Characteristics Of Atmospheric Boundary Layer Research Articles

Investigation of the atmospheric boundary layer characteristics over a high altitude station in North East India using measurements and reanalysis datasets

Investigation of the atmospheric boundary layer characteristics over a high altitude station in North East India using measurements and reanalysis datasets

Analysis of Atmospheric Boundary Layer Characteristics on Different Underlying Surfaces of the Eastern Tibetan Plateau in Summer

The atmospheric boundary layer is a key region for human activities and the interaction of various layers and is an important channel for the transportation of momentum, heat, and various substances between the free atmosphere and the surface, which has a significant impact on the development of weather and climate change. During the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) in June 2022, utilizing the comprehensive stereoscopic observation experiment of the “Plateau Low Vortex Network”, this study analyzed the variation characteristics and influencing factors of the atmospheric boundary layer height (ABLH) at three stations with different underlying surface types on the Qinghai–Tibet Plateau (QTP): Qumalai Station (grassland), Southeast Tibet Observation and Research Station for the Alpine Environment (SETORS, forest), and Sieshan Station (cropland). The analysis utilized sounding observation data, microwave radiometer data, and ERA5 reanalysis data. The results revealed that the temperature differences between the sounding observation data and microwave radiometer data were minor at the three stations, with a notable temperature inversion phenomenon observed at Sieshan Station. Regarding water vapor density, the differences between the sounding observation data and microwave radiometer data were relatively small at Sieshan Station. The relative humidity increased with height at Sieshan Station, whereas it increased and then decreased with height at SETORS and Qumalai Station. The ABLH at all sites reached its maximum value around noon, approximately 1500 m, and exhibited mostly convective boundary layer (CBL) characteristics. During the night, the ABLH mostly showed a stable boundary layer (SBL) pattern, with heights around 250 m. In summer, latent heat flux (LE) and sensible heat flux (H) in the eastern plateau were generally lower than those in the western plateau except at 20:00, where they were higher. Vertical velocity (w) in the eastern plateau was greater than in the western plateau. Among Sieshan Station and SETORS, LE, and H had the most significant impact on ABLH, while at Qumalai Station, ABLH was more influenced by surface long-wave radiation (Rlu). These four influencing factors showed a positive correlation with ABLH. The impact of different underlying surface types on ABLH primarily manifests in surface temperature variations, solar radiation intensity, vegetation cover, and terrain. Grasslands typically exhibit a larger range of ABLH variations, while the ABLH in forests and mountainous cropland areas is relatively stable.

Surface-atmosphere energy exchanges and their effects on surface climate and atmospheric boundary layer characteristics in the forest-tundra ecotone in northwestern Canada

The circumpolar forest-tundra ecotone is experiencing rapid changes in vegetation composition and structure. Collectively, these changes modify surface-atmosphere energy exchanges and thus characteristics of the atmospheric boundary layer (ABL). Here, we characterize differences in bulk surface properties and resulting energy balance components using multi-year eddy covariance and supporting measurements made at a mineral upland tundra and a nearby subarctic woodland site between 2013 and 2022. The two sites are characteristic of the forest-tundra ecotone of northwestern Canada. A mixed-layer slab model, in combination with radiosonde observations, was used to gain more insights into differences in ABL characteristics. Compared to the tundra, the tree cover of the woodland led to an enhanced ability to transfer heat into the atmosphere, a higher resistance to evapotranspiration and a stronger coupling between surface and atmosphere. Sensible heat flux (H) was generally higher at the woodland than at the tundra. The largest difference in daily mean H was observed in late winter and spring when the albedo of the snow-covered landscape at the woodland was reduced by 45% compared to the tundra. Both sites experienced similar latent heat flux throughout the year. At the woodland, modelled afternoon air temperature in the mixed layer was up to 9 °C higher in spring and up to 3 °C higher in summer compared to the tundra. In accordance with model results, radiosonde observations indicated a deeper ABL and higher air temperature at the woodland. The presence of trees in the southern part of the forest-tundra ecotone increases air temperature throughout the year and has a drying effect in spring. Consequently, vegetation shifts in the forest-tundra ecotone can be expected to modify local surface climate change patterns, which must be considered when assessing climate change impacts in the region.

