Abstract. The preconditioning of the atmosphere for a shallow-to-deep convective
transition during the dry-to-wet season transition period (August–November)
is investigated using Department of Energy (DOE) Atmospheric Radiation
Measurement (ARM) GoAmazon2014/5 campaign data from March 2014 to November
2015 in Manacapuru, Brazil. In comparison to conditions observed prior to
shallow convection, anomalously high humidity in the free troposphere and
boundary layer is observed prior to a shallow-to-deep convection transition.
An entraining plume model, which captures this leading dependence on lower
tropospheric moisture, is employed to study indirect thermodynamic effects
associated with vertical wind shear (VWS) and cloud condensation nuclei (CCN)
concentration on preconvective conditions. The shallow-to-deep convective
transition primarily depends on humidity, especially that from the free
troposphere, which tends to increase plume buoyancy. Conditions preceding
deep convection are associated with high relative humidity, and
low-to-moderate CCN concentration (less than the 67th percentile,
1274 cm−3). VWS, however, shows little
relation to moisture and plume buoyancy. Buoyancy estimates suggest that the
latent heat release due to freezing is important to deep convective growth
under all conditions analyzed, consistent with potential pathways for aerosol
effects, even in the presence of a strong entrainment. Shallow-only
convective growth, however, shows an association with a strong (weak) low
(deep) level VWS and with higher CCN concentration.