Yign Noh

A New Vertical Diffusion Package with an Explicit Treatment of Entrainment Processes

Abstract This paper proposes a revised vertical diffusion package with a nonlocal turbulent mixing coefficient in the planetary boundary layer (PBL). Based on the study of Noh et al. and accumulated results of the behavior of the Hong and Pan algorithm, a revised vertical diffusion algorithm that is suitable for weather forecasting and climate prediction models is developed. The major ingredient of the revision is the inclusion of an explicit treatment of entrainment processes at the top of the PBL. The new diffusion package is called the Yonsei University PBL (YSU PBL). In a one-dimensional offline test framework, the revised scheme is found to improve several features compared with the Hong and Pan implementation. The YSU PBL increases boundary layer mixing in the thermally induced free convection regime and decreases it in the mechanically induced forced convection regime, which alleviates the well-known problems in the Medium-Range Forecast (MRF) PBL. Excessive mixing in the mixed layer in the presenc...

IMPROVEMENT OF THE K-PROFILE MODEL FOR THE PLANETARY BOUNDARY LAYER BASED ON LARGE EDDY SIMULATION DATA

Modifications of the widely used K-profile model of the planetary boundary layer (PBL), reported by Troen and Mahrt (TM) in 1986, are proposed and their effects examined by comparison with large eddy simulation (LES) data. The modifications involve three parts. First, the heat flux from the entrainment at the inversion layer is incorporated into the heat and momentum profiles, and it is used to predict the growth of the PBL directly. Second, profiles of the velocity scale and the Prandtl number in the PBL are proposed, in contrast to the constant values used in the TM model. Finally, non-local mixing of momentum was included. The results from the new PBL model and the original TM model are compared with LES data. The TM model was found to give too high PBL heights in the PBL with strong shear, and too low heights for the convection-dominated PBL, which causes unrealistic heat flux profiles. The new PBL model improves the predictability of the PBL height and produces profiles that are more realistic. Moreover, the new PBL model produces more realistic profiles of potential temperature and velocity. We also investigated how each of these three modifications affects the results, and found that explicit representation of the entrainment rate is the most critical.

Simulations of temperature and turbulence structure of the oceanic boundary layer with the improved near-surface process

An improved model for the oceanic boundary layer is presented in view of the recent observation of the microstructure of the upper ocean including the high dissipation rate near the sea surface. In the new model the surface boundary conditions for both the turbulent kinetic energy flux and the roughness length scale are modified. The parameterization of stratification effects on turbulence is improved, and the convective process is reformulated on the basis of the observation of uniform temperature and velocity profiles within the convective mixed layer. Evolutions of the profiles of both the dissipation rate and temperature of the observation data Patches Experiment as well as the time series of the sea surface temperature over the observation days, are successfully simulated during a diurnal cycle for the first time. It is also shown that the model reproduces various important features of the oceanic boundary layer, for example, the formation of a diurnal thermocline, the profiles of buoyancy flux, and the magnitudes of the buoyancy gradients both within the mixed layer and at the diurnal thermocline. Performance of the model is compared with that of the widely used Mellor-Yamada model.

Large Eddy Simulation of the Ocean Mixed Layer: The Effects of Wave Breaking and Langmuir Circulation

Abstract Large eddy simulation (LES) of the ocean mixed layer was performed in which both wave breaking and Langmuir circulation are realized. Wave breaking was represented by random forcing consistent with the observed near-surface turbulence, and Langmuir circulation was realized by the Craig–Leibovich vortex force. High- resolution simulations were carried out using parallel computing with or without each contribution, wave breaking and Langmuir circulation, with an aim to clarify their respective roles in the ocean mixed layer. The effects of wave breaking were found to be mainly limited to the near-surface zone of the upper few meters. Langmuir circulations below it are not significantly modified, although they become somewhat weakened and less coherent. Under the influence of wave breaking, however, the turbulence production in the upper-ocean mixed layer becomes dominated by the turbulent kinetic energy flux, contrary to the case of the atmospheric boundary layer where it is dominated by shear prod...

The North Pacific Climate Transitions of the Winters of 1976/77 and 1988/89

Abstract This paper examines characteristic changes in North Pacific sea surface temperature (SST) variability during the boreal winter (December–February) for two subperiods (1956–88 and 1977–2009) during which the 1976/77 and the 1988/89 climate transitions occurred. It is found that the Pacific decadal oscillation (PDO)-like SST variability plays a dominant role in the 1976/77 climate transition, while both the North Pacific Gyre Oscillation (NPGO)-like and PDO-like SST variability contribute to the 1988/89 climate transition. Furthermore, the leading mode changes from PDO-like SST variability during the period 1956–88 to NPGO-like SST variability during the period 1977–2009, indicative of an enhancement of NPGO-like SST variability since 1988. Changes in sea level pressure across the 1976/77 climate transition project strongly onto the Aleutian low pressure system. But sea level pressure changes across the 1988/89 climate transition project primarily onto the North Pacific Oscillation, which is associ...

