Pao K. Wang

Moisture plumes above thunderstorm anvils and their contributions to cross‐tropopause transport of water vapor in midlatitudes

Water vapor in the lower stratosphere may play significant roles in the atmosphericradiative budget and atmospheric chemistry; hence it is important to understand itstransport process. The possibility of water vapor transport from the troposphere to thestratosphere by deep convection is investigated using three-dimensional, nonhydrostatic,quasi-compressible simulations of a Midwest severe thunderstorm. The results show thatthe breaking of gravity waves at the cloud top can cause cloud water vapor to be injectedinto the stratosphere in the form of plumes above a thunderstorm anvil. Meteorologicalsatellites and aircrafts have observed such plumes previously, but the source of watervapor and the injection mechanism were not identified. The present results reveal thatthere are two types of plumes, anvil sheet plumes and overshooting plumes, in thisinjection process and that the process is diabatic. A first-order estimate of this plumetransport of water vapor per day from the upper troposphere to the lower stratosphere wasmade assuming that all thunderstorms behave the same as the one simulated. Other tracechemicals may also be similarly transported by the same mechanism.

An Experimental Determination of the Efficiency with Which Aerosol Particles are Collected by Water Drops in Subsaturated Air

Abstract Experiments have been carried out to determine the efficiency with which aerosol particles of 0.25 μm radius are collected due to Brownian diffusion, and due to hydrodynamic, phoretic and electrical effects by water drops of 150 to 2500.μm equivalent radius falling in subsaturated air. In the absence of electrical effects it was found that with increasing drop size the collection efficiency decreases to a minimum and then rises again as the collection due to phoretic forces is overcompensated by the collection due to hydrodynamic forces. With further increase in drop size the collection efficiency was found to rise to a maximum, This rise was attributed to hydrodynamic effects in the rear of the drop which increase as the stagnant eddy at the downstream end of the falling drop increases in size, but progressively decrease as the drop assumes a size, and thus a Reynolds number, large enough for turbulent eddies to be shed from the rear of the drop. The present results are qualitatively consistent ...

Collision Efficiencies of Ice Crystals at Low–Intermediate Reynolds Numbers Colliding with Supercooled Cloud Droplets: A Numerical Study

Abstract The efficiencies with which ice crystals at low–intermediate Reynolds numbers collide with supercooled cloud droplets are determined numerically. Three ice crystal habits are considered here: hexagonal ice plates, broad-branch crystals, and columnar ice crystals. Their Reynolds numbers range from 0.1 to slightly beyond 100. The size of cloud droplets range from a few to about 100 μm in radius. The collision efficiencies are determined by solving the equation of motion for a cloud droplet under the influence of the flow field of the falling ice crystal. The flow fields of the falling ice crystals were determined previously by numerically solving the unsteady Navier–Stokes equations. Features of these efficiencies are discussed. The computed efficiencies are compared with those obtained by previous investigators and improvements are indicated. New results fit better with the observed riming droplet sizes and cutoff riming ice crystal sizes.

Numerical Simulations of the 2 August 1981 CCOPE Supercell Storm with and without Ice Microphysics

Abstract The Wisconsin Dynamical-Microphysical Model is used in two simulations of the 2 August 1981 supercell that passed through the Cooperative Convective Precipitation Experiment in southeastern Montana. The first simulation uses liquid water-only microphysics and is denoted as the liquid water model (LWM). The second includes both liquid water and ice microphysics and is designated as the hail category model (HCM). Results from the two simulations show that the inclusion of ice significantly alters the dynamics, kinematics, thermodynamics, and distributions of water in the storm, especially at the lower levels. Supercell features such as a rotating intense updraft, bounded weak-echo region, large forward overhanging anvil, and hooklike structure in the low-level rainwater field are present in both simulations. These features are generally more pronounced, however, and have a longer lifetime in the HCM. Hail embryo and graupel particles make up more than 85% of the total hail mass during the steady-st...

Pressure Drop and Interception Efficiency of Multifiber Filters

ABSTRACT The viscous flow fields around multifiber filters have been investigated in a previous paper. The results of the previous work show that the flow becomes periodic immediately after the first fiber array downstream from the entrance if the fibers are arranged uniformly along the flow direction. The characteristics of such flow fields enable the pressure drop and the particle interception efficiency of a multifiber filter to be represented by single-fiber models. The total filtration efficiency, however, cannot be so represented since fibers interact during filtration processes. In this study, the pressure drop and the interception efficiency were investigated by making use of the viscous flow fields modeled in the previous research. The fiber separation ratio was found to have significant effects on pressure drop and efficiency. At a given volume fraction, changes in the fiber separation ratio will result in changes to the patterns of fluid flow and aerosol particle motion. Therefore, the fiber se...

