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A persistent shortwave radiative bias of Southern Ocean (SO) clouds in climate models is strongly associated with incorrect cloud phase representation, which impacts precipitation. Measurements characterizing precipitation in low‐level mixed‐phase clouds, which frequently form over the SO, are rare, and our understanding of precipitation efficacy within these clouds remains limited. The simulated surface precipitation bias has an indirect effect on determining global climate sensitivity and a direct impact on the hydrological cycle. This study investigates the representation of low clouds, cloud variability, and precipitation statistics over the SO in real‐case Icosahedral Nonhydrostatic (ICON) simulations at the kilometer scale. The simulations are contrasted with 48 hr of continuous shipborne observations of open and closed‐cell stratocumuli, south of Tasmania. Our simulations show the significance of heavily rimed particle formation, their in‐cloud growth, and subcloud melting to capture the observed cloud‐precipitation vertical structure. In addition, supercooled drizzle formation impacts the vertical structure and precipitation statistics. ICON captures the observed intermittency of precipitation even at a standard vertical resolution of 200 m in the boundary layer but only captures the observed sparse distribution of intense precipitation (>1 mm hr−1) when the maximum vertical resolution is reduced to 100 m. However, the simulations of the 2‐day accumulated precipitation and the radiative effect are largely insensitive to the vertical resolution. The cloud reflectivity of the broken cloud deck is underestimated due to negative biases in cloud optical depth. Stratocumulus (Sc) clouds cover a large portion of the Southern Ocean (SO), where they substantially cool the ocean surface. Our understanding of the complex physics of these clouds, which include both liquid and ice remains incomplete. Accordingly, the representation of these clouds in global climate and weather models remains biased. In particular, their timing, frequency of occurrence, cloud phase and distribution, cloud cover, and precipitation characteristics are still associated with uncertainties. This results in biases in the energy balance over the SO and global equilibrium climate sensitivity. We use measurements from the Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRn oceaN (CAPRICORN) voyage south of Tasmania, to evaluate the representation of broken cloud fields, the dominant ice processes, and the precipitation characteristics in high‐resolution numerical simulations. Our results suggest that, in addition to capturing the observed discrete cloud events, graupel‐like particle formation, its growth in the cloud layer, and subsequent melting in the subcloud layer are critical processes in accurately representing the SO broken low cloud fields and precipitation characteristics during CAPRICORN. Additionally, compared to observations, the simulated clouds are too reflective. In‐cloud heavy riming, subcloud melting, and supercooled processes are vital to capture observed SO low‐cloud‐precipitation structureA finer vertical grid spacing of 100 m or less is needed to capture the frequency of observed intense surface precipitation (>1 mm hr−1) eventsWhile the simulated precipitation statistics agree with observations, the clouds are associated with a negative radiative bias In‐cloud heavy riming, subcloud melting, and supercooled processes are vital to capture observed SO low‐cloud‐precipitation structure A finer vertical grid spacing of 100 m or less is needed to capture the frequency of observed intense surface precipitation (>1 mm hr−1) events While the simulated precipitation statistics agree with observations, the clouds are associated with a negative radiative bias

The cloud properties and governing processes in Southern Ocean marine boundary layer clouds have emerged as a central issue in understanding the Earth's climate sensitivity. While our understanding of Southern Ocean cloud feedbacks have evolved in the most recent climate model intercomparison, the background properties of simulated summertime clouds in the Southern Ocean are not consistent with measurements due to known biases in simulating cloud condensation nuclei concentrations. This paper presents several case studies collected during the Capricorn 2 and Marcus campaigns held aboard Australian research vessels in the Austral Summer of 2018. Combining the surface‐observed cases with MODIS data along forward and backward air mass trajectories, we demonstrate the evolution of cloud properties with time. These cases are consistent with multi‐year statistics showing that long trajectories of air masses over the Antarctic ice sheet are critical to creating high droplet number clouds in the high latitude summer Southern Ocean. We speculate that secondary aerosol production via the oxidation of biogenically derived aerosol precursor gasses over the high actinic flux region of the high latitude ice sheets is fundamental to maintaining relatively high droplet numbers in Southern Ocean clouds during Summer. The amount of warming the Earth will experience because of increasing carbon dioxide levels in the atmosphere is sensitive to the properties of clouds that occur over the Southern Ocean. The atmosphere over the circumpolar Southern Ocean is poorly understood and presents significant challenges to climate models. Here we document the properties of the ubiquitous Southern Ocean low‐level clouds that exert a strong influence on the albedo of this region. We find that high cloud droplet number concentrations are associated with air masses that have taken paths over the high‐altitude ice sheets. The chemistry of the aerosol on which the cloud droplets form suggests that aerosols that have recently condensed from gasses emitted by phytoplankton in the highly productive waters near Antarctica are an important component of the cloud properties that must be correctly simulated in models. High cloud droplet number concentrations are associated with air masses that have recently passed over continental AntarcticaCloud droplet number concentrations decrease with time as clouds evolve in over‐water trajectories due to scavenging by precipitationKatabatic flows bring high concentrations of cloud condensation nuclei to the marine boundary layer where they influence cloud properties High cloud droplet number concentrations are associated with air masses that have recently passed over continental Antarctica Cloud droplet number concentrations decrease with time as clouds evolve in over‐water trajectories due to scavenging by precipitation Katabatic flows bring high concentrations of cloud condensation nuclei to the marine boundary layer where they influence cloud properties

