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Donald R. MacGorman and Kurt E. Nielsen

Abstract

On 8 May 1986, the National Severe Storms Laboratory (NSSL) collected Doppler radar and lightning ground strike data on a supercell storm that produced three tornadoes, including an F3 tornado in Edmond, Oklahoma, approximately 40 km north of NSSL. The Edmond storm formed 30 km ahead of a storm complex and produced its first and most damaging tornado just as the storm complex began to overtake it from the west. In the mesocyclone that spawned the tornado, low-level cyclonic shear peaked as the first tornado dissipated and a second tornado began. As low-level cyclonic shear initially increased, negative cloud-to-ground lightning flash rates also increased, reaching a peak of 11 min−1 a few minutes after the peak in cyclonic shear. During this period lightning strike locations tended to concentrate just north of the mesocyclone, near and inside a 50-dBZ reflectivity core. As cyclonic shear decreased from its peak during and after the second tornado, negative ground flash rates also decreased, and strike locations became more scattered. Positive ground flashes began just before the storm became tornadic, and positive flash rates peaked during the tornadic stage of the storm.

The evolution of cloud-to-ground lightning in the Edmond storm differed considerably from the evolution of lightning in the Binger tornadic storm of 22 May 1981 that was studied previously. In the Binger storm, ground flash rates were negatively correlated with cyclonic shear and peaked 15–20 min later than low-level shear and intracloud lightning. It is suggested that the very strong mesocyclone and updraft in the Binger storm enhanced intracloud flash production and delayed ground flashes by causing the initial height of negative charge to be higher than in most storms. It is also suggested that weaker updrafts and a weaker, shallower mesocyclone in the Edmond storm resulted in higher negative ground flash rates when the Edmond mesocyclone was still strong, because negative charge near the mesocyclone was at the lower heights common to most thunderstorms.

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Ryan J. Harris, John R. Mecikalski, Wayne M. MacKenzie Jr., Philip A. Durkee, and Kurt E. Nielsen

Abstract

Within cumulus cloud fields that develop in conditionally unstable air masses, only a fraction of the cumuli may eventually develop into deep convection. Identifying which of these convective clouds is most likely to generate lightning often starts with little more than a qualitative visual satellite analysis. The goal of this study is to identify the observed satellite infrared (IR) signatures associated with growing cumulus clouds prior to the first lightning strike, or lightning initiation (LI). This study quantifies the behavior of 10 Geostationary Operational Environmental Satellite-12 (GOES-12) IR fields of interest in the 1 h in advance of LI. A total of 172 lightning-producing storms, which occurred during the 2009 convective season, are manually tracked and studied over four regions: northern Alabama, central Oklahoma, the Kennedy Space Center, and Washington, D.C. Four-dimensional and cloud-to-ground lightning array data provide a total cloud lightning picture (in-cloud, cloud-to-cloud, cloud-to-air, and cloud-to-ground) and thus precise LI points for each storm in both time and space. Statistical significance tests are conducted on observed trends for each of the 10 LI fields to determine the unique information each field provides in terms of behavior prior to LI. Eight out of 10 LI fields exhibited useful information at least 15 min in advance of LI, with 35 min being the average. Statistical tests on these eight fields are compared for separate large geographical areas. Median IR temperatures and 3.9-μm reflectance values are then determined for all 172 events as an outcome, which may be valuable when implementing a LI prediction algorithm into real-time satellite-based systems.

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Qingfu Liu, Yefim L. Kogan, Douglas K. Lilly, Douglas W. Johnson, George E. Innis, Philip A. Durkee, and Kurt E. Nielsen

Abstract

The LES model is applied for studying ship track formation under various boundary layer conditions observed during the Monterey Area Ship Track experiment. Simulations in well-mixed and decoupled boundary layers show that ship effluents are easily advected into the cloud layer in the well-mixed convective boundary layer, whereas their transport may be suppressed by the subcloud transitional layer in the decoupled case. The clear difference between the well-mixed and decoupled cases suggests the important role of diurnal variation of solar radiation and consequent changes in the boundary layer stability for ship track formation. The authors hypothesize that, all other conditions equal, ship track formation may be facilitated during the morning and evening hours when the effects of solar heating are minimal.

