Moon glint in AMSU Nigel Atkinson and Roger Saunders Met Office (UK)
Moon glint in AMSU • Moon passes through space calibration view in AMSU-A and AMSU-B at certain seasons of year • Typically affects ~10 minutes per orbit for ~10 consecutive orbits • Tends to affect tropics (inclination of moon's orbit is low) • Magnitude: 2K for AMSU-A; 30K for AMSU-B • Errors maximum at cold scenes; zero for ambient scenes. • Various strategies have been devised to correct for this.
Impact on Earth view radiances Moon in space view AMSU-A channel 8 No data bias degK Insert cardiogram here sdev Global O-B stats for 2001
AMSU-B Hot Target View AMSU-B has 4 space view samples, each separated by 1º. Normally only 1 or 2 views are contaminated (occasionally 3) Platform 4 space views (within SV3) Moon Edge of Earth’s disk 90 Earth views
AMSU-B moon-glint corrections • NESDIS 1B files: A moon glint correction was introduced in December 2000 (following Met Office recommendation). If the space views fail the normal consistency checks, the 2 or 3 samples with the lowest space view counts are used. • AAPP local 1B files: Contaminated data fail the consistency checks and are flagged 'do not use' • Code is available for AAPP to implement the NESDIS correction. However, at mid-latitudes the problem is rare.
AMSU-A • AMSU-A has only one space view. Therefore normal quality checks are passed. • Moon glint events are detectable as a degradation in NWP model statistics. • Met Office has implemented prototype 'patch' to AAPP: • Implemented at 1B-1C converter, thus works on global and local datasets, and also on historical data. • Position of moon is calculated using standard astronomical formula, and hence angle between moon and space view • Space view data rejected if angle <3.5º. Gain interpolated between end points. Use offset from hot black body. • Zero error at ambient scenes; <0.1K at 200K.
Corrected Raw Met Office interpolation scheme Space view Hot target 290K 3K Plots show brightness temperature for fixed raw count, i.e. calibration variation around an orbit
NESDIS scheme • NESDIS are working on a correction scheme at 1B level • Involves correcting the measured space view readings on the basis of pre-launch antenna patterns and known moon surface temperature • No interpolation, so consistent with the architecture of the NESDIS pre-processor • NESDIS are using the method to derive post-launch antenna patterns
Results • Correction Algorithm Works Well on A1-2 using pre-launch antenna pattern. Ripples on A1-1 and A2 A1-2 A2
Post-launch antenna pattern measurement NESDIS interpolation using Smooth Curve
Preliminary NESDIS Results NOAA-16 AMSU-A Pre-launch Post-launch
Conclusions • The moon can affect AMSU calibration and hence Earth view radiances for NWP assimilation • For AMSU-B correction code is ready to be included in AAPP. Used at Met Office. • For AMSU-A Met Office interpolation scheme implemented operationally and tested with several events. Could easily be included in AAPP. • The NESDIS scheme could in principle be implemented in AAPP also, but would require more effort.