PNNL

Abstract

Marine cold-air outbreak (MCAO) clouds commonly form convective rolls then transition into broken cellular convection downstream. Understanding the factors controlling cloud morphology transition is an important step forward towards more realistic representations of MCAO clouds in climate models. In this study, we investigate how cloud ice number concentrations (N_i) affect cloud evolutions. We focus on a case during the COMBLE (Cold-Air Outbreaks in the Marine Boundary Layer Experiment) campaign. Our large eddy simulation is driven by ERA5 (European Center for Medium-Range Weather Forecasting (ECMWF) atmospheric reanalysis) reanalysis extracted along the back trajectory in a Lagrangian framework, with different N_i. All the simulations capture the observed cloud patterns well. Roll clouds in simulations with higher N_i breakup earlier. Surface precipitation rates are similar to each other while cloud base precipitation rates are higher in higher N_i with earlier precipitation initiation. The stronger precipitation evaporation leads to weaker boundary layer vertical mixing and weaker coupling. Increased sink (precipitation) and decreased source (vertical transport) of water result in earlier cloud breakup in higher N_i. This mechanism remains valid under different MCAO scenarios from simulations with different sea surface temperature (SST). The cloud organization is more sensitive to SST changes in low N_i environment. This work highlights the importance of accurate representation of ice processes in simulating MCAO clouds and suggests the needs for observational constraints of ice nucleating particles and N_i over the mixed-phase cloud regimes.