Mixing Height Determination Research Articles

Establishing a national standard method for operational mixing height determination

Since the 1960s the Holzworth method has remained a primary tool for operational mixed-layer height determination. The air volume through which ground-based pollutants vertically disperse defines the mixed layer. The appeal of this method rests on the simple mechanics of making a forecast where knowledge of the surface air temperature in concert with the background vertical structure is sufficient. The National Weather Service routinely issues forecasts using this method for air-quality and wildland fire activities. Methods of this type that are based principally on the static stability structure of the atmosphere and exclude vapor content or dynamical processes (e.g., advection and wind shear) can misrepresent the mixing height calculation. Systematic errors, such as the height being too low or high, can complicate wildland fire activities (e.g., go/no-go burn decisions). Motivation for the present study emerges from this premise, and thus examines the mixing height computed from four methods. Mixing height methods employed in this study include Holzworth, Stull, bulk Richardson number, and turbulent kinetic energy—where the latter two include dynamical processes. Mixing height also was derived from satellite-based lidar data to provide an observed proxy and validation. Results from a method intercomparison show that turbulent kinetic energy is the most robust and well suited as a national standard method for operational use—having both thermodynamic and dynamic processes incorporated. The bulk Richardson number and Stull methods are other possibilities because their calculations are not model dependent and heights are consistent with those from turbulent kinetic energy.

Mixing height determination by tethered balloon-based particle soundings and modeling simulations

Vertical profiles of particle number concentration, potential temperature and relative humidity were measured in the Po Valley using an optical particle counter and a portable meteorological station attached to a tethered balloon. The field campaign covered the period 2006–2008, providing an extended dataset of vertical profiles in both stable and convective boundary-layer conditions. These vertical profiles were used to estimate an experimentally retrieved mixing height (MH). The MM5 meteorological model was also used to simulate the atmospheric dispersion characteristics for the same period, using a variety of different boundary-layer and land surface parameterization schemes (Medium-Range Forecast; high-resolution Blackadar; Gayno–Seaman; and Pleim–Chang). The model simulated MHs were compared among themselves, and then with that measured from balloon soundings. MRF parameterization represented the best compromise solution to simulate increasing MHs in the Po Valley. The MM5 simulations showed the regional character of meteorological forcing on PM ground-concentrations in the Po Valley.

Parameterization of vertical diffusion and the atmospheric boundary layer height determination in the EMEP model

Abstract. This paper introduces two changes of the turbulence parameterization for the EMEP (European Monitoring and Evaluation Programme) Eulerian air pollution model: the replacement of the Blackadar in stable and O'Brien in unstable turbulence formulations with an analytical vertical diffusion profile (K(z)) called Grisogono, and a different mixing height determination, based on a bulk Richardson number formulation (RiB). The operational or standard (STD) and proposed new parameterization for eddy diffusivity have been validated in all stability conditions against the observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO42−) concentrations at different EMEP stations during the year 2001. A moderate improvement in the correlation coefficient and bias for NO2 and SO2 and a slight improvement for sulphate is found for the most of the analyzed stations with the Grisogono K(z) scheme, which is recommended for further application due to its scientific and technical advantages. The newly extended approach for the mechanical eddy diffusivity is applied to the Large Eddy Simulation data focusing at the bulk properties of the neutral and stable atmospheric boundary layer. A summary and extension of the previous work on the empirical coefficients in neutral and stable conditions is provided with the recommendations to the further model development. Special emphasis is given to the representation of the ABL in order to capture the vertical transport and dispersion of the atmospheric air pollution. Two different schemes for the ABL height determination are evaluated against the radiosounding data in January and July 2001, and against the data from the Cabauw tower, the Netherlands, for the same year. The validation of the ABL parameterizations has shown that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL height scheme based on the RiB number.

Characterisation of the mixing height temporal evolution by means of a laser dial system in an urban area – intercomparison results with a model application

Abstract. Measurements of vertical and temporal variations in ozone and aerosol as extinction over an urban area in Segovia, central Spain, were performed during two summer months in 2004 by means of a commercial Nd:YAG laser DIAL remote sensing system. The Differential Absorption Lidar (DIAL) technique was applied and its description is given. From the profile data, a practical determination of mixing height may be derived. A diurnal evolution for the whole dataset is observed, the highest mean mixing height being reached at 16:00 GMT, 2150 m. The presence of a double-layer structure at night was observed and the layers can be considered residual. On average, the lower layer is formed at 670 m and the upper layer yielded mean heights ranging between 1270 and 1390 m. The estimated mixing heights during the day are also compared with those obtained from the Lagrangian HYSPLIT model. The results show good statistical agreement between both approaches, mainly in the early afternoon, with correlation coefficients around 0.7.