Characteristics of atmospheric boundary layer and its relation with PM2.5 during winter in Shihezi, an Oasis city in Northwest China

Based on low-level atmospheric meteorological elemental and pollutant data in the vertical direction observed during a tethered sonde sounding experiment from December 27, 2019 to January 14, 2020 this study analysed the atmospheric boundary layer vertical structure and its influence on winter PM2.5 concentrations in Shihezi City, Xinjiang, using Richardson number (Ri) profiling and correlation analysis to better understand the correlation between the atmospheric boundary layer structure and continuous heavy pollution in Shihezi City during winter. The results showed that heavy pollution continued during the observational period, and the diurnal variation in the atmospheric boundary layer disappeared. The atmospheric boundary layer structure tended to remain stable, and the average boundary layer height was only 109 m. The lengthy temperature inversion lifespan, high humidity, and low-wind layer near the surface primarily contributed to the sustained heavy pollution during winter in Shihezi City. Temperature and wind speed during heavy pollution events were negatively correlated with the PM2.5 concentrations in the near-surface layer. Stable atmospheric thermodynamic stratification with a cold lower layer and a warm upper layer in the atmospheric boundary layer weakens vertical atmospheric pollutant diffusion. Moreover, increased relative humidity (RH) is beneficial for hygroscopic aerosol particle growth and liquid-phase chemical reactions, thus worsening heavy pollution.

Numerical investigation of neutral atmospheric boundary layer flows over flat terrain and three-dimensional hills considering the effects of Coriolis force

The characteristics of neutral atmospheric boundary layer (ABL) flows can be strongly affected by terrain and earth rotation-induced Coriolis force. The purpose of this study is to ascertain the effects of Coriolis force on neutral ABL flows over flat terrain and isolated three-dimensional hills. Based on large-eddy simulations (LES), a new formula for the maximum turbulence length scale lmax in the limited-length-scale (LLS) k-ε model is proposed to reproduce neutral ABL flows over flat terrain with different exposure conditions. It is demonstrated that lmax can be reasonably expressed as a function of the geostrophic wind speed Ug, the Coriolis parameter fc and the roughness length z0. The effects of Coriolis force on atmospheric flows over single isolated hills are subsequently elucidated from the perspective of flow patterns and turbulent statistics. The wake trajectory deflection and asymmetric vortex structures are identified on the lee side of hills owing to the wind veer. Additionally, lateral wind shear associated with the Coriolis force contributes to a faster wake recovery and larger turbulence fluctuations downstream of steep hills. Moreover, the speed-up factor at the hilltop is significantly enhanced with increasing hill slopes and height ratios, while it is not sensitive to the Coriolis effects.

Thermodynamic characteristics of marine atmospheric boundary layer across frontal regions of the Indian Ocean Sector of the Southern Ocean based on three field campaigns

The Southern Ocean (SO) marine atmospheric boundary layer (MABL), regulated by continuous air-sea exchanges, plays a key role in transporting exchanges between tropics and poles. Among SO sectors, the Indian Ocean sector of SO (ISSO) remained least explored in terms of MABL characterization and is examined in this study. In ISSO, occurrence of sharp oceanic thermohaline fronts regulates the vertical thermodynamic structure of MABL, clouds, and inversions. In this study, these properties of ISSO MABL are investigated over three oceanic domains (Sub-Tropical Indian Ocean (STIO), ISSO, and High-Latitude SO (HLSO)). To achieve this, near-sea surface air-sea exchanges along with profiles of meteorological parameters generated between 25°S and 68°S and 57°E to78°E during three field campaigns conducted in the austral summers of 2017, 2018, and 2020 were utilized. Results showed strong SST-Tair variability across the study region. In STIO, positive SST-Tair indicated Low-Level Cold Air Advection (LLCAA)-induced destabilized and coupled MABLs capped by multiple inversions (>3INV's of strength∼0.35 K m−1 above∼1200 m) and thin mid/high-altitudes clouds (cloud-base∼830 m, cloud-top∼2309 m, and cloud-thickness∼758 m). Over ISSO and northern HLSO, weak and negative SST-Tair indicated Low-Level Warm Air Advection (LLWAA)-induced stratified and decoupled MABLs. Aided by advective mixing of multiple air-masses, low-level thick multilayered clouds (>2 layers, average cloud-base∼604 m, cloud-top∼2288 m, and cloud-thickness∼1314 m) and multiple strong high-level inversions (>2INV's, strength>0.4 K m−1,∼1836 m) was observed. In HLSO, weakly positive SST-Tair indicated LLCAA-induced weakly destabilized MABLs capped by mid-altitude inversions (strength∼0.22 K m−1,∼1632 m) and mid-altitude clouds (cloud-base∼979 m, cloud-top∼2465 m, and cloud-thickness∼1263 m) supported by sublimation leading to virga conditions.

