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S U M M A R YA study of the dynamical formation mechanisms of atmosphcric boundary-layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with somc frequency 111 thc abscncc of visible flow tracers. For example. in 1997 MacPherson and Betts pointed out instrument observations of invi\ible boundary-layer vertical vortices nver the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterited by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) OF mean wind\. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations.A Large-Eddy Simulation ol' the convective boundary layer is performed. without mean winds, with the purpose of examining vertical vortex-formation mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity-generating mechanisms. The results indicate that vertical vortices form at some, but not all. of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the rurface. It is suggested that the vertical vorticity initially available to the vortices conies from the tilting of horizontal vorticity associated with local unidirectional chears due to the convective-cell circulations. with convective-cell asymmetries being important to the process. Subsequent evolution apparently results in the co-location of vertical vorticity centres with local updraught maxima. K E Y

Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate the plume is the M£teotron, an array of 105 fuel oil burners with a total heat output of approximately 1000 MW. Three types of vortices have been observed: 1) large counter-rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the surface on the downwind side of the plume, and 3) very large columnar vortices produced when the lower portion of the plume goes into rotation as a whole. Three mechanisms leading to the concentration of vorticity necessary to produce these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environmental wind field, generation of vorticity within the plume by the action of buoyancy and drag forces, and convergence of preexisting background vorticity from the environment. It is concluded, based on these observations and physical considerations, that the generation of vortices of moderate intensity is to be expected in large plumes, be their source a forest fire or an industrial operation.

a dust devil census was conducted at the Permanent High Explosive Test Site on White Sands Missile Range, New Mexico. In a 260-km 2 observation area, a total of 3134 dust devils were counted on 97 days (out of 112 observing days).Within the observation area, the dust devils were concentrated in a relatively small area and showed strong canrelation with roadways and other cleared areas. However, from the stunmet of 1986 to the spring of 1987, the center of activity shifted from a relatively undisturbed region to an area whose surface had been recently modified. This shift is attributed to changes in soil moisture and surface thermal properties. It is suggested that the co-location of the center of activity in 1987 with the modified surface is an example of inadvertent weather modification on a very local scale. The dust devils had a daily distribution very similar to that found by Sinclair (1969) in comparable terrain in the vicinity of Tucson, Arizona. The time of the first observed dust devil was very regular on days with dry surface conditions at sunrise and clear skies through the morning. The time of the first dust devil was delayed and the degree of activity reduced if precipitation had occurred on a previous day in the observation area. The production of dust devils often ended abruptly with the onset of an aftemoon thundershower in the observation area. l. INTRODUCrION An observational study of dust devils was conducted during late spring and summer 1986 and spring 1987 at a site on White Sands Missile Range (WSMR) in south-central New Mexico. The goals of this study were (1) to determine spatial and temporal distributions of dust devils in a 260-km 2 region and (2) to relate these to background meteorology. Since the first goal involved the counting of a large number of dust devils, this portion of the study is termed a "dust devil census" after Sinclair [1969].The pro'poses of this initial paper are twofold. The first is to present spatial and temporal distributions of dust devils determined from the census data. The second is to discuss apparent relationships between the dust devil observations, general regional meteorology, and local surface conditions. Novel aspects of this census are the large area considered, the long durations of the observing periods, and the large number of dust devils observed. C•ssus LOCATION2.1. The Site The census was conducted in the northern section of WSMR at the Permanent High Explosive Test Site (PHETS) of the Defense Nuclear Agency (DNA). PHETS is located along the eastern edge of a broad basin, commonly termed the Jomado del Muerto, that is part of the climatological division termed the Central Valley of New Mexico [Houghton, 1972]. In terms of general climate and vegetative cover, the region can be classified as medium elevation, semi-arid ranch land. (This portion of WSMR is similar in its surface and climatic features to the area around Tucson, Arizona, at the time S/nc/air [1969] conducted his census.) For the census, the designated observation area was the 13-kil...

A 4 m wide and 7 m tall tornado vortex generator (including exhaust fan and duct work) has been constructed at Purdue University that operates on a principle similar to that of the earlier machine modeled by Ward (1972). Characteristics of the Purdue simulator are described, as well as the corresponding modifications and improvements that have been made to Ward's machine. Selected photographs of vortex configurations obtained in the simulator demonstrate the ability of the machine to achieve vortex breakdown and multiple vortex configuration. A radial-axial profile of velocity magnitudes (using hot-film anemometry) has been obtained for the state of vortex breakdown characterized by two interlocking helical spiral vortices. This preliminary result shows the potential that the experimental system offers for obtaining quantitative information about the flow field of selected vortex configurations. Multiple vortex phenomena in the thunderstorm-tornado system are examined in light of the laboratory simulation and the similarity concept.