Rutherford Appleton Laboratory Requirements Consolidation of the Near-Infrared Channel of the GMES-Sentinel-5 UVNS Instrument: Initial trade-off: Height-resolved aerosol retrieval R.Siddans MTR: Estec, 1 October 2013

Overview Follows work in Eumetsat, ESA Camelot, ESA S4 studies to define instrument requirements Uses same optimal estimation retrieval simulation scheme employed in the previous studies. These updated to simulate S5. Retrieval scheme based on OE Aerosol optical properties (single-scat albedo, phase fn) assumed Extinction profile retrieved (2km vertical grid) Wavelength shift retrieved Results integrated to various layer aerosol optical depths (AOD) for comparison to user requirements Requirement on BL and free-trop column AOD=0.05 km spatial resolution) A priori error on surface albedo 0.01 assumed Estimated standard deviation (ESD) from measurement noise derived from solution covariance Errors to be assessed by performing “linear mapping” of error spectra (Δy) into L2 error (Δx): Δx = G Δy

Extension of retrieval scheme Albedo retrieved with linear wavelength dependence (2 terms) For Concept A, spectral range nm omitted and linear dependence fitted in both oxygen A and B bands (4 terms) Error in albedo variation with 2 nd order spectral dependence mapped Peak intensity of both Fluorescence Gaussians mapped or optionally fitted Retrieval of ILS width implemented as option Retrieval conditions now based on SWIR study scenarios For aerosol just defines surface albedo assumed and spatial grid of scenarios

Instrument model Simulations re-run with latest information from ESA on noise model and spectral response functions. Noise model specifies Signal:Noise (SNR) as funciton of radiance (L): Two instrument concepts defined: Concept A: 0.39nm resolution, nm, O2 A+B bands, H2O Concept B: 0.12nm resolution, m, only O2 A-band Concept B has assumes the “tropical-dark” noise model. Similar to requirement of SNR 500 in continuum of tropical dark (with b=0) Both concepts simulated + concept B scaled to different resolutions

Simulated instruments

Geophysical scenarios Aerosol retrieval performance very dependent on view / solar geometry due to variations in aerosol phase function, light path for aerosol light-path for O 2 absorption etc. Surface albedo Assumed aerosol type + size (asymmetry, single scatter albedo etc) Earlier simulations show that 0.05 requirement cannot be met for height- resolved quantities in all (most) S5 observing conditions Difficult to concisely summarise performance and optimise inst/L1 requirements without considering many conditions Aim to provide realistic along-orbit simulations of retrieval peformance Based on S5 orbit model + linear retrieval simultions for range of conditions: Solar zenith angle: 30, 45, 60, 70.,75, 80 o View zenith angle: 0, 30, 50, 60, 70 o Relative azimuth angle: 0, 30, 60, 90,120,150,180 o Surface albedo: 0.01,0.1,0.2,0.3,0.5,0.7,0.9 Aerosol profile: Camelot Mid-latitude background and “tropical dust ocean” conditions (total AOD 0.2 and 0.67, respectively)

Instrumental errors simulated Instrument noise: The impact of instrument noise on the estimated precision of aerosol layer optical depth is estimated via the ESD as described above. Sensitivity to errors in the spectral response function or instrument line shape (ILS): This is determined by linearly mapping a 1% error in the width of the assumed ILS. Sensitivity to errors in radiometric gain is determined by linearly mapping the impact of a 10% gain error, i.e. multiplying the observed spectrum by a factor 1.1 Sensitivity to an additive absolute radiometric accuracy requirement (ARA) Mapped MTRD specification as gain (~3%) Mapped proposed relaxation as a separate radiometric offset. Other representations of the error possible / more realistic ? Intra-band co-registration Mapping spatial variation in albedo associated with spatial shift, with 2 nd order wavelength dependence in band First order would be handled by linear retrieval of albedo

Absolute Radiometric Accuracy (ARA) (MR-LEO-UVN-160) MRTD: At the MAG the requirement was relaxed to apply only over a given signal level (Tropical dark at 755nm);

Co-registration requirements These identified as challenging at S4/5 MAG, proposal to relax to 0.3 inter (keep 0.1 intra for NIR)

Change in albedo at 858nm for 20% shift in Spatial response

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90) With H 2 O modelled and retrieved

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90) With fluorescence retrieved

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90) With fluorescence and spectral response function width retrieved

Weighting functions

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)

S5B (with fluorescence and ILS fitted)

S5A A-band (with fluorescence and ILS fitted)

S5A A+B band (with fluorescence and ILS fitted)

S5B (with fluorescence and ILS fitted)

Conclusions Height resolved aerosol retrievals improve with increasing (finer) spectral resolution, even considering an instrument with fixed total throughput. Dependence on geometry large Purely on grounds of theoretical precision both instrument concepts A and B considered here are competitive Option A compensates for lower resolution via including B-band (but this involves modelling aerosol spectral properties) more favourable noise model Concept B definitely preferred if only A-band used. Sensitivity to spectral response function errors seems well mitigated by retrieval. Issue now is spectral dependence in ILS knowledge/stability Relaxation to ARA leads to significant error (when mapped as offset) Mapping of 2 nd order albedo variations generated by 20% intra-band co- registration, gives large errors Amplitude vs spectral dependence of intra-band co-registration needs to be better understood to estimate realistic effect

S5B 0.06nm resolution (with fluorescence and ILS fitted)