Electromagnetic fields and electrical currents in deep turbulent convective clouds
ABSTRACT Charge separation and lightning formation in a thunderstorm is explicitly simulated usin... more ABSTRACT Charge separation and lightning formation in a thunderstorm is explicitly simulated using spectral bin microphysics the Hebrew University Cloud Model (HUCM) with resolution of 50 m. The model microphysics is based on solving equations for eight size distribution functions for aerosols, drops, three types of ice crystals, aggregates, graupel and hail. Each size distribution is defined on a mass grid containing 43 bins. The model describes the processes of nucleation of cloud particles, diffusion growth, collisions between all types of hydrometeors, differential sedimentation, freezing, melting, breakup of droplets and aggregates, etc' using the equations basing on the first principles, without any parameterization assumptions. Turbulence effects on droplet collisions are taken into account. Charge separation is calculated by collisions between graupel, hail and ice crystals in the presence of liquid water. The charge obtained by particles as a result of collisions depends on the particle size, the temperature, the presence of liquid water, following laboratory results by Takahashi. These charges are transported by convective motions and differential sedimentation depending on mass and type of particles air density. The charges are redistributed between different hydrometeors in course of particle collisions, as well as during freezing, melting and breakup. These charge transformations create time dependent electricity field. The field of electrical potential is determined by solving the Poisson equation. The recursive procedure similar to that developed by Mansell (2002) is used to calculate the lightning path with connects zones where the potential gradients exceeded the breakdown threshold. The electric currents in the clouds are being calculated. The magnetic field near and inside the clouds are shown. The relationship between lightning intensity and cloud microstructure is investigated. It is shown, for instance, that increase in aerosol concentration leads to increase of mass of super cooled water aloft, as well as to increase in the ice crystal concentration at the upper levels. In continental clouds aerosols foster formation of hail.
A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebre... more A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebrew University Cloud Model (HUCM) with a detailed description of time-dependent melting and freezing. In addition to size distributions of drops, plate-, columnar-, and branch-type ice crystals, snow, graupel, and hail, new distributions for freezing drops as well as for liquid water mass within precipitating ice particles were implemented to describe time-dependent freezing and wet growth of hail, graupel, and freezing drops. Simulations carried out using different aerosol loadings show that an increase in aerosol loading leads to a decrease in the total mass of hail but also to a substantial increase in the maximum size of hailstones. Cumulative rain strongly increases with an increase in aerosol concentration from 100 to about 1000 cm−3. At higher cloud condensation nuclei (CCN) concentrations, the sensitivity of hailstones’ size and surface precipitation to aerosols decreases. The phys...
Effects of aerosols on drop freezing, size and structure of hail: application of the theory of time -dependent freezing and hail growth in a spectral bin microphysics cloud model
The theory of wet growth of hail is extended to the case of the inhomogeneities of surface temper... more The theory of wet growth of hail is extended to the case of the inhomogeneities of surface temperature and of liquid coverage over the surface of the particle. The theory treats the heat fluxes between its wet and dry components and from the sponge radially through the liquid skin to the air. The new theory can parameterize the spheroidal shape of hail. Gradual internal freezing of liquid soaking the hail or graupel particle's interior during dry growth (riming') is treated as well. Results of the new scheme for time-dependent freezing are compared against laboratory measurements. Entire experiments performed for single hailstones in a wind-tunnel are simulated off-line for comparison. Good agreement of the theoretical prediction with observations is obtained. The algorithms of time-dependent freezing were implemented in a spectral bin microphysics cloud model of the Hebrew University Cloud Model (HUCM). Simulations of hail storms under different aerosol concentrations are p...
The Simulation of a Destructive Squall-Line Event: Sensitivity to Fundamental Microphysical Processes
The development of a classic squall-line event on July 10th, 2013 severely impacted Ohio and vici... more The development of a classic squall-line event on July 10th, 2013 severely impacted Ohio and vicinity. Strong winds and lightning knocked out power over many locations, and power remained out for more than a day. The squall-line formed from the complex interactions of smaller convective storms and convective gust fronts. We use Spectral (bin) Microphysics in WRF to examine the potential importance of difficult to simulate, but important microphysical processes of melting of ice hydrometeors and shedding of graupel and hail hydrometeors on squall line structure. Spectral (bin) Microphysics explicitly predicts the spectrum of liquid and hydrometeor sizes, as well as their interactions through collisions, and accurate fall velocities of each. The model is also sensitive to initial aerosol concentrations, as initial drop size formation and concentration depends on the availability of condensation nuclei. We conclude i) that aerosol impacts on hail formation, and ii) melting and shedding...
Hurricane Irene (2011) moved northward along the eastern coast of the United States and was expec... more Hurricane Irene (2011) moved northward along the eastern coast of the United States and was expected to cause severe wind and flood damage. However, the hurricane weakened much faster than was predicted. Moreover, the minimum pressure in Irene occurred, atypically, about 40 h later than the time of maximum wind speed. Possible reasons for Irene’s weakening and the time shift between maximum wind and minimum central pressure were studied in simulations using WRF with spectral bin microphysics (WRF-SBM) with 1-km grid spacing and ocean coupling. Both ocean coupling and aerosol distribution/concentration were found to influence Irene’s development. Without ocean coupling or with ocean coupling and uniform aerosol distribution, the simulated maximum wind occurred at about the same time as the minimum pressure. With ocean coupling and nonuniform spatial aerosol distributions caused by aerosols from the Saharan air layer (band) and the continental United States, the maximum wind occurred ...
