Vertical Turbulent Transport Research Articles

Turbulent transport of sea-spray in the coastal region

A realistic three-dimensionnal large-eddy simulation is performed for the study of turbulent transport of sea spray aerosol in the coastal region of Le Croisic, France. A new transport model for the aerosol is implemented in the ARPS code. Simulation results are compared with field measurements, both for the mean wind field and aerosol concentration. The numerical results fit well with the observations. The mean vertical concentration profile takes an exponential shape when the data is averaged over sufficiently long timeframe, whereas the 15-min averaged profiles vary and deviate from the theory. The transport of the aerosols is analyzed in relation to the sea-land transition and the changes in thermal stability of the atmosphere during the diurnal cycle. Turbulence is found to play an important role in the mixing of aerosols in the unstable surface layer. The turbulent vertical transport of aerosols is enhanced through convective cells over the land during the day, whereas, at night, aerosols remain trapped near the surface and are transported over appreciable horizontal distances under quasi-neutral or stable thermal conditions.

Relationship between Fine Layering of Stratified Water Environment and Vertical Turbulent Mass Transport

Relationship between Fine Layering of Stratified Water Environment and Vertical Turbulent Mass Transport

Parameterization of Vertical Turbulent Transport in the Inner Core of Tropical Cyclones and Its Impact on Storm Intensification. Part I: Sensitivity to Turbulent Mixing Length

Abstract In the inner core of a tropical cyclone, turbulence not only exists in the boundary layer (BL) but can also be generated above the BL by eyewall and rainband clouds. Thus, the treatment of vertical turbulent mixing must go beyond the conventional scope of the BL. The turbulence schemes formulated based on the turbulent kinetic energy (TKE) are attractive as they are applicable to both deep and shallow convection regimes in the tropical cyclone (TC) inner core provided that the TKE production and dissipation can be appropriately determined. However, TKE schemes are not self-closed. They must be closed by an empirically prescribed vertical profile of mixing length. This motivates this study to investigate the sensitivity of the simulated TC intensification to the sloping curvature and asymptotic length scale of mixing length, the two parameters that determine the vertical distribution of a prescribed mixing length. To tackle the problem, both idealized and real-case TC simulations are performed. The results show that the simulated TC intensification is sensitive to the sloping curvature of mixing length but only exhibits marginal sensitivity to the asymptotic length scale. The underlying reasons for such sensitivities are explored analytically based on the Mellor and Yamada level-2 turbulence model and the analyses of azimuthal-mean tangential wind budget. The results highlight the uncertainty and importance of mixing length in the numerical prediction of TCs and suggest that future research should focus on searching for physical constraints on mixing length, particularly in the low- to midtroposphere, using observations and large-eddy simulations. Significance Statement The parametric representation of subgrid-scale turbulent mixing is one of the major sources of uncertainty in numerical predictions of tropical cyclones (TCs). This study investigates how the numerical prediction of TC intensification is affected by the turbulent mixing length, a length scale that is required to close a turbulence scheme formulated based on the turbulent kinetic energy (TKE). The research highlights the uncertainty and importance of mixing length in numerical prediction of TCs and suggests that future research should focus on searching for physical constraints on the mixing length, particularly in the low- to midtroposphere, using observations and large-eddy simulations.

Parameterization of Vertical Turbulent Transport in the Inner Core of Tropical Cyclones and Its Impact on Storm Intensification. Part II: Understanding TC Intensification in a Generalized Sawyer–Eliassen Diagnostic Framework