Simulation of More Realistic Upper-Ocean Processes from an OGCM with a New Ocean Mixed Layer Model

A new ocean mixed layer model (OMLM) was embedded into an ocean general circulation model (OGCM) with the aim of providing an OGCM that is ideal for application to a climate model by predicting the sea surface temperature (SST) more accurately. The results from the new OMLM showed a significant improvement in the prediction of SST compared to the cases of constant vertical mixing and the vertical mixing scheme by Pacanowski and Philander. More accurate prediction of the SST from the new OMLM reduces the magnitude of the restoring term in the surface heat flux and thus provides a simulated ocean that can be coupled to the atmospheric general circulation model more naturally. The new OMLM was also shown to improve various other features of the OGCM such as the mixed layer depth and the equatorial circulation.

A new method for large-eddy simulations of clouds with Lagrangian droplets including the effects of turbulent collision

In this paper, a new Lagrangian cloud model (LCM) is introduced in which the flow field is simulated by large-eddy simulation, and the droplets are treated as Lagrangian particles responding to the simulated flow field. In order to handle the extremely large number of droplets within a cloud, the concept of a super-droplet, which represents a large number of real droplets of the same size, is introduced, and the number of contributing real droplets is called the weighting factor. A novel method is developed to realize the collision/coalescence of droplets, in which the consequent variation of the droplet spectrum is represented in terms of the modification of the radius and weighting factor of super-droplets, while keeping the number of super-droplets unchanged. Using an idealized single cloud and trade wind cumuli, the LCM is shown to reproduce the general features of shallow cumulus clouds in agreement with traditional bulk models. The droplet spectrum simulated by the LCM, using collision kernels with and without the effects of turbulence, also shows a pattern consistent with the spectral bin model. Furthermore, the sensitivity of the LCM to two model parameters, the time step and the number of super-droplets, is examined.

The transition in the sedimentation pattern of a particle cloud

Experiments were carried out to study the sedimentation of a two‐dimensional particle cloud. When a large number of particles (glass beads) of uniform size are released from a two‐dimensional opening into a column of fresh water, the mixture initially descends as a thermal; however, after some time, the particles start settling individually, thus leaving the parent fluid behind. For a given type of particle, the critical depth zc at which this transition occurs, measured from a virtual origin, was found to change as zcws/ν∼(Q/νws)α, with α≂0.3, where ws is the terminal velocity of a single particle, ν is the kinematic viscosity, and Q is the total buoyancy of the released particles per unit length. The descending velocity and the spatial growth of the particle cloud were found to depend on its sedimentation characteristics.

Dynamics of diurnal thermocline formation in the oceanic mixed layer

Abstract Diurnal thermocline formation in the oceanic mixed layer under a stabilizing buoyancy flux is studied by numerical simulation of a turbulence model in which the interaction between turbulence structure and density stratification is taken into consideration, and the mechanism for its formation is clarified based on the results. From the simulations, the flux of turbulent kinetic energy is a dominant source of turbulence in the upper mixed layer and plays an indispensable role for the formation of a diurnal thermocline; below the diurnal thermocline, turbulence is maintained by shear production, which causes the growth of diurnal thermocline thickness. The flux Richardson number at the diurnal thermocline maintains a constant value (about unity), regardless of the shear stress and buoyancy flux at the sea surface, and the diurnal thermocline depth grows more slowly than predicted by the Monin-Obukhov length scale. The model results are compared with the observational data, and the assumptions intro...

Interaction of a negatively buoyant line plume with a density interface

Laboratory experiments were carried out to investigate the interaction between turbulent line buoyant plumes and sharp density interfaces, with the aim of using the results to interpret oceanic observations pertinent to crack openings in the polar ice-cap (leads). These openings take the form of long narrow channels, and are often modeled as line bouyant plumes. The plumes descend as in a homogenoous fluid, impinge on the density interface, and then spread horizontally as gravity currents. Depending on the Richardson number Ri = Δ bld/q023, where Δb is the buoyancy jump across the interface, lD is the half-width of the plume before the impingement and q0is the buoyancy flux per unit length of the source, different flow patterns were identified. When Ri 10, the penetration was small and a sharp-nosed gravity current emerged some time after the impact. Measurements were made on the penetration depth, the velocities of the gravity current and the subsurface flow towards the plume, the entrainment rate and other wave parameters. Possible implications of the results for oceanic cases are also discussed.