Global characteristics of gravity waves in the lower stratosphere are examined using a GCM with high resolution in both the horizontal (T106, corresponding to about 120 km) and the vertical (ϳ600 m). The bottom boundary condition of the model is that of an aquaplanet with perpetual February sea surface temperature. The simulated gravity waves are in good agreement with mesosphere-stratosphere-troposphere (MST) radar observations at a middle latitude on the gravity wave structure and on the frequency spectra as a function of height. The frequency spectra of simulated wind and temperature fluctuations are also examined as a function of latitude. Large values of spectral density are observed at frequencies higher than the inertial frequency (f) in a weak wind region around 20 km, which is consistent with the characteristics of internal gravity waves. An isolated peak is observed near f for horizontal wind spectra at latitudes higher than 10Њ, while the energy is distributed in a wide range of frequency at lower latitudes where f approaches zero. Further analysis is performed of those fluctuations having periods shorter than 24 h and those having vertical wavelengths smaller than 5 km. These are frequently analyzed as gravity waves using observation data. The distribution of energy and momentum fluxes in the latitude-height section is examined. The result indicates that short-period waves mostly propagate upward and poleward from the equatorial region. The wave energy reaches about 50Њ lat at the 27-km altitude. A negative (positive) maximum of vertical flux of meridional momentum (ЈwЈ) is seen above the subtropical jet in the Northern (Southern) Hemisphere for small vertical-scale gravity waves. This is consistent with the preferred equatorward propagation of the wave indicated by a statistical analysis based on MST radar observations. The ratio of potential to kinetic energy maximizes over the equator and decreases poleward. The Eliassen-Palm flux divergence associated with gravity waves is decelerative above the subtropical jet, albeit small.
The water retention curve (WRC), which shows the relationship between the volumetric liquid water content,θv, and suction,h, is a fundamental part of the characterization of hydraulic properties. Therefore, the formulation of the WRC as a function of snow characteristics is essential for establishing a model of water movement through the snow cover. In this study, we measured the WRC of several snow samples, which had different characteristics (grain size, bulk dry density and grain type), using a gravity drainage column experiment and then analysed these data using the Van Genuchten soil physics model (VG model). The shape of the WRC depended strongly on both the sample grain size,d, and bulk dry density,ρ. Therefore, we introduced the parameterρ/dto model the WRC of snow. The relationships between the parametersαandnof the VG model andρ/dchange with grain type. For melt forms,α, which is related to the inverse value of the air-entry suction, increases quickly asρ/ddecreases, whereasn, which is related to the gradient ofθvvsh, increases withρ/d. Conversely, neither of these parameters of the VG model for rounded grains showed obvious dependence onρ/d. These results suggest that water movement through snow cover can be modelled using grain size, bulk dry density and grain type based on the soil physics model.
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