The NCEP and ERA-Interim reanalysis temperature and humidity errors and their relationships with atmospheric boundary layer in the Tibetan Plateau

The lack of sounding observations in the western Tibetan Plateau (TP), the highest terrain in the world, has resulted in few efforts to evaluate the quality of the atmospheric reanalysis results in this region. Using the sounding observations from the Third Tibetan Plateau Atmospheric Scientific Experiment during 2013–2015, the NCEP and ERA-Interim reanalysis temperature and humidity fields in the TP are evaluated and the characteristics of the reanalysis Atmospheric Boundary Layer (ABL) height are utilized to explain the reasons for the differences in temperature and humidity between the western and eastern TP. The results show that the NCEP and ERA-Interim reanalysis temperature and humidity products generally have larger errors at low level (such as 500 hPa) in the western TP (WTP) than in the eastern one (ETP) at 12:00 UTC. However, this difference is small at 00:00 UTC. Further analysis reveals that the temporal and spatial variations of temperature and humidity errors at low level are closely associated with the differences in the terrain and ABL between the western and eastern TP. In the early morning when the ABL height is low over the TP, the 500 hPa pressure level in both the WTP and ETP is significantly above the top of ABL, with weak spatial variations of temperature and humidity errors. However, in the late afternoon when there is a larger increase in ABL height over the WTP than over the ETP, the 500 hPa pressure level is located inside the ABL in the WTP and is still above the ABL in the ETP, which causes significant regional differences in these errors.

The Impact of Lead Patterns on Mean Profiles of Wind, Temperature, and Turbulent Fluxes in the Atmospheric Boundary Layer over Sea Ice

In the polar regions, the atmospheric boundary layer (ABL) characteristics are strongly influenced by convection over leads, which are elongated channels in the sea ice covered ocean. The effects on the ABL depend on meteorological forcing and lead geometry. In non-convection-resolving models, in which several leads of potentially different characteristics might be present in a single grid cell, such surface characteristics and the corresponding ABL patterns are not resolved. Our main goal is to investigate potential implications for such models when these subgrid-scale patterns are not considered appropriately. We performed non-eddy-resolving microscale simulations over five different domains with leads of different widths separated by 100% sea ice. We also performed coarser-resolved simulations over a domain representing a few grid cells of a regional climate model, wherein leads were not resolved but accounted for via a fractional sea ice cover of 91% in each cell. Domain size and mean sea ice concentration were the same in all simulations. Differences in the domain-averaged ABL profiles and patterns of wind, temperature, and turbulent fluxes indicate a strong impact of both the leads and their geometry. Additional evaluations of different turbulence parameterizations show large effects by both gradient-independent heat transport and vertical entrainment.

Investigation of Atmospheric Boundary Layer characteristics using Ceilometer Lidar, COSMIC GPS RO satellite, Radiosonde and ERA-5 reanalysis dataset over Western Indian Region

Atmospheric Boundary Layer (ABL) characteristics are investigated using a Ceilometer Lidar over Ahmedabad, a semi-arid region in western India. Strong diurnal variations of ABL are observed during 2019, the observation period. There is a stark winter-summer difference in ABL, with summer Boundary Layer Height (BLH) exceeding winter BLH by 1–1.5 km. ABL usually collapses during monsoon and is equivocal due to the presence of thick clouds on top of ABL. The ABL is thicker during the onset of monsoon in contrast to active monsoon, rises again during the withdrawal of monsoon. Lidar observed ABL has been compared with satellite, radiosonde, and European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA5) dataset. ERA5 shows good agreement with differences within 500 m; radiosonde observations have under-estimated ground-based measurements, especially during summer. Satellite observations highly overestimated BLH. This comparative study reveals the importance of ground-based lidars in continuous monitoring of ABL at high resolution because radiosonde, satellite, and reanalysis datasets have coarser resolutions and sparse observations. Such quantitative evaluation of ABL is formerly unavailable over this region, which can now be used to improve the representation in numerical models and thereby estimates of radiative and climate effects due to ABL.

