Giorgio Caranti

Power spectrum of a gravity wave propagating in a shearing background wind

We re-analyze the effect on the spectral tail of a gravity wave propagating in a shearing background. The contribution to the spectrum of horizontal wind perturbations for low vertical wavenumbers comes from the Doppler shifting with a -1 slope and for high wavenumbers from the leakage effect with slopes ranging between -4 to -2. If leakage is not present, it becomes necessary to consider the termination of the wave in order to account for the spectral tail. We show that the decreasing of the wave amplitude when the overturning starts, leads to a spectral slope of -3, and an amplitude proportional to N 2 (the square of the Brunt Vaisala frequency).

Cloud electrification by fracture in ice-ice collisions: a 3D model

A study to evaluate the electrification caused by only one kind of noninductive mechanism, namely ice-ice collisions with fractures is presented. Its ability to reproduce the most important features of cloud electrification is discussed. The study is carried out numerically and use is made of recent laboratory data. The model has the dynamical and electrical developments uncoupled. The particle spectrum is discrete with eight categories and the fragments increase the number of only the smallest of them. It is shown that high fields (250 kV/m) are produced by fracture charging in times of about 3 min. It is also found that positive and negative charge distributions overlap considerably and their superposition results in a highly stable stratification forming a vertical tripole structure. The charge on individual particles so obtained are in good accord with the experimental data reported by airborne in situ measurements.

A comparative study on hailstone trajectories using different motion equations, drag coefficients and wind fields

Abstract The growth of hydrometeors to hailstone sizes is simulated by computer. The trajectories are calculated using the results of a three-dimensional cloud model as input. The model simulates the time evolution of the wind, the liquid water content and the air temperature fields in a cubic region of 16 km per side. These fields are recorded and later used in another program that calculates the hail growth and the aerodynamical forces involved. Embryos of 0.25, 0.5 and 1.00 mm of initial radii are released at 676 locations uniformly distributed in a region of 6×6×4 km3 around the updraft center. The trajectories of all the particles are followed all the way to the 0°C altitude. The final radii and residence times in the cloud of all particles are also stored. A systematic study on the changes brought about by the use of different expressions for the drag coefficient and for the equation of motion on both the computed trajectories and on the final size of hailstones is performed. The study includes the location of the most favored regions for the initiation of hail in each case. The effect of using stationary and time dependent fields is also considered in detail. A comparison of the results obtained using a realistic equation of motion and those employing an approximation frequently used by others show the respective final radii differing by up to 30%. It is observed that expressions predicting values for the drag coefficient that are not nearly as scattered as those found in nature can lead to final radii differing by as much as 50%. The use of stationary wind fields can produce an important overestimation of the maximum sizes achieved by hail inside the cloud.

On the relation of the terminal fall speed of hydrometeors to environmental conditions and particle size and density

Expressions are derived which relate the terminal fall speed of hydrometeors to their size and density and to the environmental parameters of the air, and for extrapolating terminal velocity measurements under known conditions to other conditions and altitudes. The formulation is an extension of previous work and is based on the assumption that on average the drag coefficient is related to the Reynolds number by a simple power law over a reasonable range of Reynolds numbers. This is shown to imply a power law dependence between the terminal fall speed and diameter of a particle, as is often observed, and also to be consistent with empirical power law relations between the Best and Reynolds numbers of falling particles. A set of terminal fall speed parameters is presented which is representative of a variety of precipitation types.

Spectral Tail of a Gravity Wave Train Propagating in a Shearing Background

Abstract The causes of the appearance of a tail in the power spectrum of a gravity wave train in a shearing background terminating under the Hodges condition are studied. The power spectrum of this train for a wavenumber greater than the cutoff value has amplitudes and slopes similar to the observed in actual wind profiles. It is shown that the Fourier transform for large wavenumbers can be expressed as an inverse power series of the wavenumber, where the first two terms are dominant (k−1 and k−2) with a power spectrum slope from −2 to −4 in the tail. The main features that produce the tail are the discontinuities in the profile. An observed profile is analyzed showing that the power spectral amplitudes do not necessarily come from the waves contained in the profile; they can arise from irregularities, nonperiodic jumps that could be interpreted as discontinuities.

Comparison Between Two Attenuation Models and Precipitation Evaluation With Ground Validation

In this paper, we present an algorithm to correct the horizontal reflectivity factor <inline-formula> <tex-math notation=LaTeX>

AXISYMMETRIC NUMERICAL STUDY OF CONVECTIVE CLOUD ELECTRIFICATION

Z_h

The effect of the cloud‐droplet spectrum on electrical‐charge transfer during individual ice‐ice collisions

</tex-math></inline-formula> for attenuation by absorption plus scattering of a polarimetric radar located at the Experimental Station of INTA, in Oro Verde, Entre Rios, Argentina. Data correspond to two storms which occurred on November 18, 2009. The correction obtained was compared with models from the literature. Results show good agreement for regions with reflectivity factor lower than 55xa0dBZ while for regions that go over this limit, the corrections are different. Comparisons with rain gauges indicate the need of more statistics to improve the correlation.

Droplet charging by high voltage discharges and its influence on precipitation enhancement

Abstract An axisymmetric numerical model of convective electrification is presented. The model includes aerosols and small ions, and it takes into account the difference in mobilities between positive and negative small ions. The microphysics is represented by a bulk parameterization including cloud ice, cloud water, hail and rain. Although the results obtained with this model give the polarity structure most frequently observed in clouds, the characteristic times for charging appear to be longer than observed with electric fields too weak to produce lightning. The maximum electric field in a typical run reaches 8 kV/m in about 20 min. The charge conversion among particle categories arises as an important aspect previously overlooked in convective electrification. Based on these results it appears unlikely that the convective mechanism could be considered as the primary charging mechanism.