Marine cold air outbreaks (MCAOs) drive significant evolutions in marine boundary layer clouds and play a crucial role in high-latitude climate systems. This study examines the variability of cloud particle size distributions (PSDs) and the representation of the Gamma shape parameter μ for high-latitude MCAO clouds in the Northern Hemisphere, focusing on the initial stratocumulus stages and cumulus regime transitions. Aircraft in-situ measurements from 20 flights with identified MCAO conditions were collected during two recent field campaigns: M-Phase conducted over the Labrador Sea in March 2022, and Arctic Cold-Air Outbreak (ACAO) conducted over the Nordic Seas in October to November 2022. Results show that high-latitude MCAO clouds in the Northern Hemisphere exhibit narrow PSDs, characterized by higher μ values (mean μ=20) that imply more reflective clouds compared to the fixed μ=2.5 assumption in some bulk microphysics schemes. Cloud PSDs narrow and μ values increase with height in near-adiabatic stratocumulus clouds, while their patterns are more variable in broken cumulus clouds. Liquid water content correlates more strongly with μ variability than cloud number concentrations, suggesting its better predictability as a prognostic variable for PSD variability in these cloud systems. Both the μ=20 and its derived relation with cloud liquid water content can be applied in bulk microphysics schemes to better represent the microphysical and radiative properties of high-latitude MCAO clouds. The proposed high μ values for MCAO clouds are applicable mainly to typical horizontal resolutions of numerical weather prediction and regional climate models.

Large satellite discrepancies and model biases in representing precipitation over the Southern Ocean (SO) are related directly to the region’s limited surface observations of precipitation. To help address this knowledge gap, the study investigated the precipitation characteristics and rain rate retrievals over the remote SO using ship-borne data of the Ocean Rainfall And Ice-phase precipitation measurement Network disdrometer (OceanRAIN) and dual-polarimetric C-band radar (OceanPOL) aboard the Research Vessel (RV) Investigator in the Austral warm seasons of 2016 to 2018. Seven distinct synoptic types over the SO were analyzed based on their radar polarimetric signatures, surface precipitation phase, and rain microphysical properties. OceanRAIN observations revealed that the SO precipitation was dominated by drizzle and light rain, with small-sized raindrops (diameter < 1 mm) constituting up to 47 % of total accumulation. Precipitation occurred most frequently over the warm sector of extratropical cyclones, while concentrations of large-sized raindrops (diameter > 3 mm) were prominent over synoptic types with colder and more convectively unstable environments. OceanPOL observations complement and extend the surface precipitation properties sampled by OceanRAIN, providing unique information to help characterize the variety of potential precipitation types and associated mechanisms under different synoptic conditions. Raindrop size distributions (DSD) measured with OceanRAIN over the SO were better characterized by analytical DSD forms with two-shape parameters than single-shape parameters currently implemented in satellite retrieval algorithms. This study also revised a rainfall retrieval algorithm for C-band radars to reflect the large amount of small drops and provide improved radar rainfall estimates over the SO.

This study investigates the representation of stratocumulus (Sc) clouds, cloud variability, and precipitation statistics over the Southern Ocean (SO) to understand the dominant ice processes within the Icosahedral Nonhydrostatic (ICON) model at the kilometer scale using real case simulations. The simulations are evaluated using the shipborne observations as open-cell stratocumuli were continuously observed during two days (26th -27th of March 2016), south of Tasmania. The radar retrievals are used to effectively analyze the forward- simulated radar signatures from Passive and Active Microwave TRAnsfer (PAMTRA). We contrast cloud-precipitation statistics, and microphysical process rates between simulations performed with one-moment (1M) and two-moment (2M) microphysics schemes. We further analyze their sensitivity to primary and secondary ice-phase processes (Hallett–Mossop and collisional breakup). Both processes have previously been shown to improve the ice properties of simulated shallow mixed-phase clouds over the SO in other models. We find that only simulations with continuous formation, growth, and subsequent melting of graupel, and the effective riming of in-cloud rain by graupel, capture the observed cloud-precipitation vertical structure. In particular, the 2M microphysics scheme requires additional tuning for graupel processes in SO stratocumuli. Lowering the assumed graupel density and terminal velocity, in combination with secondary ice processes, enhances graupel formation in 2M microphysics ICON simulations. Overall, all simulations capture the observed intermittency of precipitation irrespective of the microphysics scheme used, and most of them sparsely distribute intense precipitation (>1mm h-1 ) events. Furthermore, the simulated clouds are too reflective as they are optically thick and/or have high cloud cover.