Abstract An analytical method for diagnosing the interaction between the primary and secondary circulations of a tropical cyclone (TC) and vortex intensification is developed. It includes a diagnostic equation describing the mean secondary circulation of a TC in an unbalanced framework by including the radial eddy forcing in the analytical system. It is an extension of the Sawyer–Eliassen equation (SEE) developed from the strict gradient-wind balance. This generalized SEE (GSEE) remediates some of the limitations of SEE and can be used to diagnose both balanced and unbalanced dynamical processes during the TC evolution. Using GSEE, this study investigates how the tangential and radial eddy forcing affects the TC intensification simulated by the Hurricane Weather Research and Forecasting Model (HWRF) with differently parameterized turbulent mixing. The diagnostic results show that the supergradient component of radial eddy forcing contributes positively to the acceleration of the peak tangential wind, whereas the subgradient component of the radial eddy forcing tends to lower the height of peak tangential wind. The relative importance of negative and positive effects of tangential eddy forcing on TC intensification varies depending on the details of turbulence parameterization. For a turbulent kinetic energy (TKE) scheme used in this study, a large sloping curvature of mixing length in the low troposphere causes the tangential eddy forcing to produce a net positive tangential wind tendency near the location of the peak tangential wind. In contrast, a small sloping curvature of mixing length generates a net negative tangential wind tendency at the peak tangential wind. Significance Statement The interaction between the primary and secondary circulations of a tropical cyclone (TC) plays a key role in TC evolution. Historically, the secondary circulation induced by turbulence and convection is often described by a so-called Sawyer–Eliassen equation (SEE). While SEE has provided much insight into the TC dynamics in the past, the assumption of gradient-wind balance used by SEE prevents it from understanding TC unbalanced dynamics. To remediate the limitation, we extended the analytical framework into the unbalanced regime by including radial eddy forcing in the analytical system and derived a generalized SEE (GSEE). Using GSEE, this study investigates how tangential and radial eddy forcing affects TC intensification. The result highlights the importance of multiple roles that turbulence plays in the intensification of TCs.

Flow Field Characteristics at the Spanwise Edge of a Vegetative Canopy Model

This study investigates the complex flow field across a spanwise vegetative model canopy edge focusing on turbulent transport processes. Utilizing stereoscopic particle image velocimetry, the three velocity components were measured in wall-parallel planes at various elevations within canopy across the spanwise canopy edge. Conventional ensemble averaged results were contrasted with those obtained by conditionally averaged flow properties across instantaneous internal interfaces in the flow to understand their contribution to the ensemble average. The conditional average captured the strong gradients in mean velocities, Reynolds stresses, vorticity, swirling strength, and turbulent kinetic energy production across the dynamically changing instantaneous interface. In contrast, the conventional ensemble average smeared out the strong gradients. Small magnitudes of advective terms in the turbulent kinetic energy transport equation suggested weak secondary transverse flows in the present model canopy. The turbulent flow structure across the spanwise canopy edge was further investigated using Quadrant-Hole analysis for both averaging approaches. Conventional ensemble averaged results indicated a shift from sweep to ejection dominance when moving from canopy into the open patch, while the conditional average showed only sweep dominated transport. In contrast to a homogeneous canopy layout, below canopy height at the canopy edge, sweeps and ejections lose their dominance in vertical turbulent transport. The present results show that the dynamics of internal interfaces govern the ensemble averaged results and a possible implementation into existing models is proposed. The present results are expected to increase understanding of spanwise turbulent transport and aid in developing strategies to mitigate desertification.

Investigating the Potential of Using Mixdown Altitudes to Forecast Peak Wind Gusts

Abstract Forecast models based on the gust factor, the ratio of peak gust to sustained wind speed, have shown promise in predicting peak wind gusts in recent years. These models assume that turbulent vertical transport forced by wind flowing over upstream terrain mixes high-momentum air aloft down to the surface. A recently constructed database of hourly peak gusts, together with approximately coincident, nearby upper-air wind observations, is used to identify mixdown altitudes, the altitudes from which peak gusts descend, during different weather scenarios at 16 locations across the United States. Median mixdown altitudes generally ranged from 50 to 450 m AGL with occasional exceptions, particularly for convective gusts and at mountainous locations where terrain effects are likely to amplify gusts. A mixdown model in which surface peak gusts are predicted by obtaining forecast upper-air winds within this altitude interval was developed and tested. Our results suggest that a mixdown model methodology for forecasting peak gusts may be feasible at locations and during weather conditions where terrain-forced turbulent mixing is the principal cause of wind gusts.

