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Roger M. Lhermitte

Abstract

A Doppler radar has been used in central Oklahoma to probe the motion of invisible targets usually referred to as “angels.” The large density targets detected on certain days suggests the presence of a dense atmospheric “plankton” drifting with the air. Uniform motion for all the targets in the area surveyed by the radar beam is confirmed by the systematic pattern of target radial motions as a function of radar beam azimuth. Target horizontal motion direction and speed are derived from the radial velocity-azimuth patterns and interpreted as horizontal wind. The small vertical motion of targets is also estimated from the data. This technique is applied to the analysis of wind variance and the study of a low-level jet.

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Roger M. Lhermitte

Abstract

The paper discusses the use of airborne pulse-Doppler techniques for the sensing of particle motion inside convective storms. A downward-looking beam method providing observations of particle vertical velocity distributed in a vertical plane generated by the aircraft's motion is presented in detail. The possible use of a horizontal beam, which is perpendicular to the aircraft's trajectory and senses horizontal motion, is briefly mentioned. Included is a discussion of the application of the method to the probing of vertical air motion in convective storms and the observation of drop-size distributions in widespread rain. The real-time processing and recording of the Doppler signals by an on-line digital computer installed aboard the airplane is also described. The paper discusses the accuracy of the velocity measurements as related to radar system parameters and the aircraft ground speed.

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Roger M. Lhermitte and Marc Gilet

Abstract

This paper presents dual-Doppler radar observations and analysis of the kinematics of a convective storm which developed under sea breeze conditions over the area of Miami, Fla. The method, which is based on simultaneous observations of the precipitation particles' velocities by two Doppler radars installed at two different locations, is discussed. The corrections on the estimate of vertical air velocity due to the terminal velocity of the precipitation particles and the change of air density with altitude are also discussed.

The observational results show that in the early stages of its development, the storm system is composed of many small cells with apparent disorganization of the wind fields. The observed structures are complex and bear little resemblance to those predicted by numerical models of an isolated cell. There is always indication of convergence regions of a transitory nature between the precipitation cells. The motion fields become more organized in time and then exhibit extensive and persistent regions of convergence and divergence, with the convergence most likely found at altitudes above 2.5 to 3 km and the divergence below. At the time of maximum organization of the storm, a substantial increase in radar reflectivity is observed in the vicinity of the freezing level which is associated with a drastic increase of the inflow condition at this level, suggesting that the latent heat release due to freezing plays an important part in the storm dynamics at this stage of the storm development.

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Roland J. Boucher, Raymond Wexler, David Atlas, and Roger M. Lhermitte

Abstract

Doppler wind soundings were taken in the lower 4.5 km of the atmosphere at 12 minute intervals during a seven hour period in a snowstorm over eastern Massachusetts. A time-height cross section of the wind revealed numerous small scale, short period changes in the wind structure.

Initially there was a region of strong wind shear near 4 km, above the warm front zone. Periodic break-downs in the wind shear appeared to result in a downward transfer of momentum. In this way, winds at lower levels increased by as much as 10 m sec−1 within a half hour period. The series of three breakdowns occurred at about hourly intervals at successively lower levels, until by the end of the period the wind speed at all levels had increased by about a factor of two, and the wind shear zone was confined to the lowest few hundred meters.

A time-height cross section of vertical motions indicated that each breakdown was preceded by a down-draft, followed by a turbulent region of successive updrafts and downdrafts of 2 to 4 m sec−1. These turbulent regions may be responsible for much of the short period change in structure of “uniform” precipitation.

A comparative analysis, using hourly rawinsondes during a rainstorms with an analogous wind structure also revealed similar breakdowns, although the absence of resolution precluded delineation of the smaller scale turbulence which the Doppler observations so clearly reveal.

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