MIPAS Observations of CFC Trends

Transcription

1 MIPAS Observations of CFC Trends Alastair Burgess Anu Dudhia, Don Grainger AOPP, University of Oxford Page 1

2 What is MIPAS? Michelson Interferometer for Passive Atmospheric Sounding wavenumber (4-16 micron) range, cm -1 resolution design Fourier Transform Limb Thermal Emission Infrared Spectrometer Launched 1 st March 2002 as a part of Envisat Polar orbit of 100 minutes at 800 km Page 2

3 CFC-11 (CCl 3 F) and CFC-12 (CCl 2 F 2 ) F11 Region Profile F12 Region Profile Sources Anthropogenic: Industry, refrigerants etc. Sinks Mesospheric and Stratospheric Mainly photolysis by UV radiation. Some radical processes. Long lifetime (F11/Strat 45 years, F12/Strat 100 years) Distribution ~250pptv (F11/Trop) and ~500pptv (F12/Trop) Significant stratospheric height and latitude dependance Well mixed (Trop) - variability reflecting circulation and source distributions. Page 3

4 Reduced Resolution L1B 0.025cm cm -1 Why the change? Observe large numbers of drive unit errors. Observe rest periods allow self-healing and reduction in error rate. Observe a reduction in corner cube slide distance improves reliability. Resulting change in data: Rest periods introduce regular, planned breaks in observations. Reduced slide movement reduces interferometer resolution. Change Measurement Patterns as RR sweeps take less time. Page 4

5 Microwindow Error Analyses (F11) 0.025cm cm -1 Retrieval Sensitive to Temperature accuracy, pressure accuracy. Strong radiance gradients and radiometric calibration Reduction in resolution (RR modes) Increased significance of random noise: Use Coaddition Over sampling the atmosphere expected to increase random error Over km region of interest, total error terms worsen from ~25% to ~50%. (MWs have been re-selected for all operational species, too.) Page 5

6 RR Interfering Species Retrieval Nominal RR Scan pattern 27 scans, 1.5-5km spacing, 6-68km + variable offset, May 2005 UTLS1 RR Scan Pattern 19 scans, 1.5-5km spacing, 6-45km, February 2006 Validation (Full resolution retrieval ( MORSE ) already well validated.) Quantitative comparison with Climatologies Compare error characteristics with validated full resolution retrievals. Direct comparisons (e.g. HiRDLS) planned. 1 Orbit Temperature, Nominal RR 1 Orbit Methane, UTLS1 RR 1 Orbit Ozone, Nominal RR 1 Orbit NO2, UTLS1 RR Page 6

7 Literature discussion of trends Treaties governing production and use Significant Reduction in Emission, especially industrialised nations. Both F11 and F12 regarded as Greenhouse Gases (Intergovernmental Panel on Climate Change) Ozone Depleting Substances (Montreal Protocol) F11 is the reference species Ozone Depleting Potential of 1.0 Size of trends F11 estimated at -5 pptv yr -1 Or 2% (see figure, Blake 05) F12 estimated at ~0 pptv yr -1 Finally stopped increasing? D. CFC-113) Blake in Chlorofluorocarbons Whole-air Samples. (CFC-11, CFC-12, and CFC-113) in Whole-air Samples. Quick Estimate: Could MIPAS observe this? 3 days: Zonal mean Lat. band has ~50 profiles => random noise reduced 7x. Single scan error bars in UTLS to Mid Strat about 35% for F11. 1/7 of this is ~5% [Random component, assume systematics constant] D. Blake Chlorofluorocarbons (CFC-11, CFC-12, and Page 7

8 Testing for a trend Dec02 Sep03 Sep02 Mar03 Mar04 May05RR Feb06RR MIPAS measurements (L1B) exist from mid-2002 to Feb 2006 Select a wide distribution for maximum sensitivity Demonstrate RR retrieval of arbitrary species. Processing constraints exist for RR data Operational species must be retrieved first. Full resolution data processing made use of ESA L2 Selected 3-day blocks as processing time trade-off Allows coaddition to improve accuracy (reduce random noise) Page 8

9 Nominal Resolution F11 & F12 Zonal Means F11 Dec02 Small North : South asymmetry Mainly observed for the tropical maximum Anthropogenic emission variability not really seen Suspicious variability in tropical troposphere Cloud Saved processing time using operational ESA L2 pt and species Altitudes: 68,60,52,47,42,39,36,33,30,27,24,21,18,15,12,9,6 km F12 Page 9

10 RR Nominal Pattern F11 & F12 Zonal Means F11 May 05 New measurement pattern ( 1.5km). Atmosphere is over sampled Do not get a 2x improvement in vertical resolution. More like ~2km. Possible to observe finer structure in retrievals Altitudes:70,66,62,58,54,50,46,43,40,37,34,31,29,27,25,23,21,19.5,18,16.5,15,13.5,12,10.5,9,7.5,6 km, variable base F12 Page 10

11 RR Nominal Pattern F11 & F12 Errors F12 Stddev of Mean May 05 %RndErr from retrieval F11 Page 11

12 RR UTLS1 Pattern F11 & F12 Zonal Means F11 Feb 06 F12 UTLS1 RR Mode concentrates in the UTLS region Not problematic as restrict altitude range for comparisons to <30km UTLS region is oversampled again. More scans per orbit (~100) vs the nominal modes (~70) Altitudes:51,47.5,43,38.5,34,31,28,26,24,21.5,20,18.5,17,15.5,14,12.5,11,9.5,8 km, variable base. Page 12

13 Trends by latitude band F11 Colours represent latitude bands: 90N-65N Red 65N-20N Orange 65S-90S Violet 20S-65S Blue 20N-Eq Yellow/Green Eq-20S Green Values are from a simple lower stratospheric (15-30km) mean F12 3 days of measurements (~30 operational orbits => ~250 scans) Page 13

14 Accuracy and Factors influencing trend Random Error on the means significant. F11: Equatorial 220->170pptv over 3.5 years. Global, likewise, has of 50pptv. Gives -15ppt yr -1, about -7.5 % yr -1 Trend range between -5 and -25 ppt yr -1 F12: Equatorial 460->450pptv. Global, neglecting Feb06, same Gives -2.8pptv yr -1, about -0.6%. Trend range between ~+4 and -8 ppt yr -1 Variations in Systematics over time a possibility E.g. Radiometric calibration Interfering species change in significance - RR CFCs more susceptible But notice F11 trend seems similar between full and RR measurements Page 14

15 Conclusions Demonstration of operational Reduced Resolution retrievals. MW Error characteristics change. Changes in coverage and over sampling / 1.5km step. CFCs are a bonus to the baseline mission expectations Estimated CFC-11 and CFC-12 trends (-7.5%, -0.6% yr -1 ) Global, height resolved fields. Co addition to improve random errors Far from perfect, however. F11 trend a bit high; but: Correct sign and visible before RR mode start. F12 may show a small negative trend Further work: Compare / validate with other groups (initially full resolution) Degrade nominal resolution data to and compare result Use more days to reduce random error further. Add other months, increasing confidence in observed trends. The latest developments: Page 15