Mesoscale Ocean–Atmosphere Coupling Effects on the North Pacific Subtropical Mode Water

Abstract Subtropical mode water (STMW) is a thick layer of water mass characterized by homogeneous properties within the main pycnocline, important for oceanic oxygen utilization, carbon sequestration, and climate regulation. North Pacific STMW is formed in the Kuroshio Extension region, where vigorous mesoscale eddies strongly interact with the atmosphere. However, it remains unknown how such mesoscale ocean–atmosphere (MOA) coupling affects the STMW formation. By conducting twin simulations with an eddy-resolving global climate model, we find that approximately 25% more STMW is formed with the MOA coupling than without it. This is attributable to a significant increase in ocean latent heat release primarily driven by higher wind speed over the STMW formation region, which is associated with the southward deflection of storm tracks in response to oceanic mesoscale imprints. Such enhanced surface latent heat loss overwhelms the stronger upper-ocean restratification induced by vertical eddy and turbulent heat transport, leading to the formation of colder and denser STMW in the presence of MOA coupling. Further investigation of a multimodel and multiresolution ensemble of global coupled models reveals that the agreement between the STMW simulation in eddy-present/rich coupled models and observations is superior to that of eddy-free ones, likely due to more realistic representation of MOA coupling. However, the ocean-alone model simulations show significant limitations in improving STMW production, even with refined model resolution. This indicates the importance of incorporating the MOA coupling into Earth system models to alleviate biases in STMW and associated climatic and biogeochemical impacts. Significance Statement North Pacific subtropical mode water (STMW) is a distinct pycnostad within the main thermocline located south of the Kuroshio Extension. As short-term heat and carbon silos, STMW is traditionally thought to be driven by the basin-scale atmospheric forcing. The role of air–sea interactions at mesoscales residing in the Kuroshio Extension region has been overlooked. Here, we demonstrate that the strong thermal feedback of mesoscale sea surface temperature anomalies is not negligible for the STMW formation. This is achieved by accelerating wind and consequently promoting ocean latent heat release. Our results pinpoint the significance of accounting for the role of oceanic mesoscale feedback in improving the simulation of STMW as well as its climatic and biogeochemical impacts in Earth system models.

Investigation of Hydrostatic Imbalance with Field Observations

Abstract The hydrostatic equilibrium addresses the approximate balance between the positive force of the vertical pressure gradient and the negative gravity force and has been widely assumed for atmospheric applications. The hydrostatic imbalance of the mean atmospheric state for the acceleration of vertical motions in the vertical momentum balance is investigated using tower, the global positioning system radiosonde, and Doppler lidar and radar observations throughout the diurnally varying atmospheric boundary layer (ABL) under clear-sky conditions. Because of the negligibly small mean vertical velocity, the acceleration of vertical motions is dominated by vertical variations of vertical turbulent velocity variances. The imbalance is found to be mainly due to the vertical turbulent transport of changing air density as a result of thermal expansion/contraction in response to air temperature changes following surface temperature changes. In contrast, any pressure change associated with air temperature changes is small, and the positive vertical pressure-gradient force is strongly influenced by its background value. The vertical variation of the turbulent velocity variance from its vertical increase in the lower convective boundary layer (CBL) to its vertical decrease in the upper CBL is observed to be associated with the sign change of the imbalance from positive to negative due to the vertical decrease of the positive vertical pressure-gradient force and the relative increase of the negative gravity force as a result of the decreasing upward transport of the low-density air. The imbalance is reduced significantly at night but does not steadily approach zero. Understanding the development of hydrostatic imbalance has important implications for understanding large-scale atmosphere, especially for cloud development. Significance Statement It is well known that the hydrostatic imbalance between the positive pressure-gradient force due to the vertical decrease of atmospheric pressure and the negative gravity forces in the vertical momentum balance equation has important impacts on the vertical acceleration of atmospheric vertical motions. Vertical motions for mass, momentum, and energy transfers contribute significantly to changing atmospheric dynamics and thermodynamics. This study investigates the often-assumed hydrostatic equilibrium and investigate how the hydrostatic imbalance is developed using field observations in the atmospheric boundary layer under clear-sky conditions. The results reveal that hydrostatic imbalance can develop from the large-eddy turbulent transfer of changing air density in response to the surface diabatic heating/cooling. The overwhelming turbulence in response to large-scale thermal forcing and mechanical work of the vast Earth surface contributes to the hydrostatic imbalance on large spatial and temporal scales in numerical weather forecast and climate models.

