This work studies nonhydrostatic effects (NHE) on the momentum flux of orographic gravity waves (OGWs) forced by isolated three-dimensional orography. Based on linear wave theory, an asymptotic expression for low horizonal Froude number ( where (U, V) is the mean horizontal wind, γ and a are the orography anisotropy and half-width and N is the buoyancy frequency) is derived for the gravity wave momentum flux (GWMF) of vertically-propagating waves. According to this asymptotic solution, which is quite accurate for any value of Fr, NHE can be divided into two terms (NHE1 and NHE2). The first term contains the high-frequency parts of the wave spectrum that are often mistaken as hydrostatic waves, and only depends on Fr. The second term arises from the difference between the dispersion relationships of hydrostatic and nonhydrostatic OGWs. Having an additional dependency on the horizontal wind direction and orography anisotropy, this term can change the GWMF direction. Examination of NHE for OGWs forced by both circular and elliptical orography reveals that the GWMF is reduced as Fr increases, at a faster rate than for two-dimensional OGWs forced by a ridge. At low Fr, the GWMF reduction is mostly attributed to the NHE2 term, whereas the NHE1 term starts to dominate above about Fr = 0.4. The behavior of NHE is mainly determined by Fr, while horizontal wind direction and orography anisotropy play a minor role. Implications of the asymptotic GWMF expression for the parameterization of nonhydrostatic OGWs in high-resolution and/or variable-resolution models are discussed.
Preparation of a high-efficiency, low-cost, and environmentally
friendly non-precious metal catalyst for the oxygen reduction reaction
(ORR) is highly desirable in fuel cells. Herein, a Fe–Fe
3
C-functionalized few-layer graphene sheet (Fe/Fe
3
C/FLG) nanocomposite was fabricated through the vacuum heat treatment
technique using ferric nitrate and glucose as the precursors and exhibited
a high-performance ORR electrocatalyst. Multiple characterizations
confirm that the nanosized Fe particles with the Fe
3
C interface
are uniformly distributed in the FLGs. Electrocatalytic kinetics investigation
of the nanocomposite indicates that the electron transfer process
is a four-electron pathway. The formation of the Fe
3
C interface
between the Fe nanoparticles and FLGs may promote the electron transfer
from the Fe to FLGs. Furthermore, the Fe/Fe
3
C/FLG nanocomposite
not only exhibits high ORR catalytic activity but also displays desirable
stability. Consequently, the obtained Fe/Fe
3
C/FLG nanocomposite
might be a promising non-precious, cheap, and high-efficiency catalyst
for fuel cells.
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