During the oxidative degradation of 2,4-dimethylaniline (2,4-xylidine) by means of the H2O2/UV method, a series of hydroxylated aromatic amines are formed, this result confirming the role of the hydroxyl radical as an initiator of the oxidative chain reaction. Thermal or photochemically enhanced Fenton reactions in the presence of 2,4-dimethylaniline (2,4-xylidine) yield primarily 2,4-dimethylphenol as an intermediate product, the genesis of which may only be explained by an electron transfer mechanism. Experimental evidence for such a mechanism is presented, and values for the quantum yields of the photochemically enhanced reduction of iron(III) to iron(II) in aqueous solutions of 2,4-xylidine are given.

New Evidence against Hydroxyl Radicals as Reactive Intermediates in the Thermal and Photochemically Enhanced Fenton Reactions

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

The aquatic photochemical behavior of a class of sulfa drugs containing six-membered heterocyclic substituents (sulfamethazine, sulfamerazine, sulfadiazine, sulfachloropyridazine, and sulfadimethoxine) was investigated. Photodegradation of the sulfa drugs in a natural water sample was significantly enhanced relative to the degradation in deionized water, with the exception of sulfadimethoxine. This indicated an indirect photochemical process that was identified through the use of quenchers to be attributable to interaction with triplet excited-state dissolved organic matter (3DOM). The direct photolysis rate constant and quantum yield for both the neutral and anionic species of each sulfa drug were calculated using matrix deconvolution methods. The quantum yield values range from 0.01 x 10(-3) for the neutral form of sulfadimethoxine to 5 x 10(-3) for the anionic form of sulfamethazine and are significantly lower than those observed in a previous study for sulfa drugs containing five-membered heterocyclic substituents, although the rate constants are of similar magnitude. The primary product formed in both direct and indirect photodegradation for all five compounds was identified as a sulfur dioxide extrusion product. The predicted environmental half-lives solely attributable to direct photolysis range from 8.6 h in midsummer at 30 degrees latitude in pH 7 surface water for sulfachloropyridazine to 420 h in midwinter at 45 degrees in pH 7 surface water for sulfadimethoxine. These half-lives, except for sulfadimethoxine, will be decreased by interaction with 3DOM.

Triplet-sensitized photodegradation of sulfa drugs containing six-membered heterocyclic groups: identification of an SO2 extrusion photoproduct.

Most atmospheric motions of different spatial scales and precipitation are closely related to phase transitions in clouds. The continuously increasing resolution of large-scale and mesoscale atmospheric models makes it feasible to treat the evolution of individual clouds. The explicit treatment of clouds requires the simulation of cloud microphysics. Two main approaches describing cloud microphysical properties and processes have been developed in the past four and a half decades: bulk microphysics parameterization and spectral (bin) microphysics (SBM). The development and utilization of both represent an important step forward in cloud modeling. This study presents a detailed survey of the physical basis and the applications of both bulk microphysics parameterization and SBM. The results obtained from simulations of a wide range of atmospheric phenomena, from tropical cyclones through Arctic clouds using these two approaches are compared. Advantages and disadvantages, as well as lines of future development for these methods are discussed. (Less)

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014RG000468

Representation of microphysical processes in cloud-resolving models: Spectral (bin) microphysics versus bulk parameterization

An enhanced hail detection algorithm (HDA) has been developed for the WSR-88D to replace the original hail algorithm. While the original hail algorithm simply indicated whether or not a detected storm cell was producing hail, the new HDA estimates the probability of hail (any size), probability of severe-size hail (diameter ≥19 mm), and maximum expected hail size for each detected storm cell. A new parameter, called the severe hail index (SHI), was developed as the primary predictor variable for severe-size hail. The SHI is a thermally weighted vertical integration of a storm cell’s reflectivity profile. Initial testing on 10 storm days showed that the new HDA performed considerably better at predicting severe hail than the original hail algorithm. Additional testing of the new HDA on 31 storm days showed substantial regional variations in performance, with best results across the southern plains and weaker performance for regions farther east.

/pdf/an-enhanced-hail-detection-algorithm-for-the-wsr-88d-284acq7ecd.pdf

An Enhanced Hail Detection Algorithm for the WSR-88D

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

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%.

The Kinetic Energy of Hailfalls. Part I: Hailstone Spectra

Physical factors influencing winter precipitation chemistry

A classification of the degree of riming of snow crystals is established, based on a visual rating and expressed on a scale ranging from 0 (unrimed) to 5 (graupel). For 84 individual snow crystals collected from six cases over three winters, the size distribution of the accreted cloud droplets is studied. Relationships between the degree of riming and quantitative parameters, namely, the total number and mass of accreted cloud droplets and especially the rimed mass fraction of a snow crystal, are presented. These new relationships may be a powerful tool to estimate the aforementioned parameters by rating only the degree of riming of a snow crystal.

An Analysis of Accreted Drop Sizes and Mass on Rimed Snow Crystals

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 (ΔZe) and the intercept and slope parameters of the Marshall-Palmer drop size distribution (N0, ∇) 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 ΔZe, whereas flat spectra with relatively few small drops and with large drops exhibit a well-defined bright band with a large ΔZe. Considering ΔZe 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%.

A. Waldvogel

Papers

Comparison of Traditional and Newly Developed Thunderstorm Indices for Switzerland

The Kinetic Energy of Hailfalls. Part I: Hailstone Spectra

Physical factors influencing winter precipitation chemistry

An Analysis of Accreted Drop Sizes and Mass on Rimed Snow Crystals

Raindrop Size Distributions and the Radar Bright Band.