Amazon Coastal Squall Lines. Part I: Structure and Kinematics

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.