An Experimental Study of the Scavenging of Aerosol Particles by Natural Snow Crystals

Abstract An experimental study of the scavenging of aerosol particles of mean radius 0.75 μm by natural snow crystals of a few milimeters is carried out. Aerosol particles are spherical indium acetylacetonate particles generated by a modified La Mer generator. Snow crystals are obtained during natural snowfalls. Shapes of snow crystsals include needless column broad-branched crystals stellar crystals, and hexagonal plates. Aerosol particles are dispersed into an aerosol chamber and snow crystals fall through the chamber to scavenge aerosol particles. The collection efficiency of aerosol particles by snow crystals is found to decrease with increasing crystal size for all shapes. This can be explained by the relative strength of the inertial force of particles and the hydrodynamic drag force created by the fall of the snow crystal. Large crystals would create greater drags during the Call and force the aerosol particle to follow more closely to streamlines and hence reduce the collection efficiency.

A study of microphysical processes in the 2 August 1981 CCOPE supercell storm

Abstract A three-dimensional, time-dependent, non-hydrostatic model is used to simulate the microphysical processes in an intense supercell storm that passed through the Cooperative Convective Precipitation Experiment (CCOPE) network on 2 August 1981. The simulation utilizes the hail parameterization model (HPM) version of the Wisconsin Dynamical/Microphysical Model (WISCDYMM), in which the precipitation fields are all assumed to follow exponential size distributions, while cloud water and ice are assumed to be monodispersed. The simulation is carried out for 7200 s and is found to exhibit dynamic and thermodynamic features characteristic of the observed storm, including an intense and persistent updraft with strong rotation in the lower and mid levels, an undiluted updraft core, and a cool pool and gust front which propagates along with the storm beneath the subcloud region. A supercellular structure is also depicted in the microphysical fields by a large overshooting top, large forward overhanging anvil, low-level hook-like feature and distinct bounded weak-echo region. Calculations of the total integrated hydrometeor mass for the entire domain show that more than 80% of the storm hydrometeor mass is ice, with graupel and hail being the most predominant type. The total mass of ice, snow, graupel and hail increase rapidly with storm intensity, while the total liquid water mass consisting of cloud water and rain increase much more slowly as shown by time-dependent distributions of the hydrometeor types. Production and depletion curves depicting microphysical processes for each precipitating hydrometeor indicate that rain is produced primarily from the melting and shedding of graupel and hail, snow is primarily initiated by the Bergeron-Findeisen process and grows predominately from the accretion of cloud ice and cloud water, and graupel and hail is primarily initiated through rain-snow collisions and has largest mass gains through accretions of cloud water, rain and snow. The largest sinks are evaporation and accretion by graupel and hail for rain water, sublimation and accretion by graupel and hail for snow, and melting and shedding to form raindrops for graupel and hail.

An Experimental Test of a Theoretical Model to Determine the Rate at which Freely Falling Water Drops Scavenge SO2 in Air

Abstract An experimental method involving the UCLA Rain Shaft is described. This method allows determining the rate at which SO2 is scavenged from air by freely falling water drops. In the present experiment water drops of radii near 300 μm were allowed to pass through a chamber filled with SO2 whose partial pressure was determined by an infrared spectrometer. By varying the length of the gas compartment, the drops could be exposed to SO2 for different intervals of time. An electrochemical method verified by three quantitative chemical methods was used to determine the total amount of sulfur taken up by the drops falling through the gas compartment. The present experimental results were compared with the results from our theoretical model (Baboolal et at., 1981), which was evaluated for the present experimental conditions. Satisfactory agreement between experiment and theory was found.

A Wind Tunnel Investigation of the Effect of an External, Vertical Electric Field on the Shape of Electrically Uncharged Rain Drops

Abstract Results are presented of a recent wind tunnel experiment in which electrically unchanged water drops of 1000–3000 μm equivalent radius were freely suspended in the vertical air stream of the UCLA Cloud Tunnel. During their suspension, the drops were exposed to external, vertical electric fields of 0–90 volts cm-1. The change in the drop shape with drop size and with electric field strength was noted and is discussed in the light of theoretical work cited in literature which does not take into account the feedback effects between the electric forces of an external electric field and the hydrodynamic forms due to the flow past the drop. In contrast, the present wind tunnel study, documented by photographs from a 16 mm motion picture film, recorded the shape of the water drop in response to both hydrodynamic as well as electric forces.

A Numerical Study of Cirrus Clouds. Part I: Model Description

Abstract This article, the first of a two-part series, presents a detailed description of a two-dimensional numerical cloud model directed toward elucidating the physical processes governing the evolution of cirrus clouds. The two primary scientific purposes of this work are (a) to determine the evolution and maintenance mechanisms of cirrus clouds and try to explain why some cirrus can persist for a long time; and (b) to investigate the influence of certain physical factors such as radiation, ice crystal habit, latent heat, ventilation effects, and aggregation mechanisms on the evolution of cirrus. The second part will discuss sets of model experiments that were run to address objectives (a) and (b), respectively. As set forth in this paper, the aforementioned two-dimensional numerical model, which comprises the research tool for this study, is organized into three modules that embody dynamics, microphysics, and radiation. The dynamic module develops a set of equations to describe shallow moist convectio...