Sylvain M. Joffre

Review and intercomparison of operational methods for the determination of the mixing height

Abstract The height of the atmospheric boundary layer (ABL) or the mixing height (MH) is a fundamental parameter characterising the structure of the lower troposphere. Two basic possibilities for the practical determination of the MH are its derivation from profile data (measurements or numerical model output) and its parameterisation using simple equations or models (which only need a few measured input values). Different methods suggested in the literature are reviewed in this paper. The most important methods have been tested on data sets from three different sites in Europe (Cabauw – NL, Payerne – CH, Melpitz – D). Parcel and Richardson number methods applied to radiosonde profiles and the analysis of sodar and wind profiler data have been investigated. Modules for MH determination implemented in five currently used meteorological preprocessors for dispersion models have been tested, too. Parcel methods using a revised coefficient for the excess temperature and Richardson number methods using a surface excess temperature worked well under convective conditions. Under stable conditions, the inherent difficulties call for a combination of several methods (e.g., mast and sodar). All the tested parameterisation schemes showed deficiencies under certain conditions, thus requiring more flexible algorithms able to take into account changing and non-classical conditions. Recommendations are formulated regarding both the analysis of profile measurements and the use of parameterisations and simple models, and suggestions for the preprocessor development and for future research activities are presented.

Momentum and heat transfers in the surface layer over a frozen sea

Wind and temperature profiles from a 10-m mast were measured over the frozen Baltic Sea. The Monin-Obukhov similarity theory applies well to runs carefully selected according to stationarity criteria. This provides relatively low-scattered results with the roughness length z0 ≈ 0.04 cm, the drag coefficient CD≈ 1.5 × 10-3 and the Stanton number CH≈ 1.00 × 10-3 under near-neutral conditions. The roughness length, however, behaves in a peculiar way under intense stratification conditions. The reasons proposed for this could lead to an extension of the theory. The mechanisms for momentum and heat transfers are also examined, revealing that there are two different regimes, the smooth and the rough, for the wall friction but not for the heat transfer. Further, the scatter of the data for momentum and heat transfer under the aerodynamically rough regime can be explained by the distinction between the type of thermal stability conditions.

VARIABILITY OF THE STABLE AND UNSTABLE ATMOSPHERIC BOUNDARY-LAYER HEIGHT AND ITS SCALES OVER A BOREAL FOREST

Radiosondes releases during the NOPEX-WINTEX experiment carried out in late winter in Northern Finland were analysed for the determination of the height h of the atmospheric boundary layer. We investigate various possible scaling approaches, based on length scales using micrometeorological turbulence surface measurements and the background atmospheric stratification above h. Under stable conditions, the three previously observed turbulence regimes delineated by values of z/L (L is the Obukhov length) appears as a blueprint for understanding the departures found for the suitability of the Ekman scaling based on LE = u★/f (u★ is the friction velocity and f the Coriolis parameter). The length scale LN = u★/N (where N is the Brunt–Väisälä frequency) appears to be a useful scale under most stable conditions, especially in association with L. Under unstable conditions, shear production of turbulence is still significant, so that the three scales L, LN and LE are again relevant and the dimensionless ratios μN = LN/L and LN/LE = N/f describe well the WINTEX data. Furthermore, in the classical scaling framework, the unstable domain may also be divided into three regimes as reflected by the dependence ofu★/f on instability (z/L).

Investigating the Surface Energy Balance in Urban Areas – Recent Advances and Future Needs

Recent advances in understanding of the surface energy balance of urban areas, based on both experimental investigations andnumerical models, are reviewed. Particular attention is directedto the outcome of a COST-715 Expert Meeting held in April 2000,as well as experiments initiated by that action. In addition, recentcomplete parameterisations of urban effects in meso-scalemodels are reviewed. Given that neither the surface energybalance, nor its components, normally are directly measuredat meteorological stations, nor are there guidelines for theset-up of representative meteorological stations in urbanareas, this paper also provides recommendations to closethese gaps.