Analysis on the Characteristics and Cause of a Compound Pollution Process in Changsha in 2017

Using the conventional meteorological observations, the air Air quality monitoring data, the L-band wind-profiling radar data, the NECP reanalysis data, and the simulation results of the HYSPLIT4 model, the compound pollution process from November 2 to 5, 2017 in Changsha is comprehensively analyzed in this study, including the sources and transport paths of pollutions, the Meteorological conditions and atmospheric boundary layer characteristics. The results showed that the process of air pollution in Changsha is a compound pollution process with high concentration of O3 and fine PM2.5 coexisting. It reached serious pollution from 08:00 to 11 : 00 on 4, PM2.5 was 269.3 ug·m−3 at 09:00 on 4 November. The characteristics of the two phases of this pollution process were clearly different. The first phase was mainly result of accumulation of local pollutants, and the second one was mainly result of outside resource. The maximum wind were light breezein on this pollution process, gentle breeze mainly occurred in the second stage of pollution, and the maximum wind velocity was 4.4m/s. It appearred multi-layer inversion usually, temperature stratification was stabled, and the inversion layer thickness reached the maximum value 909m at 20:00 on 3 November. The boundary layer convergenced and ascented movement, horizontal transport, vertical wind field distribution and atmospheric layer provided the favorable conditions for the air pollution process. The results showed that short-range transport of pollutants is the maily reason based on the HYSPLIT4 model, the northeast airflow was dominant at 100-1000m height. The vary of wind profile data and CO and SO2 emission ratio showed that the different pollution characteristics of two phases, the maximum ratio was more than 100. It has an important indicator for weather forecast in Changsha.

Validating CFD Predictions of Flow over an Escarpment Using Ground-Based and Airborne Measurement Devices

Micrometeorological observations from a tower, an eddy-covariance (EC) station and an unmanned aircraft system (UAS) at the WINSENT test-site are used to validate a computational fluid dynamics (CFD) model, driven by a mesoscale model. The observation site is characterised by a forested escarpment in a complex terrain. A two-day measurement campaign with a flow almost perpendicular to the escarpment is analysed. The first day is dominated by high wind speeds, while, on the second one, calm wind conditions are present. Despite some minor differences, the flow structure, analysed in terms of horizontal wind speeds, wind direction and inclination angles shows similarities for both days. A real-time strategy is used for the CFD validation with the UAS measurement, where the model follows spatially and temporally the aircraft. This strategy has proved to be successful. Stability indices such as the potential temperature and the bulk Richardson number are calculated to diagnose atmospheric boundary layer (ABL) characteristics up to the highest flight level. The calculated bulk Richardson values indicate a dynamically unstable region behind the escarpment and near the ground for both days. At higher altitudes, the ABL is returning to a near neutral state. The same characteristics are found in the model but only for the first day. The second day, where shear instabilities are more dominant, is not well simulated. UAS proves its great value for sensing the flow over complex terrains at high altitudes and we demonstrate the usefulness of UAS for validating and improving models.

Atmospheric boundary layer height and surface parameters: Trends and relationships over the west coast of India

Trends and features of Atmospheric Boundary Layer (ABL) over southern part of the west coast of India has been studied using MERRA datasets for 1980–2018 during different seasons incorporating the marine and continental influences. A significant decrease in the ABL height (ABLH) has been observed in the westerly regimes of the west coast. The shallow ABL is expected to cause amplified response in the atmosphere and enhance the formation of extreme events over the region. It is also observed that its variation is related with other surface layer parameters such as specific humidity, temperature, surface zonal wind, Richardson number, CAPE (Convective Available Potential Energy) and sensible & latent heat fluxes. A significant increasing (decreasing) trend in specific humidity (surface zonal wind) is observed in pre-monsoon, monsoon and post monsoon seasons and this trend has significant correlation with the ABL height. The CAPE has a significant increasing trend in daytime of pre-monsoon season, which can lead to formation of intense thunderstorms. A significant reduction in the surface dynamic instability due to decrease in surface westerly zonal wind is observed and it has a significant role in the decreasing trend of ABL height. Surface sensible heat flux possesses a significant decreasing (increasing) trend during the pre-monsoon and post monsoon seasons (monsoon season). Surface latent heat flux shows a decreasing trend during the monsoon which may be attributed to the combined effect of decrease in ABL height and trapping of moisture at lower levels together with an increase in surface temperature.

