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

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Weighting Functions

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Drifts in Measurement Time Descending node is 12 hours earlier

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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.

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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.

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MSU Channel 4 Diurnal Cycles from CCM3 Second harmonic appears to be important and larger in the tropics Are these reasonable???????

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Impact of the CCM3 Diurnal Adjustment on MSU4/AMSU9 CCM3-based adjustment is fairly small, but this doesnt mean its correct! Global average, 50S to 50N

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Validating the Diurnal Cycle/Adjustment I.Radiosondes – Solar heating problems well documented. Probably cant be trusted to see the small diurnal signal. II.Ascending minus Descending satellite comparisons mostly sensitive to first harmonic – useful as a sanity check in the troposphere but less so for the stratosphere. III.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) IV.Or ……?????

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

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

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Error Model and Pentad Difference Equations 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 T 2 s probably related to non-linearity in the radiometer Error model includes intersatellite offsets, plus a dependence on target temperature anomaly (Spencer and Christy)

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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!

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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.

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

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Map of MSU 4/AMSU 9 Trends

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Comparison to UAH Results Reasons for the Difference not yet known, but probably a combination of 1.Differences in the Diurnal Adjustment 2.Differences in Merging Parameters, esp. Target Factors. 3.UAH is currently revamping their diurnal adjustment and treatment of AMSU

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Extrapolating MSU4 upward MSU4, near-nadir, Trend = -0.36K/Dec. MSU4, near-limb, Trend = -0.44K/Dec. 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) SSU 15X

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Other AMSU Channels AMSU 9 AMSU 10 AMSU 11 AMSU 12 AMSU 13 SSU 15x SSU 25 SSU 26 SSU 27 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

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

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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.

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MSU Operation The MSU is a cross-track scanning microwave sounder operating at 4 frequencies near the oxygen absorption line at 57 GHz. Channel 1Channel 2Channel 3Channel 4 Frequency (GHz) 50.3053.7454.9657.05 Altitude of peak weight Surface4-7 km10-12 km17-19 km

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