On the turbulent fluxes of buoyancy, heat and moisture at the air-sea interface at low wind speeds
On the turbulent fluxes of buoyancy, heat and moisture at the air-sea interface at low wind speeds
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DOI：
10.1029/91jc02015
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发表时间：
1991-12
影响因子：
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通讯作者：
J. S. Godfrey;A. Beljaars
中科院分区：
文献类型：
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作者：
J. S. Godfrey;A. Beljaars
在低风、不稳定的水面条件下，Liu、Katsaros 和 Businger (LKB) 对进入大气的浮力通量的估计形式为 ν1/3 ΔB4/3 g(xi)，其中 22.7ψ 是 Monin-Obukhov 长度与静水粗糙度长度的比率； ΔB 是海洋表面上方的空气与上方环境空气之间的浮力差，ν 是粘度。对于低风，获得了 g(xi) 的解析表达式； g(xi) 的最小值约为 xi=40（对应于热带海洋上的风速约为 0.3–0.6 m s−1）。对于较低的 x 值，LKB 算法的假设不再有效。对于无量纲常数的合理选择，g(xi) 在 40<xi<200 范围内仅变化 8%，略高于最小值。因此，LKB 通量估计在其适用性下限几乎与风速无关；通过零风时通量的实验室观察来确定 LKB 最小通量似乎是合理的。这些表明浮力通量由 C* Pr−⅔ ν1/3 ΔB4/3 给出，其中 C* 的测量值约为 0.14±0.01（Golitsin 和 Grachev，1986）。对于空气，Pr=0.71，这意味着 g(0) = 0.19 — 完全在 g(xi) 最小值的允许值范围内。然而，g(xi) 的最小值变化为 (γAT)1/3，其中 γ 是 Dyer-Hicks-Businger 稳定性剖面中的无量纲常数，AT 是低风时标量粗糙度长度中的常数；这可用于修复 AT。在现场应用中，行星边界层中的浮力运动确保 r.m.s.即使对于干对流，地表上方的风速 wG 通常也大于 g(xi) 最小值时的风速，但如果行星边界层通常为 100 m（而不是 1000 m）厚，则情况并非如此。在批量公式中，通过将平均风速 U 替换为 (U2+wG2)½ 来考虑这种“阵风”。
Under low-wind, unstable conditions over water, the Liu, Katsaros and Businger (LKB) estimate of the buoyancy flux into the atmosphere has the form ν1/3 ΔB4/3 g(ξ), where 22.7ξ is the ratio of the Monin-Obukhov length to the calm-water roughness length; ΔB is the buoyancy difference between air just above the ocean surface and the ambient air above, and ν is viscosity. An analytic expression for g(ξ) is obtained, for low winds; g(ξ) has a minimum at about ξ=40 (corresponding over tropical oceans to wind speeds of about 0.3–0.6 m s−1). For lower values of x the assumptions underlying the LKB algorithm are no longer valid. For a reasonable choice of dimensionless constants, g(ξ) varies only by 8% over 40<ξ<200, being just above the minimum. Thus the LKB flux estimates are nearly independent of wind speed at their lower limit of applicability; it seems reasonable to identify this LKB minimum flux with laboratory observations of fluxes at zero wind. These show that the buoyancy flux is given by C* Pr−⅔ ν1/3 ΔB4/3, where measured values of C* are about 0.14±0.01 (Golitsin and Grachev, 1986). With Pr=0.71 for air, this implies that g(0) = 0.19 — well within the allowable range of values for the minimum of g(ξ). However, the minimum in g(ξ) varies as (γAT)1/3, where γ is the dimensionless constant in the Dyer-Hicks-Businger stability profile and AT the constant in the scalar roughness length for low winds; this can be used to fix AT. In field applications, buoyant motions in the planetary boundary layer ensure that r.m.s. wind speeds wG just above the surface are generally greater than the wind speed at the minimum of g(ξ), even for dry convection — though this would not be true if the planetary boundary layer were typically 100 m thick, instead of 1000 m. This “gustiness” is allowed for by replacing mean wind speed U by (U2+wG2)½ in the bulk formulae.