SUMMARYCollection efficiencies of falling water drops ranging from -& to 1 mm in radius for spores of Lycoperdon (mean radius i: = 2.25 p), spores of black rust (i: = 2.6 p), and grains of Paper Mulberry pollen (i: = 6.4 p) have been determined. The experimental results are in fair agreement with calculations based on the assumption of potential flow around the larger drops. With both kinds of small spore the collection efficiencies reach maximum values for drops of about 400 p radius and fall sharply for smaller drops. In this latter regime, where the potential flow solution is inappropriate and the viscous flow solution gives zero values, a sharp fall in collection efficiency is to be expected. Aggregates of two or three spores are collected slightly more efficiently than single spores, but the results show very similar trends in all cases.Some preliminary results have been obtained on the collection of pollen grains by circular, hexagonal and star-shaped paper discs that simulate snow crystals. The collection efficiencies, as deduced from the numbers of particles caught on their leading faces, are usually < 0.05, but the appearance of similar numbers of particles on the trailing faces suggests that particle capture in the wake of the disc is important at these Reynolds numbers of about 100.

A new linked mode of operation has been used to obtain virtually simultaneous measurements by a fully instrumented aircraft and a ground-based high power radar within a train of large amplitude Kelvin-Helmholtz billows in the optically clear atmosphere. Both sets of data were analysed to show the detailed distribution of air motion and turbulence within the billows. The resulting pattern was consistent with a train of vortices each of which was characterized by a vertical wind shear of 6 m s-l over 100 m, with downward motion of about 1 m s-l on the downshear side and upward motion of about 1 m s-l on the upshear side. Between the vortices the vertical shear decreased to almost half the maximum value. The most intense radar echoes occurred within inclined layers connecting the top of one region of maximum vorticity to the bottom of the next. The aircraft, which flew through the upper parts of the billows, encountered turbulence of up to moderate intensity as it penetrated the crests of the billows. Between successive penetrations of the billow crests the aircraft flew close to but just above the region of intense radar echo and encountered smooth air with a slowly varying vertical component of motion.

Abstract. The high-power Defford radar has been upgraded to provide Doppler information regarding the motion of echoes from weak refractive index inhomogeneities within the optically clear atmosphere. A case study is presented in which data from the radar are used to derive the detailed velocity structure in and above the planetary boundary layer. These data are analysed to show how convective circulations in the boundary layer can perturb the height of a shallow inversion overlying it, thereby producing local enhancements of wind shear and a decrease in dynamic stability within the inversion. The measurements were obtained as part of a Boundary-Layer Project in which simultaneous measurements were made using fast-response instruments suspended from a tethered balloon within the region scanned by the radar. The balloon-borne probes showed that the most intense turbulence and fluctuations of temperature and refractivity were encountered when radar-detected hummocks in the height of the inversion were advected through the probes. The fine-scale turbulence measurements within the perturbed inversion are consistent with the existence of Kelvin-Helmholtz billows.

Large amplitude Kelvin-Helmholtz billows observed by radar at a height of almost 11 km are shown to have been associated with clear air turbulence which produced a vertical aircraft acceleration of 0.65g.

A very high-sensitivity 107 mm radar has been brought into operation for meteorological studies of airflow within the optically clear (or cloudy) atmosphere. One application of this radar has been to observe the detailed three-dimensional structure of lee waves, using backscatter from both refractive index inhomogeneities and cirrus 'clouds' as means of tracing the pattern of airflow. This paper presents case studies showing how the radar has revealed the structure of lee waves downwind from the Welsh mountains. One of the case studies illustrates a steady-state wave pattern, another illustrates an unsteady pattern with waves varying in orientation and wavelength, another illustrates a strongly damped wave pattern, and a final one illustrates two families of waves co-existing at different altitudes. The radar method is shown to provide a direct and elegant technique for measuring lee wave properties. Although it depends on the presence of naturally occurring targets to trace the airflow pattern, such targets are found to be abundant in situations of strong lee wave activity. ' Two-dimensional mountain wavesa method for the rapid computation of lee wavelengths and vertical velocities,' Ibid., 88, pp. 271-285. ' Multiwavelength backscatter from the dear atmosphere,' J Geophys. Res.i7l, pp. 1,5374,552. ' Theory of waves in the lee of mountains,' Quart. J.R. Met. ' High-power radar for meteorological studiea in clear air,' ' The airflow over mountains,' W.M.O. Tech. Note No. 34. Edited by Akka, M.A. 135 pp. ' Detailed observations of mountain lee waves and a comparison with theory,' Colorado State University. Atmot. Sci. Paper No. 138. SOC., 75, pp. 41-56. PCOG. ZEE, 118, pp. 519-528.