Vertical Structure Of Boundary Layer Research Articles

Types of Vertical Structure of the Nocturnal Boundary Layer

The vertical structure of the observed stable boundary layer often deviates substantially from textbook profiles. Even over flat homogeneous surfaces, the turbulence may not be completely related to the surface conditions and instead generated by elevated sources of turbulence such as low-level jets and transient modes. In stable conditions, even modest surface heterogeneity can alter the vertical structure of the stable boundary layer. With clear skies and low wind speeds, cold-air drainage is sometimes generated by very weak slopes and induces a variety of different vertical structures. Our study examines the vertical structure of the boundary layer at three contrasting tower sites. We emphasize low wind speeds with strong stratification. At a given site, the vertical structure may be sensitive to the surface wind direction. Classification of vertical structures is posed primarily in terms of the profile of the heat flux. The nocturnal boundary layer assumes a variety of vertical structures, which can often be roughly viewed as layering of the heat-flux divergence (convergence). The correlation coefficient between the temperature and vertical velocity fluctuations provides valuable additional information for classification of the vertical structure.

Two Inversion Layers and Their Impacts on PM2.5 Concentration over the Yangtze River Delta, China

AbstractAn integrated winter field campaign was conducted to investigate the atmospheric boundary layer structure and PM2.5 concentration at three sites over the Yangtze River delta (YRD) in China: Shouxian (a rural area), a site in a northern suburb of Nanjing, and Dongshan (a residential area). Two temperature inversion layers and air pollution events occurred simultaneously from 30 to 31 December 2016, local time, over the YRD. It was found that the two inversion layers were related to the presence of a high pressure system, resulting in divergence in the upper boundary layer and radiative cooling near the ground at night. Dominated by agricultural and residential biomass burning, the surface emission sources from the Shouxian rural area were moderately strong. After the formation of the two inversions, the vertical distribution of PM2.5 concentration below the upper inversion layer was uniform as a result of thorough boundary layer mixing in the earlier hours. During nighttime at the Nanjing site, air pollutant plumes from nearby elevated point sources could not easily diffuse downward/upward between the two inversion layers, which led to a distinct peak in the PM2.5 concentration. At the Dongshan site, the emission sources were weak and the nighttime PM2.5 concentration above 100 m was high. The surface PM2.5 concentration gradually increased from early morning to noon, which was attributed to emissions related to the local residents. The results indicated that the vertical distribution of pollutants was affected by a combination of local emissions, vertical boundary layer structure, and horizontal and vertical transports.

Investigation of the atmospheric boundary layer during an unexpected summertime persistent severe haze pollution period in Beijing

Persistent severe haze pollution seldom happens in summer in Beijing. In this study, a persistent severe haze pollution event, defined as daily PM2.5 concentration higher than 150 μg m−3 on 5 consecutive days, was observed in Beijing from 26 to 30 in July 2010. It was not caused by crop residue burning during the harvest period. Weak pressure systems dominated at surface and implicated weak advection. Regional weak southerly winds with speeds of 2–3 m s−1 consistently brought pollutants from southern large-emission regions to Beijing. Surface convergence resulted from northerly winds prevailing in northern regions induced pollutants remaining in Beijing, which contributed to the maximum daily PM2.5 concentration on 26 in July. A continental high-pressure system persisted in the northwest of Beijing at 500 hPa, which led to significant sinking motion. Elevated inversion was found in the form of subsidence inversion, and this was confirmed by a skew T-logp diagram and vertical velocity analysis. It is much different from the vertical boundary layer structure in wintertime pollution period that surface-based inversion dominates. The subsidence inversion was an extremely stable layer with an average depth of hundred meters and strength of 1.4 °C. The capping effect of the inversion layer at low altitude of average 720 m limited vertical diffusion of pollutants and trapped them in a shallow layer, and thus, extremely high concentration of PM2.5 remained. WRF-Chem model simulation demonstrated that about 70% of PM2.5 was transported to Beijing from its southern regions.

