Mixing Layer Height Research Articles

Air-quality monitoring by optical and acoustic radars at Pune, India

The co-located optical (argon-ion lidar) and acoustic (Doppler sodar) radar systems at the Indian Institute of Tropical Meteorology (IITM), Pune (, , 559 m AMSL), India, have been employed to study the nocturnal aerosol pollution dynamics or air quality. Both the systems have been operated simultaneously in order to sample the common atmospheric volume. The results of the experiments thus conducted on some typical nights are presented in this paper. The time - height cross section of the mean, horizontal and vertical components of the wind field derived from the sodar observations indicate that the wind is either easterly or northerly with its vertical component negative (downdraft) for most of the observational period. The interesting feature is that the winds are either calm or relatively weaker in the night-time boundary layer compared with those in the region aloft. This is more evident in the north - south direction in which the lidar and sodar equipment are located. Furthermore, the regions where stable/elevated layer formation is active are found to be associated with the smallest wind gradients, leading to accumulation of aerosol particles or larger temperature structure parameters. The time evolution of the mixed-layer heights and the associated ventilation coefficients, determined from the combined lidar - aerosol and sodar - wind observations, are found to be useful to explain the transport and diffusion of pollutants across the elevated temperature inversions over the experimental station. The details of the experimental techniques are presented and their value for application in the study of regional air quality and pollution potential is discussed.

Atmospheric diffusion model of secondary pollutants with settling

A time‐dependent mathematical model of chemically reactive atmospheric primary pollutants and their byproduct in a protected zone above the surface layer has been developed to study the impact of removal of larger particles when the heavy admixture is present. The different meteorological and source parameters in the surface layer (SL), including a settling velocity term, have been taken into account through appropriate boundary conditions. The variation of surface layer and mixing layer heights on atmospheric stability, geostrophic drag by terrain and several other meteorological parameters are also treated. The results have been analysed for stable and neutral atmospheric conditions. It is shown that the effect of settling velocity on concentration of secondary pollutants is significant in a stable layer compared to the neutral case. Further, the effect of chemical reaction of a primary pollutant is to increase the concentration of secondary pollutant throughout the protected zone for all values of chemical reaction rate and the effect of settling is to increase the concentration in the lower part of the protected zone for certain ranges of settling velocity. In the upper part, the effect of settling is to decrease the concentration for all values of settling velocity. It is interesting to note that the effect of chemical reaction rate has virtually no impact on the formation of secondary pollutant at short travel time but has significant effect at large travel time.

Seasonal variation of the global electrical circuit

The effects of boundary layer aerosol particles on the electric field measurement of the DC global circuit are considered. Aitken (condensation) nuclei concentrations are found to have systematic local seasonal variations which obscure the global behavior of the DC circuit. These local variations appear to be the result of several seasonal factors, including variations in atmospheric mixed layer heights, variations in the productions rates of anthropogenic aerosols, and variations in surface wind speed. Air‐Earth conduction current measurements made by W. Cobb at Mauna Loa (1977–1983), a site remote from sources of pollution and mostly above the boundary layer, appear to be relatively free of aerosol particle effects. The Mauna Loa data are examined and the air‐Earth current is found to peak in the northern hemisphere summer, consistent with the peak of the global thunderstorm activity in the same season. A reanalysis of the entire Carnegie and Maud ocean data set as well as ongoing Schumann resonance results support this finding. However, the general absence of a distinct semiannual signal remains unresolved.

