The Study Of The Parameterization Of The Stratified Upper Ocean Mixing

The upper ocean is the main place of ocean water motion where the exchange of movement,heat and moisture between the atmosphere and the ocean take place.The mixing of the vertical momentum,heat and material caused by turbulence is an important physical process in the upper ocean.This process can not only affects the horizontal and vertical exchange of momentum,heat and material,but also affects the energy transfer among different scale motions as well as the energy transform between kinetic energy,potential energy and thermal energy.The vertical mixing motion will further affect the generation and development of the ocean fronts as well as the temporal and spatial distribution and variation of potential vorticity and stratification structure of the ocean.The vertical mixing has an important influence on the temporal and spatial variation of sea surface temperature,the meridional heat transport and the upper ocean heat content,and then affects the oceanic thermohaline circulation.This will further influence the variation of climate and environment.Therefore,the study of the upper ocean vertical mixing process is of profound significance to the study of the ocean circulation and climate change.The vertical eddy viscosity coefficient and vertical eddy diffusion coefficient are used to represent the momentum,heat and vertical mixing intensity in the numerical ocean model.Turbulent motion should be resolved using a sub-grid parameterization because the ocean model resolution is too coarse for turbulent motion to be modeled effectively.Therefore,the parameterization of the vertical mixing intensity has a direct impact on the simulation results of the numerical model.The turbulent mixing induced momentum transport mostly are shear and convergence transfer,but most of the second-moment turbulent closure model only reflect the shear transfer of momentum.The Langmuir circulation induced momentum transfer is similar to the magnetic flux generated by a current circuit,it has a significant effect to the vertical mixing and also need to introduced in the parameterization of the vertical mixing coefficients.To overcome these defects,a more reasonable expressions of the vertical eddy viscosity and diffusivity than the available ones is proposed in this work which to reduce these errors in the simulation of the upper ocean turbulence mixing processes.A modified k-? model(include the turbulent kinetic energy and turbulent frequency transport equations)was developed to include the Langmuir turbulent effect.The new derived vertical eddy viscosity and diffusivity stability function based on a second-moment model which containing the influence of Langmuir turbulence used in this modified model.At the same time,a parametric approach is used to modify the pressure covariance by introducing buoyancy,anisotropic and vorticity productions induced by velocity shear.In the modified model,Lagrangian mean velocity is used to instead of the Eulerian mean velocity in the model,and the Langmuir turbulent production are included in the turbulent kinetic energy equation as well as the turbulent frequency transport equation.One-dimensional numerical experiments for horizontal homogeneous flows were carried out and the results were compared with Large-eddy simulations to analyze the difference between simulation results of the new model and some existing models.Results indicate that the new model is more accurate than other models in terms of the vertical eddy viscosity,vertical turbulence velocity variance,kinetic energy and mean velocity profile,when compared with large eddy simulation results.The simulated sea surface temperature(SST)and mixing layer depth of the new model were also compared with observations at Papa ocean weather station.When considering Langmuir turbulent effect,the modified k-? model can overcome the shortcoming that the simulated SST is higher than observations.The k-? model performs better than the MY25 model,although both of them considered the Langmuir turbulent effect.In order to study the effect of the turbulent buoyancy flux to the upper ocean mixing,the sensitivity experiment are done in conditions of with and without the buoyancy production term respectively using the modified k-? model which include the Langmuir turbulence effect.These tests were carried out for four different typical regions.The stratification stability of these four regions are different,the buoyancy frequency value is high and the mixed layer is deep in high latitude,the buoyancy frequency value is low and the mixed layer is shallow in the middle and low latitude regions.But the stratified stabilities are weak in high latitude and strong in low latitude.The simulation results show that: in condition of considering the buoyancy production term,the vertical eddy viscosities varied.The vertical mixing rate can affect the numerical model simulated total energy.When the vertical mixing rate increased,the simulated total energy increase and vice versa.The buoyancy production tensor has little effect to the vertical profiles of mean velocity and turbulence kinetic energy.Considered the buoyancy production tensor can improve the simulated temperature and salinity profile and made them more consistent with the observation profile.Also,this improved k-? second-moment closure model was introduced into the three-dimensional Regional Ocean Modeling System(ROMS).The influence of Langmuir circulation induced mixing scheme on ocean circulation model was tested with the ROMS.Numerical experiment results indicated that the proposed model which includes the Langmuir circulation can improve the simulation accuracy of the mixed layer depth,sea surface temperature(SST).It is also found that the variation of the vertical mixing strength can influence the Sverdrup transport in baroclinic condition since the motion of sea water obeys the conservation law of potential vorticity.When the vertical mixing intensity increases,the pole-ward transport became stronger.There is obvious correlation between vertical eddy viscosity and meridional Sverdrup transport in the equatorial region.The new derived mixing coefficient also can improve the simulation of typhoon path.