The role of a low-level jet for stirring the stable atmospheric surface layer in the Arctic

Abstract. In this study, we analyze the transition of a stable atmospheric boundary layer (ABL) with a low-level jet (LLJ) to a traditional stable ABL with a classic Ekman helix in the late-winter central Arctic. Vertical profiles in the ABL were measured with a hot-wire anemometer on a tethered balloon during a 15 h period in March 2018 in northeast Greenland. The tethered balloon allows high-resolution turbulence observations from the ground to the top of the ABL. The core of the LLJ was observed at about 150 m altitude, and its height and strength were associated with the temperature inversion. Increased turbulence was observed in the vicinity of the LLJ, but most of the turbulence does not reach down to the surface, thus decoupling the LLJ from the surface. Only when the LLJ collapses and the ABL again exhibits a more classical Ekman spiral is a coupling to the surface re-established. The LLJ might enhance both advective and turbulent vertical transport of passive tracers such as aerosol particles or moisture in the often stably stratified Arctic ABL.

Three distinct scales dominate the role of eolian electric fields in dust turbulent transport

Although previous studies have shown that eolian electric fields significantly alter the lifting and dynamics of dust particles, they are limited to mean fields. The effects of eolian electric fields on the dust turbulent transport have not been reported before. Here, by combing the observational data and wavelet-based spectral analysis, we find that eolian electric fields enhance the vertical turbulent transport of dust particles in the near-surface layer and exhibit three distinct crucial scales. Specifically, the eolian electric fields exhibit a dominant promoting effect at the kilometer-sized synoptic scale, a secondary suppressive effect at the hectometer-sized very-large-scale motion scale, and a negligible effect at the decameter-sized turbulent integral scale. Such scale-dependent electrical effects can be explained by the fact that the linear coupling between vertical eolian electric field and dust concentration is strongest at the synoptic scale, followed by the very-large-scale motion scale, and is weakest at the turbulent integral scale.

What Controlled the Low-Level Moisture Transport during the Extreme Precipitation in Henan Province of China in July 2021?

Abstract A record-breaking precipitation event occurred in the Henan province of China in July 2021 (217HP). To identify the moisture source of the event, ensemble experiments with 120 members were conducted in this study. Results show that the precipitable water during this extreme event was primarily contributed by the low-level southeasterly (LLSE) water vapor transport. The LLSE was largely enhanced by the pressure gradient force maintained by the western Pacific subtropical high and further amplified by the latent heat release in the rainfall system over Henan. The positive moisture advection by the LLSE and evaporative water occurred below 950 hPa and was redistributed into higher levels by the LLSE jet-enhanced subgrid vertical turbulent transport. As a result, the combination of the enhanced LLSE centered around 950 hPa and the increase of moisture below 850 hPa were the main drivers for the continuous strengthening of LLSE moisture transport, with the former playing the dominant role. It is also found that not only the presence but also the intensity of the LLSE jet were important for reproducing the extreme rainfall. The impact of binary tropical cyclones In-Fa and Cempaka on the low-level moisture transport was also examined. We found that In-Fa (2021) presented an uncertain impact on 217HP, while Cempaka (2021) was found to be unfavorable for 217HP. Different from the LLSE water vapor transport, Cempaka mainly acted to weaken the southwesterly wind to the southwest of Henan by reducing the pressure gradient and impeding the water vapor transport toward Henan.

