Drop Spectra Research Articles

Nucleation Scavenging of Smoke Particles and Simulated Drop Size Distributions over Large Biomass Fires

A prescribed burn experiment was conducted in Hardiman Township, Ontario, Canada in August 1987. The fire was of adequate intensity to force the formation of a cumulus cloud, and much of the smoke passed through this cloud. The smoke plume and the fire-driven cloud were observed using ground- and aircraft-based instrumentation. In this paper, the authors present calculations of the evolution of the smoke and drop size distributions within this cloud due to nucleation and condensation from a detailed microphysical entraining model. Initial conditions for the model simulations were specified using a three-dimensional warm-cloud dynamical model. The authors investigate the changes of parcel characteristics caused by the effect of entrainment, as well as the sensitivity of nucleation scavenging and drop number concentration to the different background environmental aerosol distributions. The fraction of smoke particles nucleated depends on the chemical nature of the particles, their size distribution, the quantity of smoke present at cloud base, the updraft velocity, and the environmental conditions. The predicted drop size distribution and the measured data above the fire are in good agreement for drop sizes that correspond to condensation on the measured smoke spectra. The results from this study can be used to develop amore » parameterization that relates the mass of smoke scavenged by nucleation to the updraft velocity and the initial smoke mass loading. The authors plan to extend this study to predict the effects of aerosols on drop spectra for use in climate models. 36 refs., 13 figs.« less

The Simulation of a Convective Cloud in a 3-D Model With Explicit Microphysics. Part I: Model Description and Sensitivity Experiments

Abstract A three-dimensional nonhydrostatic anelastic numerical model of a convective cloud with an explicit description of microphysical processes has been developed. Two distribution functions are considered in the model—one for cloud condensation nuclei (19 categories from 0.0076 to 7.6 microns) and another for cloud droplets and raindrops (30 categories on a logarithmic scale from 4 to 3250 microns). The prognostic kinetic equations for these distribution functions enable the calculation of the aerosol and drop spectra starting from activation and culminating in rain formation. The warm rain microphysical processes studied include nucleation, condensation/evaporation, coalescence, breakup and sedimentation. Analysis of supersaturation evolution with time shows that it does not experience growth with the onset of coalescence and correlates well with values derived from quasi-steady assumptions. Sensitivity tests of the accuracy of the supersaturation calculations and the effect of the salt factor in th...

Measurement of drop spectra from rotary cage aerial atomizers

The drop spectra produced by various examples of Micronair AU3000 rotary cage atomizer were measured using a laser-based Particle Measuring Systems optical array probe. To simulate aerial application by fixed-wing aircraft, the atomizers were mounted in an open jet wind tunnel operating in the range 130–225 km h −1. An empirical model which predicted the volume median diameters (VMDs) and volume quartiles of the spectra was developed. The model showed a high degree of correlation with the experimental results. The model took into account not only the speed of rotation of the cage, but its diameter, the liquid flow rate, and the air speed of the aircraft. The model showed general agreement with research into rotary disc atomization. Although a water-based liquid was used in the tests, it was concluded that the model would be suitable for use as a performance guide for most pesticide formulations.

Coalescence and Breakup-Induced Oscillations in the Evolution of the Raindrop Size Distribution

Models or the coalescence/breakup process yield drop number distributions that approach equilibrium but the number density often is not a monotonic function of time. In some cases, the small-raindrop portion of the distribution rapidly attains high concentration levels before settling back toward an equilibrium position. An eigenanalysis of the coalescence/breakup equation is performed to gain an understanding of the solution behavior near equilibrium. The analysis reveals that the departure of the solution from equilibrium can be expressed as a linear combination of basis functions of the form eλj where Re(λj) < 0 so that the equilibrium drop distribution is asymptotically stable. The exponential basis functions feature a wide range of decay rates, and since Im (λj) ≠ 0 in some cases, the functions provide evidence of oscillations in the drop spectrum. It is shown that one particular damped oscillation can combine with a rapidly decaying transient to describe very well the nonmonotonic behavior characteristic of model-generated drop spectra. While the physical mechanism behind the oscillation is not yet understood, the initial reversal in the small-drop peak may be explained as rapid response due to filament breakup followed by a slower response due to coalescence. A particular sequence of observed raindrop distributions is found to exhibit a reversal in the spectral peak similar to that produced by the model.

