Excessive Convective Precipitation Formation Caused by Warm‐Phase Bulk Microphysical Scheme Biases: A Multi‐Platform Observational Diagnosis

Abstract Recent work has demonstrated that convection‐permitting models often produce a greater number of precipitating convective cells compared to radar observations. Congestus‐depth cells contribute most to the bias, which are colloquially termed “popcorn convection.” This study explores warm‐phase microphysical contributions to the bias using simulations spanning a field campaign hosted in the mountains of central Argentina. Observations from a C‐band radar, satellite, in situ aircraft, and surface disdrometers are used to evaluate a 3‐km grid spacing, 6.5‐month simulation using bulk aerosol‐aware microphysics and case studies with higher model resolutions. Compared to observations, clouds precipitate too frequently (by up to a factor of 2.5), particularly for congestus‐depth clouds. These cells precipitate under high aerosol loadings (>4,000 cm −3 ) and most are purely liquid upon initiation of precipitation. Comparison with in situ aircraft observations shows that simulations shift too much bulk liquid mass to precipitation‐sized drops, especially for low liquid water contents (LWCs), indicating potential issues with parameterized autoconversion. This leads to high‐biased in‐cloud radar reflectivity of 8–14 dBZ for LWCs < 10 −1 g m −3 , producing radar reflectivity signals that contribute to convective cell identification and cell‐number biases. Case studies using 500 and 100 m horizontal grid spacings do not ameliorate these biases, implying that under‐resolved updrafts and entrainment are not first‐order culprits. Attributing warm‐phase microphysical processes to convective cell‐number and precipitation frequency biases motivates a more detailed understanding of precipitation formation pathways in continental convective clouds, their sensitivity to aerosols, and novel methods capable of relieving modern structural limitations in bulk microphysics schemes.