Orientation cues for high-flying nocturnal insect migrants: do turbulence-induced temperature and velocity fluctuations indicate the mean wind flow?

Migratory insects flying at high altitude at night often show a degree of common alignment, sometimes with quite small angular dispersions around the mean. The observed orientation directions are often close to the downwind direction and this would seemingly be adaptive in that large insects could add their self-propelled speed to the wind speed, thus maximising their displacement in a given time. There are increasing indications that high-altitude orientation may be maintained by some intrinsic property of the wind rather than by visual perception of relative ground movement. Therefore, we first examined whether migrating insects could deduce the mean wind direction from the turbulent fluctuations in temperature. Within the atmospheric boundary-layer, temperature records show characteristic ramp-cliff structures, and insects flying downwind would move through these ramps whilst those flying crosswind would not. However, analysis of vertical-looking radar data on the common orientations of nocturnally migrating insects in the UK produced no evidence that the migrants actually use temperature ramps as orientation cues. This suggests that insects rely on turbulent velocity and acceleration cues, and refocuses attention on how these can be detected, especially as small-scale turbulence is usually held to be directionally invariant (isotropic). In the second part of the paper we present a theoretical analysis and simulations showing that velocity fluctuations and accelerations felt by an insect are predicted to be anisotropic even when the small-scale turbulence (measured at a fixed point or along the trajectory of a fluid-particle) is isotropic. Our results thus provide further evidence that insects do indeed use turbulent velocity and acceleration cues as indicators of the mean wind direction

Urban roughness mapping validation techniques and some first results

Because of measuring problems related to evaluation of urban roughness parameters, a new approach using a roughness mapping tool has been tested: evaluation of roughness length z(o) and zero displacement z(d) from cadastral databases. Special attention needs to be given to the validation of the tool, and the roughness formulas therein. After a roughness formula validation using laboratory data, three other validation tests have been applied sucessfully, using roughness classifications, comparison with an established roughness model, and spatial averaging consistency tests. The latter is an interesting, new approach which allows to rank the performance of the otherwise largely similar models. Further results of interest are the sensitivity of the Raupach model to some of its parameters, and the need to include the drag of forest, town and obstacle group edges in an effective, spatially averaged roughness. (C) 1998 Elsevier Science Ltd. All rights reserved

Factors Determining Particle Dynamics over the Air-Sea Interface

Work done in the framework of the ASE subproject, topic 5, on factors determining particle dynamics over the air-sea interface, is briefly reviewed. Emphasis is on the cooperative efforts between the authors, covering a period of roughly 8 years, from 1988 until 1996 [1–16], which in part were carried out in cooperation with other institutes. The work has focused on the description of the aerosol life cycle in the marine atmosphere, including production of sea spray aerosol, transport of sea spray aerosol from the surface into the surface layer and the mixed layer, the dispersal of aerosol throughout the marine atmospheric boundary layer (MABL), and the removal at the surface and at the top of the MABL. Continental aerosol, advected over the oceans in off-shore winds, is an important factor in the MABL, and processes involving aerosols of non-marine origin were included in our studies, in particular as regards their deposition on water surfaces

The influence of thermal effects on flow and dispersion in street canyons

The thermal effects due to the variable heating of the building walls by solar radiation and the heating produced by the vehicles on the airflow within a street canyon were studied by this group. The full-scale and wind tunnel measurements suggest that the overall effect of the heated walls on the street canyon flow dynamics is smaller than in 2-D numerical simulations. Thermal effects may generate a thin thermal convective flow close to the heated wall. As the flow in the wall boundaries carries air from the street level upwards, while normally cleaner air is transported from above, thermal effects may still be important for the air quality at pedestrian level and for the pollution transfers indoor. The heated walls affect the three components of the wind close to the wall and, therefore, this topic must be dealt with 3-D numerical calculations.The heat flux close to the heated wall is an important issue of these studies. In numerical calculations the use of heat flux boundary conditions is certainly more appropriate than the use of temperature boundary conditions. The results of the Nantes\u201999 campaign together with the new experiment Nantes 2000 documenting temperature and wind speed close to the wall are expected to give further input to the refinement of the treatment of wall heating effects and to the formulation of the heat flux boundary condition in CFD codes. Traffic heat-induced turbulence can be comparable in magnitude to that due to traffic mechanically-induced turbulence (TPT), especially for low vehicle speeds, and so needs to be considered for inclusion in models