Daytime Mixed Layer Research Articles

Applied model for the growth of the daytime mixed layer

A slab model is proposed for developing the height of the mixed layer capped by stable air aloft. The model equations are closed by relating the consumption of energy (potential and kinetic) at the top of the mixed layer to the production of convective and mechanical turbulent kinetic energy within the mixed layer. By assuming that the temperature difference at the top of the mixed layer instantaneously adjusts to the actual meteorological conditions without regard to the initial temperature difference that prevailed, the model is reduced to a single differential equation which easily can be solved numerically. When the mixed layer is shallow or the atmosphere nearly neutrally stratified, the growth is controlled mainly by mechanical turbulence. When the layer is deep, its growth is controlled mainly by convective turbulence. The model is applied on a data set of the evolution of the height of the mixed layer in the morning hours, when both mechanical and convective turbulence contribute to the growth process. Realistic mixed-layer developments are obtained.

A field study of photochemical smog formation under stagnant meteorological conditions

Photochemical air pollution episodes during 15–20 July 1981, covering the Tokyo Metropolitan Area under the stagnant meteorological conditions, were analyzed. Three-dimensional (3-D) pollutant and meteorological observations showed that polluted air masses followed sea and land breeze circulation patterns. Aged secondary pollutants were detected aloft from midnight until early morning. These pollutants subsequently entrained into the daytime mixed layer and affected O 3 formation. To understand this stagnating system, a simple trajectory model was used. Simulated results showed good qualitative agreement with observations, but the model overestimated the surface concentration and underestimated the aircraft observations. This was mainly caused by the assumption of vertical uniformity within the model.

Doppler Lidar Measurement of Profiles of Turbulence and Momentum Flux

A short-pulse CO2 Doppler lidar with 150-m range resolution measured vertical profiles of turbulence and momentum flux. Example measurements are reported of a daytime mixed layer with strong mechanical mixing caused by a wind speed of 15 m/sec, which exceeded the speed above the capping inversion. The lidar adapted an azimuth scanning technique previously demonstrated by radar. Scans alternating between two elevation angles allow determination of mean U-squared, V-squared, and W-squared. Expressions were derived to estimate the uncertainty in the turbulence parameters. A new processing method, partial Fourier decomposition, has less uncertainty than the filtering used earlier.

Generation of Convective Storms over the Escarpment of Northeastern South Africa

Abstract The generation of convective storms over the escarpment of the northeastern region of South Africa is examined in terms of synoptic-, meso-, and local forcing. Severe storm days are defined and contrasted with no-storm days over a 5-year period. Storms are associated with midtropospheric troughs in the westerlies which interact with a major topographic boundary. A mesoscale capping inversion is shown to exist in the lee of the escarpment on storm days. Formation of the inversion is enhanced by a diurnal shift in wind direction across the escarpment. The subsidence inversion anchored to the escarpment is deepened during the day by the advection of the daytime mixed layer off an elevated beat source. Development of severe storms occurs selectively in time and space where easterly low-level moist inflow can penetrate the capping inversion. There appears to be a critical relationship between the larger-scale synoptic circulations and the meso- and local-effects for severe storms to develop.

Gas-to-particle conversion of sulfur in power plant plumes—II. Observations of liquid-phase conversions

The data from 5 days of plume measurements under wet conditions are presented for the transport of the Labadie plume (St. Louis, Missouri) and the Cumberland and Johnsonville plumes (Tennessee). Collectively, the data include instances of liquid-phase reactions in wetted aerosols, as well as in rain, cloud and fog droplets. Detailed case studies are presented for two days of the Tennessee measurements when considerable plume-cloud interactions occurred, and on one of these days, the plumes were also sampled in light rain conditions. The data indicate evidence of variable contributions of liquid-phase reactions to sulfate formation, depending on the type of liquid environment, the extent of plume exposure to it, and the state of dilution of the plume. Sulfate formation appears to be particularly rapid in light rain. In clouds, sulfate forms more rapidly in dilute aged plumes than in coherent fresh plumes. Tall-stack plumes emitted into stable elevated layers may experience a burst of sulfate formation if they pass through a cloud layer at the inversion base during entrainment into the growing daytime mixing layer. A larger set of the MISTT and STATE data indicates that 25 ± 15% of the SO 2 emissions from large midwestern power plants may ordinarily be converted to sulfate on a summer day (24 h), and that the liquid-phase contribution may be 40% or more of this, on average.

