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Figure 1. Location of the 74 global summary of day (GSOD) meteorological stations used in the
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analysis.
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Figure 2. For a selection of 4 stations, shown are: left column: Tdew from observations (blue) over
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the period 1990-1991, Tdew calculated by VMS (red), and Tmin for comparison (black). Middle
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column: coherence squared between the observed and VMS-calculated Tdew time series over the
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period 1975-2009. The dotted lines show the 95% confidence interval; the dashed line shows the
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95% significance level. Frequency is in units of day-1. Right column: phase (radians) between
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observed and VMS-calculated Tdew. Frequency is in units of day-1. Red dots are plotted at
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frequencies where the coherence**2 is statistically significant at the 95% level.
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Figure 3. Mean value of Tdew estimated by VMS minus Tdew from observations (C) at the
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meteorological stations, by season. The period covered is 1975-2009.
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Figure 4. As in Fig. 3, but for the RMS error (deg C).
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Figure 5. Histograms of the mean error in Tdew (VMS calculated value minus observed) at all 74
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meteorological stations in the western U.S., averaged over the indicated season, using data over
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the period 1975-2009. The vertical axis is count of stations. The blue line shows the error
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calculated using Kimball et al.'s original parameterization, while the red line shows the error
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using the parameterization as modified in VMS.
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Figure 6. Red dots: observed ratio of Tdew/Tmin - 0.0006 DTR (in degrees K, where DTR is the
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diurnal temperature range) as a function of the EF aridity parameter from Kimball et al. (1997).
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The blue line shows the correction suggested in that work.
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Figure 7. Mean Tdew bias (degrees C) in the five warmest years in the 1975-2009 record minus
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the mean bias in the five coldest years. Stratified by season (ALL = all months).
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Figure 8. Percentiles of RH trends (percentage points decade-1), annual and by season, found in a
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set of 17 CMIP3 simulations with the SRES A1B forcing scenario. Contour interval: 0.1.
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Figure 9. Number of models (from the set of 17) that show negative relative humidity trends at
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each location, by season.
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Figure 10. RH trends (percentage points decade-1) for the 8 model runs analyzed in detail. Left
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column: trend from global model run. Second column: trend produced by VMS. Third column:
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trend produced by VMS after bias correction (BC). Right column: trend produced by VMS after
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bias correction and downscaling (d/s). (Figure continues.)
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Figure 10, continued.
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Figure 11. Errors in the VIC-simulated relative humidity trend field (percentage points
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decade−1), with respect to the trend in the original global model. Contour interval is 0.1.
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Figure 12. Histograms of changes (shifts) in the estimated RH trend (percentage points decade-1)
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at all gridpoints in the western U.S. Left column (blue): RH trend in VMS minus that found in
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the global model. Center column (yellow): trend after bias correction minus trend before bias
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correction. Right column (green): trend after bias correction and downscaling minus that found
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after bias correction alone. Y axis is number of gridcells.
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Figure 13. Trend in global model Tdew and Tavg (C decade-1) for the 8 model simulations.
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Figure 14. Trend in Tdew estimated by VMS (C decade-1) for the 8 model simulations, along with
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the error in the estimated trend, i.e., the VMS trend minus the global model trend. Note that the
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error panels use a different color scale than the VMS-estimated trend panels.
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Figure 15. Tmin trend (C decade-1) for the 8 model simulations.
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Figure 16. Contribution (percentage points decade-1) to global model relative humidity trend
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arising from global model trends in Tdew and Tavg, as noted in the panel titles.
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Figure 17. The relative humidity trend (percentage points decade-1) simulated by the global
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model (left column), full VMS algorithm (middle column), and using Tmin only (right column).
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(Figure continues.)
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Figure 17, continued.
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Figure 18. Diurnal temperature range (DTR) trend, C decade-1.
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Figure 19. Trend in Kimball et. al’s EF parameter (0.001 decade-1).
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Figure 20. Trend in Kimball et. al’s EF parameter (0.001 decade-1) vs. the error in VMS-
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estimated Tdew trend (C decade-1). Also shown in blue is the best-fit linear regression (drawn as a
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solid line where the slope excludes zero at the 95% level, and dashed otherwise).
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Figure 21. Changes in annual runoff (%) given an imposed, fixed 1.5 C decrease in Tdew. Note
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the nonlinear color scale.
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