U.S. HCN Temperature Trends: A brief overview

1. U.S. HCN Temperature Trends:A brief overview

2. Some Topics • The U.S. HCN Datasets • What is climate data homogenization? • Some examples • Analysis of U.S. temperature trends • Representativeness of temperature trends versus temperature means

3. U.S. HCN Version 1 • Originally released in 1987 • 1221 stations • Addresses • Time of observation bias (Karl et al. 1986; Vose et al. 2003) • Station History Changes (Karl and Williams 1987) • MMTS instrument change (Quayle et al. 1991) • Urbanization (Karl et al. 1988)

4. U.S. HCN Version 2 • 1218 stations • Addresses • Time of observation bias (Karl et al. 1986; Vose et al. 2003) • Station History and Undocumented Changes (Menne and Williams, Journal of Climate, in review)

5. The Time of Observation Bias

6. The Time of Observation Bias • Since the 1950s, observation times have shifted from afternoon/ evening to morning 1960s 1950s 1970s 1980s 1990s 2000- 2006 Hour of observation histograms at bottom of each U.S. decadal map (Figure courtesy of Xioamoa Lin, University of Nebraska)

7. Impact of Time of Observation Changes Average year by year difference over the conterminous United States between the Time of Observation Bias (TOB)-adjusted data and the unadjusted (raw) data.

8. Homogenization

9. Simulated temperature series with random shifts caused by station moves/site changes (Annual Averages) σ (°C) • Series in red treated as the target in subsequent figures • All shifts are considered to be undocumented • True “climate” trend in all simulated series is zero

10. Target series and differences with neighbors before adjustment for undocumented shifts Case 7 unadjusted

11. Target series and differences with neighbors following adjustment by pairwise algorithm

12. Simulated temperature series following adjustment by pairwise algorithm (Annual Averages) σ (°C) • Original Target Series in Red • Adjusted Target Series in Green • Adjusted Neighbor Series in Black

13. An Example: Reno, Nevada From: http://wattsupwiththat.wordpress.com/2008/01/10/how-not-to-measure-temperature-part-46-renos-ushcn-station/

14. Reno, Nevada Move to Airport Likely urban warming ASOS Equip. Moves • Mean annual TOB and fully adjusted (TOB+Pairwise) minimum temperatures at Reno, Nevada • Difference between minimum temperatures at Reno and the mean from its 10 nearest neighbors.

15. U.S. HCN Version 2 Temperature Trends (1895 to 2006) Maximum Temperature Unadjusted Adjusted Minimum Temperature Unadjusted Adjusted °C dec-1

16. Impact of Documented and Undocumented Changes Year by year differences averaged over the conterminous United States between the fully adjusted (TOB+pairwise homogenization algorithm) HCN V2 data and the TOB-only adjusted data.

17. What about siting?

18. SurfaceStations.Org Stations in red have a classification >= 3 Station in black have a classification of 1 or 2

19. U.S. HCN Version 2 Temperature Trends (1895 to 2006) Maximum Temperature All Stations Rating 3, 4 or 5 Minimum Temperature All Stations Rating 3, 4 or 5 °C dec-1

20. U.S. HCN Version 2 Temperature Trends (1895 to 2006)

21. Impact of Siting on Trends U.S. annual and seasonal temperature trends (°C dec-1) 1895 to 2006

22. Trends Versus Means Here is a not-so-well maintained or well sited USHCN station at Marysville

23. Can station measurements be unrepresentative, and the (adjusted) trend be representative? Difference between Annual Minimum Temperature at Marysville, CA and Surrounding Stations Documented and undocumented shifts identified by pairwise algorithm (using monthly values) indicated by vertical bars °C Raw data in red Adjusted data in green

24. Adjusted Minimum Temperature Series at Marysville, CA