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Stratospheric Microwave Sounders: Current Methods, Problems, and Limitations

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signe-donaldson
21 diapositives · 313 Views · Nov 21, 2014

This study focuses on current methods for measuring stratospheric conditions using microwave sounders, specifically MSU4/AMSU9. It explores diurnal adjustments, merging techniques, issues, and limitations. The impact of CCM3 Diurnal Adjustment on MSU4/AMSU9 is assessed, along with the need for independent relative calibration. Various topics like error modeling, data merging from multiple satellites, and drifts in measurement time are discussed. Trends, differences, and adjustments in AMSU channels are examined, providing insights into the reliability of stratospheric measurements.

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  1. Stratospheric Measurements:Microwave Sounders I. Current Methods – MSU4/AMSU9 Diurnal Adjustment Merging II. Problems and Limitations III. Other AMSU Channels

  2. Weighting Functions

  3. Drifts in Measurement TimeDescending node is 12 hours earlier

  4. Harmonic Sensitivity of Diurnal Correction Since measurements are separated by ~12 hours, the first harmonic of the diurnal cycle is mostly cancelled if we average the ascending and descending nodes together. Instead, the second harmonic dominates.

  5. Calculating the MSU Channel 4 Diurnal Cycles from CCM3 • 5 Years of hourly output from the Community Climate Model 3 (CCM3), run in “reanalysis” mode (1980-1984). • Use a radiative transfer model to calculate a simulated MSU Channel 4 brightness temperature time series for each point on earth. • Average together to compute mean monthly diurnal cycles for grid point.

  6. MSU Channel 4 Diurnal Cycles from CCM3Second harmonic appears to be important and larger in the tropics Are these reasonable???????

  7. Impact of the CCM3 Diurnal Adjustment on MSU4/AMSU9 “Global” average, 50S to 50N CCM3-based adjustment is fairly small, but this doesn’t mean it’s correct!

  8. Validating the Diurnal Cycle/Adjustment • Radiosondes – Solar heating problems well documented. Probably can’t be trusted to see the small diurnal signal. • Ascending minus Descending satellite comparisons mostly sensitive to first harmonic – useful as a sanity check in the troposphere but less so for the stratosphere. • Multiple satellites at different measurement times (I.e. NOAA-15, NOAA-16, NOAA-17 and AQUA and now, I hope, Metop-A) but need independent relative calibration to < 0.1K. Maybe the “simultaneous nadir overpass” (SNO) calibration scheme can help. (Cheng-Zhi Zou et al) • Or ……?????

  9. MSU4/AMSU9 Merging Antenna Temperature from 11 Satellites Each dot is a 5-day average, 50S to 50N

  10. MSU4/AMSU9 Merging Intersatellite differences show offsets, trends, and wiggles.

  11. Error Model and Pentad Difference Equations Error model includes intersatellite offsets, plus a dependence on target temperature anomaly (Spencer and Christy) For each pentad where 1 or more satellites have a good observation, we can from a difference equation for each satellite pair Typically > 1300 valid pentad pairs/equations Solve equations to minimize SdT2 a’s probably related to non-linearity in the radiometer

  12. MSU4/AMSU9 Merging MSU-only differences are fit well by the empirical error model AMSU-MSU differences include additional variation partly due (presumably) to small differences in the weighting function. AMSU appears to drift relative to MSU!

  13. MSU and AMSU Weighting Functions Black: MSU4 Red: AMSU9 nadir Blue: AMSU9 near limb We use a near-limb set of AMSU views to help match MSU4. (views 4 - 7 and 24 - 27) Not much impact on variability of MSU minus AMSU.

  14. MSU minus AMSU Difference Red: Unadjusted Blue: Mean seasonal cycle removed empirically Trend of ~0.1K decade still present

  15. Map of MSU 4/AMSU 9 Trends

  16. Comparison to UAH Results • Reasons for the Difference not yet known, but probably a combination of • Differences in the Diurnal Adjustment • Differences in Merging Parameters, esp. Target Factors. • UAH is currently revamping their diurnal adjustment and treatment of AMSU

  17. Extrapolating MSU4 upward SSU 15X MSU4,limb-nadir, Trend = -0.52K/decade FOV_Weights = (0.5,0.5,0.0,0.0,-0.33,-0.33,-0.33,0.0,0.0,0.5,0.5) MSU4, near-limb, Trend = -0.44K/Dec. MSU4, near-nadir, Trend = -0.36K/Dec.

  18. Other AMSU Channels SSU channels can reconstructed from AMSU data. Sanity check AND diurnal adjustment. Upper channels are on very narrow lines – small drifts in LO frequency could cause big problems – need to check stability SSU 27 AMSU 13 SSU 26 AMSU 12 SSU 25 AMSU 11 AMSU 10 SSU 15x AMSU 9

  19. AMSU Channel 12 Measurement Bands 30,10,3 hPa Absorbtivity (kg-1) Frequency (GHz)

  20. Quick Conclusions • MSU 4 probably accurate to ~0.1K/decade • MSU 4 sees no evidence of the upward trend in SSU 15X after 1998 • All Stratospheric probably channels need to be corrected for diurnal effect. (Maybe AMSU can help here.) • AMSU can probably be used for a sanity check for SSU channels after mid-1998. But there is a good chance that AMSU (too?) is drifting.

  21. MSU Operation The MSU is a cross-track scanning microwave sounder operating at 4 frequencies near the oxygen absorption line at 57 GHz.