The solution of electromagnetic scattering by a homogeneous prolate (or oblate) spheroidal particle with an arbitrary size and refractive index is obtained for any angle of incidence by solving Maxwell's equations under given boundary conditions. The method used is that of separating the vector wave equations in the spheroidal coordinates and expanding them in terms of the spheroidal wavefunctions. The unknown coefficients for the expansion are determined by a system of equations derived from the boundary conditions regarding the continuity of tangential components of the electric and magnetic vectors across the surface of the spheroid. The solutions both in the prolate and oblate spheroidal coordinate systems result in a same form, and the equations for the oblate spheroidal system can be obtained from those for the prolate one by replacing the prolate spheroidal wavefunctions with the oblate ones and vice versa. For an oblique incidence, the polarized incident wave is resolved into two components, the TM mode for which the magnetic vector vibrates perpendicularly to the incident plane and the TE mode for which the electric vector vibrates perpendicularly to this plane. For the incidence along the rotation axis the resultant equations are given in the form similar to the one for a sphere given by the Mie theory. The physical parameters involved are the following five quantities: the size parameter defined by the product of the semifocal distance of the spheroid and the propagation constant of the incident wave, the eccentricity, the refractive index of the spheroid relative to the surrounding medium, the incident angle between the direction of the incident wave and the rotation axis, and the angles that specify the direction of the scattered wave.

Light Scattering by a Spheroidal Particle

The characteristics of a prototype optical disdrometer are presented. Particles are detectable in the diameter range from 0.3 to 30 mm having velocities of up to 20 m s−1. Advantages of the new system are (i) it is easy to handle, robust, and low cost, allowing a cluster of instruments to investigate the spatial and temporal fine-scale structure of precipitation; (ii) it provides reliable detection of the range of small drops; and (iii) it allows the possibility of snow measurements. Results of rain measurements are compared with data from a Joss–Waldvogel disdrometer and a Hellmann rain gauge. Furthermore, some snow measurements are presented and compared with results of a research spectrometer. The overall agreement is good. The repeatability of particle size estimation was checked in the diameter range between 1.4 and 8.0 mm and yielded a standard deviation of less than 5%. For drop velocities the standard deviation varies between 25% (0.3-mm drops) and 10% (5-mm drops). The optical disdromete...

An Optical Disdrometer for Measuring Size and Velocity of Hydrometeors

Operational early warning systems for water-related hazards in Europe

On the Caltech Active Strand Cloudwater Collectors

. The phase of precipitation when it reaches the ground is a first-order driver of hydrologic processes in a watershed. The presence of snow, rain, or mixed-phase precipitation affects the initial and boundary conditions that drive hydrological models. Despite their foundational importance to terrestrial hydrology, typical phase partitioning methods (PPMs) specify the phase based on near-surface air temperature only. Our review conveys the diversity of tools available for PPMs in hydrological modeling and the advancements needed to improve predictions in complex terrain with large spatiotemporal variations in precipitation phase. Initially, we review the processes and physics that control precipitation phase as relevant to hydrologists, focusing on the importance of processes occurring aloft. There is a wide range of options for field observations of precipitation phase, but there is a lack of a robust observation networks in complex terrain. New remote sensing observations have the potential to increase PPM fidelity, but generally require assumptions typical of other PPMs and field validation before they are operational. We review common PPMs and find that accuracy is generally increased at finer measurement intervals and by including humidity information. One important tool for PPM development is atmospheric modeling, which includes microphysical schemes that have not been effectively linked to hydrological models or validated against near-surface precipitation-phase observations. The review concludes by describing key research gaps and recommendations to improve PPMs, including better incorporation of atmospheric information, improved validation datasets, and regional-scale gridded data products. Two key points emerge from this synthesis for the hydrologic community: (1) current PPMs are too simple to capture important processes and are not well validated for most locations, (2) lack of sophisticated PPMs increases the uncertainty in estimation of hydrological sensitivity to changes in precipitation phase at local to regional scales. The advancement of PPMs is a critical research frontier in hydrology that requires scientific cooperation between hydrological and atmospheric modelers and field scientists.

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Rain or Snow: Hydrologic Processes, Observations, Prediction, and Research Needs

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 de...

Comparison of Traditional and Newly Developed Thunderstorm Indices for Switzerland

This paper deals with the ability of a vertically pointing, FM–CW Doppler radar to measure quantitatively raindrop size distributions and rainfall intensity. The wavelength of the radar is 12.5 mm (K band). To improve estimates of the rainfall intensity, radar-received noise, Mie scatter, and radar calibration corrections are applied to the radar data. The electronic noise correction rendered the radar particle size retrievals below 0.7-mm drop diameters invalid; exponential extrapolation of the spectrum below drop diameters of 0.7 mm decreased the rain intensity up to 20%. Proper Mie corrections to the particle backscatter cross sections decreased the rain intensity between 5% and 30%. Calibration of the radar against a Joss–Waldvogel disdrometer led to a decrease in rain intensity of 49%. The electronic noise, Mie scatter, and calibration corrections yielded a correlation coefficient of 0.94 in a comparison of the radar and disdrometer data for a case study. The daily rain sums over half a year...

On the Performance of a Low-Cost K-Band Doppler Radar for Quantitative Rain Measurements

Further evidences of deep convective vertical transport of water vapor through the tropopause

Wet deposition plays an important role as a mechanism for cleansing of the polluted troposphere Fluxes of pollutants from the atmosphere to earth's surface can affect soil and aquatic ecosystems The investigation of the pollutant incorporation into wet deposition is therefore of considerable interest for environmental research

Scientific goals and experiments of the project “winter precipitation at Mount Rigi”: An overview

RainCast, a new method nowcasting with radar is presented. The method has been developed at ETH for extrapolating radar images. Forecasted images and forecasts of probability of precipitation up to 1-2 hours in advance are available every 5 min. The procedure can be applied to summer and winter precipitation. The use in winter precipitation is discussed here. The properties of the radar estimates in rain, melting snow and snowfall, and their impacts on the short-term forecasts are summarized. In the second part we analyze a sudden snowstorm that was responsible for a series of accidents and chaotic situations in the Swiss morning traffic. The propagating frontal line, associated with the snowstorm, is reliably predicted with RainCast. For effective warning, “snow signatures”, retrieved from ground data of temperature and precipitation must be recognized. Incorporated in the RainCast system, this leads to a significant improvement of the effectiveness of storm warnings based on extrapolated radar data.

Willi Schmid

Papers

Comparison of Traditional and Newly Developed Thunderstorm Indices for Switzerland

On the Performance of a Low-Cost K-Band Doppler Radar for Quantitative Rain Measurements

Further evidences of deep convective vertical transport of water vapor through the tropopause

Scientific goals and experiments of the project “winter precipitation at Mount Rigi”: An overview

Nowcasting Winter Precipitation with Radar