The Helsinki Testbed: A Mesoscale Measurement, Research, and Service Platform

Abstract The Finnish Meteorological Institute and Vaisala have established a mesoscale weather observational network in southern Finland. The Helsinki Testbed is an open research and quasi-operational program designed to provide new information on observing systems and strategies, mesoscale weather phenomena, urban and regional modeling, and end-user applications in a high-latitude (~60°N) coastal environment. The Helsinki Testbed and related programs feature several components: observing system design and implementation, small-scale data assimilation, nowcasting and short-range numerical weather prediction, public service, and commercial development of applications. Specifically, the observing instrumentation focuses on meteorological observations of meso-gamma-scale phenomena that are often too small to be detected adequately by traditional observing networks. In particular, more than 40 telecommunication masts (40 that are 120 m high and one that is 300 m high) are instrumented at multiple heights. Oth...

Modelling the dry deposition velocity of highly soluble gases to the sea surface

Abstract A model for estimating dry deposition of highly reactive gases to natural waters is developed using available knowledge of the aerodynamic variability of the flow over developing waves. The aerodynamic resistance decreases strongly with increasing wind velocity. On the other hand, the significance of molecular diffusion increases as the wind strengthens, by increasing the total resistance to transfer. The effect of thermal stability is taken into account, coupled with the variations of the boundary-layer height. As stable stratification strengthens, the resistance in the core of the boundary layer becomes the main rate-limiting process for downward transfer of effluents. Depletion rates along trajectories are very sensitive to the value of the boundary layer height and are probably also affected by entrainment fluxes from aloft.

The refinement of a meteorological pre-processor for the urban environment

The meteorological pre-processor used routinely at the Finnish Meteorological Institute (FMI) has been modified in order better to represent urban conditions. We have re-evaluated the roughness length, introduced the zero displacement height and divided the surface layer into a roughness sub-layer and an inertial sub-layer. The friction velocity and Monin-Obukhov length are evaluated using an empirically developed, exponential, Reynolds stress profile in the roughness sub-layer. The effect of these modifications has been studied by computing the dispersion parameters used in the urban dispersion modelling system UDM-FMI and comparing the revised parameters with the previous model computations.

Meteorological variability of the wet and dry deposition of sulphur and nitrogen compounds over the baltic sea

The dry and wet deposition of N and S compounds to the Baltic Sea Basin were assessed using daily concentration measurements of air and precipitation concentrations, and actual meteorological data on precipitation, daily wind speed and wind direction, as well as the buoyant fluxes at the air- sea interface. The data cover the period 1980–86. Both concentrations and deposition levels show distinct south-north gradients and strong seasonal variability. Dry deposition is an important contribution in the case of S (25 to 80%) but is less significant for the N compounds (10 to 30%). The contribution of particles to dry deposition is negligible for S, equivalent to the gaseous contribution for oxidized N and about 10 to 20% for reduced nitrogen compounds. The obtained total annual deposition of about 1.4 to 1.8 g(S) m−2 for the S compounds, and 1 g(N) m−2 for the N compounds are comparable with, but on the upper side of, previous empirical and model estimates.

An Overview of the Urban Boundary Layer Atmosphere Network in Helsinki

The Helsinki Urban Boundary-Layer Atmosphere Network (UrBAN: http://urban.fmi.fi) is a dedicated research-grade observational network where the physical processes in the atmosphere above the city are studied. Helsinki UrBAN is the most poleward intensive urban research observation network in the world and thus will allow studying some unique features such as strong seasonality. The networks key purpose is for the understanding of the physical processes in the urban boundary layer and associated fluxes of heat, momentum, moisture, and other gases. A further purpose is to secure a research-grade database, which can be used internationally to validate and develop numerical models of air quality and weather prediction. Scintillometers, a scanning Doppler lidar, ceilometers, a sodar, eddy-covariance stations, and radiometers are used. This equipment is supplemented by auxiliary measurements, which were primarily set up for general weather and/or air-quality mandatory purposes, such as vertical soundings and t...

Evaluation of inversion strengths and mixing heights during extremely stable atmospheric stratification

We have compiled data from Finnish sounding stations and classified the inversions according to their total depth and the stability of the boundary layer. We studied the persistence of the inversions and the influence of cloudiness and wind speed on the temperature gradient. Additionally, we addressed the specific problems of an urban area, using measurements from a meteorological mast situated in the Helsinki metropolitan area and comparing them to those from the radiosonde profiles at the rural site of Jokioinen. We also compared several common schemes for the height of the stable boundary layer to results from our own Finnish Meteorological Institute (FMI) method. Based on this analysis a modification to the FMI method was suggested.