96/06111 Structured cabling for IT systems

Volatile organic compounds (VOC) are important precursors of secondary organic aerosols (SOA). The pollution processes in Beijing were investigated from 18th October to 6th November 2013 to study the characteristics, SOA formation potential and contributing factors of VOC during hazy episodes. The mean concentrations of VOC were 67.4 ± 33.3 μg m−3 on clear days and have 5–7-fold increase in polluted periods. VOC concentrations rapidly increased at a visibility range of 4–5 km with the rate of 25%/km in alkanes, alkenes and halocarbons and the rate of 45%/km in aromatics. Analysis of the mixing layer height (MLH); wind speed and ratios of benzene/toluene (B/T), ethylbenzene/m,p-xylene (E/X), and isopentane/n-pentane (i/n) under different visibility conditions revealed that the MLH and wind speed were the 2 major factors affecting the variability of VOC during clear days and that local emissions and photochemical reactions were main causes of VOC variation on polluted days. Combined with the fractional aerosol coefficient (FAC) method, the SOA formation potentials of alkanes, alkenes and aromatics were 0.3 ± 0.2 μg m−3, 1.1 ± 1.0 μg m−3 and 6.5 ± 6.4 μg m−3, respectively. As the visibility deteriorated, the SOA formation potential increased from 2.1 μg m−3 to 13.2 μg m−3, and the fraction of SOA-forming aromatics rapidly increased from 56.3% to 90.1%. Initial sources were resolved by a positive matrix factorization (PMF) model. Vehicle-related emissions were an important source of VOC at all visibility ranges, accounting for 23%–32%. As visibility declined, emissions from solvents and the chemical industry increased from 13.2% and 6.3% to 34.2% and 23.0%, respectively. Solvents had the greatest SOA formation ability, accounting for 52.5% on average on hazy days, followed by vehicle-related emissions (20.7%).

Measurements of atmospheric SO 2 and SO 42−, and determination of the wet scavenging coefficient of sulfate aerosols for the winter monsoon season over the sea of Japan

In order to determine the scavenging coefficient of oxides of sulfur in winter monsoon over the Sea of Japan both air and precipitation concentrations of sulfur dioxide and sulfate were measured on Sado Island on the Sea of Japan coast. The measurements were taken during the period between 28 January and 3 February 1992. The atmospheric concentrations of SO 4 2− were 0.57–1.11 μg S m −3 whereas those of SO 2, were below 0.27 μg S m −3, much lower than SO 4 2− concentration. A back trajectory analysis showed that these sulfur species would have been transported from Eurasian continent over the Sea of Japan. A layer-average scavenging coefficient, k, for sulfate by precipitation including snow, graupel and rain was determined on the basis of the equation k = HP/ h, where H, P and h are the scavenging ratio, precipitation intensity and mixing layer height, respectively, under the assumption of a uniformly mixed layer over the warm sea current along the Sea of Japan coast. The scavenging coefficients were calculated to be 3.5 × 10 5̄−2.9 × 10 −4S −1 for precipitation intensities of 0.13–3.1 mm h −1. A regression analysis of these datasets gave a k value for submicron SO 4 2− aerosol over the Sea of Japan of k = 1.38 × 10 −4 P 0.74, where p is precipitation intensity. These values are very close to a recently obtained result by Jylhä for radioactive aerosols. Summarizing the measurements taken by the authors and other investigators the wet scavenging coefficient k may be represented by k = aP b where a is around 10 −4, and b is between 0.67 and 0.76.

A laboratory study and improved pdf model of fumigation into a growing convective boundary layer

A saline convection tank was used to study the fumigation of a plume as it is entrained into and rapidly mixed through a growing convective boundary layer. Dispersion results in the form of crosswind-integrated concentrations for the whole field were obtained using a quantitative video digitization technique. Good agreement was obtained with a probability density function (PDF) dispersion model when it was modified to properly account for the spatial variability of the local mixed layer height. A wide range of boundary layer growth rates from very slow ( 0.002w ∗ ) to fast ( 0.15w ∗ , where w ∗ is the convective velocity) was investigated to determine the influence of growth rate on ground-level concentrations (GLCs), duration of the fumigation episode, and thickness of the boundary layer entrainment zone. Within this range of growth rates, maximum crosswind-integrated GLCs were found to be only very weakly dependent on the entrainment rate. The duration of a fumigation episode was found to be proportional to the sum of the entrainment zone thickness and the initial plume spread and inversely proportional to the boundary layer growth rate. The experimental and model results are shown to be applicable to fumigation into a thermal internal boundary layer developing inland from a shoreline as well as to nocturnal inversion breakup fumigation.

