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R. R. Rogers

Abstract

Statistical properties of the field backscattered from a weather radar target are related to the velocity distribution of the scatterers comprising the target. The complete distribution of radial velocity components can be derived from the received radar signal using coherent Doppler techniques. The standard deviation, and in special cases the complete form of this distribution, can be obtained from incoherent radar measurements.

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R. R. Rogers

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R. R. Rogers and B. R. Tripp

Abstract

The Doppler spectrum of snow contains information about the turbulent wind field in which the snow is embedded. The average spread of the spectrum is proportional to the kinetic energy per unit mass of air in turbulent eddies that are generally smaller than the radar-sampled volume. The time variations in the mean value of velocity in the Doppler spectrum can be analyzed to determine the kinetic energy in eddies that are larger than the sampled volume. Thus from the time behavior of Doppler signals from snow can be estimated the total turbulent energy and the partitioning of this energy between large and small scales.

Root-mean-square turbulent velocities computed on the basis of this technique were found to range between 0.4 and 2 m sec−1, with most of the values less than 1 m sec−1. Approximately three-quarters of the energy is usually in wavelengths shorter than 100 m. The data on the partitioning of eddy energy show considerable scatter, but there is some resemblance to the relationship expected from a Kolmogorov spectral law.

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R. R. Rogers and R. J. Pilié

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H. G. Leighton and R. R. Rogers

Abstract

The growth of cloud droplets by condensation and coalescence in a strong updraft is investigated for different cloud conditions and different initial droplet distributions. Growth by coalescence is determined by solving the stochastic collection equation, and growth by condensation is calculated from the diffusion equation.

The results indicate that under suitable conditions, starting from an initial droplet distribution centered at about 8 µm radius with a dispersion of 0.2, an appreciable number of cloud droplets will grow to precipitation sizes in times of the order of 10–15 min. The results are sensitive to the initial droplet concentration and moisture content of the cloud. The importance of combining coalescence and condensation raises the question of the validity of calculations of rain formation by coalescence only.

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R. R. Rogers and A. P. Schwartz

Abstract

During a 7-week period in the winter of 1986, a six-channel ground-based microwave radiometer was operated at a site near Halifax, Nova Scotia, as part of the Canadian Atlantic Storms Program. One of the quantities measured by this instrument was the vertically integrated water-vapor content of the atmosphere, called the columnar vapor. Time histories of the columnar vapor show variability on many scales. This paper focuses on the fluctuations with frequencies between 0.2 and 2 cycles per hour (periods between 30 min and 5 h), which we denote as mesoscale fluctuations. Power spectra of the columnar vapor fluctuations were calculated for seven records of at least 30 h length. Three of these records were found to have well-defined spectral peaks in the mesoscale range; three had weaker peaks; and one had no significant peak in this range, the spectrum failing off smoothly with approximately an f −2.5 power law. In all cases analyzed, the weather was generally fair, with a stable layer extending approximately between 900 and 700 mb and with Ins stable conditions from the surface to 900 mb. An investigation of the average temperature profiles during the seven periods analyzed showed that the mesoscale spectral peaks tended to be more prominent in cases with stronger low-level instability. From this association between the existence of the spectral peaks and the temperature profile, we tentatively conclude that the mesoscale fluctuations in columnar vapor arise from convection-induced waves in the stable layer between 900 and 700 mb. Thew waves modulate the thickness of the mixed layer and show up as time variations in the columnar vapor as the waves propagate and are advected over the observing site.

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R. R. Rogers and Gene B. Walker

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Ray C. Boston and R. R. Rogers

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M. K. Yau and R. R. Rogers

Abstract

Given a long, continuous record of rain rate measured at a point on the ground, it is straightforward to select a threshold value of rate and note the duration of each event in which the threshold is exceeded. The distribution of durations for any prescribed threshold can be transformed to a distribution of linear extents or lengths over which the threshold rate is exceeded by employing the translation velocity of the rain pattern and invoking the time-to-distance conversion, known as Taylor's hypothesis. The conversion from a distribution of lengths to a corresponding distribution of areas in which the threshold rate is exceeded is a more difficult step, but one that can be carried out if the shapes of the rain regions are specified.

Using a ten-year record of rain rate at Montreal, we have generated the size distributions of rain regions in which a number of thresholds between 1 and 100 mm h−1 are exceeded, assuming the shapes to be circular. Results were compared with size distributions determined from a large sample of radar records at an altitude of 2 km. It was found that there is reasonable approximate agreement between the observed and computed distributions if an allowance is made for the bias against small echoes in the radar observations arising from limited spatial resolution.

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R. R. Rogers and W. O. J. Brown

On 23 May 1996, a Montreal suburban paint factory containing several hundred thousand gallons of paints, solvents, and other chemicals burned to the ground in a spectacular fire. The smoke plume from the fire was readily detected by three radars operated by McGill University for routine observations of the atmosphere. An S-band (10-cm wavelength) scanning radar provided a plan view of the plume from the time of its initial appearance over the plant until the fire was finally extinguished. These data reveal the history of the plume, showing how it meandered and spread as it was advected downwind. The plume passed directly over the site of two vertically pointing radars, one a high-resolution X-band radar (3-cm wavelength) and the other a UHF (33-cm wavelength) wind profiler. Doppler spectra of the smoke echoes in the vertical beam of the profiler indicated predominantly downward velocities, but it was not possible to distinguish in the spectra between scattering by settling particles and scattering by the refractively perturbed air. The reflectivity of the plume in the vertical beam of the wind profiler, expressed in terms of the rain-equivalent reflectivity factor, had values up to 40 dBZ. At the shorter wavelength of the X-band radar, the reflectivity factors were less by amounts ranging from 20 to more than 30 dBZ. The difference in reflectivity can probably be accounted for by a combination of 1) the presence in the plume of particles on the order of 10 mm in diameter, which are too large to satisfy the Rayleigh scattering approximation at the shorter wavelength, and 2) a strongly perturbed structure of atmospheric refractivity, caused by the heating and turbulent mixing generated by the fire and creating a strong echo at the longer wavelength.

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