A Linear Radar Reflectivity–Rainrate Relationship for Steady Tropical Rain

Modeling by Gillesple and List established the evolution to equilibrium of raindrop number distributions in one-dimensional shaft models. Donaldson and List et al. (LDS) then demonstrated mathematically that equilibrium distributionsƒ for raindrop number concentration N consist of a product of the rainfall rate R and a shape function Δ (which is independent of Donaldson et al. and LDS) showed that such spectra contain three at diameters of ∼0.3, 0.8 and 1.8 mm, whereas Brown obtained only the two smaller ones. For equilibrium distributions, the present shows that the mass concentration M and the radar reflectivity Z are also proportional to the rainfall rate R, with Z=CzR and that N,M, R and Z are mutually proportional. Calculation provided Cz=742μm3 (mm h−1)−1, which is suggested to be a universal constant for steady tropical rain. Data from a tropical rain experiment, carried out jointly by the Malaysian Meteorological Service and the University of Toronto, confirmed the equilibrium peaks at diameters of ∼0.3 (estimate), 0.9 and 1.9 mm. Due to basic deficiencies of the disdrometer only study state events could be used to assess radar reflectivities. The four available cases of steady rain consist of 112 one-minute spectra with different types and origins of MID (warm clouds, clouds containing ice particles, convective or stratiform clouds). As in the model, the radar reflectivity Z as calculated from measured drop spectra, was found to be proportional to rainfall rate.

Landes-Fronts 84: Analyse granulométrique au sol d'une averse de pluie convective

Ground measurements of raindrop spectra have been made during a convective shower. It appears that this shower is likely to be divided in two phases: the first one (growth stage), characterized by large drop spectra with an exponential form is followed by a second one (dissipation stage) with spectra of rather small drops. During the transitory stage, a mean spectrum showing a slope break is observed (spectrum with double slope — Donnadieu, 1985). During the growth stage of the shower, the classic relationship Z = AR b presents a parameter A which is about twice as large as that obtained during the dissipation stage. For the transitory stage, the two parameters A and b show very uncommon values. At last it is observed that the Z − R points present a time evolution like a loop. These results obtained at ground level are quite comparable to those found by Carbone and Nelson (1978) in raindrop measurements below cloud base during precipitation stages of cloud development and are in agreement with their numerical model.

Aerosols in Polluted versus Nonpolluted Air Masses: Long-Range Transport and Effects on Clouds

To assess the influence of anthropogenic aerosols on the physics and chemistry of clouds in the northeastern United States, aerosol and cloud-drop size distributions, elemental composition of aerosols as a function of size, and ionic content of cloud water were measured on Whiteface Mountain, NY, during the summers of 1981 and 1982. In several case studies, the data were cross-correlated with different air mass types - background continental, polluted continental, and maritime - that were advected to the sampling site. The results are the following: (1) Anthropogenic sources hundreds of kilometers upwind cause the small-particle (accumulation) mode number to increase from hundreds of thousands per cubic centimeter and the mass loading to increase from a few to several tens of micrograms per cubic meter, mostly in the form of sulfur aerosols. (2) A significant fraction of anthropogenic sulfur appears to act as cloud condensation nuclei (CCN) to affect the cloud drop concentration. (3) Clouds in Atlantic maritime air masses have cloud drop spectra that are markedly different from those measured in continental clouds. The drop concentration is significantly lower, and the drop size spectra are heavily skewed toward large drops. (4) Effects of anthropogenic pollutants on cloud water ionic composition are an increase of nitrate by a factor of 50, an increase of sulfate by more than one order of magnitude, and an increase of ammonium ion by a factor of 7. The net effect of the changes in ionic concentrations is an increase in cloud water acidity. An anion deficit even in maritime clouds suggests an unknown, possibly biogenic, source that could be responsible for a pH below neutral, which is frequently observed in nonpolluted clouds.