Seasonal Characteristics of Atmospheric Boundary Layer and its Associated Dynamics over Central India

Atmospheric boundary-layer studies have applications in lower tropospheric processes, air pollution meteorology, weather prediction and climate monitoring. Despite this, very few studies are available over tropics, especially over central India. Here, we analyse, high resolution vertical profiles of meteorological parameters corresponding to fully developed mixed layer obtained using balloon borne radiosonde to study characteristics of atmospheric boundary layer over Nagpur (21.15 °N, 79.15 °E). Study reveals that, within the mixed layer, variation in RH is very less (within 20%) while above mixed layer, large variations are seen. Back trajectory analysis confirms that the moist days (dry days) are associated with the transport of air masses from the Arabian Sea (from arid locations). Mixed layer heights derived through gradients in Virtual Potential Temperature (VPT) showed highest values during Pre monsoon (2564 ± 867 m) and lowest (788 ± 477 m) in Monsoon season. Specific Humidity varies in the range from 0 to 10 g/kg during pre-monsoon and 0–20 g/kg during monsoon season. D-theta exhibits lowest values in monsoon and highest values during pre-monsoon season while Parcel saturation pressure difference exhibits highest values in monsoon and lowest values during pre-monsoon season. During the monsoon and early post monsoon there is a presence of high moist static energy (Theta-e ~ 365 K) at the surface and moderate during mid-winter. We have examined, for the first time, the spatial (vertical) coherence of VPT using a novel technique of auto correlation analysis. Coherence length scales, derived with this method are found to be Maximum (1129 ± 48 m) in winter and minimum (694 ± 281 m) in Pre monsoon season.

Comparison of CFD Simulation to UAS Measurements for Wind Flows in Complex Terrain: Application to the WINSENT Test Site

This investigation presents a modelling strategy for wind-energy studies in complex terrains using computational fluid dynamics (CFD). A model, based on an unsteady Reynolds Averaged Navier-Stokes (URANS) approach with a modified version of the standard k-ε model, is applied. A validation study based on the Leipzig experiment shows the ability of the model to simulate atmospheric boundary layer characteristics such as the Coriolis force and shallow boundary layer. By combining the results of the model and a design of experiments (DoE) method, we could determine the degree to which the slope, the leaf area index, and the forest height of an escarpment have an effect on the horizontal velocity, the flow inclination angle, and the turbulent kinetic energy at critical positions. The DoE study shows that the primary contributor at a turbine-relevant height is the slope of the escarpment. In the second step, the method is extended to the WINSENT test site. The model is compared with measurements from an unmanned aircraft system (UAS). We show the potential of the methodology and the satisfactory results of our model in depicting some interesting flow features. The results indicate that the wakes with high turbulence levels downstream of the escarpment are likely to impact the rotor blade of future wind turbines.

Characteristics of the Marine Atmospheric Boundary Layer Over the Southern Ocean in Response to the Synoptic Forcing

AbstractThe characteristics of the marine atmospheric boundary layer (MABL) in relation to synoptic meteorology over the Southern Ocean are examined using upper‐air soundings and surface precipitation at Macquarie Island (54.62°S, 158.85°E), with a primary focus on the post‐cold‐frontal environment where large cloud and radiative biases are presented in a multitude of climate models. Thermodynamic profiles from the European Centre for Medium‐Range Weather Forecasts ERA‐Interim reanalyses are compared with the observations to evaluate their representation of the MABL characteristics. Observations confirm that boundary layer clouds over the Southern Ocean commonly reside within a shallow MABL under the influence of frequent midlatitude cyclones and fronts. The evaluation of MABL height shows that, for both observations and reanalysis, the inversion is higher northward of the low center and under postcold front conditions. Under cold frontal passages, however, the main inversions are not well represented by ERA‐Interim, which is featured by an underestimate of the MABL height by 22%. Significant differences are found in the moisture profiles within the MABL between the observations and ERA‐Interim soundings within the context of cold frontal passages. The moisture in the ERA‐Interim is found to be too confined to the surface layer, which is consistent with the shallower MABL represented by the ERA‐Interim. Analysis of the surface precipitation shows that ERA‐Interim overestimates the amount of precipitation over Macquarie Island in the vicinity of cyclone cores but underestimates the precipitation not immediately associated with cold fronts, leading to an overall underestimate of the annual precipitation by 11%.