Boundary layer structure and scavenging effect during a typical winter haze-fog episode in a core city of BTH region, China

The vertical distribution of PM2.5 and meteorological parameters from ground to upper levels were observed simultaneously using meteorological tower, tethered balloons and aerosol laser radar in Dec of 2016 in the urban area of Tianjin and its southern district, Jinghai. The influence of the vertical structure of boundary layer on a typical haze-fog episode was analyzed. There existed long distance transport of PM in the high layers before the haze formed in Tianjin and the downward airflows brought the PM from the high layer to the ground. In the early stages of this episode, periodic temperature inversions occurred, leading to conspicuous diurnal variations in the vertical profile of the PM2.5. In the middle and late stages of this episode, strong inversion and thick humidity layer were sustained below 400 m, and there were no big daily changes in the vertical profiles of the PM2.5. During the rapid formation period of the fog, the inversion layer was damaged and turbulence was strengthened. During the stationary phase of the fog process, wind and turbulence in the boundary layer became weak again. Rime was the main weather-related, wet cleaning mechanism that lowered pollutants concentration during this fog episode. High concentrations of water soluble ions in the rime samples and the concentrations of those ions in ambient PM2.5 appeared significant decrease during the rime period, which illustrated the scavenging effect of rime.

Evaluation of Near-Surface Variables and the Vertical Structure of the Boundary Layer in CMIP5 Models

Abstract The diurnal cycles of near-surface variables and turbulent heat fluxes are evaluated in 16 models from phase 5 of CMIP (CMIP5) and compared with observations from 26 flux tower sites. The diurnal cycle of 2-m temperature agrees well in general with what is observed. The amplitude of the diurnal cycle of wind speed shows a large intermodel spread and is often overestimated at midlatitude grassland sites and underestimated at midlatitude forest sites. There is a substantial systematic negative bias in the nighttime net surface radiative flux, which is partly compensated for by the turbulent heat fluxes. Four models (CESM1, BCC_CSM1.1, HadGEM2-A, and IPSL-CM5A) are evaluated in more detail, including the vertical structure of the atmospheric boundary layer, at the ARM Southern Great Plains site in Oklahoma. At that site, all models tend to frequently overestimate the boundary layer depth and the wind turning in the boundary layer reveals large intermodel differences. In summer, these models exhibit a substantial warm bias with particularly high daytime temperatures. These high temperatures are associated with very small latent heat fluxes, indicating that the soil is too dry, which is likely to impact climate change scenarios.

Evaluation of the WRF PBL Parameterizations for Marine Boundary Layer Clouds: Cumulus and Stratocumulus

Abstract The performance of five boundary layer parameterizations in the Weather Research and Forecasting Model is examined for marine boundary layer cloud regions running in single-column mode. Most parameterizations show a poor agreement of the vertical boundary layer structure when compared with large-eddy simulation models. These comparisons against large-eddy simulation show that a parameterization based on the eddy-diffusivity/mass-flux approach provides a better performance. The results also illustrate the key role of boundary layer parameterizations in model performance.

Comparison of three weather prediction models with buoy and aircraft measurements under cyclone conditions in Fram Strait

Operational model analyses of the European Centre for Medium-Range Weather Forecast (ECMWF), the German Weather Service (DWD) and the Norwegian Meteorological Institute (HIRLAM) are compared to data measured by 16 ice buoys and by a research aircraft in eight flight missions during the Fram Strait cyclone experiment FRAMZY in March/April 2007. The sea level pressure (SLP) compares well with the buoys for all models, SLP correlations are >0.99, SLP tendency correlations are >0.95 and spatial SLP correlations are >0.94 on the average. The aircraft measurements are used for comparison of the horizontal and vertical boundary layer structure in ECMWF and HIRLAM under cyclonic conditions. Horizontal SLP gradients are slightly underestimated. The surface air temperature (SAT) is too low on the warm side and too high on the cold side of the synoptic systems. The temperature inversion on the cold side is not realistically captured with respect to its base and strength in the ECMWF model and with respect to its thickness in HIRLAM. This results in a wrong vertical structure of humidity and wind. The SAT errors hint at model deficits in the representation of the surface energy balance and the inversion errors hint at deficits in the parametrization of the vertical mixing.

