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A. Waldvogel and W. Schmid

Abstract

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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W. Schmid and A. Waldvogel

Abstract

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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J. Bader, W. A. Stahel, and W. Schmid

Abstract

The data obtained in Grossversuch IV about hail prevention triggered the hypothesis that only the first rocket launched into a potential hail cell decreases hail kinetic energy in an effect-time interval around 10 min after launching time. Several variations of a randomization test were applied to substantiate this hypothesis. All showed a tendency to confirm the hypothesis, the most significant result being a p value of 0.2%. However, since the test was applied to the data that had been explored to generate the hypothesis, the significant result should be interpreted as a strong hint rather than as a statistical proof.

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

Abstract

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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A. Waldvogel, W. Schmid, and B. Federer

Abstract

The relationship between radar reflectivity and kinetic energy flux (Z-Ė) is investigated by means of analyses of hailstone size distributions measured with hail spectrometers. Semi-empirical relations assuming monodisperse or exponential and untruncated hailstone size distributions are compared with real relations obtained from 175 measured spectra. It is found that 1) the semi-empirical relations agree very well with real ones, the deviation in Ė in the case of exponential spectra being less than 15% for Z values between 50 and 70 dBZ, 2) the Z-Ė relations from different storms show a surprisingly similar behavior, and 3) the total kinetic energy in one point calculated from radar reflectivity factors agrees with the real energy value to better than 25%.

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L. Li, W. Schmid, and J. Joss

Abstract

Motion vectors of radar echo patterns can be obtained by applying a cross-correlation method (e.g., the TREC method) to radar data collected several minutes apart Here an extension of TREC, called COTREC, is presented. Based on constraints and a variational technique, this extension is an efficient objective analysis method for smoothing the motion vectors and forcing them to fulfill the continuity equation. COTREC corrects the apparently wrong vectors that are often caused by failures of TREC. This allows us to identify regions of growth and decay of radar echoes.

For different types of precipitation (convective and widespread), radar data were collected for evaluation of COTREC in complex orography. A comparison between the radial velocity components of retrieved fields of echo motion and the measured Doppler velocity has been made. A marked reduction of the differences with respect to the measured Doppler field was obtained for COTREC, as compared to TREC vectors.

A retardation of COTREC-derived motion compared to Doppler-derived motion was found in orographic precipitation. This retardation may have two causes: 1) a tendency of radar patterns to become stationary (triggered) on upsloping orography; and 2) the influence of ground clutter and shielding, also highly correlated with orography. While the first reflects the fact that propagation of echoes (by growth/decay) and translation of echoes (with the wind) are two different phenomena, the second cause is an artifact produced by the method of observation (radar) but mitigated with Doppler techniques (by suppressing the stationary ground clutter).

COTREC may be useful for nowcasting, especially in orographically complex areas: for orographic precipitation as well as for severe convective storms, the technique predicts the echo development approximately 20 min ahead, and there is good hope to extend the forecasting period.

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

Abstract

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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A. Waldvogel, B. Federer, W. Schmid, and J. F. Mezeix

Abstract

The kinetic energy of hailfalls is measured with a 10 cm radar. For comparison of the results a total of 204 hailpad stations was used covering an area of 780 km2, i.e., one pad per 3.8 km2. The experimental setup is such that the largest distance between any hailpad and the radar is less than 30 km. The radar is accurately calibrated with raindrop disdrometers and hailstone spectrometers. The radar data consist of PPI pictures obtained at a constant elevation angle of 5.5° with a time resolution of 1 min and a reflectivity resolution of 5 dBZ. The radar reflectivity values are transformed into kinetic energies by means of empirical Z–Ė relations obtained from measured time-resolved hailstone spectra.

The data of six severe hailstorms are studied and the following results are obtained: 1) The correlation coefficient of corresponding hailpad and radar derived kinetic energies at single points of a hailstorm is 0.7, a value which can be increased to 0.8 if four-point averages are taken; 2) the global kinetic energy Eg(Z) of a hailfall calculated from radar data agrees to better than 20% with the corresponding value Eg deduced from hailpad information; and 3) the agreement between Eg(Z) and Eg of a hailstorm is better than a few percent for the two cases for which individual Z–Ė relations are available.

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A. Waldvogel, H. H. Schiesser, W. Schmid, and J. F. Mezeix

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

Abstract

The preconvective environment on thunderstorm days in Switzerland north of the Alps has been investigated during a 5-yr period (1985–89). Thermodynamic and kinematic parameters calculated from the radiosounding in Payerne (started at 0000 and 1200 UTC) were used to characterize the initiation of convection. The best parameters were evaluated by using three methods: 1) skill scores, 2) probability distributions, and 3) mean temperature soundings and hodographs. For the decision whether a thunderstorm day was expected or not, the best results were obtained at 0000 UTC with the original Showalter index and at 1200 UTC with the SWEAT index. In addition, to decide whether an isolated or widespread thunderstorm day was expected, the most successful parameter was the modified CAPECCL. Furthermore, the best thermodynamic and kinematic parameters were combined to create new thunderstorm indices, similar to the calculations of the SWEAT index in the United States. The new thunderstorm indices especially designed for northern Switzerland were jointly called the “SWISS index” (combined stability and wind shear index for thunderstorms in Switzerland). All of the traditional and new indices were verified with independent data from 3 yr (1990, 1992, and 1993), showing the best results for the new combined indices.

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