North Atlantic subtropical mode water formation controlled by Gulf Stream fronts.

The North Atlantic Ocean hosts the largest volume of global subtropical mode waters (STMWs) in the world, which serve as heat, carbon and oxygen silos in the ocean interior. STMWs are formed in the Gulf Stream region where thermal fronts are pervasive and result in feedback with the atmosphere. However, their roles in STMW formation have been overlooked. Using eddy-resolving global climate simulations, we find that suppressing local frontal-scale ocean-to-atmosphere (FOA) feedback leads to STMW formation being reduced almost by half. This is because FOA feedback enlarges STMW outcropping, attributable to the mixed layer deepening associated with cumulative excessive latent heat loss due to higher wind speeds and greater air-sea humidity contrast driven by the Gulf Stream fronts. Such enhanced heat loss overshadows the stronger restratification induced by vertical eddies and turbulent heat transport, making STMW colder and heavier. With more realistic representation of FOA feedback, the eddy-present/rich coupled global climate models reproduce the observed STMWs much better than the eddy-free ones. Such improvement in STMW production cannot be achieved, even with the oceanic resolution solely refined but without coupling to the overlying atmosphere in oceanic general circulation models. Our findings highlight the need to resolve FOA feedback to ameliorate the common severe underestimation of STMW and associated heat and carbon uptakes in earth system models.

The Effect of Advection on the Three Dimensional Distribution of Turbulent Kinetic Energy and Its Generation in Idealized Tropical Cyclone Simulations

AbstractThe distribution of turbulent kinetic energy (TKE) and its budget terms is estimated in simulated tropical cyclones (TCs) of various intensities. Each simulated TC is subject to storm motion, wind shear, and oceanic coupling. Different storm intensities are achieved through different ocean profiles in the model initialization. For each oceanic profile, the atmospheric simulations are performed with and without TKE advection. In all simulations, the TKE is maximized at low levels (i.e., below 1 km) and ∼0.5 km radially inward of the azimuthal‐mean radius of maximum wind speed at 1‐km height. As in a previous study, the axisymmetric TKE decreases with height in the eyewall, but more abruptly in simulations without TKE advection. The largest TKE budget terms are shear generation and dissipation, though variability in vertical turbulent transport and buoyancy production affect the change in the azimuthal‐mean TKE distribution. The general relationships between the TKE budget terms are consistent across different radii, regardless of storm intensity. In terms of the asymmetric distribution in the eyewall, TKE is maximized in the front‐left quadrant where the sea surface temperature (SST) is highest and is minimized in the rear‐right quadrant where the SST is the lowest. In the category‐5 simulation, the height of the TKE maximum varies significantly in the eyewall between quadrants and is between ∼400 m in the rear‐right quadrant and ∼1,000 m in the front‐left quadrant. When TKE advection is included in the simulations, the maximum eyewall TKE values are downwind compared to the simulations without TKE advection.

Vertical turbulent nutrient transport in the western north Pacific and its marginal seas

Vertical turbulent nutrient transport in the western north Pacific and its marginal seas

Stable Numerical Implementation of a Turbulence Scheme with Two Prognostic Turbulence Energies

