Amazon Coastal Squall Lines. Part II: Heat and Moisture Transports

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 Q1 and Q2 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 Q1 and Q2 profiles indicates a coupling of Q1 and Q2 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.