Pressure-driven Ventilation of Homogenous Seasonal Snow Layers

Surface snow is a "gateway" between the atmosphere and snowpack that mediates the potential energy source for air movement in snow and firn. Air pressure changes above snow elicit in-snow pressure gradients that spawn air movement in snowpack. This process, termed "pressure-pumping" or "windpumping" may enhance sublimation from the snow surface for high-frequency pressure changes and broaden the snow-air age difference at depth for low-frequency pressure changes. The magnitudes of the various effects of this pressure-pumping process are not well characterized but are thought to depend on several factors including permeability, heterogeneity, and frequency-dependent amplitude of the pressure changes. We present field-based experimental results of high-frequency pressure measurements acquired in a vertical column of homogenous surface snow layers. Previous experiments have shown that layering has a first-order effect on in-snow pressure changes so we deployed the measurement system exclusively in the topmost, new-fallen snow layer for 14 cases that span a broad range of snow density and permeability. Results indicate that high-frequency pressure changes have greater amplitude at a given wind speed than that diagnosed using Colbeck's (1989) empirical formula. High-frequency pressure changes attenuate monotonically with depth through a homogenous snow layer and attenuation with depth is greater than theory suggests. The implications of this research are that it is feasible that pressure changes on the order of seconds to minutes have sufficient amplitude and frequency to increase sublimation rate when the vapor pressure deficit directly above the snow is sufficiently large. Perturbation pressure attenuation was not evident in any of the cases at frequencies below 4 × 10-2 Hz. This result suggests that low-frequency pressure changes transmit across a homogenous, permeable snow layer with negligible attenuation.