Atmospheric thermic structure studied by acoustic echo sounder, boundary layer model, and direct measurements

In this work, the thermic structure of the atmospheric boundary layer is analyzed by means of direct measurements with radiosonde equipment, remote exploration with a three-monostatic Doppler sodar, and a boundary layer model of order one-and-a-half. Intercomparisons of radiosonde data, sodar data, and model results are made through the study of radiative nocturnal inversion, subsidence inversion, development and height of the mixing layer, and calculus of the temperature structure parameter. The ability of sodar to find the mixing layer height and to detect stable layers is enhanced when these layers are low enough.

Thickness of the dry convection and large-scale subsidence above deserts

Thickness of dry convection above various deserts of the world is obtained from aerological data, and assimilated data from ECMWF. A mixed layer develops up to a height of about 1 km above the central Sahara, where strong subsidence occurs. However, above many other deserts in Africa and Asia, a deep mixed layer develops up to 4–6 km. These mixed layers develop to a high altitude because the daytime mixed layer links with an existing weakly stratified, near-neutral layer above. Large-scale subsidence does not reach the surface throughout the day, except in the winter season. Mixed-layer height is shallower in the Southern Hemisphere than in the Northern Hemisphere in the summer season.

Comparison of Atmospheric Boundary Layer Structure Mesured with a Microwave Temperature Profiler and a Mie Scattering Lidar

Measurements of the atmospheric boundary layer using a microwave temperature profiler and a Mie scattering lidar were conducted simultaneously. The atmospheric mixed layer height obtained from the aerosol concentration measured using the lidar agreed very well with the height of the unstable layer measured using the microwave temperature profiler.

A method for computing single trajectories representing boundary layer transport

For many purposes it is necessary to represent the transport of air pollutants by a single trajectory. Examples are models which simulate the physical and chemical processes within a box moving along a trajectory or the receptor-oriented statistical modelling of air pollutants. Especially within the planetary boundary layer strong vertical wind shear poses serious problems for describing transport processes with a single trajectory. This paper describes a new method for calculating boundary layer trajectories which uses a pollutant mass-weighted transport vector. For evaluating this vector it is assumed that. the pollutant is evenly distributed within the mixed layer. During periods of reduced mixed layer depth (e.g. nighttime) a residual layer may form above the mixed layer and most of the pollutant is transported aloft. However, this residual layer transport is only of interest if the mixed layer height rises again and pollutants are fumigated back to ground-level. Thus, estimation of the most appropriate pollutant transport depth requires the knowledge of mixing heights along the whole trajectory from the starting point to the receptor point. As this information is not a priori available, an iterative scheme was developed which uses the mixing heights along a preliminary trajectory as an input for an improved guess of the final trajectory. Experiments have shown that this method converges very fast. The computation of a first guess trajectory with fixed transport depth and one iteration is sufficient.

Development and testing of meteorology and air dispersion models for Mexico City

Los Alamos National Laboratory and Instituto Mexicano del Petróleo are completing a joint study of options for improving air quality in Mexico City. We have modified a three-dimensional, prognostic, higher-order turbulence model for atmospheric circulation (HOTMAC) and a Monte Carlo dispersion and transport model (RAPTAD) to treat domains that include an urbanized area. We used the meteorological model to drive models which describe the photochemistry and air transport and dispersion. The photochemistry modeling is described in a separate paper. We tested the model against routine measurements and those of a major field program. During the field program, measurements included: (1) lidar measurements of aerosol transport and dispersion, (2) aircraft measurements of winds, turbulence, and chemical species aloft, (3) aircraft measurements of skin temperatures, and (4) Tethersonde measurements of winds and ozone. We modified the meteorological model to include provisions for time-varying synoptic-scale winds, adjustments for local wind effects, and detailed surface-coverage descriptions. We developed a new method to define mixing-layer heights based on model outputs. The meteorology and dispersion models were able to provide reasonable representations of the measurements and to define the sources of some of the major uncertainties in the model-measurement comparisons.

