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Jefrey Stith, John Scala, Roger Reinking, and Brooks Martner

Abstract

The results from three methods for studying transport and dispersion in cumuli are compared. These three methods include two tracer techniques and a numerical simulation. The tracers, SF6 and radar chaff, were simultaneously released below the base of a convective storm. The SF6 was measured in situ by two research aircraft and the chaff was followed using TRACIR (tracking air with circular-polarized radar), a method that measures the circular depolarization ratio (CDR) of the chaff, which is much stronger than that of most hydrometeors. TRACIR allows the CDR signal from the chaff to be measured and traced even when the reflectivity from the chaff is much less than that from the cloud. The behavior of the two-tracer release was compared with the trajectories of air from a two-dimensional simulation of the storm, using a nonhydrostatic cloud model, the National Aeronautics and Space Administration/Goddard Cumulus Ensemble Model. By combining information from the three techniques, their individual shortcomings are alleviated, and a more complete documentation of transport and dispersion is provided.

The tracers were followed during a 32-min period as they were transported 6 km vertically by the storm at an average rate of 2.6 m s−1. This was within the ranges of the vertical transport rate of trajectories in the model simulation. The maximum updraft speed measured by the aircraft was 18 m s−1, which agreed well with the maximum updraft in the simulators of 20 m s−1. Both the simulation and the chaff observations show that portions of the released material were incorporated into the cloud and other portions were not.

The main area of downward transport was located in the lower third of the simulated cloud where the rainfall was the heaviest. Major downdrafts were not found in the upper regions of the storm where the aircraft were sampling. The simulation suggests that the precipitation-induced downdraft played the major role in determining the trajectories of air from the cloud base, at least at the mature stage of the storm. Interactions between cloud-base air and downdrafts took place in the lower third of the storm where the bulk of the precipitation was located.

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Steven Greco, John Scala, Jeffrey Halverson, Harold L. Massie Jr., Wei-Kuo Tao, and Michael Garstang

Abstract

The column response to propagating deep convection over the central Amazon Basin is investigated with rawinsonde data from the Amazon Boundary Layer Experiment (ABLE 2B). Heat and moisture budgets are calculated from a relatively small surface network (1000 km2) to determine the distribution of heating within the convective and stratiform regions of three Amazon coastal squall lines (ACSL) in varying degrees of maturity. Portable Automated Mesonet instrumentation, satellite imagery, and radar data are used to partition the large-scale system into distinct cloud and rainfall components. The dimensions of the surface network enable an evaluation of the collective effects of an ensemble of convective elements that are considered to be representative of the synoptic-scale system.

Calculations of Q 1 and Q 2 from the ABLE 2B network follow the methods used by Johnson and Young and Gallus and Johnson. The computations are performed over intervals of 3–6 h using composite soundings derived from a network average. The distribution of heating and drying for the 1 May 1987 ACSL and its variation in time are shown to be similar to the results of other studies, particularly those of West African squall lines. Peak heating occurs between 500 and 550 mb, and peak drying is concentrated between 450 and 650 mb. A lack of separation between the peaks in the convective Q 1 and Q 2 profiles indicates a coupling of Q 1 and Q 2 and suggests the presence of significant stratiform processes in the absence of pronounced eddy transports.

The vertical eddy flux of total heat (F) is calculated by assuming the horizontal eddy flux term is small relative to the net vertical transports. Even though the horizontal transfer of heat and moisture may not be negligible in this study, the area encompassed by the surface network is large relative to the area occupied by active portions of convective clouds. From a network perspective, these cloud-scale fluxes are considered small relative to the vertical eddy flux of total heat. The distribution of vertical eddy flux compares favorably with a mesoscale calculation performed by Gallus and Johnson for a midlatitude squall line suggesting the assumptions regarding the net contribution of the horizontal fluxes may be reasonable.

Convective transports of heat are equalled by transports occurring within the stratiform region of the system. The heat transported by a single ACSL when extrapolated to the ACSL as a whole represents a significant contribution to the global heat balance.

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Michael Garstang, Harold L. Massie Jr., Jeffrey Halverson, Steven Greco, and John Scala

Abstract

Mesoscale to synoptic-scale squall lines that form along the northeastern coast of South America as sea-breeze-induced instability lines and propagate through the Amazon Basin are investigated using data collected during the April–May 1987 Amazon Boundary Layer Experiment (ABLE 2B).

These systems, termed “Amazon coastal squall lines” (ACSL), have been noted by others, but details of the structure and evolution of the ACSL are limited. The present paper uses Geostationary Operational Environmental Satellite, radar, upper-air rawinsonde, and surface Portable Automated Mesonet data to describe the structure, dynamics, and life cycle of the ACSL. Twelve ACSL were sampled during ABLE 2B, and three cases are discussed in detail.

The ACSL are discontinuous lines of organized mesoscale cloud clusters that propagate across the central Amazon Basin at speeds of 50–60 km h−1. The ACSL undergo six possible life cycle stages: coastal genesis, intensification, maturity, weakening, reintensification, and dissipation. Analysis also indicates that mesoscale clusters within the ACSL are composed of three distinct cloud components: a prestorm region that often contains towering cumulus, leading edge convection (LEC), and multiple, precipitating cloud layers in the trailing stratiform region (TSR).

Divergence and vertical velocity calculations indicate deep vertical ascent in the LEC and a region of midlevel convergence (≈500 mb) in the TSR. The latter midlevel convergence is associated with a weak updraft above 500 mb and an unsaturated downdraft below. Vertical motions in the TSR are an order of magnitude smaller than in the LEC.

Substantial shear in the low-level inflow occurs in all three case studies and, as suggested by model simulations, may play an important role in the longevity (24–48 h) of the ACSL. Profiles of equivalent potential temperature θe, taken from the prestorm, leading edge convection and trailing stratiform regions demonstrate that the ACSL stabilize the troposphere in their wake and remove a tropospheric minimum of θe. It is hypothesized that the removal of this minimum is accomplished both by direct mixing via vertical motions in the LEC ("hot towers") and also through detrainment in the multiple-layered TSR. Part I describes the structure and kinematics of the ACSL, while Part II deals with the heat and moisture transports of these systems.

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David A. R. Kristovich, George S. Young, Johannes Verlinde, Peter J. Sousounis, Pierre Mourad, Donald Lenschow, Robert M. Rauber, Mohan K. Ramamurthy, Brian F. Jewett, Kenneth Beard, Elen Cutrim, Paul J. DeMott, Edwin W. Eloranta, Mark R. Hjelmfelt, Sonia M. Kreidenweis, Jon Martin, James Moore, Harry T. Ochs III, David C Rogers, John Scala, Gregory Tripoli, and John Young

A severe 5-day lake-effect storm resulted in eight deaths, hundreds of injuries, and over $3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30–90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic-scale, mesoscale, and microscale processes.

An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake-Induced Convection Experiment (Lake-ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake-effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of these collaborative projects. Preliminary findings are given with illustrative examples of new state-of-the-art research observations collected. Analyses associated with Lake-ICE and SNOWBAND hold the promise of greatly improving our scientific understanding of processes involved in these important wintertime phenomena.

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