Environmental Characteristics of Convective Systems during TRMM-LBA

Abstract

In this paper, data collected from 51 days of continual upper-atmospheric soundings and the Tropical Ocean Global Atmosphere (TOGA) radar at Anglo?Brazilian Amazonian Climate Observation Study (ABRACOS) Hill during the Tropical Rainfall Measuring Mission component of the Brazilian Large Scale Biosphere?Atmosphere (TRMM-LBA) experiment are used to describe the mean thermodynamic and kinematic airmass properties of wet season convection over Rondonia, Brazil. Distinct multiday easterly and westerly lower-tropospheric wind regimes occurred during the campaign with contrasting airmass characteristics. Westerly wind periods featured modest CAPE (1000 J kg?1), moist conditions (>90% RH) extending through 700 mb, and shallow (900 mb) speed shear on the order of 10?4 s?1. This combination of characteristics promoted convective systems that featured a relatively large fraction of stratiform rainfall and weak convection nearly devoid of lightning. In contrast, easterly regime convective systems were more strongly electrified and featured larger convective rain rates and reduced stratiform rainfall fraction. These systems formed in an environment with larger CAPE (1500 J kg?1), drier lower- and midlevel humidities (<80% RH), and a wind shear layer that was both stronger (10?3 s?1) and deeper (700 mb). The time series of low- and midlevel averaged humidity exhibited marked variability between westerly and easterly regimes and was characterized by low-frequency (i.e., multiday to weekly) variations. In addition to its importance in stratiform rain formation, the humidity content directly influenced cloud cover and, thus, the degree of thermal instability present during regimes. The synoptic-scale origins of these moisture fluctuations are examined. The results reported herein provide an environmental context for ongoing dual-Doppler analyses and numerical modeling case studies of individual TRMM-LBA convective systems.