On the Trigger and Time-Scales of Shallow-to-Deep Convection in Amazonia

Deep atmospheric convection over Tropical forests continues to be a challenge, partially because it is a complicated process that involves a large range of spatial and time scales, and partially because of the lack of continuous observations with high resolution. As a consequence, the shallow-to-deep convective transition is often not properly represented in numerical models, hampering our ability to trust, for instance, important studies about climate change impacts on the hydrological cycle.

To further our understanding of the s-t-d transition we used 2-years of data from the GoAmazon 2014/5 experiment (Martin et al., 2017) to study the evolution of convective events over the T3 site, in the central Amazonia (3° 12' 47.88" S, 60° 35' 55.32" W). Cloud-top brightness temperature (CTT) from GOES13 was used to identify 151 afternoon transition events as in Adams et al. (2017). In an Eulerian approach, we built composites (centered at time of minimum CTT, t₀) to investigate the transition timescales and evaluate the thermodynamic and environmental conditions.

According to our analysis, the typical s-t-d transition with the sunrise. The boundary layer height and the lifting condensation level (LCL) rise from 200m (t₀-8h) to 600m (t₀-4h), at the same time as the level of free convection (LFC) drops from 1800m to 600m, and CAPE increased from 200 J to 1000 J. During this period, warm-cloud fraction (CTT > 0^oC) is about 25% and deep-cloud fraction is negligible (<3%, CTT < -38^oC). After the atmospheric trigger (LFC = LCL), shallow clouds grow into congestus (t₀-4h to t₀-2h), with warm-cloud fraction decreasing to 15% and cold- fraction increasing to 15%. Since sunrise (t₀-8h), column water vapor increased from 5.5 to 5.8 cm. The next phase is congestus organizing into deep convection, which happens from t₀-2h to t₀, at the expense of CAPE consumption. CTT drops from 280K to 220K. Evaporation of rain, which started at t₀-2h, moistens the PBL, increasing RH and lowering the LCL, which decouples from the LFC that started to rise, as CAPE is reduced. Precipitation persists until t₀+2h. Warm-cloud fraction reaches a minimum of 8% around t₀+1h, when cold- fraction is maximum (60%).