Examples of the use of remote sensing with relevance for the urban boundary layer are presented as an inspiration for future observational studies in urban areas. The temporal and spatial variability of meteorological parameters within the boundary layer in urban areas constitute the major difficulties in the interpretation of the observations, the simulations with different models and the comparisons between models and measurements. Other examples involve measurement of: the vertical wind profile and horizontal variability of the mean wind speed over a forest; and the depth of the marine boundary layer using different remote sensing instruments. It is noted that research radiosoundings are an indispensable tool in observational campaigns and help to interpret the remote sensing data.

The use of a remote sensing for routine measurements of the boundary layer height is an interesting challenge. The ceilometer is a new not yet fully explored instrument for boundary layer depth measurements. It is an inexpensive and sturdy instrument originally developed for routine cloud height observations, consisting of a vertically pointing laser and a receiver in the same location. It determines profiles by measuring the time required for a pulse of light to be scattered back from the particles in the air. Since the instrument will note any returns, it is possible to determine particle profiles by looking at the whole pattern of returned energy. This has been developed in research and could be applied for operational purposes. Assuming that particles within the boundary layer originate from the ground and that the particle concentration above the boundary layer is comparatively small, the height of the boundary layer can be determined from particle profiles measures by a ceilometer.

In the Galathea expedition (http://www.galathea.nu/) a ceilometer was used successfully to determine the depth of the marine boundary layer. The measurements were performed in the up welling zone west of Namibia. The marine boundary layer is shallow, having a depth of about 200 m. Part of the time atmospheric waves form on the top of the boundary layer were observed. Obviously the sea-spray is confined to the boundary layer and does not penetrate up into the free atmosphere, constituting near ideal conditions for measurements of the depth of the boundary layer by use of a ceilometer. The traditional way of measuring the boundary layer height is to look for jumps in the profiles from radiosoundings; especially in the potential temperature, but also humidity and wind speed and direction, and the variability of the wind direction. In the radiosoundings performed outside Namibia the top of the boundary layer is clearly marked by a 15 K increase of potential temperature and a simultaneous decrease of the wind speed from 11 to 3 m s-1, ideal conditions for the formation of waves.

The use of ceilometer for boundary layer height measurements over land and especially over urban areas is not yet proved to be comparably successful.

Wind LiDAR installed on a mast above forest or urban canopy can be used for vertical and horizontal wind profile measurements. This was proved successful over forest, but is not yet tested properly over urban areas.

The work is related to activities of the authors within COST-728 , COST-732, and COST-735.