Dutch Atmospheric Large-Eddy Simulation Research Articles

Open boundary conditions for atmospheric large-eddy simulations and their implementation in DALES4.4

Abstract. Open boundary conditions were developed for atmospheric large-eddy simulation (LES) models and implemented into the Dutch Atmospheric Large-Eddy Simulation model. The implementation was tested in a “Big Brother”-like setup, in which the simulation with open boundary conditions was forced by an identical control simulation with periodic boundary conditions. The results show that the open boundary implementation has minimal influence on the solution. Both the mean state and the turbulent structures are close to the control simulation, and disturbances at the in- and outflow boundaries are negligible. To emulate a setup in which the LES is coupled to a coarser model, the influence of coarse boundary input was tested by smoothing the output of the periodic control simulation both temporally and spatially before feeding it as input to the simulation with open boundary conditions. When smoothing is applied over larger spatial and longer temporal scales, disturbances start to form at the inflow boundary and an area exists where turbulence needs to develop. Adding synthetic turbulence to the smoothed input reduces the size of this area and the magnitude of the disturbances.

Momentum Transport in Organized Shallow Cumulus Convection

Abstract This study investigates momentum transport in shallow cumulus clouds as simulated with the Dutch Atmospheric Large Eddy Simulation (DALES) for a 150 × 150 km2 domain east of Barbados during 9 days of EUREC4A. DALES is initialized and forced with the mesoscale weather model HARMONIE–AROME and subjectively reproduces observed cloud patterns. This study examines the evolution of momentum transport, which scales contribute to it, and how they modulate the trade winds. Daily-mean momentum flux profiles show downgradient zonal momentum transport in the subcloud layer, which turns countergradient in the cloud layer. The meridional momentum transport is nontrivial, with mostly downgradient transport throughout the trade wind layer except near the top of the surface layer and near cloud tops. Substantial spatial and temporal heterogeneity in momentum flux is observed with much stronger tendencies imposed in areas of organized convection. The study finds that while scales < 2 km dominate momentum flux at 200 m in unorganized fields, submesoscales carry up to 50% of the zonal momentum flux in the cloud layer in organized fields. For the meridional momentum flux, this fraction is even larger near the surface and in the subcloud layer. The scale dependence of the momentum flux is not explained by changes in convective or boundary layer depth. Instead, the results suggest the importance of spatial heterogeneity, increasing horizontal length scales, and countergradient transport in the presence of organized convection.

Cloud Botany: Shallow Cumulus Clouds in an Ensemble of Idealized Large‐Domain Large‐Eddy Simulations of the Trades

AbstractSmall shallow cumulus clouds (<1 km) over the tropical oceans appear to possess the ability to self‐organize into mesoscale (10–100 km) patterns. To better understand the processes leading to such self‐organized convection, we present Cloud Botany, an ensemble of 103 large‐eddy simulations on domains of 150 km, produced by the Dutch Atmospheric Large Eddy Simulation model on supercomputer Fugaku. Each simulation is run in an idealized, fixed, larger‐scale environment, controlled by six free parameters. We vary these over characteristic ranges for the winter trades, including parameter combinations observed during the EUREC4A (Elucidating the role of clouds–circulation coupling in climate) field campaign. In contrast to simulation setups striving for maximum realism, Cloud Botany provides a platform for studying idealized, and therefore more clearly interpretable causal relationships between conditions in the larger‐scale environment and patterns in mesoscale, self‐organized shallow convection. We find that any simulation that supports cumulus clouds eventually develops mesoscale patterns in their cloud fields. We also find a rich variety in these patterns as our control parameters change, including cold pools lined by cloudy arcs, bands of cross‐wind clouds and aggregated patches, sometimes topped by thin anvils. Many of these features are similar to cloud patterns found in nature. The published data set consists of raw simulation output on full 3D grids and 2D cross‐sections, as well as post‐processed quantities aggregated over the vertical (2D), horizontal (1D) and all spatial dimensions (time‐series). The data set is directly accessible from Python through the use of the EUREC4A intake catalog.

