Future humidity trends over the western United States in the CMIP5 global climate models and variable infiltration capacity hydrological modeling system

D W Pierce,A L Westerling,J Oyler

doi:10.5194/hess-17-1833-2013

Abstract

Abstract. Global climate models predict relative humidity (RH) in the western US will decrease at a rate of about 0.1–0.6 percentage points per decade, albeit with seasonal differences (most drying in spring and summer), geographical variability (greater declines in the interior), stronger reductions for greater anthropogenic radiative forcing, and notable spread among the models. Although atmospheric moisture content increases, this is more than compensated for by higher air temperatures, leading to declining RH. Fine-scale hydrological simulations driven by the global model results should reproduce these trends. It is shown that the MT-CLIM meteorological algorithms used by the Variable Infiltration Capacity (VIC) hydrological model, when driven by daily Tmin, Tmax, and precipitation (a configuration used in numerous published studies), do not preserve the original global model's humidity trends. Trends are biased positive in the interior western US, so that strong RH decreases are changed to weak decreases, and weak decreases are changed to increases. This happens because the MT-CLIM algorithms VIC incorporates infer an overly large positive trend in atmospheric moisture content in this region, likely due to an underestimate of the effect of increasing aridity on RH. The result could downplay the effects of decreasing RH on plants and wildfire. RH trends along the coast have a weak negative bias due to neglect of the ocean's moderating influence. A numerical experiment where the values of Tdew are altered to compensate for the RH error suggests that eliminating the atmospheric moisture bias could, in and of itself, decrease runoff up to 14% in high-altitude regions east of the Sierra Nevada and Cascades, and reduce estimated Colorado River runoff at Lees Ferry up to 4% by the end of the century. It could also increase the probability of large fires in the northern and central US Rocky Mountains by 13 to 60%.

Highlights

The Earth’s climate is warming due to the accumulation of human-produced greenhouse gases in the atmosphere (IPCC, 2007)
Warmer surface temperatures will likely lead to increased evaporation and greater specific humidity, but an approximately constant relative humidity (RH); the greater concentration of water vapor will in turn warm the surface further, since water vapor is a potent greenhouse gas (GHG) (e.g., IPCC, 2007; Dessler and Sherwood, 2009)
We show future changes in surface RH over the western US projected by global climate models (Sect. 3.1), and evaluate how well these changes are captured by VIC modeling system (VMS) when it is driven by Tmin, Tmax, and P from the global model simulations (Sect. 3.2)

Summary

Introduction

The Earth’s climate is warming due to the accumulation of human-produced greenhouse gases in the atmosphere (IPCC, 2007). Warmer surface temperatures will likely lead to increased evaporation and greater specific humidity, but an approximately constant relative humidity (RH); the greater concentration of water vapor will in turn warm the surface further, since water vapor is a potent greenhouse gas (GHG) (e.g., IPCC, 2007; Dessler and Sherwood, 2009). It seems unlikely that RH will remain constant in locations far from large open bodies of water and where annual evaporative demand considerably exceeds annual precipitation, such as the arid regions of the western US Irrigation can increase surface relative humidity locally (Kueppers et al, 2007), but we do not include the effects of irrigation in this work

Objectives

Results

Conclusion