stratospheric measurements: microwave sounders i. current methods – msu4/amsu9 diurnal adjustment...

Stratospheric Measurements:Microwave Sounders

I. Current Methods – MSU4/AMSU9Diurnal Adjustment

Merging

II. Problems and Limitations

III. Other AMSU Channels

Weighting Functions

Drifts in Measurement TimeDescending node is 12 hours earlier

Harmonic Sensitivity of Diurnal Correction

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

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.

MSU Channel 4 Diurnal Cycles from CCM3Second harmonic appears to be important and larger in the tropics

Are these reasonable???????

Impact of the CCM3 Diurnal Adjustment on MSU4/AMSU9

CCM3-based adjustment is fairly small, but this doesn’t mean it’s correct!

“Global” average, 50S to 50N

Validating the Diurnal Cycle/Adjustment

I. Radiosondes – Solar heating problems well documented. Probably can’t 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 ……?????

MSU4/AMSU9 Merging

Antenna Temperature from 11 SatellitesEach dot is a 5-day average, 50S to 50N

MSU4/AMSU9 Merging

Intersatellite differences show offsets, trends, and wiggles.

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

, , , ,MEAS MEAS TARGET TARGETi j i j i i j jT T A A T T T

Typically > 1300 valid pentad pairs/equations

Solve equations to minimize T2

’s probably related to non-linearity in the radiometer

, ,0MEAS TARGETi i i i iT T A T

Error model includes intersatellite offsets, plus a dependence on target temperature anomaly (Spencer and Christy)

MSU4/AMSU9 MergingMSU-only differences are fit well by the empirical error modelAMSU-MSU differences include additional variation partly due (presumably) to small differences in the weighting function.

AMSU appears to drift relative to MSU!

MSU and AMSU Weighting Functions

Black: MSU4Red: AMSU9 nadirBlue: 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.

MSU minus AMSU Difference

Red: UnadjustedBlue: Mean seasonal cycle removed empirically

Trend of ~0.1K decade still present

Map of MSU 4/AMSU 9 Trends

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

Extrapolating MSU4 upward

MSU4, near-nadir, Trend = -0.36K/Dec.

MSU4, near-limb, Trend = -0.44K/Dec.

MSU4,limb-nadir, Trend = -0.52K/decadeFOV_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

Other AMSU Channels

AMSU 9

AMSU 10

AMSU 11

AMSU 12

AMSU 13

SSU 15x

SSU 25

SSU 26SSU 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

AMSU Channel 12 Measurement Bands

Frequency (GHz)

30,10,3 hPa

Abs

orbt

ivit

y (k

g-1)

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.

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

Channel 1 Channel 2 Channel 3 Channel 4

Frequency (GHz)

50.30 53.74 54.96 57.05

Altitude of peak weight

Surface 4-7 km 10-12 km 17-19 km