No abstract

Abstract. Pressure pumping at the Earth's surface is caused by short-period atmospheric turbulence, longer-period barometric changes, and quasi-static pressure fields induced by wind blowing across irregular topography. These naturally occurring atmospheric pressure variations induce periodic fluctuations in airflow through snowpacks, soils, and any other porous media at the Earth's surface. Consequently, the uptake or release of trace gases from soils and snowpacks is a combination of molecular diffusion and advection forced by pressure pumping. Using model-estimated fluxes, this study attempts to quantify the influence that turbulent pressure fluctuations with periods between 0.1 and 1000 s can have on the rate of exchange of CO2, N20, and CH 4 through a seasonal snowpack. Data for this study were collected at a forested subalpine meadow site in the Rocky Mountains of southern Wyoming, during February 1995 when the snowpack was distinctly layered and approximately 1.4 m deep. The data include mole fraction of CO2, N20, and CH 4 just above and at the base of the snowpack, several profiles of CO2, N20, and CH 4 mole fraction in the top 1 m of the snowpack, and a profile of snowpack density and tortuosity. Turbulent atmospheric pressure-pumping fluctuations, sampled at approximately 11 Hz for several hours, were obtained with a fast response differential pressure sensor. A onedimensional steady state diffusion model and one-and three-dimensional time-dependent pressure-pumping models are used to estimate the gas fluxes through the snowpack. Boundary conditions are provided by grab samples just above the snowpack and at the soil/snow interface. The pressure-pumping models are driven by the observed pressure fluctuations, and all models include the observed layering of the snowpack. As with previous studies the present results indicate that the effects of pressure pumping are diminished with increasingly strong gradients. Furthermore, we conclude that unless pressure pumping influences the gas concentrations at the boundaries of the snowpack, it appears unlikely that it can significantly impact the rate of gaseous diffusion through the snowpack. Even two-and three-dimensional effects, which can have a significant shortterm impact on the fluxes and concentration profiles, are nearly eliminated when averaged over a period of hours. It is also suggested that vertical layering is important for threedimensional pressure-pumping studies and that the time-dependent temperature term, which is traditionally ignored when modeling dynamic pressure variations, may in fact be dominant in some situations and probably should be incorporated in future modeling studies of pressure pumping.

[1] We report on the interannual variability of evapotranspiration (E) and energy exchange of an annual grassland in the Mediterranean climate zone of California. They were measured directly with the eddy covariance technique over a 6-year period that spanned between July 2001 and June 2007 and experienced a large range in precipitation (376 mm to 888 mm). Despite a two-fold range in precipitation, annual E ranged much less, between 266 mm and 391 mm. We found that pronounced energy-limited and water-limited periods occurred within the same year. In the water-limited period, monthly integrated E scaled negatively with solar radiation and was restrained by precipitation. In the energy-limited period, on the other hand, the majority of E scaled positively with solar radiation (R g ) and was confined by potential E (E p ). E was most sensitive to the availability of soil moisture during the transition to the senescence period rather than onset of the greenness period, causing annual E to be strongly modulated by growing season length. Bulk surface conductance scaled consistently with Priestley-Taylor a coefficient regardless of interannual and seasonal variability of precipitation, E, and solar radiation.

No abstract

High rural concentrations of ozone (O3) are thought to be either stratospheric in origin, advected from upwind urban sources, or photochemically generated locally as a result of natural trace gas emissions. Ozone is known to be transported vertically downward from the above‐canopy atmospheric surface layer and destroyed within stomata or on other biological and mineral surfaces. However, here we report winter‐time eddy correlation measurements of vertical O3 flux above a subalpine canopy of Picea engelmannii and Abies lasiocarpa in the Snowy Range Mountains of Wyoming that indicate anomalous upward O3 fluxes Upward fluxes of 0.5 µg m−2 s−1 (11 kg km−2 day−1) were routinely measured during the 1991–92 winter season. Decreasing O3 concentration from several hours to several days that relate to increasing positive O3 flux magnitudes and visa versa, suggest O3 may be temporarily stored in the snow base.