info: <50 3. completely wrong temperature profile: < % bias in O 3 column (variability for worst-case 200 latitude, for most months above 45 km) 5. interannual variability, (July=worst-case, [Keating <200 et al., 1990]) 6. Neglect of diurnal variation below 53 km ([Keating et al.]) since LT at poles is not LT at equator <50 7. Radiative transfer accuracy planetary and gravity wave activity affecting p at 46 km [Barnett and Labitzke, 1990]: worst-case: hi lat winter 1280 best-case: equator 20-40

Worst-case total error (added in quadrature): ~1300 m at the equator: ~330 m

Application to Level 2: NO 2 profile intercomparison with SAGE III SAGE III pointing for solar occultation is accurate to ~100 m. Vertical column (21-37 km) agreement improves marginally from 6.5% to 3.8% but profile agreement improves from 21% to 11% as sharp features are captured only after TH correction of 2.6 km