In a series of experiments, the authors also studied the effects of additional buoyancy caused by the heat from the ship engine exhaust, the strength of the subcloud transitional layer, and the subcloud layer saturation conditions. The authors conclude that additional heat from ship engine and the increase in ship plume buoyancy may indeed increase the amount of the ship effluent penetrating into the cloud layer. The result, however, depends on the strength of the stable subcloud transitional layer. Another factor in the ship effluent transport is the temperature of the subcloud layer. Its decrease will result in lowering the lifting condensation level and increased ship plume buoyancy. However, the more buoyant plumes in this case have to overcome a larger potential barrier. The relation between all these parameters may be behind the fact that ship tracks sometimes do, and sometimes do not, form in seemingly similar boundary layer conditions.

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Kevin J. Noone, Elisabeth Öström, Ronald J. Ferek, Tim Garrett, Peter V. Hobbs, Doug W. Johnson, Jonathan P. Taylor, Lynn M. Russell, Richard C. Flagan, John H. Seinfeld, Colin D. O’Dowd, Michael H. Smith, Philip A. Durkee, Kurt Nielsen, James G. Hudson, Robert A. Pockalny, Lieve De Bock, René E. Van Grieken, Richard F. Gasparovic, and Ian Brooks

Abstract

The effects of anthropogenic particulate emissions from ships on the radiative, microphysical, and chemical properties of moderately polluted marine stratiform clouds are examined. A case study of two ships in the same air mass is presented where one of the vessels caused a discernible ship track while the other did not. In situ measurements of cloud droplet size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aerosol in the two ship plumes are discussed;these indicate that combustion-derived particles in the size range of about 0.03–0.3-μm radius were those that caused the microphysical changes in the clouds that were responsible for the ship track.

The authors examine the processes behind ship track formation in a moderately polluted marine boundary layer as an example of the effects that anthropogenic particulate pollution can have in the albedo of marine stratiform clouds.

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Kevin J. Noone, Doug W. Johnson, Jonathan P. Taylor, Ronald J. Ferek, Tim Garrett, Peter V. Hobbs, Philip A. Durkee, Kurt Nielsen, Elisabeth Öström, Colin O’Dowd, Michael H. Smith, Lynn M. Russell, Richard C. Flagan, John H. Seinfeld, Lieve De Bock, René E. Van Grieken, James G. Hudson, Ian Brooks, Richard F. Gasparovic, and Robert A. Pockalny

Abstract

A case study of the effects of ship emissions on the microphysical, radiative, and chemical properties of polluted marine boundary layer clouds is presented. Two ship tracks are discussed in detail. In situ measurements of cloud drop size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside-cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to remotely sensed measurements of cloud radiative properties.

The authors examine the processes behind ship track formation in a polluted marine boundary layer as an example of the effects of anthropogenic particulate pollution on the albedo of marine stratiform clouds.

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Peter V. Hobbs, Timothy J. Garrett, Ronald J. Ferek, Scott R. Strader, Dean A. Hegg, Glendon M. Frick, William A. Hoppel, Richard F. Gasparovic, Lynn M. Russell, Douglas W. Johnson, Colin O’Dowd, Philip A. Durkee, Kurt E. Nielsen, and George Innis

Abstract

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as “ship tracks.” Airborne measurements are used to derive emission factors of SO2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ∼15–89 and ∼2–25 g kg−1 of fuel burned, respectively. By contrast a steam turbine–powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ∼6 g kg−1.

Various types of ships, burning both MFO and navy distillate fuel, emitted from ∼4 × 1015 to 2 × 1016 total particles per kilogram of fuel burned (∼4 × 1015–1.5 × 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (∼0.03–0.05 μm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ∼0.02 μm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine–powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis.

The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN.

The fluxes of heat and water vapor from ships are estimated to be ∼2–22 MW and ∼0.5–1.5 kg s−1, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

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