Abstract
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.
Highlights
Deep convection is the primary source of global precipitation over the tropics and midlatitudes (Houze, 2004) and has a large influence on extreme rainfall events like floods and droughts (Houze et al, 2015)
To identify favorable atmospheric conditions before shallow and deep convective systems form, we evaluate differences in the mixing ratio averaged over all BSHDP (BSH) conditions relative to such averages over all the clear sky conditions, denoted mr, in all seasons
This study employs a suite of ground-based measurements from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) mobile facility in Manacapuru, Brazil, as part of the GOAmazon campaign to investigate associations between meteorological parameters and cloud condensation nuclei (CCN) concentrations on an entraining plume’s buoyancy before the formation of shallow or deep convective clouds during the transition season
Summary
Deep convection is the primary source of global precipitation over the tropics and midlatitudes (Houze, 2004) and has a large influence on extreme rainfall events like floods and droughts (Houze et al, 2015). Investigating the meteorological parameters and suitable environmental conditions favoring the formation and evolution of deep convection is of interest to more accurately predict rainfall in climate models. Shallow and congestus convection transports moisture from the atmospheric boundary layer (BL) to the lower and middle troposphere, allowing for the development of deep convection (Zhuang et al, 2017; Del Genio and Wu, 2010; Jensen and Del Genio, 2006). There are a number of factors that can potentially dictate whether shallow convection will develop into deep, precipitating convection, such as free tropospheric moisture, vertical wind shear, cold pool formation, cloud–aerosol interactions, and the diurnal cycle
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