Morning boundary layer conditions for shallow to deep convective cloud evolution during the dry season in the central Amazon

Abstract

Abstract. Observations of the boundary layer (BL) processes are analyzed statistically for dry seasons of 2 years and in detail, as case studies, for 4 shallow convective days (ShCu) and 4 shallow-to-deep convective days (ShDeep) using a suite of ground-based measurements from the Observation and Modeling of the Green Ocean Amazon (GoAmazon 2014/5) Experiment. The BL stages in ShDeep days, from the nighttime to the cloudy mixing layer stage, are then described in comparison with ShCu days. Atmospheric thermodynamics and dynamics, environmental profiles, and surface turbulent fluxes were employed to compare these two distinct situations for each stage of the BL evolution. Particular attention is given to the morning transition stage, in which the BL changes from stable to unstable conditions in the early morning hours. Results show that the decrease in time duration of the morning transition on ShDeep days is associated with high humidity and well-established vertical wind shear patterns. Higher humidity since nighttime not only contributes to lowering the cloud base during the rapid growth of the BL but also contributes to the balance between radiative cooling and turbulent mixing during nighttime, resulting in higher sensible heat flux in the early morning. The sensible heat flux promotes rapid growth of the well-mixed layer, thus favoring the deeper BL starting from around 08:00 LST (UTC−4 h). Under these conditions, the time duration of morning transition is used to promote convection, having an important effect on the convective BL strength and leading to the formation of shallow cumulus clouds and their subsequent evolution into deep convective clouds. Statistical analysis was used to validate the conceptual model obtained from the case studies. Despite the case-to-case variability, the statistical analyses of the processes in the BL show that the described processes are very representative of cloud evolution during the dry season.

How weather events modify aerosol particle size distributions in the Amazon boundary layer

“…N ACC and N AIT decrease during the night; the nocturnal BL height is normally above 60 m (Carneiro and Fisch, 2020), so this decrease is likely related to the interaction between the surface and the BL. Henkes et al (2021) discuss the shallow and deep convective events. For both types of events, the nocturnal boundary layer has nearly zero and sometimes negative latent and sensible surface flux, as well as nearly zero turbulence kinetic energy, consequently no effective mechanism to support exchange between BL and free atmosphere.…”

“…At this time, the BL evolves from the stable stage (nocturnal BL) to the convective BL (mixed layer). Following Stull (1988) classification of the stages of BL evolution, Henkes et al (2021) show in detail the BL height evolution and surface fluxes for these stages of the development of the convective BL in central Amazonian region during the dry season. The growing stage begins at sunrise and is determined by the time when the surface heat flux passes above zero and the turbulent kinetic energy promotes rapid growth of the convective BL.…”

Quantifying vertical wind shear effects in shallow cumulus clouds over Amazonia

“…In our study, we have focused mostly on the quantification of the VWS in the cloud properties (i.e., direct effect), which show an overall inhibition of convective development under sheared environments. However, both our simulations and the results fromHenkes et al (2021) point to a more complex relation between VWS and convective cloud formation and growth. Of course, the natural occurrences of VWS over the region are not linear as in our simulations.…”

“…On average, the horizontal cloud cover reaches a maximum of about 3% between 1400 m and 1500 m (not shown), indicating that the effects of evaporation should be small on the entire domain. There are also other processes that can cause invigoration of clouds by VWS, including the intensification of turbulence at the boundary layer (Henkes et al, 2021). The identification of processes that can lead to cloud invigoration due to increased VWS in our simulations will be the subject of future studies.…”

“…Another potential indirect effect of VWS on the cloud field development is on the early formation of the convective boundary layer. Recently,Henkes et al (2021) analyzed 2 years of observations of the boundary layer (BL) conditions previous to shallow-only (ShCu) convection and to shallow-to-deep convective transition (ShDeep) in the Amazonian dry season. In their conceptual model, the wind shear close to the top of the BL plays an important role on determining the morning BL evolution and setting up the conditions for deeper convection.…”