Abstract In this paper, we present a new and more stable numerical implementation of the two-energy configuration of the Third Order Moments Unified Condensation and N-dependent Solver (TOUCANS) turbulence scheme. The original time-stepping scheme in TOUCANS tends to suffer from spurious oscillations in stably stratified turbulent flows. Because of their high frequency, the oscillations resemble the so-called fibrillations that are caused by the coupling between turbulent exchange coefficients and the stability parameter. However, our analysis and simulations show that the oscillations in the two-energy scheme are caused by the usage of a specific implicit–explicit temporal discretization for the relaxation terms. In TOUCANS, the relaxation technique is used on source and dissipation terms in prognostic turbulence energy equations to ensure numerical stability for relatively long time steps. We present both a detailed linear stability analysis and a bifurcation analysis, which indicate that the temporal discretization is oscillatory for time steps exceeding a critical time-step length. Based on these findings, we propose a new affordable time discretization of the involved terms that makes the scheme more implicit. This ensures stable solutions with enough accuracy for a wider range of time-step lengths. We confirm the analytical outcomes in both idealized 1D and full 3D model experiments. Significance Statement The vertical turbulent transport of momentum, heat, and moisture has to be parameterized in numerical weather prediction models. The parameterization typically employs nonlinear damping equations, whose numerical integration can lead to unphysical, time-oscillating solutions. In general, a presence of such numerical noise negatively affects the model performance. In our work, we address numerical issues of the recently developed scheme with two prognostic turbulence energies that have more realism and physical complexity. Specifically, we detect, explain, and design a numerical treatment for a new type of spurious oscillations that is connected to the temporal discretization. The treatment suppresses the oscillations and allows us to increase the model time step more than 4 times while keeping an essentially non-oscillatory solution.

Sensitivity of large eddy simulations of tropical cyclone to sub-grid scale mixing parameterization

The surface wind structure and vertical turbulent transport processes in the eyewall of hurricane Isabel (2003) are investigated using six large-eddy simulations (LESs) with different horizontal grid spacing and three-dimensional (3D) sub-grid scale (SGS) turbulent mixing models and a convection permitting simulation that uses a coarser grid spacing and one-dimensional vertical turbulent mixing scheme. The mean radius-height distribution of storm tangential wind and radial flow, vertical velocity structure, and turbulent kinetic energy and momentum fluxes in the boundary layer generated by LESs are consistent with those derived from historical dropsonde composites, Doppler radar, and aircraft measurements. Unlike the convection permitting simulation that produces storm wind fields lacking small-scale disturbances, all LESs are able to produce sub-kilometer and kilometer scale eddy circulations in the eyewall. The inter-LES differences generally reduce with the decrease of model grid spacing. At 100-m horizontal grid spacing, the vertical momentum fluxes induced by the model-resolved eddies and the associated eddy exchange coefficients in the eyewall simulated by the LESs with different 3D SGS mixing schemes are fairly consistent. Although with uncertainties, the decomposition in terms of eddy scales suggests that sub-kilometer eddies are mainly responsible for the vertical turbulent transport within the boundary layer (~1 km depth following the conventional definition) whereas eddies greater than 1 km become the dominant contributors to the vertical momentum transport above the boundary layer in the eyewall. The strong dependence of vertical turbulent transport on eddy scales suggests that the vertical turbulent mixing parameterization in mesoscale simulations of tropical cyclones is ultimately a scale-sensitive problem.

Recovery processes in a large offshore wind farm

Abstract. Wind turbines in a wind farm extract energy from the atmospheric flow and convert it into electricity, resulting in a localized momentum deficit in the wake that reduces energy availability for downwind turbines. Atmospheric momentum convergence from above, below, and the sides into the wakes replenishes the lost momentum, at least partially, so that turbines deep inside a wind farm can continue to function. In this study, we explore recovery processes in a hypothetical offshore wind farm with particular emphasis on comparing the spatial patterns and magnitudes of horizontal- and vertical-recovery processes and understanding the role of mesoscale processes in momentum recovery in wind farms. For this purpose, we use the Weather Research and Forecasting (WRF) model, a state-of-the-art mesoscale model equipped with a wind turbine parameterization, to simulate a hypothetical large offshore wind farm with different wind turbine spacings under realistic initial and boundary conditions. Different inter-turbine spacings range from a densely packed wind farm (case I: low inter-turbine distance of 0.5 km ∼ 5 rotor diameter) to a sparsely packed wind farm (case III: high inter-turbine distance of 2 km ∼ 20 rotor diameter). In this study, apart from the inter-turbine spacings, we also explored the role of different ranges of background wind speeds over which the wind turbines operate, ranging from a low wind speed range of 3–11.75 m s−1 (case A) to a high wind speed range of 11–18 m s−1 (case C). Results show that vertical turbulent transport of momentum from aloft is the main contributor to recovery in wind farms except in cases with high-wind-speed range and sparsely packed wind farms, where horizontal advective momentum transport can also contribute equally. Vertical recovery shows a systematic dependence on wind speed and wind farm density that is quantified using low-order empirical equations. Wind farms significantly alter the mesoscale flow patterns, especially for densely packed wind farms under high-wind-speed conditions. In these cases, the mesoscale circulations created by the wind farms can transport high-momentum air from aloft into the atmospheric boundary layer (ABL) and thus aid in recovery in wind farms. To the best of our knowledge, this is one of the first studies to look at wind farm replenishment processes under realistic meteorological conditions including the role of mesoscale processes. Overall, this study advances our understanding of recovery processes in wind farms and wind farm–ABL interactions.