On the estimate of aerosol mixing layer height on the base of correlation of aerosol scattering coefficient

On the estimate of aerosol mixing layer height on the base of correlation of aerosol scattering coefficient

Comparison of Methods for Estimating Mixing Height Used during the 1992 Atlanta Field Intensive

Abstract During the summer of 1992, measurements of the boundary layer mixing height were conducted at five locations around the city of Atlanta, Georgia, as part of the 1992 Atlanta Field Intensive of the Southern Oxidants Research Program on Ozone Non-Attainment. These measurements were made during a series of “high-ozone-event days” for the purpose of acquiring information about the temporal evolution of the convective mixed layer. The information acquired from these systems was included in a database of meteorological variables for use in the photochemical modeling efforts associated with the study. The following measurement systems were selected for use in this study by organizers of the 1992 Atlanta Field Intensive: one rawinsonde system, four radar wind profiler–RASS (radar acoustic sounding system) systems, and two lidar systems. A comparison of the mixing-height estimates from each of the measurement systems used during the 1992 Atlanta Field Intensive was performed in an effort to evaluate the c...

Sensitivity of Regional Oxidant Model Predictions to Prognostic and Diagnostic Meteorological Fields

Abstract Objective analysis and diagnostic methods are used to provide hourly meteorological fields to many air quality simulation models. The viability of using predictions from the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 4 (MM4) together with four-dimensional data assimilation, technique to provide meteorological information to the U.S. EPA Regional Oxidant Model (ROM) was studied. Two numerical simulations were performed for eight days using the ROM for a domain covering the eastern United States. In the first case, diagnostically analyzed data were used to provide meteorological conditions, while in the second case the MM4's prognostic data were used. Comparisons of processed diagnostic and prognostic meteorological data indicated differences in dynamical, thermodynamical, and other derived variables. Uncertainties and forecast errors present in the predicted vertical temperature profiles led to estimation of lower mixed-layer heights (∼ 30%–50%) ...

Synoptic Classification of the Mixed-Layer Height Evolution

Around 270 meteorological soundings launched during 1991 and 1992 in the Madrid basin have been studied. Using the vertical profiles of the main meteorological variables obtained from these soundings, an analysis of mixed-layer (ML) height evolution has been carried out. It has been found that the synoptic conditions exert a strong influence on the daily ML evolution, type of mixing, and maximum ML height. The mechanical and convective contributions to the ML development have been analyzed considering a synoptic classification of weather types affecting the Iberian Peninsula. For each weather type, the mean ML evolution along the day has been estimated using a nonlinear regression model. The results have shown that the diurnal variability of the mechanical mixing heights is substantially small compared with the convective ML evolution. In addition, the temporal ML growth from the daily minimum to the daily maximum has been calculated for each synoptic type. Finally, a stepwise variable procedure has been used to determine the main meteorological variables involved in the ML development processes under cloud cover and clear sky conditions.

Response of an Oceanic Bottom Boundary Layer on a Slope to Interior Flow. Part II: Time-Dependent Interior Flow

The behavior of time-dependent oceanic boundary layers on a sloping bottom in the presence of stratification is investigated by the method of direct numerical simulations. The Navier–Stokes equations are decomposed into mean and turbulent components with the mean equations involving a slope Burger number: SN22f2 where N is the buoyancy frequency, θ is the bottom slope angle, and f is the Coriolis parameter. The influence of the turbulent fluctuations is parameterized as eddy coefficients of viscosity and diffusivity. The interior alongslope flow is allowed to be time dependent, and boundary conditions are derived for two possibilities: one where the interior alongslope responds to an interior pressure gradient, and the second where the flow is in balance with other model prognostic variables. Sinusoidal forcing is used to drive the model. The response of the system to the two forms of forcing is found to be quite different and consistent with the forms of the forcing. When the frequency of forcing ω is much less than N, the system exhibits the asymmetric response in mixed layer height observed in oceanic regimes. When ω is of tidal frequency, the measured buoyancy fluxes and upslope transport working against mean background gradients of buoyancy produce secondary mixing rates of the same order as maximum prescribed values of diffusivity. It is concluded that this mechanism is probably not vigorous enough to generate observed oceanic interior mixing rates.

