Study of the Turbulence Characteristics of the Bottom Ekman Layer in the Persian Gulf using Large Eddy Simulation Approach

In this study, the turbulent properties of the bottom Ekman layer of the Persian Gulf is studied, using a Parallelized Large-Eddy Simulation Model (PALM). Three numerical experiments were carried out with emphasis on stratification effects. A reference experiments (EXP C) without vertical gradients of the potential temperature and salinity and two experiments with vertical gradient of the potential temperature and salinity. The initial values of the surface potential temperature and salinity and their vertical gradients, provided from in situ data, were chosen according to August (EXP A) and November (EXP N) condition of the Persian Gulf. The eastern part of the Iranian coastal area of the Persian Gulf near the Hormuz Strait was chosen because there is a considerable western current during the year in this area. Also, the sea is deep enough to observe a distinctive pycnocline layer which separates surface and bottom mixed layer. The domain size is 200m×200m×100m in x, y and z directions respectively. A pycnocline layer with 40m and 20m deep was considered for August and November, respectively. A Geostrophic current with 0.15m s^(-1) speed is supposed to flow in x direction over the rough sea bed. The bottom boundary condition of the momentum flux was set to Dirichlet in order to create a no-slip condition at the sea bed. The simulations were carried out for 48h to include at least two inertial periods and avoid inertial oscillations. The results showed that the stratification limits the bottom Ekman layer depth and it does not grow with time. While in EXP C, where the fluid is neutral, a rapid growth of the bottom Ekman layer is obvious during the first 20h and its maximum depth reaches 60m. The Ekman cross-stream current component cannot entrain into pycnocline layer and it vanished at the bottom of the pycnocline layer. In autumn in which the pycnocline layer is thinner, the Ekman spiral is broadened and the magnitude of the Ekman cross-stream current component is 25% larger in compare to summer. The maximum value of the Ekman cross-stream current component is about 0.04m s^(-1) in EXP C and EXP A while it is about 0.05m s^(-1) in EXP N. The hodograph of the horizontal velocity in EXP C is more similar to Ekman theoretical solution. The stream-wise component of the horizontal velocity decrease with the same rate near the sea bed in all experiments which implies that the stratification does not have much effect on bottom stress. It is concluded that when an intense pycnocline exists and the bottom mixed layer is thin, less time is needed to trigger the turbulence. The bulk turbulent kinetic energy in all experiments is the same. Since the bottom boundary is assumed adiabatic and there is no heat flux from the bottom, the heat budget in the neutral BBL is approximately conserved (molecular diffusion is not considerable compared). Then a pycnocline is necessary to maintain heat conservation after the formation of a mixed layer. These intensified pycnocline can be observed as distinctive peaks in vertical profiles of the buoyancy frequencies near the top and bottom of the stratified layer. The thickness of the intensified pycnocline grows with time and at the end of simulation reaches about few meters. These intensified pycnocline layers in November is thicker than that of August. Also at the bottom interface of the stratified, the shear stress increases.