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W. David Rust and Thomas C. Marshall

Abstract

In order to place instruments for measuring meteorological and electrical parameters into thunderstorms, we developed an inexpensive apparatus that allows us to inflate, transport, and launch balloons in high winds. The launching apparatus is a cylinder of “bubble” plastic that is made by joining the sides of the cylinder together with a VELCRO ”rip strip.” We launch a balloon by pulling the rip strip rapidly. This allows the balloon to pop upward into the ambient low-level wind and carry its instrumentation aloft. We construct different-sized launch tubes to accommodate particular sizes of balloons and have successfully launched balloons in winds of about 20 m s−1.

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Thomas C. Marshall and W. David Rust

Electric field (E) soundings in the stratiform regions and transition zones of mesoscale convective systems (MCSs) are reported. Most of the E soundings were made during the 1991 Cooperative Oklahoma Profiler Studies (COPS-91). Multiple E soundings were made in several MCSs. All of the E soundings collected here can be grouped into one of two types. Within each type the soundings and the inferred charge structures are remarkably similar from one place in an MCS to another and from one MCS to another. The charge regions inferred from the E soundings are hundreds of meters thick and have charge densities up to 5.3 nC m−3. Typically, the maximum E in the soundings is about 100 kV m−1 Here, E soundings from three classes of MCSs are discussed. The bow-echo MCSs have simpler vertical charge structures with four main charge regions, while squall-line MCSs and predominantly stratiform MCSs have five main charge regions. In all of the E soundings there is a substantial region of charge and a large E at or near 0°C.

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Tommy R. Shepherd, W. David Rust, and Thomas C. Marshall

Abstract

Earlier studies of mesoscale convective system stratiform regions have shown that large electric fields and charge densities are found near the 0°C level. Here 12 soundings of the electric field were analyzed through the 0°C level in various types of electrified stratiform clouds. For each electric field sounding, the thermodynamic sounding and supporting radar data were also studied. For comparison, five soundings not from stratiform clouds were included. Charge densities were found at or near 0°C in the stratiform clouds of at least 1 nC m−3 in eight of the soundings, and four of those had charge densities of at least 2 nC m−3. Of the stratiform soundings, 11 had an electric field magnitude of greater than 50 kV m−1 near 0°C, and 7 of those had an electric field magnitude of at least 75 kV m−1. The evidence suggests that melting may be the primary cause of the charge density found at and below 0°C in electrified stratiform clouds. In all 12 of the stratiform soundings, positive charge density was found at or near 0°C, and 11 of those had weaker negative charge density below. The evidence further suggests these two features do not exist in the absence of a bright band and (usually) an associated quasi-isothermal layer.

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Monte G. Bateman, W. David Rust, and Thomas C. Marshall

Abstract

A new balloon-borne instrument created by the authors and referred to as the q-d instrument that measures the charge q and size d of precipitation particles is discussed. The instrument measures charge with an induction cylinder size with an optical sensor, and fall speed by the time difference between the two. A second induction cylinder at the top serves as the entry point and detects precipitation that splashes off the entry. In this way, particles contaminated by splashing are removed from the data. It is capable of measuring particle sizes ranging from 0.8 to 8.0 mm in diameter and charges ranging from ±4 to ±400 pC. Since the size is measured optically, one can detect uncharged particles and measure their size. The q-d instrument does not show evidence of corona at its extremities until the electric field is as large as 100 kV m−1 at 700 mb.

