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Trends in Marine Winds

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Trends in Marine Winds

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  1. TRENDS IN MARINE WINDS ADJUSTED FOR CHANGES IN OBSERVATION METHOD, 1980-2002Bridget R. Thomas1, Elizabeth C. Kent2, Val R. Swail3 and David I. Berry21 and 3Meteorological Service of Canada, Climate Research Branch, 1Dartmouth, NS & 3Downsview, ON2National Oceanography Centre, Southampton, UK

  2. Trends in Marine Winds • Aim: Identify and remove spurious trends in ICOADS Winds • Cardone et al. (1990, On trends in historical marine data. J. Climate) found that in the period 1946 to 1984, trends could be removed by adjusting winds using observing method metadata. Is this still true? • Data • ICOADS winds and observation method flag • Metadata on observing heights (WMO Pub. 47) • Method • Use only data of known method (restricts to 1980 - 2002) • Adjust for anemometer height and observing method • Compare datasets of 5˚ x 5˚ area monthly mean winds for subsets defined by sufficient metadata • Results • Using metadata and adjusting for height removes most but not all of the increasing trend • Spurious trend in visual winds increases following adjustment

  3. ICOADS Quasi-global (all available data) mean wind speed, no adjustments applied for measurement methods

  4. The Evolving Marine Wind Observing System • Changing observing method: • transition from visual estimates based on sea state to more measurements by anemometers • Changing ships: • Ships are getting bigger over time • higher anemometers • higher observing platforms for visual winds • as a result, flow distortion biases may change over time (see poster by Moat et al.)

  5. The Evolving Marine Wind Observing System (2) • Changing logging methods (measured winds): • Anemometer "Eye-ball" average of dial or readout (biased to gusts?) • Average of continually logged data stream (true mean) • TurboWin height correction (only for a few years; stopped at 2002) • Changing method to calculate true wind from the relative wind and the ship’s motion

  6. Partial Solutions to Account for Changes • ICOADS contains a wind indicator flag (WI) which indicates measured or estimated, and original units; WMO Pub. 47 gives ship metadata • For wind reports identified as measured, we adjust the winds for anemometer height, to a 10 m reference level • For wind reports identified as estimated, we adjust the winds using the Lindau (1995) improved Beaufort equivalent scale

  7. Correction Methods for Wind Observations • Visual Winds (or estimated: WI = 0, 2, 3, 5) • Beaufort Scale adjustments applied following Lindau (1995): A new Beaufort equivalent scale, Proceedings of the International COADS Winds Workshop, Kiel, Germany, 232-252. • Anemometer Winds (or measured: WI = 1, 4, 7, 8) • Adjusted to 10m height assuming neutral stability • Over much of the ocean a good approximation • In unstable conditions winds will be slightly over corrected • In stable conditions winds will be under corrected, possibly significantly, but these conditions are not common • Individual anemometer heights from WMO Pub. 47 • Anemometer wind data were not used in this analysis if no height was available

  8. Comparison of Winds of Different Methods 1) Before adjustment to 10m

  9. Comparison of Winds of Different Methods 2) After adjustment to 10m

  10. Comparison of Winds of Different Methods 3) Comparison to Reanalyses 10m winds

  11. Results so far • Adjusting the winds to 10m reduces the overall trend from 0.7 ms-1 to 0.5 ms-1 over 23 years: 1980 - 2002 • Agreement between the trend in visual and anemometer winds worsens over the period • The trend in the visual winds actually increases (the correction applied increases as the winds increase) • NCEP1 and ERA40 Reanalysis 10m winds show little or no increase over the period for this area • 10m anemometer winds increase by 0.2 ms-1, the visuals by 0.7 ms-1 • What's going on?

  12. What could affect temporal trends in estimated winds? • Ships getting larger • Expect visual estimates to get smaller (observers are further from the sea surface) • However visual estimates are getting larger • And estimated winds are higher on bigger ships than on smaller ships, the opposite of what we expect • Are observers influenced by the presence of anemometer onboard? • This would explain the trends seen: visual winds are getting closer to the uncorrected anemometer winds over time • Is there evidence for this? • The VSOP-NA showed that visual winds at night on ships carrying anemometers were significantly higher than on those ships without anemometers

  13. Annual mean day-night differences are larger for estimated than measured winds; differences decrease over time

  14. What could affect temporal trends in anemometer winds? • Flow distortion • as ships increase in size, the geometry of the anemometer location changes (see Moat et al. poster) • a change in fleet composition could lead to trends (see Thomas et al. poster) • Thomas et al. (2005, CLIMAR Special Issue of IJC) found that 10m ship anemometer winds were 6% higher than co-located 10 m buoy winds over the period 1980 - 1995. • Reduction in true wind calculation errors • Due to the introduction of automatic coding and logging software • Automated averaging of winds removes human tendency to report gusts • Mixture of winds at observation height and 10m • Some versions of the TurboWin logging software apply wind reduction to 10m, unfortunately we don't know which reports were corrected. More work is needed to identify them.

  15. Conclusions • Adjustment of ICOADS wind speed to 10m reduces trends in the period 1980 to 2002 • Metadata on both observation method and instrument height is vital to make the necessary corrections • But the visual winds show greater trends than either the anemometer winds or the Reanalyses • There is some evidence that observers making visual wind estimates are "influenced" by the (uncorrected for height) anemometer wind • Future work .......

  16. Future work • Extend analysis back in time to 1950 • Requires improved assignment of measurement method flag • And forward in time • Requires identification of winds already corrected to 10m by TurboWin coding software after 2001 • Develop corrections to remove spurious trends in visual winds • Time varying Beaufort Scale? • Identify individual ships which report anemometer winds as visuals? • 'Calibrate' visual winds using daily pressure fields in well sampled regions? (using dataset described by Kent and Berry poster) • Quantify the effects of flow distortion on anemometer winds • Variations with ship type • and ship size, geometry, wind direction ..... etc. • Produce corrected wind speed dataset from ICOADS

  17. Other slides, if needed during questions

  18. Wind Speed Indicator from ICOADS Wind speed is stored in tenths of ms-1. WI shows the method used and units in which wind speed was originally recorded (0, 1, 3, 4 follow WMO code for Wind Indicator).

  19. Frequency distribution of global wind speed (in m/s) for each ICOADS Wind Indicator, for Jan1955 • in January 1955, most wind reports have an ambiguous wind indicator; they could be estimated or the method is unknown • the second most frequent method indicator is for estimated winds, originally reported as Beaufort force; nearly all values are at midpoints of the Beaufort intervals • measured winds show a more continuous distribution than estimated

  20. Can we assume WI = 6 are all visual reports? • The histograms show the distributions of wind speeds from 3 individual ships in the WI = 6 (visual OR unknown) category in January 1950. • One is clearly in Beaufort intervals and therefore visual, one is probably using an anemometer, the one on the right is unclear. • Conclusion: Exclude WI = 6 from analysis and start in 1980. • In the future we must try and understand the winds with unknown method.

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