The interactions of aerosol and planetary boundary layer over a large city in the Mongolian Plateau

The interactions of aerosol and planetary boundary layer (PBL) play a crucial role in deteriorating the air quality in vast urban agglomeration areas in eastern China. However, there remains a lacking of report regarding their performance in the hazy events in Mongolian Plateau cities in northern China. In this study, half-month (01–16 January 2020) physical and material datasets of the PBL measured by multi instrumentations mounted in downtown Hohhot, a largest Mongolian Plateau city, are statistically analyzed. Results demonstrate that the aerosol-PBL feedback is of particular importance in promoting the hazy outbreak and the statistical relationship follows PBLH = −76.14 × ln(PM2.5) + 820.61. The non-linear fitting implies that there exists a potential threshold of 76.14 μg m−3 for PM2.5, below which the PBLH decrease rapidly along with the increasing of air pollutants, defined as strong aerosol-PBL interaction phase, while beyond which there is minimal decrease for PBLH even when PM reaches to a high value, i.e., the hazy accumulation phase. Using a large-eddy simulation model named as Dutch Atmospheric Large-Eddy Simulation (DALES) initialized with the synergetical observations in a representative hazy process in 11 January 2020, we found that the DALES is effective to capture the diurnal PBLH in this region. The existence of atmospheric aerosols is essential for lowering PBLH by 51.4 % from 1090 m of clean scenario to 530 m of polluted condition, postponing the development time, and advancing the afternoon lapse time. The enhancement of aerosol absorption ability strengthens the aerosol heating rate, thereby weakening the sensible heat flux, and inhibiting the development of PBLH. While opposite elevation on PBLH is found for the scattering aerosols. These findings highlight the importance of aerosol-PBL interaction in motivating the hazy episodes in Mongolian Plateau cities, which provide scientific references for the local policy-making towards pollution reductions in future.

Quantifying vertical wind shear effects in shallow cumulus clouds over Amazonia

Abstract. This study analyses and quantifies the effects of vertical wind shear (VWS) on the properties of shallow cumulus cloud fields over Central Amazonia. We perform idealised simulations with high resolution (50 m horizontally and 20 m vertically) using the Dutch Atmospheric Large-Eddy Simulation (DALES) model, changing the initial conditions and large-scale forcing of VWS. The resulting cloud field is analysed by applying a cloud tracking algorithm to generate Lagrangian datasets of the life cycle of individual clouds as well as their time-varying core and margin dimensions. The reference run has no wind speed or directional shear and represents a typical day in the local dry season. Numerical experiments with moderate and high wind speed shear are simulated by adding linear increases in the wind speed of 1.2 and 2.4 m s−1 km−1, respectively. Three additional runs are made by adding 90∘ of wind rotation between the surface and the top of the domain (5 km) on top of the three wind speed shear conditions. We find that clouds developing in a sheared environment have horizontal equivalent diameter increased by up to 100 m on average, but the cloud depth is reduced. Our quantification shows that VWS tends to increase the size of the cloud cores but reduces their relative area, volume, and mass fractions compared to the overall cloud dimensions. The addition of 2.4 m s−1 km−1 of VWS decreases the relative core area by about 0.03 (about 10 % of the overall average) and its volume and mass ratios by about 0.05 (10 %–25 % in relative terms). Relevant for the cloud transport properties is that the updraught speed and the liquid water content are lower within the cores, and consequently so is the upward mass flux. All quantifications of mean cloud properties point to the inhibition of convective strength by VWS, therefore hampering the shallow-to-deep transition. However, open questions still remain given that the individually deepest clouds were simulated under high environmental shear, even though they occur in small numbers. This could indicate other indirect effects of VWS that have opposite effects on cloud development if found to be significant in the future.

