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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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