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Roland J. Boucher

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Roland J. Boucher

Abstract

More than 1000 hr of radar records taken with the APS-34 1.25-cm vertical-beam radar during a four-year period have been readily classified into four clearly recognizable types. A limited number of precipitation growth mechanisms are thereby suggested, and the echo types representing them are shown to be well correlated to three basically different classes of synoptic situations. A generalized echo cyclone model is derived. Maximum attainable hourly rates of precipitation are empirically related to the depth of detectable echo. The average vertical distance between the warm-front surface and that of the top of the radar echo above is 12,000 ft with no significant correlation. A marked increase in the frequency of precipitation echoes is found in the temperature interval −11C to −15C.

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Roland J. Boucher

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Sequential analyses of data from a network of eight special rawinsonde stations 100 km apart demonstrate the important effects of mesoscale disturbances in a 2½-hr episode of light-to-moderate clear air turbulence (CAT). The most significant feature was the development of two turbulent fields below and above a 2-km region of a wind minimum which passed over Wallops Island, Va., near 1530 GMT 4 February 1970. The small scale of these features preclude their detection in the standard 12-hr observation sequence. The rawinsonde network utilized here incorporating fine-scale details in both space and time is adequate to provide a very useful current CAT depiction and an excellent source for the preparation of a short range forecast of CAT.

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Roland J. Boucher

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Seventeen thunderstorm complexes, observed by TIROS satellite over the United States, were investigated. The diameter of the conspicuously bright storm cloud shield, composed of integrated thunderstorm anvils, has been found to be a useful index of severity. Seven out of ten complexes with shield diameters greater than 60 n mi were accompanied by severe local storms. Another, but less reliable, index of severity is the presence of cirrus streamers or “blowoffs.” The line structure, commonly observed by radar, is generally masked by the cirrus shield.

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Roland J. Boucher and Hans Ottersten

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Sinusoidal variations in the longitudinal speed of the wind in the planetary boundary layer are observed with a C-band multiple-gate Doppler radar using snow as a wind tracer. These undulations in the horizontal wind are believed to be orographically generated wave phenomena. The observed wind structures have average scale lengths of 300 m and their amplitude decreases with altitude. The propagation speeds of the, wind structures have been determined with a spatial correlation technique and have, at times, been found to depart appreciably from the mean ambient wind. Under stable atmospheric conditions the wind structures persist for a considerable period. In one extreme case the structures preserved identifiable characteristics during advection over a distance of 9 km. This remarkable persistence resulted in a Lagrangian-Eulerian time-scale ratio of 45.

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Roland J. Boucher and Raymond Wexler

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Analysis of radar film from New England and the midwest has shed additional light on the relation of precipitation-line motion to upper winds, the direction of motion, the line lifetimes and the regious of appearance and disappearance. The mechanics of line motion are discussed, and an expression is derived by taking into account advection and development. A series of objective tests on the line-motion predictability leads to a suggested technique for forecasting the arrival of a line at a particular point.

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Roland J. Boucher and Ralph J. Newcomb

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Twenty-one vortex cloud patterns from TIROS I wide angle photographs were related to the pertinent features of the corresponding synoptic analyses. A five-stage model depicting the evolution of a cyclonic vortex pattern from the initial frontal wave to the final stage of the decaying cyclone is presented with appropriate examples from the TIROS pictures.

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Roland J. Boucher and James G. Wieler

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A unique method that provides for relating radar-measured reflectivity factors to snowfall rates at the ground is presented. Data were provided by a CPS-9, 3.2 cm radar from six 1978 Massachusetts snowstorms A best-fit power-law relationship between hourly-averaged snowfall rate (depth) and radar reflectivity factor is determined. This derived relationship is found to be Z = 5.07S 1.65 with a correlation coefficient of 0.91. With this relationship hourly snowfall rates are found to be accurate to within +41 to −29% and total storm snowfall accumulation to within +44 to −31%.

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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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Kenneth M. Glover, Roland J. Boucher, Hans Ottersten, and Kenneth R. Hardy

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The results of simultaneous studies of clear air turbulence (CAT) in the lower 15 km of the atmosphere by multi-wavelength radar, jet aircraft and special rawinsondes at the JAFNA radar facility at Wallops Island, Va., are reported. The most important finding is that for the particular aircraft and velocity used in these experiments, every clear air radar echo above 3 km is associated with aircraft reports of at least some perceptible degree of turbulence. Between the altitudes of 3 and 6 km, all CAT is detected by the radars; however, the ability of the radars to detect weak CAT decreases with increasing altitude and only the more intense turbulence is detected above 12 km. The indications are that strong CAT at high altitudes in the free atmosphere is generally associated with zones of increased refractive index variability and enhanced radar backscattering. Therefore, if radars of extreme sensitivity are employed, the useful range for CAT detection may be extended considerably and may possibly satisfy the requirements of an operational ground-based CAT detecting radar system. The vertical vector wind shear appears to be the most significant meteorological factor in specifying turbulent regions. A wind shear criterion ≥ 0.8 × 10−2 sec−1 applied to rawinsonde data specifies the presence or absence of turbulence correctly in 77% of all cases, including 100% of the cases involving CAT greater than light.

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