Comparison of three weather prediction models with buoy and aircraft measurements under cyclone conditions in Fram Strait

Operational model analyses of the European Centre for Medium-Range Weather Forecast (ECMWF), the German Weather Service (DWD) and the Norwegian Meteorological Institute (HIRLAM) are compared to data measured by 16 ice buoys and by a research aircraft in eight flight missions during the Fram Strait cyclone experiment FRAMZY in March/April 2007. The sea level pressure (SLP) compares well with the buoys for all models, SLP correlations are >0.99, SLP tendency correlations are >0.95 and spatial SLP correlations are >0.94 on the average. The aircraft measurements are used for comparison of the horizontal and vertical boundary layer structure in ECMWF and HIRLAM under cyclonic conditions. Horizontal SLP gradients are slightly underestimated. The surface air temperature (SAT) is too low on the warm side and too high on the cold side of the synoptic systems. The temperature inversion on the cold side is not realistically captured with respect to its base and strength in the ECMWF model and with respect to its thickness in HIRLAM. This results in a wrong vertical structure of humidity and wind. The SAT errors hint at model deficits in the representation of the surface energy balance and the inversion errors hint at deficits in the parametrization of the vertical mixing.

Some Observational and Modeling Studies of the Atmospheric Boundary Layer at Mississippi Gulf Coast for Air Pollution Dispersion Assessment

Coastal atmospheric conditions widely vary from those over inland due to the land-sea interface, temperature contrast and the consequent development of local circulations. In this study a field meteorological experiment was conducted to measure vertical structure of boundary layer during the period 25-29 June, 2007 at three locations Seabee base, Harrison and Wiggins sites in the Mississippi coast. A GPS Sonde along with slow ascent helium balloon and automated weather stations equipped with slow and fast response sensors were used in the experiment. GPS sonde were launched at three specific times (0700 LT, 1300 LT and 1800 LT) during the experiment days. The observations indicate shallow boundary layer near the coast which gradually develops inland. The weather research and forecasting (WRF) meso-scale atmospheric model and a Lagrangian particle dispersion model (HYSPLIT) are used to simulate the lower atmospheric flow and dispersion in a range of 100 km from the coast for 28-30 June, 2007. The simulated meteorological parameters were compared with the experimental observations. The meso-scale model results show significant temporal and spatial variations in the meteorological fields as a result of development of sea breeze flow, its coupling with the large scale flow field and the ensuing alteration in the mixing depth across the coast. Simulated ground-level concentrations of SO2 from four elevated point sources located along the coast indicate diurnal variation and impact of the local sea-land breeze on the direction of the plume. Model concentration levels were highest during the stable morning condition and during the sea-breeze time in the afternoon. The highest concentrations were found up to 40 km inland during sea breeze time. The study illustrates the application of field meteorological observations for the validation of WRF which is coupled to HYSPLIT for dispersion assessment in the coastal region.

On the boundary layer structure of differentially heated cavity flow in a stably stratified porous medium

This paper considers two-dimensional flow generated in a stably stratified porous medium by monotonic differential heating of the upper surface. For a rectangular cavity with thermally insulated sides and a constant-temperature base, the flow near the upper surface in the high-Darcy–Rayleigh-number limit is shown to consist of a double horizontal boundary layer structure with descending motion confined to the vicinity of the colder sidewall. Here there is a vertical boundary layer structure that terminates at a finite depth on the scale of the outer horizontal layer. Below the horizontal boundary layers the motion consists of a series of weak, uniformly stratified counter-rotating convection cells. Asymptotic results are compared with numerical solutions for the cavity flow at finite values of the Darcy–Rayleigh number.

