Contrasting convective regimes over the Amazon: Implications for cloud electrification

Abstract

Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol‐rich October and aerosol‐poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the “green ocean” westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…The transition from the dry to wet season is influenced by complex interactions between smoke-derived aerosols and deep convective clouds (Albrecht et al, 2011). Although the seasonal variability in the average convective available potential energy (CAPE) is small, the tail of the CAPE seasonal distribution (computed as the surface parcel) exhibits relatively higher values during the dry to wet season transition than during the wet season (Williams et al, 2002). During the dry season, the aerosols produced by biomass burning in central South America impact a larger area, reaching the tropical Pacific, subtropical South America, and the South Atlantic Freitas et al, 2005Freitas et al, , 2017Camponogara et al, 2014).…”

“…During the wet season, the atmosphere is mostly clean and convective, and the landscape is referred to as the Green Ocean (Roberts et al, 2001;Williams et al, 2002;Andreae et al, 2004) because the convection there resembles storms over blue oceans, where the warm phase in clouds generally produces rain. Large urban areas, however, introduce perturbations into the pristine air .…”

“…The ZDR that largely reflects ice orientation contains a greater number of near-zero or negative signals during the dry season compared to the wet season. This is likely associated with crystal orientation by the electrical field as more lightning occurs (see Williams et al, 2002), and more vertically oriented ice shapes such as graupel may occur during the dry season than the wet season. The KDP distribution shows considerably larger values in the warm layer during the dry compared to the wet season, indicating that the higher RRs and greater number of positive values in the mixed phase are probably associated with intense updrafts, as shown by Giangrande et al (2016).…”

Overview: Precipitation Characteristics and Sensitivities to the Environmental Conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…During the transition from the dry to the wet season is influenced by complex interactions between smokederived aerosols and deep convective clouds occurs (Albrecht et al, 2011). Although the average Convective Available Potential Energy (CAPE) seasonal variability is small, the tail of the CAPE seasonal distribution exhibit relatively higher 15 values during dry to wet season transition than during the wet season (Williams et al, 2002). During the dry season, the aerosols produced by biomass burning in central South America impact a larger area, reaching the tropical Pacific, subtropical South America and South Atlantic (Andreae et al 2001;Freitas et al, 2005, Camponogara et al, 2014.…”

“…During the wet season, 5 the atmosphere is mostly clean and convective, and the landscape is referred to as the Green Ocean (Roberts et al, 2001, Williams et al, 2002 and Andreae et al, 2004) because convection resembles storms over blue oceans, where the warm phase in clouds generally produces rain. Large urban areas, however, introduce perturbations into the pristine air (Martin et al , 2017).…”