Representing Cloud Mesoscale Variability in Superparameterized Climate Models

AbstractIn atmospheric modeling, superparameterization (SP) has gained popularity as a technique to improve cloud and convection representations in large‐scale models by coupling them locally to cloud‐resolving models. We show how the different representations of cloud water in the local and the global models in SP lead to a suppression of cloud advection and ultimately to a systematic underrepresentation of the cloud amount in the large‐scale model. We demonstrate this phenomenon in a regional SP experiment with the global model OpenIFS coupled to the local model Dutch Atmospheric Large Eddy Simulation, as well as in an idealized setup, where the large‐scale model is replaced by a simple advection scheme. As a starting point for mitigating the problem of suppressed cloud advection, we propose a scheme where the spatial variability of the local model's total water content is enhanced in order to match the global model's cloud condensate amount. The proposed scheme enhances the cloud condensate amount in the test cases, however a large discrepancy remains, caused by rapid dissipation of the clouds added by the proposed scheme.

Explicit aerosol–cloud interactions in the Dutch Atmospheric Large-Eddy Simulation model DALES4.1-M7

Abstract. Large-eddy simulation (LES) models are an excellent tool to improve our understanding of aerosol–cloud interactions (ACI). We introduce a prognostic aerosol scheme with multiple aerosol species in the Dutch Atmospheric Large-Eddy Simulation model (DALES), especially focused on simulating the impact of cloud microphysical processes on the aerosol population. The numerical treatment of aerosol activation is a crucial element for simulating both cloud and aerosol characteristics. Two methods are implemented and discussed: an explicit activation scheme based on κ-Köhler theory and a more classic approach using updraught strength. Sample model simulations are based on the Rain in Shallow Cumulus over the Ocean (RICO) campaign, characterized by rapidly precipitating warm-phase shallow cumulus clouds. We find that in this pristine ocean environment virtually all aerosol mass in cloud droplets is the result of the activation process, while in-cloud scavenging is relatively inefficient. Despite the rapid formation of precipitation, most of the in-cloud aerosol mass is returned to the atmosphere by cloud evaporation. The strength of aerosol processing through subsequent cloud cycles is found to be particularly sensitive to the activation scheme and resulting cloud characteristics. However, the precipitation processes are considerably less sensitive. Scavenging by precipitation is the dominant source for in-rain aerosol mass. About half of the in-rain aerosol reaches the surface, while the rest is released by evaporation of falling precipitation. The effect of cloud microphysics on the average aerosol size depends on the balance between the evaporation of clouds and rain and ultimate removal by precipitation. Analysis of typical aerosol size associated with the different microphysical processes shows that aerosols resuspended by cloud evaporation have a radius that is only 5 % to 10 % larger than the originally activated aerosols. In contrast, aerosols released by evaporating precipitation are an order of magnitude larger.

Regional Superparameterization in a Global Circulation Model Using Large Eddy Simulations

AbstractAs a computationally attractive alternative for global large eddy simulations (LESs), we investigate the possibility of using comprehensive three‐dimensional LESs as a superparameterization that can replace all traditional parameterizations of atmospheric processes that are currently used in global models. We present the technical design for a replacement of the parameterization for clouds, convection, and turbulence of the global atmospheric model of the European Centre for Medium‐Range Weather Forecasts by the Dutch Atmospheric Large Eddy Simulation model. The model coupling consists of bidirectional data exchange between the global model and the high‐resolution LES models embedded within the columns of the global model. Our setup allows for selective superparameterization, that is, for applying superparameterization in local regions selected by the user, while keeping the standard parameterization of the global model intact outside this region. Computationally, this setup can result in major geographic load imbalance, because of the large difference in computational load between superparameterized and nonsuperparameterized model columns. To resolve this issue, we use a modular design where the local and global models are kept as distinct model codes and organize the model coupling such that all the local models run in parallel, separate from the global model. First simulation results, employing this design, demonstrate the potential of our approach.