Intercomparison of Nocturnal Lower-Atmospheric Structure Observed with Lidar and Sodar Techniques at Pune, India

Coordinated experiments to study the nocturnal lower atmosphere were conducted on selected nights during April–August 1991 using an argon ion lidar and a Doppler sodar at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m MSL), India. The lidar and the sodar have been operated simultaneously so as to detect the nocturnal atmospheric structure in the common air volume sampled by both the techniques. By analyzing the thermal and aerosol structures in the vertical profiles of the sodar and the lidar signal intensity, the nocturnal mixed-layer height or ground-based inversion height and the stably stratified or multiple elevated layers aloft have been determined. The top of the nocturnal ground-based inversion observed in the sodar records is taken as the height above the ground where the negative vertical gradient in aerosol concentration first reaches a maximum in the lidar records. The results of the study indicate an agreement between the lidar-derived mixing depth and the sodar-derived heights of the ground-based inversion and the low-level wind maximum.

Toward Evaluation of Heat Fluxes in the Convective Boundary Layer

Abstract This article demonstrates that vertical profiles of the heat flux in the convective boundary layer can be diagnosed through an integration over height of the time change rates of observed potential temperature profiles. Moreover, the basic characteristics of the convective boundary layer, such as the mixed-layer height zi, the depth of the interfacial (entrainment) layer, and the beat flux zero-crossing height h0 can be uniquely evaluated based on a time evolution of potential temperature profiles in the lower atmosphere.

Relation of Na+ concentration and δ18O in winter precipitation with weather conditions

A chemical study of the winter precipitation was conducted in Tohkamachi, central Japan. The ratios of Cl−/Na+ and Mg2+/Na+ found in the winter precipitation samples are nearly equal to those of sea water. This result suggests that Na+, Mg2+, and Cl− found in the winter precipitation are originated from sea water. A relationship was determined between the Na+ concentration in the winter precipitation sample and the height of the convective mixing layer. Cloud droplets containing sea salt particles are believed to be captured by snow crystals through the rain‐out process. The results indicate that the higher the convective mixing layer heights, the larger the amount of sea salt particles captured by the snow crystals. A relationship between the δ18O values in the winter precipitation and the air temperature at the top of convective mixing layer was determined for several synoptic weather conditions that produced winter precipitation. The δ18O values in samples collected under conditions of winter monsoon are larger than those under other weather conditions at the same temperature. A good relationship is observed between the temperatures at the top of convective mixing layer and the δ18O of samples of winter precipitation under conditions of the winter monsoon.

An applied model for the height of the daytime mixed layer and the entrainment zone

A model is presented for the height of the mixed layer and the depth of the entrainment zone under near-neutral and unstable atmospheric conditions. It is based on the zero-order mixed-layer height model of Batchvarova and Gryning (1991) and the parameterization of the entrainment zone depth proposed by Gryning and Batchvarova (1994). However, most zero-order slab type models of mixed-layer height may be applied. The use of the model requires only information on those meteorological parameters that are needed in operational applications of ordinary zero-order slab type models of mixed-layer height: friction velocity, kinematic heat flux near the ground and potential temperature gradient in the free atmosphere above the entrainment zone. When information is available on the horizontal divergence of the large-scale flow field, the model also takes into account the effect of subsidence, although this is usually neglected in operational models of mixed-layer height owing to lack of data. Model performance is tested using data from the CIRCE experiment.