In this paper we consider 15 cases of line convection at ana‐cold fronts which have been observed by radar. Although line convection can be a simple two‐dimensional feature, it is usually broken up into line elements having lengths from a few to many tens of kilometres separated by somewhat smaller gaps. The passage of a line element is accompanied by a short burst of heavy precipitation, a temperature drop of about 2degC, a sudden veer and decrease in wind speed, and a pressure jump of 1 to 2 mb. If a gap between line elements passes over a station, however, the changes can occur gradually over a period of up to an hour. Despite the variable size of the line elements, they are found to share common characteristics. They are always oriented slightly clockwise with respect to the synoptic cold front. They all move with the same velocity as their neighbours on a given front, with a component parallel to the front in the direction of the strong low‐level flow which occurs ahead of the front. Moreover, the line elements tend to travel in a fairly predictable manner, largely unaffected by topography and with the major features having lifetimes of several hours.
It is now 15 years since radars were first used intensively to probe the atmosphere in clear‐air conditions. The first experiments were concerned with the nature of the targets and accounting for the intensity of the measured echoes. It was found that inhomogeneities in the refractive index field were responsible for a large proportion of clear‐air returns. Such returns were detected in characteristic patterns in association with sheared statically stable layers, convective thermals, air mass boundaries, and a variety of wave structures. More sensitive Doppler radars have been developed in recent years which are able to obtain usable signals from all heights in the troposphere. Such radars, as well as providing additional information about the clear‐air patterns observed previously, enable continuous measurements of wind profiles to be made. This paper describes the phenomena which have been observed and the measurements which have been made with radars under clear‐air conditions. A summary is given of the types of targets observed, and the kinds of radars used in clear‐air studies are briefly described.
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