Thermal coral bleaching events (CBEs) over the Pacific, including those over the Great Barrier Reef (GBR), have commonly been linked to the El Niño–Southern Oscillation (ENSO), with bleaching reported to be a direct result of sea surface temperature (SST) anomalies driven by El Niño. However, such a relationship cannot explain CBEs that occurred during La Niña or the neutral phase of the ENSO. Here we show that the GBR is characterized by a significant negative correlation between total cloud cover anomaly (TCCA) and lagged SST anomaly (SSTA) whose magnitude and spatial extent are greater than the SSTA-ENSO correlation. This significant negative TCCA-SSTA (lagged) correlation prevails over two-thirds of the study domain even after the ENSO signal is removed, which suggests that local-scale reduced cloud cover is a key component of the regional warm shallow water formation over the GBR and the occurrence of thermal CBEs.

The persistent Southern Ocean (SO) shortwave radiation biases in climate models and reanalyses have been associated with the poor representation of clouds, precipitation, aerosols, the atmospheric boundary layer, and their intrinsic interactions. Capitalizing on shipborne observations collected during the Clouds Aerosols Precipitation Radiation and atmospheric Composition Over the Southern Ocean (CAPRICORN) 2016 and 2018 field campaigns, this research investigates and characterizes cloud and precipitation processes from synoptic to micro scales. Distinct cloud and precipitation regimes are found to correspond to the seven thermodynamic clusters established using a K-means clustering technique, while less distinctions are evident using the cyclone and (cold) front compositing methods. Cloud radar and disdrometer data reveal that light precipitation is common over the SO with higher intensities associated with cyclonic and warm frontal regions. While multiple microphysical processes and properties are present in several cloud regimes, ice aggregation appears to be dominant in deep precipitating clouds. Mixed phase, and in some cases, riming was detected in shallow convective clouds away from the frontal conditions. Two unique clusters with contrasting cloud and precipitation properties are observed over the high-latitude SO and coastal Antarctica, suggesting distinct physical processes therein. Through a single case study, in-situ and remote-sensing data collected by an overflight of the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) were also evaluated and complement the ship-based analysis.

In this work, the characteristics of the diurnal cycle around Sumatra are examined with unprecedented detail using high-resolution satellite-derived cloud properties from the Himawari-8 Advanced Himawari Imager (AHI) data. Offshore propagation is resolved into multiple local propagation directions of cloud cells, and the interaction between propagations is shown to result in forced convection over the Strait of Malacca, Java Sea, and the Indian Ocean. The diurnal cycle of rainfall and deep convection over Sumatra show complex interactions between the land-sea-breeze system, the seasonal background wind, the local topography, and the influence of surrounding islands. We used high-resolution satellite-derived products from Himawari-8 AHI, the Geostationary Cloud Algorithm Testbed Geocat, and Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) to investigate the cloud properties of deep convection and the signatures of the diurnal cycle of rainfall and cloudiness over Sumatra. Previous studies have shown evidence of the variability of diurnally forced convection in the Maritime Continent, including the diurnal signal over land in the late afternoon and the offshore propagation of Sumatra at night (Yang and Slingo 2001). The role of gravity waves has explained the night-time propagation (e.g., Mapes 2003; Love et al. 2011; Vincent and Lane 2016; Sakaeda et al. 2020). This propagation can be modified by the influence of small islands and the interconnection of diurnal cycles between Sumatra, Malay Peninsula, Java, and Borneo (Ruppert and Zhang 2019; Ruppert and Chen 2020). In this work, we present evidence of the cloudiness and rainfall patterns propagating offshore/onshore Sumatra during five austral summers from 2016 to 2020, employing cloud properties from Himawari-8 and IMERG collections. By combining detailed satellite-based cloud properties and rainfall estimates, we highlight the strong dependency of the diurnal cycle on local modulators.

In-situ observations made over twenty flights during three Austral winters (Jun–Oct, 2013–15) were analyzed to characterize the cloud microphysical properties and natural variability of mid-latitude shallow convective clouds over the Southern Ocean (SO), with a focus on pristine conditions and the mixed-phase temperature range (MPTR, 0 to -31ºC). Liquid, mixed-phase, and ice cloud fractions were observed 39%, 44%, and 17% of the time, respectively, under various meteorological settings. Liquid phase clouds were typically characterized by low droplet number concentrations and the common presence of drizzle. Supercooled liquid water was prevalent in the MPTR, while freezing of supercooled raindrops likely formed the primary ice nucleation mechanism in these shallow clouds. Ice particles of various habits were present in the mature/maturing convective cloud cells, suggesting the operation of multiple particle growth regimes. Increased ice particle concentrations (exceeding 100 L-1), well in excess of the expected ice nuclei concentrations, were measured at temperature warmer than approximately -12℃, signaling the operation of secondary ice production mechanisms. However, these cloud segments were spatiotemporally inhomogeneous, suggesting the chaotic and turbulent nature of the secondary ice-forming processes. Accurately representing these processes in global models, while necessary, is likely a challenge. Our analysis also found marked inconsistencies between several satellite-based cloud phase products that have underpinned recent developments of model parameterization frameworks. Understanding and addressing these inconsistencies are critical towards improving the representation of SO clouds and their radiative properties in climate models.