Enhanced layer of black carbon in a north Indian industrial city

During a comprehensive aerosol field campaign as part of Indian Space Research Organization Geosphere Biosphere Programme (ISRO‐GBP), aircraft measurements of vertical profiles of aerosol black carbon (BC) were made during winter, for the first time, at Kanpur (80°20′E and 26°26′N), an urban industrial location in Northern India. Two vertical profiling from the same day (morning and afternoon) of BC showed that BC decreases with height up to ∼600 m and then increases up to 900 m before becoming more or less constant with height. Potential temperature profile, derived from concurrent measurements of temperature, shows a stable layer at the same altitude where BC shows increased concentration. This vertical structure of boundary layer was further confirmed by separate temperature and relative humidity profiles obtained from balloonsondes during December. The increased BC at ∼900 m suggests the presence of enhanced BC layer, which will have significant implications to BC radiative forcing and modifying cloud properties.

Simplified model for a weakly unstable atmospheric boundary layer capped by an inversion layer

A simplified model for a weakly unstable atmospheric boundary layer capped by a stable inversion layer at the top is presented. This type of boundary layer flow might occur frequently over high-latitude ice fields for sufficiently strong winds. The shear stress is modelled by using a linear eddy viscosity representation. The vertical boundary layer structure, the associated surface shear stress and surface shear stress direction are presented together with comparison with a data set from field observations.

Thermally driven cavity flows in porous media I. The vertical boundary layer structure near the corners

Cavity flows driven by an applied horizontal temperature gradient are considered in the high Rayleigh number limit for a fluid-saturated porous medium. The analysis is concerned with the behaviour of the vertical boundary layer equations near the corners of the cavity. Implications for the structure of the core flow are discussed. The present results, which are new, compare well with a recent numerical solution. Although the results are consistent with the standard hypothesis that the vertical boundary layers empty into the core, they are not in agreement with the corner behaviour previously suggested in the literature. The analysis of the vertical boundary layer structure is also applicable to cavity flows in electrically conducting fluids in the limit when magnetic drag is the dominant force.

Continuously Stratified Models of the Steady-State Equatorial Ocean

Abstract Two linearized, vertically diffusive steady-state models are formulated on an equatorial β-plane. The purpose is (a) to investigate the vertical boundary-layer structure in a continuously stratified ocean spanning the equator and (b) to test the sensitivity of the results to different turbulence parameterizations. Both models are analytically tractable in a horizontally unbounded basin. One is characterized by Newtonian cooling, the other has biharmonic friction. For either model, the equations are analogous to the well-known equations governing equatorial wave motion. This analogy is exploited in both obtaining and interpreting the solutions. In both models, zonal wind forcing leads to features such as the Equatorial Undercurrent, South Equatorial Current and Equatorial Intermediate Current. Structures resembling the recently discovered subsurface countercurrents are also generated. The depth, velocity and other scales are model dependent but the basic dynamics are not. Specifically, near the eq...

Hydromagnetic boundary layers in a rotating cylindrical container

Linear, steady, axisymmetric flow of an electrically conducting homogeneous fluid confined within a rigid, rotating electrically insulated cylinder is analyzed. The fluid motions are driven by differential rotation of horizontal boundaries. The applied magnetic field and the rotation vector are aligned normal to the horizontal boundaries. The magnetic Reynolds number, the Rossby number, and the Ekman number are assumed to be much less than unity. The dynamics of the inviscid interior and vertical boundary layers are investigated as functions of the rotational magnetic interaction parameter α2(=σB2/2ρΩ) which measures the ratio between the magnetic force and the Coriolis force. If α2 ≪1, the flow behaves as a nonmagnetic rotational flow to dominant order with Stewartson’s E1/3 and E1/4 double layer structures and the electric currents pumped by the Ekman–Hartmann layer return through the interior. At the other extreme, if α2≫E−1/3 the flow behaves as a strongly magnetic nonrotational flow with a single vertical layer (≈E1/4 α−1/2) of parabolic structure. The intermediate range 1≪α2≪E−1/3, which is a transition from weakly magnetic to the strongly magnetic flows, is characterized by a quadruple vertical boundary layer structure: (1) a nonmagnetic E1/3 layer, (2) a nonmagnetic E1/4α1/2 layer, (3) a magnetic α−2 layer to transport the electric current vertically, and (4) a magnetic E1/4α−1/2 layer with axial scale E1/4α3/2 to feed to feed the current from the Ekman–Hartmann layers into the α−2 layer.