The depth of the daytime mixed layer at two coastal sites: A model and its validation

A mathematical model of mixed-layer depth based on the thermodynamic analysis of Tennekes (1973) is generalized to include advection and subsidence. The effects of advection on mixed-layer depth have been modelled by setting the model equations in a Lagrangian frame, performing an approximate first integral in order to derive the spatial dependence of the model variables, and using these spatial forms to give a set of Eulerian equations. The effects of subsidence have been modelled by imposing a subsidence velocity on the top of the mixed layer as well as allowing subsidence-induced warming above that layer.

The early evening boundary layer transition

AbstractVarious mechanisms which lead to nocturnal accelerations and formation of the low‐level jet are examined by analysing Wangara data. the stress field is inferred from the wind field and equations of motion. the evolution of the stress divergence, during the evening transition from daytime mixed layer flow to nocturnal boundary layer flow, is found to increase the ageostrophic flow and subsequent nocturnal accelerations by roughly a factor of two. During this transition the stress divergence in the lowest few hundred metres increases due to the fact that the influence of decreasing boundary layer depth exceeds the effect of decreasing surface stress. This leads to temporary deceleration and rotation of the low‐level wind vector towards low pressure and thus increases the ageostrophic flow.Diurnal variation of the geostrophic wind is also found to significantly strengthen the nocturnal flow. This diurnal variation is apparently due to heating and cooling over terrain which slopes gently upward, east of the location of the Wangara experiment.

Lagrangian and Eulerian Time-Scale Relations in the Daytime Boundary Layer

Abstract Lagrangian (neutral balloon) and Eulerian (tower and aircraft) turbulence observations were made in the daytime mixed layer near Boulder, Colorado. Average sampling time was ∼25 min. Average Lagrangian time scale is ∼70 s and average ratio of Lagrangian to Eulerian time scales (β = TL/TE) is about 1.7. The ratio β is inversely proportional to turbulence intensity i. These data support the formula β = 0.7/i. Lagrangian time scale for the vertical component of turbulence at heights above ∼100 m is given by the formula TL = 0.17zi/σμ where zi is mixing depth. This formula is valid for the horizontal components of turbulence at all heights in the mixed layer. Lagrangian spectra in the inertial subrange are best represented by the formula Fr(n) = 0.2ϵn−2.

Long‐range transport model of SO2 and sulfate and its application to the eastern United States

A long‐range transport model of SO2 and sulfate has been developed. The model consists of two major programs: (1) A program that calculates the trajectories of plumes leaving from any chosen source area (s), dispersion parameters, and the parameters necessary for concentration calculations along trajectory segments using observed wind data. (2) A program that calculates the concentrations of SO2 and sulfate along trajectory segments and interpolates the plume concentrations to grid intersections. In the model the effect of the diurnal change of mixing layer height is taken into consideration. Trajectories of mean winds of the nighttime and daytime mixing layer, and trajectories of the mean wind of the layer between daytime mixing height and nighttime mixing height, are included. Removals, owing to dry deposition and precipitation, are also included. The model is applied to calculate the distributions of 24‐hour average concentrations of SO2 and sulfate over the eastern United States. The distribution patterns of calculated concentrations of SO2 and sulfate were statistically related to those of observed concentrations.

A numerical technique for the investigation of the transport and dry deposition of chemically reactive pollutants

A numerical technique to approximate solutions to the atmospheric advection-diffusion equation, which applies the concept of Locally-One-Dimensional methods, and utilizes the Crank-Nicolson Galerkin method for the diffusion-dominated vertical transport and the Egan and Mahoney method for the advection-dominated horizontal transport, is discussed. This technique is used to simulate the dynamic response of an elevated plume to the growth and dissipation of the daytime mixing layer.

A Parameterization of Diffusion into the Mixed Layer

Abstract The diffusion from a source near the ground into the daytime mixed layer has been deduced from laboratory measurements, making use of boundary-layer free-convection similarity scaling. A parameterization is presented which preserves this similarity scaling for intermediate and long-range diffusion, and which for short-range diffusion incorporates the effects of source height and of near-ground mechanically induced mixing. The parameterization can be applied in steady conditions to the pollution concentration downwind of a steady source, or in unsteady conditions to the concentration in an expanding cylindrical column advected within a multiple puff model. The parameterization applies only to chemically inert, nonbuoyant pollutants having no interaction with the ground.