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Thomas G. Konrad

Abstract

Sensitive, high-powered radar have been used to study the dynamics of the convective process in optically clear air. Various stages in the development of the convective field have been identified on the basis of distinctive radar patterns of the cells. The characteristics of the convective field as a whole and the individual cells making up the field during each stage are discussed.

During the initial formation stage, the radar returns are diffuse and mottled. Following the destruction of the surface inversion, the first doughnut or ring-shaped radar returns appear which mark the beginning of the field growth stage. At the onset of this stage, the cells are small, around 100-200 m in diameter. As time progresses, the height of the convective field increase at a rate of 4.5-6 m min− 1, while the convective cells themselves grow to several thousand meters in diameter. Based on a comparison of the radar patterns with previous aircraft measurements of the physical properties of convective cells, it is suggested that the strong, patterned radar returns from the top of the convective field are from individual cells which are cold and moist relative to a slightly stable environment.

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Thomas G. Konrad

Abstract

This study concerns the statistical modeling of rain cell characteristics based on high-resolution radar measurements of the cell structure. The fine-scale three-dimensional structure of summer rainshowers in the mid-Atlantic region has been analyzed leading to statistical descriptions of the rain cells in terms of a variety of physical cell parameters. Core reflectivity profiles, contour area and altitude extent of the cells have been generated along with the frequency of occurrence for various storm classes and categories. Rain cell classes and categories were based on the reflectivity levels at essentially ground level. Cell populations which include all the cells observed were also treated. The mean and median core reflectivity profiles are essentially constant up to altitudes of 4–6 km and then fall off with increasing altitude and follow an orderly progression with cell category. The area of the reflectivity contours about the core value was found to follow an exponential relationship.

The statistical descriptions of the rain cells for the mid-Atlantic region were compared to those from previous investigators at other geographical locations. In terms of core reflectivity profiles, the data for the various locations are rather similar. Significant differences, however, are evident in the altitude extent. Finally, simplified models of rain cells based on the statistical descriptions are developed for the different rain categories as a function of frequency of occurrence.

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Thomas G. Konrad and Freda L. Robison

Abstract

Observations of the development of a convective field in clear air were made simultaneously using radar and an instrumented aircraft. The growth of the field as seen by the radar was compared with the characteristics of the temperature and humidity measurements made by aircraft at various times and locations within the convective depth. Probability distributions of the temperature and mixing ratio fluctuations exhibit distinct, identifiable characteristics for different regions within the field. The altitude of the aircraft measurement, referenced to the maximum altitude of the convective field at the time of the measurement, was found to be a usable parameter in the comparison of aircraft data and in the characterization of convective field growth. Using the relative height parameter, it was found that the area covered by convective cells (modified air) varies linearly with height in the upper reaches of the convective field. The characteristics of individual cells as seen by the radar and as deduced from the aircraft measurements are also discussed.

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Freda L. Robison and Thomas G. Konrad

Abstract

A comparison between the fluctuations in the horizontal component of velocity measured simultaneously by Doppler radar and aircraft has been made in clear air convective conditions. This experiment is the first in which such a comparison was attempted where the radar return was from fine-scale refractivity fluctuations due to turbulent mixing and not from precipitation particles or artificial scatterers. Good agreement was found between the velocity spectra obtained from Doppler measurements of the variations in the mean wind field and the spectra of velocity fluctuations measured by a hot-wire anemometer. Although the velocity spectrum from Doppler radar measurements is limited to those scales larger than the dimensions of the pulse volume, the comparison with the hot-wire data shows that the spectrum of wind variations for scales smaller than the pulse volume may be confidently estimated by extrapolating the velocity spectrum along a −5/3 slope. This result is further supported by the good agreement found between the predicted level of the velocity spectrum based on the variance of the individual Doppler spectra and the measured velocity spectra from the mean Doppler fluctuations and hot-wire anemometer data.

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Thomas G. Konrad and John C. Howard

Abstract

An unusual case of multiple streamers or filaments with the characteristic mare's tail pattern in vertical section has been observed by radar where the generating elements were condensation trails laid by high-altitude aircraft. The contrails were laid perpendicular to the wind and as they drifted a multitude of streamers formed along each trail. The streamers extended from 9 km to the ground. Numerous contrails were observed, each of which produced a sheet of streamers. RHI and PPI photographs at X and S band taken over a 2-hr period show the three-dimensional shape of the streamers due to the wind shear. Doppler measurements were also taken. The resulting velocity spectra are very narrow indicating little or no turbulence. Reflectivity factors were measured at various altitudes and show a decrease in reflectivity with distance from the generating line. Fall velocities based on the slopes of the streamer patterns varied from 0.4 to 1.4 m sec−1. In general, the characteristics of the precipitation streamers were quite similar to those previously measured in naturally occurring cloud forms such as cirrus uncinus.

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