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W. David Rust, Thomas C. Marshall, Maribeth Stolzenburg, and James Fitzgibbon

Abstract

Meteorological radiosondes that use navigation systems to determine winds (and horizontal location) can be susceptible to data loss in thunderstorm environments. This paper reports on tests of a radiosonde that uses the Global Positioning System (GPS) for windfinding. Tests were made by flying the GPS radiosonde into three thunderstorms on free balloons that also carried an electric field meter and a long-range navigation (loran) radiosonde of a type previously tested. The GPS radiosonde performed without any significant loss of wind or thermodynamic data in in-storm maximum electric fields of up to −104 kV m−1. Also, no obvious deleterious effect on radiosonde data was found from the presence of nearby lightning. The radiosonde was further tested in a laboratory-produced electric field in an ambient atmospheric pressure of about 70 kPa, in which the radiosonde functioned normally in a vertical electric field up to 160 kV m−1 and in a horizontal electric field up to 100 kV m−1, the respective maximum applied. Radiosondes that were sprayed with water to simulate flight in rain performed correctly in an electric field of 135 kV m−1—the maximum that could be applied safely. The hypothesized reason for the excellent windfinding performance in high electric fields is partly the very short antenna length needed for GPS reception. Other factors, which could not be assessed in this study, may include the inherent low-noise susceptibility of the GPS signals and the processing circuitry. The tests showed that the GPS radiosonde obtains wind data in larger electric fields than does the loran radiosonde. It is concluded that GPS radiosondes will acquire windfinding data in most, if not all, thunderstorm and nonthunderstorm clouds that contain high electric fields. The thermodynamic data were also very good in the large electric fields.

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Marshall Shepherd, Thomas Mote, John Dowd, Mike Roden, Pamela Knox, Steven C. McCutcheon, and Steven E. Nelson

No Abstract available.

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Terry J. Schuur, W. David Rust, Bradley F. Smull, and Thomas C. Marshall

Abstract

An electric field sounding through the transition zone precipitation minimum that trailed an Oklahoma squall line on 18 June 1987 provides information about the electrical structure within a midlatitude trailing stratiform cloud. A single-Doppler radar analysis concurrent with the flight depicts a kinematic structure dominated by two mesoscale flow regimes previously identified in squall-line systems: a strong midlevel, front-to-rear flow coinciding with the stratiform cloud layer and a descending rear inflow that sloped from 6.5 km AGL at the stratiform cloud's trailing edge to 1.5 km AGL at the convective line. Electric field magnitudes as high as 113 kV m−1 were observed by the electric field sounding, which reveals an electric field structure comparable in magnitude and complexity to structures reported for convective cells of thunderstorms. The charge regions inferred with an approximation to Gauss' law have charge density magnitudes of 0.2–4.1 nC m−3 and vertical thicknesses of 130–1160 m; these values, too, are comparable to those reported for thunderstorm cells. In agreement with previous studies, an analysis of the lightning data revealed a “bipolar” cloud-to-ground lightning pattern with positive flashes being relatively more common in the stratiform region.

From the analysis, we conclude that the stratiform region electrical structure may have been advected from the squall line convective cells as the in-cloud charge regions were primarily found within the front-to-rear flow. Screening layers were found at the lower and upper cloud boundaries. In situ microphysical charging also seems to be a possible source of charge in the stratiform region. We hypothesize that the radar-derived similarities of this system to those previously documented suggests that the newly-documented stratiform electrical structure might also be representative of this type of mesoscale convective system.

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Maribeth Stolzenburg, Thomas C. Marshall, W. David Rust, and Bradley F. Smull

Abstract

Five soundings of the electric field and thermodynamic properties were made in a mesoscale convective system (MCS) that occurred in Oklahoma and Texas on 2–3 June 1991. Airborne Doppler radar data were obtained from three passes through the stratiform echo. From these electrical, kinematical, and reflectivity measurements, a conceptual model of the electrical structure of an MCS is developed.

Low-level reflectivity data from the storm's mature and dissipating stages show typical MCS characteristics. The leading convective region is convex forward, and the back edge of the stratiform echo is notched inward. The maximum areal extent of the low-level echo is about 250 km × 550 km, and the radar bright band is intense (reflectivity 45–50 dBZ) through an area of at least 50 km × 100 km. The reflectivity above the bright band is horizontally stratified with decreasing intensity and echo-top height toward the rear of the system. Analyses of the velocity data reveal a convective-line-relative flow structure of front-to-rear flow and mesoscale ascent aloft, and weak rear inflow and descent below about 5 km.