Analysis of the Low and List Drop-Breakup Formulation

An analysis of the Low and List drop-breakup formulation has uncovered several computational problems that arise in calculating both the fragment distribution function and the Bleck expansion coefficients that appear in the discrete coalcacence/breakup equation. Special procedures have been developed to deal effectively with these problems. The discrete coalescence/breakup equation has been solved using the Low and List breakup formulation and the earlier List and Gillespie formulation. Comparison shows that the Low and List model solutions approach equilibrium more slowly than do the earlier model solutions; moreover, the Low and List equilibrium drop spectra exhibit a bimodality in the small-drop end of the spectrum. The longer time constants associated with the Low and List equations ease somewhat the severe computational stability problem associated with List and Gillespie equations.

An implicit scheme for efficient solution of the coalescence/collision-breakup equation

Detailed moist-convection models of the atmosphere require solution of the coalescence/collision-breakup equation to predict change in the water-drop spectrum. Under certain meteorological conditions, computational stability problems are encountered in attempting to solve this equation by explicit techniques. A stability analysis is performed making use of empirical results that depend on the form of observed drop spectra and the nature of the equation. Information derived from the analysis is used to design a two-step implicit scheme that provides both stability and efficiency.

Drop Size Spectra from Nozzles in High-Speed Airstreams

ABSTRACT Alaser system was used to measure drop size spectra of fan and cone nozzles in a wind tunnel. Tests were conducted to evaluate the effects of flow rate, nozzle orientation angle, and airspeed on the drop size spectrum. The volume median diameter ranges from 142 to 927 jum. The range and relative span of the drop size spectra are presented. Also the potential drift hazard of the drop spectra are discussed.

Characteristics of cloud drop spectra in tropical warm clouds

Characteristics of cloud drop spectra were studied using 400 samples obtained from 120 warm cumulus clouds formed during the summer monsoon season.

A field study of nocturnal stratocumulus; III. High resolution radiative and microphysical observations

High resolution observations from a tethered balloon of nocturnal stratocumulus on three occasions are presented. Measurements of the microphysical properties of the clouds (drop-size distributions, concentrations and liquid water contents) were obtained with an Axially Scattering Spectrometer Probe (ASSP). A ground based 95 GHz radiometer was used to infer the integrated liquid water path through the cloud. In the two thick clouds studied the drop-size data show an almost monotonic increase of mean radius from cloud base to top. The liquid water content increases with height above cloud base at slightly less than the adiabatic rate and shows considerable variability towards cloud top. The observations support the concept of the inhomogeneous mixing of the cloudy air with the dry air entrained at the cloud top. Measurements were also made of infrared net radiative fluxes, and very good agreement is found with the theoretical fluxes from a high resolution radiative transfer scheme, using the ASSP drop spectra and liquid water contents, scaled by the 95 GHz radiometer data.

A field study of nocturnal stratocumulus; III. High resolution radiative and microphysical observations

AbstractHigh resolution observations from a tethered balloon of nocturnal stratocumulus on three occasions are presented. Measurements of the microphysical properties of the clouds (drop‐size distributions, concentrations and liquid water contents) were obtained with an Axially Scattering Spectrometer Probe (ASSP). A ground based 95 GHz radiometer was used to infer the integrated liquid water path through the cloud. In the two thick clouds studied the drop‐size data show an almost monotonic increase of mean radius from cloud base to top. The liquid water content increases with height above cloud base at slightly less than the adiabatic rate and shows considerable variability towards cloud top. The observations support the concept of the inhomogeneous mixing of the cloudy air with the dry air entrained at the cloud top. Measurements were also made of infrared net radiative fluxes, and very good agreement is found with the theoretical fluxes from a high resolution radiative transfer scheme, using the ASSP drop spectra and liquid water contents, scaled by the 95 GHz radiometer data.

Cloud modification by man‐made pollutants: Effects of a coal‐fired power plant on cloud drop spectra

Pollutants from coal‐fired power plants increase the drop modal radii and broaden the drop spectra of stratocumulus clouds. Additional water from the coal combustion leads to a measurable increase of the liquid water content of polluted clouds. Implied consequences are an increase of the coalescence efficiency, and an increase in acidity, of affected clouds.