A Case Study of Ozone Diurnal Variation in the Convective Boundary Layer in the Southeastern United States Using Multiple Observations and Large-Eddy Simulation

We investigated the diurnal ozone variation on 6 September 2013 in a midsize urban environment using multiple in situ and remote-sensing measurements along with the Dutch atmospheric large-eddy simulation (DALES) model coupled with a chemical module and a dry deposition module that we added for this study. Our study area was Huntsville, Alabama, USA, a typical midsize city in the Southeastern United States. The ozone variation in the convective boundary layer (CBL) resulted mainly from local emissions and photochemical production stemming from weather conditions controlled by an anticyclonic system on that day. Local chemical production contributes approximately two thirds of the ozone enhancement in the CBL and, in this case, dynamical processes including ozone transport from the free troposphere (FT) to the CBL through the entrainment processes contributed the remainder. The numerical experiments performed by the large-eddy simulation (LES) model showed acceptable agreement with the TOLNet (The tropospheric ozone lidar network)/RO3QET (Rocket-city ozone quality evaluation in the troposphere) ozone DIAL (differential absorption lidar) observations. This study indicated the need for fine-scale, three-dimensional ozone observations with high temporal and spatial resolution for air quality studies at the urban scale and smaller.

Large-Eddy Simulation Comparison of Neutral Flow Over a Canopy: Sensitivities to Physical and Numerical Conditions, and Similarity to Other Representations

The representation of a neutral atmospheric flow over roughness elements simulating a vegetation canopy is compared between two large-eddy simulation models, wind-tunnel data and recently updated empirical flux-gradient relationships. Special attention is devoted to the dynamics in the roughness sublayer above the canopy layer, where turbulence is most intense. By demonstrating that the flow properties are consistent across these different approaches, confidence in the individual independent representations is bolstered. Systematic sensitivity analyses with the Dutch Atmospheric Large-Eddy Simulation model show that the transition in the one-sided plant-area density from the canopy layer to unobstructed air potentially alters the flow in the canopy and roughness sublayer. Anomalously induced fluctuations can be fully suppressed by spreading the transition over four steps. Finer vertical resolutions only serve to reduce the magnitude of these fluctuations, but do not prevent them. To capture the general dynamics of the flow, a resolution of 10 % of the canopy height is found to suffice, while a finer resolution still improves the representation of the turbulent kinetic energy. Finally, quadrant analyses indicate that momentum transport is dominated by the mean velocity components within each quadrant. Consequently, a mass-flux approach can be applied to represent the momentum flux.

Boundary Layer Characteristics over Homogeneous and Heterogeneous Surfaces Simulated by MM5 and DALES

AbstractThe multiple-single-column approach is proposed as a new concept to study the boundary layer parameterization scheme in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results are compared with the Dutch Atmospheric Large-Eddy Simulation Model (DALES). Numerical experiments were performed over homogeneous and heterogeneous surfaces under clear convective boundary layer conditions. Identical simulations using MM5 and DALES were performed, which enabled an evaluation of the MM5 boundary layer scheme with DALES results. From the experiment with a homogeneous surface, MM5 shows a slightly shallower, colder, and moister boundary layer than DALES. This result is produced by an underestimation of turbulent mixing near the surface and less-vigorous entrainment of heat and dry air in MM5. In the heterogeneous surface experiment, the domain is divided into dry and wet patches, with the result that both models produce a mesoscale circulation. However, relative to the homogeneous case, larger differences were found between the models in the representation of the boundary layer dynamics. In DALES, the surface heterogeneity influenced the turbulent motions, making the mesoscale circulation much stronger (wmax is 6 times as large) than in MM5. Because of this stronger circulation, the boundary layer height, bulk temperature, and humidity also displayed differences in time and spatial patterns. Because of the land–atmosphere coupling in MM5, the mesoscale circulation strengthened the surface flux heterogeneity. Cold and moist air advection close to the surface from the wet patch to the dry patch increased the sensible heat flux above the dry patch and thus the induced mesoscale flow.

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

Abstract. The current version of the Dutch Atmospheric Large-Eddy Simulation (DALES) is presented. DALES is a large-eddy simulation code designed for studies of the physics of the atmospheric boundary layer, including convective and stable boundary layers as well as cloudy boundary layers. In addition, DALES can be used for studies of more specific cases, such as flow over sloping or heterogeneous terrain, and dispersion of inert and chemically active species. This paper contains an extensive description of the physical and numerical formulation of the code, and gives an overview of its applications and accomplishments in recent years.