Mixing height determination with lidar ceilometers results from Helsinki Testbed

Helsinki Testbed is a research project of mesoscale meteorology. Data from various meteorological sensors including five lidar ceilometers situated in and around Helsinki have been collected since 2005. The ceilometers report attenuated backscatter profiles that have been evaluated in respect of mixing height determination. Two different methods, a gradient method and an idealized backscatter method have been applied for this purpose. Data examples from different meteorological situations are discussed. The enhanced single lens optical concept of the ceilometers improves their ability to detect nocturnal stable layers. Residual layers are visible up to heights exceeding 2500 m. Such layers can also be seen in data from the space-borne lidar CALIOP.

Mixing height determination by ceilometer

Abstract. A novel method for estimating the mixing height based on ceilometer measurements is described and tested against commonly used methods for determining mixing height. In this method an idealised backscatter profile is fitted to the observed backscatter profile. The mixing height is one of the idealised backscatter profile parameters. An extensive amount of ceilometer data and vertical soundings data from the Helsinki area in 2002 is utilized to test the applicability of the ceilometer for mixing height determination. The results, including 71 convective and 38 stable cases, show that in clear sky conditions the mixing heights determined from ceilometer based aerosol profiles and BL-height estimates based on sounding data are in a good agreement. Rejected outlier cases corresponded to very low aerosol concentrations in the mixed layer leading to a very weak aerosol backscatter signal in the lowest layer.

Alpine radiosoundings feasible for mixing height Ddetermination?

The depth of the mixing layer, i.e. of the domain, in which pollutants are dispersed by convection or mechanical turbulence, is an essential scaling parameter for the interpretation of air pollution measurements and for air pollution modeling. For flat terrain, several procedures are available to determine the mixing height from radiosonde profiles: In the convective boundary layer, the advanced parcel method and the evaluation of the gradient of the mixing ratio of water vapor are appropriate, in stable conditions approaches based on the Richardson number and on the low level wind profile. In stable and unstable cases, the detection of a critical inversion on top of the mixing layer renders a mixing height estimate. As direct observations of the mixing height (e.g. lidar measurements) are rare, the agreement of results from different approaches is a measure for the reliability of the mixing height estimate. The main task of this study is to investigate in how far the methods for mixing height determination can also be applied to radiosonde data in an Alpine valley. This question is investigated based on radiosonde profiles from five stations which were operated in the Alpine Rhine Valley from September to November 1999 during the Mesoscale Alpine Programme (MAP). Under weak pressure gradient conditions, the mixing height is well discernable from the soundings. The top of the convective boundary layer is horizontally flat. The mixing height in stable conditions is lower at the stations on the valley floor in the lower part than in the upper part of the valley. During foehn flow, mechanical mixing increases significantly and no mixing layer in the traditional sense is detectable.

Mixing height estimation from sodar data — A critical discussion

Sodar measurements have been used to estimate the mixing height for about 20 yr, nevertheless the issue is still the subject of controversial discussion. The paper attempts to critically examine the methods and results of mixing height determination from sodar data that have been reported in the literature. The theoretical base, the methods suggested, the automatization of algorithms, and the intercomparison of sodar-based mixing height values with data from independent measurements and models are briefly discussed. An assessment is given of sodar capabilities in comparison with other profiling techniques. It is concluded that sodar data may be used to derive reliable mixing height information in many situations which are of special relevance for environmental monitoring, namely in stagnant weather situations with low mixing height values between about 50 and 500–1000 m, depending on the type of the sodar. However, the algorithms available up to now for their automatic evaluation appear not yet reliable enough to be recommended for operational purposes, and a control of the output by a trained analyst is advisable. Simultaneous operation of sodars and other remote sensing systems (lidar, wind profiler) is shown to be a promising way to overcome the range limitations of sodars and to allow continuous mixing height estimation throughout the complete diurnal cycle.