2006 PSL Seminars

ESRL/PSL - Western Water Assessment Joint Seminar

Thomas Painter

National Snow and Ice Data Center

(with Andrew P. Barrett,Core R. Lawrence, Chris C. Landry, and Jason C. Neff )

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Winter and spring storms entrain radiatively absorbing dust from desert regions and redistribute optically thick layers to the snow cover of the Rocky Mountains as wet and dry deposition. Whereas dust loading in the atmosphere temporarily decreases the surface irradiance through scattering and absorption, dust loading at the snow surface, which persists well beyond the atmospheric presence of the dust event, positively forces tropospheric temperatures through direct and indirect effects. Absorption by dust in the snow increases near-surface snowpack temperatures, decreasing the column cold content of the snowpack and increasing the energy available for melt. Enhanced absorption represents the direct effect of dust deposition on the regional radiative budget. Indirect effects occur as associated increases in snow grain size (further lowering albedo) and the more rapid snowpack ablation that reveals a darker substrate.

In the years 2003-2005, we observed 3-4 significant dust deposition events per year in winter and spring, whereas after the weak monsoon and intense drought through winter 2006 we observed 8 significant depositions including the multi-state heavy deposition on February 15 not seen in 20 years. Our monitoring of surface radiative fluxes and discharge commenced in winter 2005 at alpine and subalpine meteorological towers in the San Juan Mountains, CO. Spring season mean daily surface radiative forcings increased from 21 W m-2 in 2005 (mean desert precipitation) to 36 W m-2 in 2006 (intense drought). Snowmelt modeling showed that in 2005, dust radiative forcing affected a 20-25 day earlier melt whereas in 2006 dust radiative forcing affected a 30 day earlier ablation. With continued soil disturbance and projected drought in the southwest US under global warming scenarios (Hansen et al., 2005; Cook et al., 2004), the 2006 season may represent a new regime of dust forcing of radiative and hydrologic systems in the intermountain west.

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Wednesday 29 November, 2006
2:00 PM (Refreshments at 1:50 pm)
DSRC, PSL-South Conference Room (1D403)

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