u.s. hcn temperature trends: a brief overview

Visit with Anthony WattsApril 23, 2008

1 NOAA’s National Climatic Data Center

U.S. HCN Temperature Trends:A brief overview



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



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)



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)



The Time of Observation Bias



The Time of Observation Bias

1950s 1960s

1980s

2000-2006

1970s

1990s

• Since the 1950s, observation times have shifted from afternoon/ evening to morning

Hour of observation histograms at bottom of each U.S. decadal map(Figure courtesy of Xioamoa Lin, University of Nebraska)



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.



Homogenization



σ (°C)

Simulated temperature series with random shifts caused by station moves/site changes

•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

(Annual Averages)



Case 7 unadjustedTarget series and differences with neighbors before adjustment for undocumented shifts



Target series and differences with neighbors following adjustment by pairwise algorithm



σ (°C)

Simulated temperature series following adjustment by pairwise algorithm

•Original Target Series in Red

•Adjusted Target Series in Green

•Adjusted Neighbor Series in Black

(Annual Averages)



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

http://wattsupwiththat.wordpress.com/2008/01/10/how-not-to-measure-temperature-part-46-renos-ushcn-station/






Reno, Nevada

(a) Mean annual TOB and fully adjusted (TOB+Pairwise) minimum temperatures at Reno, Nevada(b) Difference between minimum temperatures at Reno and the mean from its 10 nearest neighbors.

Move toAirport

Likely urbanwarming

ASOS Equip.Moves



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

Unadjusted Adjusted

Minimum Temperature Adjusted

°C dec-1

Unadjusted

Maximum Temperature



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.



What about siting?



SurfaceStations.Org

Stations in red have a classification >= 3

Station in black have a classification of 1 or 2




All Stations Rating 3, 4 or 5

Minimum Temperature

°C dec-1

Maximum Temperature

All Stations Rating 3, 4 or 5



Impact of Siting on Trends

Season Maximum Temperature Minimum Temperature

All Stations Ratings 3, 4 & 5 All Stations Ratings 3, 4 & 5

Annual 0.056 0.048 0.070 0.054

D-J-F 0.094 0.074 0.101 0.073

M-A-M 0.069 0.061 0.059 0.040

J-J-A 0.036 0.033 0.064 0.060

S-O-N 0.014 0.012 0.046 0.037

U.S. annual and seasonal temperature trends (°C dec-1) 1895 to 2006



Trends Versus Means

Here is a not-so-well maintained or well sited USHCN station at Marysville



Can station measurements be unrepresentative, and the (adjusted) trend be representative?

°C

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

Raw data in red

Adjusted data in green



Adjusted Minimum Temperature Series at Marysville, CA

u.s. hcn temperature trends: a brief overview

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