The electric field soundings are similar over a period of 3 h and a horizontal scale of 100 km across the stratiform region, suggesting that the charge structure is nearly steady state and the charge regions are horizontally extensive and layered. The basic charge structure consists of four layers: a 1–3-km-deep region of positive charge (density ρ ≈ +0.2 nC m−3) between 6 and 10 km, negative charge (ρ ≈ −1.0–2.5 nC m−3) between 5 and 6 km, positive charge (ρ ≈ +1.0–3.0 nC m−3) near 0°C, and negative charge (ρ ≈ −0.5 nC m−3) near cloud base. The upper positive and densest negative charge layers could result from advection of charge from the convective region. The negative charge layer may be augmented by noninductive collisional charging in the stratiform region. The positive charge near 0°C is probably caused by one or more in situ charging mechanisms. The negative charge near cloud base is likely the result of screening layer formation. In addition to the basic four charge layers, positive charge is found below the cloud in each sounding, and in the two soundings closest to the convection (70–100 km distant) there is a low-density negative charge region near echo top.

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W. David Rust, Donald W. Burgess, Robert A. Maddox, Lester C. Showell, Thomas C. Marshall, and Dean K. Lauritsen

We have tested the NCAR Cross-Chain LORAN Atmospheric Sounding System (CLASS) in a fully mobile configuration, which we call M-CLASS. The sondes use LORAN-C navigation signals to allow calculation of balloon position and horizontal winds. In non-stormy environments, thermodynamics and wind data were almost always of high quality. Besides providing special soundings for operational forecasts and research programs, a major feature of mobile ballooning with M-CLASS is the ability to obtain additional data by flying other instruments on the balloons. We flew an electric field meter, along with a sonde, into storms on 8 of the initial 47 test flights in the spring of 1987. In storms, pressure, temperature, humidity, and wind data were of good quality about 80%, 75%, 60%, and 40% of the time, respectively. In a flight into a mesocyclone, we measured electric fields as high as −135 kV/m (at 10 km MSL) in a region of negative charge. The electric field data from several storms allow a quantitative assessment of conditions that accompany loss of LORAN data. LORAN tracking was lost at a median field of about 16 kV/m, and it returned at a median field of about 7 kV/m. Corona discharge from the LORAN antenna on the sonde was a cause of the loss of LORAN. We provided our early-afternoon M-CLASS test soundings to the National Weather Service Forecast Office in Norman, Oklahoma, in near real-time via amateur packet radio and also to the National Severe Storms Forecast Center. These soundings illustrate the potential for improving operational forecasts. Other test flights showed that M-CLASS data can provide high-resolution information on evolution of the Great Plains low-level jet stream. Our intercept of Hurricane Gilbert provided M-CLASS soundings in the right quadrant of the storm. We observed substantial wind shear in the lowest levels of the soundings around the time tornadoes were reported in south Texas. This intercept demonstrated the feasibility of taking M-CLASS data during the landfall phase of hurricanes and tropical storms.

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Graeme A. MacGilchrist, Helen L. Johnson, David P. Marshall, Camille Lique, Matthew Thomas, Laura C. Jackson, and Richard A. Wood

Abstract

A substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high-latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backward-in-time trajectories from a model-defined North Atlantic Deep Water (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major fraction of NADW ventilation results from subduction in the Labrador Sea, predominantly within the boundary current (~60% of ventilated NADW volume) and a smaller fraction arising from open ocean deep convection (~25%). Subsurface transformations—due in part to the model’s parameterization of bottom-intensified mixing—facilitate NADW ventilation, such that water subducted in the boundary current ventilates all of NADW, not just the lighter density classes. There is a notable absence of ventilation arising from subduction in the Greenland–Iceland–Norwegian Seas, due to the re-entrainment of those waters as they move southward. Taken together, our results emphasize an important distinction between ventilation and dense water formation in terms of the location where each takes place, and their concurrent sensitivities. These features of NADW ventilation are explored to understand how the representation of high-latitude processes impacts properties of the deep ocean in a state-of-the-science numerical simulation.

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