A Non‐Gradient Model of Turbulent Gas Fluxes Over Land Surfaces

AbstractA non‐gradient model is formulated for estimating gas fluxes using single‐level time‐series data of near‐surface gas concentration over land surfaces. When the vertical turbulent transport process in the atmospheric surface layer is described by a one‐dimensional diffusion equation, a gas flux is expressed as a weighted integral of the time‐history of single‐level gas concentration. The eddy‐diffusivity may be parameterized as a function of sensible heat flux based on the Monin‐Obukhov similarity theory without explicit dependence on wind speed and surface roughness. Sensible heat flux may be estimated from net radiation and surface temperature using the maximum entropy production model. Case studies at six sites with diverse vegetation covers, geographic and climatic conditions at sub‐daily and seasonal scales demonstrate the model's capability of simulating water vapor and CO2fluxes using fewer inputs than other models. The proposed method provides an alternative modeling tool for the study of water and carbon cycles over vegetated land surfaces.

Effect of Scale-Aware Nonlocal Planetary Boundary Layer Scheme on Lake-Effect Precipitation at Gray-Zone Resolutions

Abstract The effects of a nonlocal planetary boundary layer (PBL) scheme that considers scale dependency in the parameterized turbulent vertical transport are investigated for a case of wintertime lake-effect precipitation over Korea at gray-zone resolutions using a mesoscale model. An experiment using the scale-aware PBL scheme is compared with that using a conventional PBL scheme, which shows that the simulated precipitation amount at a resolution of less than 1 km is smaller with the scale-aware PBL scheme. The role of turbulent processes in simulating lake-effect precipitation is understood through interaction with microphysical processes. When the scale-aware PBL scheme is used, liquid water content is increased while ice water content is reduced. The higher cloud water content is because of enhanced condensation with stronger updrafts, attributed to the suppression of parameterized turbulent mixing. This results in higher rainwater content by enhancing autoconversion and accretion from cloud water to rainwater. The cloud ice content is reduced mainly because of the suppressed deposition and enhanced sublimation centered near the PBL top, and the snow content is reduced mainly because of the enhanced sublimation below and near the PBL top and suppressed growth of cloud ice to snow. The lower ice water content is mainly due to the drier PBL, attributed to the enhanced resolved (suppressed parameterized) turbulent moisture transport and enhanced condensation. The melting of a smaller amount of snow under dominant cold rain processes is responsible for the reduced surface precipitation with the scale-aware PBL scheme.

Dynamic one-equation-based subgrid model for large-eddy simulation of stratified turbulent flows

The locally dynamic one-equation based subgrid-scale is extended for large-eddy simulation of stably stratified turbulent flows. It is comprehensively assessed by simulating turbulent channel flow for a range of Reynolds and Richardson numbers. It captures all the key flow features, such as suppression of vertical turbulent transport, presence of internal waves and pycnocline, and decrease of the friction coefficient and Nusselt number. The turbulence statistics show good agreement with the reference results thus demonstrating its applicability to stratified flows with variable mean shear.