A Numerical Determination of the Evolution of Cloud Drop Spectra due to Condensation on Natural Aerosol Particles

The time variation in size of aerosol particles growing by condensation is studied numerically by means of an air parcel model which allows entrainment of air and aerosol particles. Particles of four types of aerosols typically occurring in atmospheric air masses were considered. The present model circumvents any assumption about the size distribution and chemical composition of the aerosol particles by basing the aerosol particle growth on actually observed size distributions and on observed amounts of water taken up under equilibrium by a deposit of the aerosol particles. Characteristic differences in the drop size distribution, liquid water content and supersaturation were found for the clouds which evolved from the four aerosol types considered.

The Evolution of Raindrop Spectra in Warm-Based Convective Storms as Observed and Numerically Modeled

Abstract Airborne optical spectrometer measurements of raindrop spectra were performed in Texas together with 10 and 3 cm radar observations. The observations reveal large drop spectra emanating from cloud base with low concentrations of small drops compared to the Marshall and Palmer (1948) distributions. Systematic patterns of exponential spectrum parameters (N0 and λ) are revealed to constitute a “parametric cycle” in time and space. Drop spectrum form is attributed principally to updraft sorting of smaller drops. Subsequent to updraft sorting, the importance of the collision breakup process with respect to spontaneous breakup appears to be greatly diminished. Recent theoretical studies by Srivastava (1971, 1978) partially support conclusions drawn with respect to the relative importance of spontaneous versus collisional breakup. A one-dimensional, time-dependent numerical model was employed for initial conditions simulating the observations. The resultant spectra emanating from the model were quantita...

Rainfall drop spectra intensities and fine structures on different time bases

Five years of comprehensive research on precipitation structures showed that the time bases and the design of the measuring equipment are most important for statistical measure of precipitation parameters. The influence of different methods, and different time bases as determined by calculations based on measurements are presented. Small differences on the time scale produce large errors (e.g. more than 100 %in rainfall rate).

AZ-RRelationship for the GATE B-Scale Array

Abstract A better understanding of how the precipitation budget operates in tropical convective systems is a prime objective of the GATE research effort. Measurement of rainfall rate with shipboard radar is the principal method by which precipitation from tropical clouds that develop within the GATE B-scale array will be determined. Knowledge of the relationship between radar reflectivity (Z) and rainfall rate (R) is essential for an accurate interpretation of precipitation data derived through the use of radar technology. The Z-R relationship is determined through application of a least-squares linear regression to data points derived by appropriate integration of the third and sixth moments of a series of raindrop size spectra. Drop spectra measurements were obtained during GATE by means of a foil impactor operated at cloud-base level on board the NOAA DC-6 aircraft. A total of 107 Z-R data points are available, representing showers occurring on 12 days. The best-fit Z-R relationship for the cloud-base ...

A Field Experiment on the Calibration of Radars with Raindrop Disdrometers

Radar reflectivity factors determined from disdrometer measurements of drop spectra are compared with simultaneous WSR-57 radar measurements in two Oklahoma thunderstorms. The possibility of using a disdrometer for an in-field calibration check of a radar is examined and found to have limited usefulness for convective precipitation sampled at long ranges.

The Evolution of Drop Spectra Due to Condensation, Coalescence and Breakup

A numerical model of warm rain processes incorporating activation of cloud condensation nuclei, drop growth by condensation and stochastic coalescence, and drop breakup is described. A collisional breakup model is compared to a spontaneous disintegration model and found not only to dominate over the spontaneous disintegration model but to produce an exponential spectrum in fair agreement with average observed drop spectra. The steady-state spectrum was found to be quite insensitive to the number of satellite drops farmed by collision or their size distribution. The effect of the finite-difference solution to the collection and breakup equations is analysed. A properly stochastic formulation for finite time steps is presented but found to differ only slightly from the simpler, “discrete” formulation. Time stops of 5 s and 45 size categories are found adequate to describe the essential quantitative features of more intensive treatments. The model results were found to be somewhat sensitive to values used for the collection efficiency.