PNNL

Abstract

A lack of routine environmental observations located in close proximity to deepening cumulus congestus clouds limits verification of important theorized or simulated updraft-environment interaction processes occurring during deep convection initiation (CI). We analyze radiosonde profiles collected during several hundred CI events near a mountain range in central Argentina during the CACTI field campaign. Statistical analyses illustrate environmental conditions supporting radar-observed CI outcomes that span a spectrum of convective cell depth and width, as well as events lacking precipitating convection. Tested environmental factors include sounding-derived measurements of convective available potential energy (CAPE), convective inhibition (CIN), moisture, terrain-relative wind velocity, vertical wind shear, and lifted parcel properties, with supplemental model reanalyses of background larger-scale ascent. CAPE and CIN metrics do not consistently differentiate CI success from failure. Only a few environmental factors contain consistent monotonic relationships among the spectrum of cloud depth achieved during CI: i) mid-level relative humidity, ii) the depth and strength of background ascent, and iii) the component of low-level flow oriented parallel to the nearby ridgeline. These metrics suggest that entrainment-driven dilution of updrafts in the lower-to-middle free troposphere, the ability of the surrounding flow to lift parcels to their LFC, and terrain-modified flow are consistently relevant processes for CI. Circumstantial evidence was found linking cell width, an important cloud property governing the probability of CI, to: LCL height, free tropospheric relative humidity, depth and magnitude of the CIN layer, and ambient wind shear.