Budgets of turbulent kinetic energy and scalar variance in the continental shelf bottom boundary layer

Abstract

A local turbulent kinetic energy (TKE) budget, in which dissipation balances production minus buoyancy flux, and a nonlocal turbulent scalar variance (TSV) budget, in which dissipation balances production minus TSV flux divergence, are tested observationally. The calculations are based on 6 week long records of velocity and sound speed measured between 0.7 and 5.4 m above bottom on the New England shelf. Estimates of fluxes and gradients are direct, while estimates of dissipation rates are indirectly obtained from inertial subrange spectra. Within 2 m of the bottom, buoyancy flux is not an important component of the TKE budget (flux Richardson number Rf < 0.05, on average). At 4.35 m above the bottom, the highest resolved elevation, the direct effect of stratification is more pronounced (Rf = 0.13, on average). The local TKE balance is essentially closed by the measurements within 2 m of the bed (r2 > 0.8 and regression coefficient = 0.89 ×/÷ 1.06 and 1.08 ×/÷ 1.05 at 1.65 and 0.74 m above the bottom, respectively). The local TKE budget at 4.35 m above the bed is not closed, however. TSV production and dissipation estimates are strongly correlated at all heights (r2 > 0.64), but dissipation is systematically greater than production, particularly at 0.74 m above the bed (regression coefficient = 2.42 ×/÷ 1.28). Including TSV flux divergence in the balance reduces, statistically significantly, the observed discrepancy. The implications of the results for turbulence closure models and microstructure diffusivity estimates are discussed. Copyright 2001 by the American Geophysical Union.