In maritime clouds over warm surface most droplets formed at the cloud base fall out without ever... more In maritime clouds over warm surface most droplets formed at the cloud base fall out without ever reaching the freezing level. Nevertheless, lightning takes place sometimes in the eye walls of hurricanes, where clouds are, supposedly, the most maritime in the world. In this study we address the following question: "Why does lightning take place in deep maritime convective clouds in the Intertropical Convergence zone and in hurricane eyewalls at all?" Numerical simulations using the spectral microphysics Hebrew University cloud model show that the formation of lightning in maritime clouds requires two conditions to be satisfied: a) significant vertical velocities and a large cloud-depth, and b) the existence of small aerosols with the radii lower than about 0.05 m μ in diameter in the cloud condensational nuclei (CCN) size spectra. Both factors are necessary components for the lightning to occur. Any absence of small aerosols would prevent in-cloud nucleation of small drops at high distances above the cloud base. Any lack of sufficiently fast updrafts prevents in-cloud nucleation and the formation of supercooled water at high levels. Small aerosols form over the ocean, supposedly, due to chemical reactions followed by particle collisions to create the aerosol accumulation mode.
A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebre... more A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebrew University Cloud Model (HUCM) with a detailed description of time-dependent melting and freezing. In addition to size distributions of drops, plate-, columnar-, and branch-type ice crystals, snow, graupel, and hail, new distributions for freezing drops as well as for liquid water mass within precipitating ice particles were implemented to describe time-dependent freezing and wet growth of hail, graupel, and freezing drops. Simulations carried out using different aerosol loadings show that an increase in aerosol loading leads to a decrease in the total mass of hail but also to a substantial increase in the maximum size of hailstones. Cumulative rain strongly increases with an increase in aerosol concentration from 100 to about 1000 cm−3. At higher cloud condensation nuclei (CCN) concentrations, the sensitivity of hailstones’ size and surface precipitation to aerosols decreases. The phys...
Simulation of aerosol effects on precipitation from green-ocean, smoky and pyro-clouds using a spectral microphysics cloud model with a precise calculation of supersaturation and diffusion growth
Effects of aerosols on drop freezing, size and structure of hail: application of the theory of time -dependent freezing and hail growth in a spectral bin microphysics cloud model
The theory of wet growth of hail is extended to the case of the inhomogeneities of surface temper... more The theory of wet growth of hail is extended to the case of the inhomogeneities of surface temperature and of liquid coverage over the surface of the particle. The theory treats the heat fluxes between its wet and dry components and from the sponge radially through the liquid skin to the air. The new theory can parameterize the spheroidal shape of hail. Gradual internal freezing of liquid soaking the hail or graupel particle's interior during dry growth (riming') is treated as well. Results of the new scheme for time-dependent freezing are compared against laboratory measurements. Entire experiments performed for single hailstones in a wind-tunnel are simulated off-line for comparison. Good agreement of the theoretical prediction with observations is obtained. The algorithms of time-dependent freezing were implemented in a spectral bin microphysics cloud model of the Hebrew University Cloud Model (HUCM). Simulations of hail storms under different aerosol concentrations are p...
The Simulation of a Destructive Squall-Line Event: Sensitivity to Fundamental Microphysical Processes
The development of a classic squall-line event on July 10th, 2013 severely impacted Ohio and vici... more The development of a classic squall-line event on July 10th, 2013 severely impacted Ohio and vicinity. Strong winds and lightning knocked out power over many locations, and power remained out for more than a day. The squall-line formed from the complex interactions of smaller convective storms and convective gust fronts. We use Spectral (bin) Microphysics in WRF to examine the potential importance of difficult to simulate, but important microphysical processes of melting of ice hydrometeors and shedding of graupel and hail hydrometeors on squall line structure. Spectral (bin) Microphysics explicitly predicts the spectrum of liquid and hydrometeor sizes, as well as their interactions through collisions, and accurate fall velocities of each. The model is also sensitive to initial aerosol concentrations, as initial drop size formation and concentration depends on the availability of condensation nuclei. We conclude i) that aerosol impacts on hail formation, and ii) melting and shedding...
At subzero temperatures, cloud particles can contain both ice and liquid water fractions. Wet gro... more At subzero temperatures, cloud particles can contain both ice and liquid water fractions. Wet growth of precipitation particles occurs when supercooled cloud liquid is accreted faster than it can freeze on impact.
The time-dependent process of raindrop freezing is described in a general form including 23 therm... more The time-dependent process of raindrop freezing is described in a general form including 23 thermodynamic effects from accretion of cloud-liquid and -ice. Freezing drops (FDs) larger than 24 80 µm (and their water mass) are represented explicitly in a cloud model with spectral bin 25 microphysics. The FDs consist of interior water covered by ice initially. Possibilities of both 26 dry (icy surface) and wet growth (surface covered by liquid) of FDs are accounted for. 27 Schemes of time-dependent freezing for rain (this Part II) and wet growth of hail and 28 graupel (Part I) were implemented in a spectral bin microphysics cloud model. The model 29 predicted that accretion of liquid produces giant FDs of 0.5-2 cm in diameter, far larger than 30 purely liquid drops can become. This growth of FDs is promoted by recirculation from the 31 downdraft back into the updraft and by cessation of internal freezing from some accreted liquid 32 remaining unfrozen (wet growth of FDs). Significant contents of FDs reach a height level of 7 33 km (-29 o C ) in the simulated storm. After FDs finish freezing and become hailstones, wet 34 growth may resume. The critical diameter separating wet and dry growth regimes is predicted to 35 increase with height for FDs, and is more vertically uniform for hail.
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Papers by Nir Benmoshe