Differences of atmospheric boundary layer characteristics between pre-monsoon and monsoon period over the Erhai Lake

The differences in planetary boundary layer characteristics, in particular atmospheric boundary layer height (ABLH), humidity, and local circulations in pre-monsoon and monsoon period over the Erhai Lake, were simulated by the lake-atmosphere coupled model WRF v3.7.1. No lake simulations were also conducted to investigate lake effects over complex topography. During pre-monsoon period, local circulation was fully developed under weak synoptic system. The ABLH ran up to 2300 m or so. During monsoon period, temperature difference between land and lake became smaller, resulting in weaker local circulations. The height of circulation reduced by 500 m, and ABLH ran up to 1100 m during the day. Enhanced soil moisture and low surface temperature due to monsoon rainfalls in July could be the main reason for the slightly lower ABLH over the Erhai Lake area. Specific humidity of the boundary layer increased 8.8 g kg−1 or so during monsoon period. The Erhai Lake enlarged thermal contrast between valley and mountain slope in the Dali Basin. The lake reduced air temperature by 2~3 °C during daytime and increased air temperature by nearly 2 °C in the evening. Due to its small roughness length and large thermal capacity, the Erhai Lake enlarged lake-land temperature difference and local wind speed. A cyclonic circulation was maintained by the combination of mountain breeze and land breeze in the south of the lake. The lake decreased air temperature, increased specific humidity, and reduced ABLH during daytime, whereas the opposite effect is presented at night.

Modeling of energy characteristics of atmospheric boundary layer

Modeling of energy characteristics of atmospheric boundary layer

An explicit algebraic model of the planetary boundary layer turbulence: test computation of the neutrally stratified atmospheric boundary layer

An explicit algebraic model of Reynolds stresses and the turbulent heat flux vector for the planetary boundary layer in a neutrally stratified boundary layer of the atmosphere above a homogeneous rough surface is tested. The version of the algebraic model under consideration is constructed on the physical principles of the RANS (Reynolds-averaged Navier−Stokes) approximation for describing stratified turbulence, it employs three forecasting equations, and a correct reproduction of the main characteristics of a neutral atmospheric boundary layer — the components of the mean wind velocity, the wind turn angle, and the turbulent statistics is shown. Test computations show that the proposed model may be used for goal-oriented investigations of the atmospheric boundary layer.

An Investigation of Wind Farm Power Production for Various Atmospheric Boundary Layer Heights

The dependency of the atmospheric boundary layer (ABL) characteristics on the ABL’s height is investigated by using large eddy simulations (LES). The impacts of ABL’s height on the wind turbine (WT) power production are also investigated by simulating two subsequent wind turbines using the actuator line method (ALM). The results show that, for the same driving pressure forces and aerodynamic roughness height, the wind velocity is higher at deeper ABL, while the wind shear and the wind veer are not affected by the depth. Moreover, the turbulence intensity, kinetic energy, and kinematic shear stress increase with the ABL’s height. Higher power production and power coefficient are obtained from turbines operating at deeper ABL.

Simulation of a 7.7 MW onshore wind farm with the Actuator Line Model

Recently, the Actuator Line Model (ALM) has been evaluated with coarser resolution and larger time steps than what is generally recommended, taking into account an atmospheric sheared and turbulent inflow condition. The aim of the present paper is to continue these studies, assessing the capability of the ALM to represent the wind turbines’ interactions in an onshore wind farm. The ‘Libertad’ wind farm, which consists of four 1.9MW Vestas V100 wind turbines, was simulated considering different wind directions, and the results were compared with the wind farm SCADA data, finding good agreement between them. A sensitivity analysis was performed to evaluate the influence of the spatial resolution, finding acceptable agreement, although some differences were found. It is believed that these differences are due to the characteristics of the different Atmospheric Boundary Layer (ABL) simulations taken as inflow condition (precursor simulations).