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A. Waldvogel

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A. Waldvogel

Abstract

Raindrop spectra are measured with an electromechanical raindrop spectrometer, responding to drops >0.3 mm in diameter. The radar reflectivity profile is measured simultaneously with a 5-cm vertically pointing radar.

Measured raindrop spectra are described by two parameters, N 0 and A, the intercept and slope respectively of an exponential distribution having the same water content and radar reflectivity factor as the observed sample.

Sudden variations in the raindrop spectra and in the reflectivity profile during widespread rain situations were observed. Due to the parameterization used, these sudden variations of the spectra can be recognized easily as N 0 jumps. It seems that the N 0 jumps indicate the transition from one mesoscale area within the precipitation field to another. The characteristics of the N 0 jumps and the related cloud physics are discussed. An empirical model is proposed for the relation between the type of raindrop spectra and the convective activity of the precipitating air mass.

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W. Schmid and A. Waldvogel

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Radar reflectivity profiles of 154 hail cells have been investigated. An average profile (the hail profile) has been derived. A global method has been used since the hail profile is designed to be applied to global hail parameters.

Maximum reflectivities are found at h 0 − 1 km, where h 0 is the height of the 0°C isotherm. The change in reflectivity reaches 3 dB km−1 at h 0 − 2 km and for 60 dBZ. This finding agrees with a theoretical profile based on the melting of hailstones. Above h 0 the radar reflectivities decrease continuously with a rate of −1 to −2 dB km−1.

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A. Waldvogel and W. Schmid

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The representativeness of hailfall kinetic energies derived from point measurements of networks is studied by means of radar data of hail cells. The 202 cells were observed within a radius of 60 km from the 10 cm radar. Different Cartesian networks with grid sizes of 0.5–4km are constructed and used to determine estimates of global kinetic energies of the hail cells. From the investigation of these estimates it is found that the normalized standard deviation (s/E) decreases with the square root of increasing global kinetic energy E and increases with decreasing density of the network: doubling the grid size causes about a factor of 3 change in (s/E). The results can be approximated by a simple relation:

E is in MJ and the grid size in km. For networks with a grid size of 2 km and large kinetic energy hailfalls (E ≈ 10 GJ), one finds a normalized standard deviation of ∼ 10%, whereas a value of ∼100% and more is found for small kinetic energy hailfalls (E ≈ 0.1 GJ). Because most of the hail cells are small (68% of the 202 cells investigated), this is an important result for the evaluation of weather modification experiments of hailstorm studies when using ground network data.

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M. Steiner and A. Waldvogel

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The multipeak behavior of raindrop size distributions has been studied. Peaks have been found for distinct drop diameters: 0.7, 1.0, 1.9, and possibly 3.2 mm. The probability is about 65% that at least one of these peaks exists in an observed size distribution. Such peaks may represent important clues for investigations into precipitation growth and binary interaction mechanisms of raindrops. The observational finding of the peaks in raindrop spectra is consistent with several theoretical models.

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J. Joss and A. Waldvogel

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B. Federer and A. Waldvogel

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Time-resolved hail and raindrop size distributions measured simultaneously during a multicellular hailstorm are presented. The time variation of the size distributions allowed a detailed analysis of the structure of the hailswath. It consisted of four hail cells with a duration of 2–3 min each. The number density of the hailstones increased and the mean diameter decreased during the lifetime of a cell. The contributions of liquid water and ice water content to the total water W were about equal; W was never larger than 5 g m−3. The size distributions were well approximated by an exponential law. The mean distribution is given by NH (D) = 12 exp (−0.42D). The distributions are compared with measurements and calculations of other authors and the differences are discussed.

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J. Joss and A. Waldvogel

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The standard deviation of the rain intensity R and the reflectivity factor Z are derived theoretically for R and Z values which are calculated from measured distributions of raindrop size. The derivation is based on the assumption that the distribution of raindrop size follows a negative exponential law. The Poisson distribution is assumed for the number of the counted drops with diameters between D and D+dD. We found that a large sample is necessary to get a good estimate of R and Z; for example, in a widespread rain with a rainfall rate of 1 mm hr−1, a filter paper with an area of ˜1 m2 must be exposed during 1 sec to obtain, with a probability of 68%, a Z value which deviates less than 20% from the mean.

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A. Huggel, W. Schmid, and A. Waldvogel

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The relationship between raindrop size distribution, measured with a disdrometer, and a radar parameter of the melting-layer bright band is investigated. The data, obtained in July 1993 in Switzerland, cover 120 h of precipitation. A good correlation (about −0.7) between the strength of the bright band (ΔZ e) and the intercept and slope parameters of the Marshall-Palmer drop size distribution (N 0, ∇) is observed for horizontally uniform precipitation of moderate intensity (one-third of total rainfall). Steep spectrum with many small drops and no large drops are associated with small values for ΔZ e, whereas flat spectra with relatively few small drops and with large drops exhibit a well-defined bright band with a large ΔZ e.

Considering ΔZ e allows a significantly better derivation of the rainfall rate from radar measurements than is possible with standard Z-R relationships. The rms errors of the 10-min averages of rainfall intensity can be reduced by 20%–40%.

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W. Schmid, H. H. Schiesser, and A. Waldvogel

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Ground-and radar-measured patterns of hail kinetic energy from eight hailstorms have been compared. The radar patterns were shifted horizontally in such a way that the correlation coefficient between the ground and radar data reaches a maximum. The correlation coefficients increased from .7 (no shift) to .9 (optimal shift) on average. The “optimal” shift vector is in good agreement with conceptual models about the kinematics of the storms.

The influence of microphysical effects and of the storm dynamics on the agreement between the hailpad-and radar-measured quantities is discussed in detail. The melting of hailstones and the shape of hailstone spectra have been considered in additional analyses, and the impact of these effects on the ground and radar measurements could be isolated. The results imply that the melting of hailstones is more pronounced in supercell storms than in single-cell storms or in multicell storms.

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