In this study, we develop statistical relationships between radar observables and drop size distribution properties in different latitude bands to inform radar rainfall retrieval techniques and understand underpinning microphysical reasons for differences reported in the literature between satellite mean zonal rainfall products at high latitudes (up to a factor 2 between products over ocean). A major assumption in satellite retrievals is the attenuation‐reflectivity relationships for convective and stratiform precipitation. They are found to systematically produce higher attenuation than our relationships with all latitudes included or within individual latitude bands (except in the tropics). The scatter around fitted curves approximating the radar reflectivity‐mass‐weighted diameter Dmrelationship and the dual‐frequency ratio (ratio of Ka‐ to Ku‐band reflectivities)‐Dmrelationships is found to be large and of the same magnitude. This result suggests that the added value of two radar frequencies to improve the Dmretrieval from space seems limited. In contrast, the relationship between Dmand the attenuation/reflectivity ratio is robust and not dependent on latitude. Direct relationships between rainfall and either reflectivity or attenuation are also found to be very robust. Attenuation‐reflectivity, Dm‐reflectivity, and rainfall rate‐reflectivity relationships in the Southern Hemisphere high latitude and Northern Hemisphere polar latitude bands are fundamentally different from those at other latitude bands, producing smaller attenuation, much larger Dm, and lower rainfall rates. This implies that specific relationships need to be used for these latitude bands in radar rainfall retrieval techniques using such relationships. The relationship between radar observables and rainfall properties is different at high latitudesThere seems to be limited value to the dual‐frequency GPM radar measurements for rainfall retrievalThese results should help improve GPM radar rainfall retrievals

In this study, we analyze an in situ shipboard global ocean drop size distribution (DSD) 8‐year database to understand the underpinning microphysical reasons for discrepancies between satellite oceanic rainfall products at high latitudes reported in the literature. The natural, latitudinal, and convective‐stratiform variability of the DSD is found to be large, with a substantially lower drop concentration with diameter smaller than 3 mm in the Southern hemisphere high latitude (S‐highlat, south of 45°S) and Northern Hemisphere polar latitude (N‐polar, north of 67.5°S) bands, which is where satellite rainfall products most disagree. In contrast, the latitudinal variability of the normalizedoceanic DSD is small, implying that the functional form of the normalized DSD can be assumed constant and accurately parameterized using proposed fits. The S‐highlat and N‐polar latitude bands stand out as regions with oceanic rainfall properties different from other latitudes, highlighting fundamental differences in rainfall processes at different latitudes and associated specific challenges for satellite rainfall retrieval techniques. The most salient differences in DSD properties between these two regions and the other latitude bands are: (1) a systematically higher (lower) frequency of occurrence of rainfall rates below (above) 1 mm h‐1, (2) much lower drop concentrations, (3) very different values of the DSD shape parameter (μ0) from what is currently assumed in satellite radar rainfall algorithms, and (4) very different DSD properties in both the convective and stratiform rainfall regimes. Overall, this study provides insights into how DSD assumptions in satellite radar rainfall retrieval techniques could be refined. The latitudinal variability of statistical oceanic rainfall properties is characterized for the first timeResults should inform improvements to the GPM radar rainfall retrievals

The Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRn oceaN (CAPRICORN) experiment was carried out in March–April 2016 onboard R/V Investigatorstudying momentum (τ), sensible heat (Hs), and latent heat (Hl) fluxes over the Australian sector of the Southern Ocean including over one cyclonic cold‐core and one anticyclonic warm‐core mesoscale oceanic eddy. The turbulence‐based flux measurements obtained with the NOAA PSD flux system employing eddy covariance (EC) and inertial dissipation (ID) methods are compared with those obtained by the Coupled Ocean‐Atmosphere Response Experiment (COARE) 3.5 bulk model, and the neutral transfer coefficients are studied. The relative uncertainty between the turbulence‐based and COARE 3.5 estimates of τ, Hs, and Hlare 22%, 70%, and 26%, respectively, at 1‐hr time scale over the Southern Ocean. Further, the variability in bulk fluxes is investigated with respect to oceanic eddies, precipitation events, atmospheric stability, and extratropical cyclones encountered during the voyage. The main observed variability is an increase in significant wave height or γw(∼33%), τ(∼89%), Hs(∼187%), and Hl(∼79%) over the warm eddy as compared to average voyage values. During the passage of six extratropical cyclones, an increase in τ(∼62% average) and a decrease in Hs(∼235%) and Hl(∼79%) is noted in the warm sector, compared to prestorm conditions, but the pattern reverses behind the cold front. COARE 3.5 bulk model gives lower mean estimates of wind stress and heat fluxes in the Southern Ocean when compared with observationsThe 10‐m‐neutral transfer coefficients of momentum and heat fluxes increase with wind speed >5 m/sHeat fluxes decrease in the warm sector of extratropical cyclones, but increase behind the cold front

Precipitation efficiency refers to the fraction of condensate in the atmosphere that reaches the surface as precipitation. A high‐quality data set of radar‐estimated precipitation rates and convective scale vertical velocity near Darwin, Australia, is used to construct the first estimate of precipitation efficiency at convective scales for a long record of observations in the tropics. It is found that precipitation efficiency increases with precipitation rate and midtropospheric humidity and decreases with increasing convective available potential energy and surface temperature. Precipitation efficiency is largest under moist monsoonal conditions and smallest during monsoon break periods, which are characterized by a drier free troposphere. However, these differences in efficiency do not translate to differences in the instantaneous precipitation rate across the synoptic regimes because of a compensating change in the net condensation rate. This is driven by variations in cloud updraft velocity, which is larger in drier environments than in moist environments. First long‐term observational estimate of precipitation efficiency at convective scales for a location in the tropics is presentedPrecipitation efficiency increases with environmental humidity and decreases weakly with instability (CAPE)Compensation between precipitation efficiency and updraft velocity results in weak dependence of precipitation intensity on synoptic regime

Accurately representing the properties and impact of tropical convection in climate models requires an understanding of the relationships between the state of a convective cloud ensemble and the environment it is embedded in. We investigate this relationship using 13 years of radar observations in the tropics. Specifically, we focus on convective cell number and size and quantify their relationship to atmospheric stability, midtropospheric vertical motion and humidity. We find several key convective states embedded in their own unique environments. The most area‐averaged rainfall occurs with a moderate number of moderate size convective cell in an environment of high humidity, strong vertical ascent, and moderate convective available potential energy (CAPE) and convective inhibition (CIN). The strongest rainfall intensities are found with few large cells. Those exist in a dry and subsiding environment with both high CAPE and CIN. Large numbers of convective cells are associated with small CAPE and CIN, weak ascent, and a moist midtroposphere. The relationship between a convective cloud ensemble and its environment is studied to improve the representation of tropical convection in modelsThe strongest rainfall intensities are found with few large cells those exist in a dry and subsiding environment with both high CAPE and CINThe convective cell number and size are quantified and their relationship to atmospheric stability, vertical motion, and humidity is studied

Inaccurate mixed‐phase cloud parameterizations over the Southern Ocean remain one of the largest sources of disagreement among global models in determining shortwave cloud radiative feedbacks. Suitable global observations supporting model improvements are currently unavailable. The conventional satellite cloud phase retrieval from passive radiometers is strongly biased toward cloud top without further information on the subcloud phase. Mixed‐phase clouds with the liquid‐top mixed‐phase (LTMP) structures are often classified simply as supercooled liquid. This paper presents a daytime multispectral detection algorithm for LTMP clouds, based on differential absorption between liquid and ice in shortwave infrared bands (1.61 and 2.25 μm). The LTMP algorithm, previously developed for polar‐orbiting sensors, is applied to Himawari‐8 Advanced Himawari Imager (the first of the next‐generation geostationary satellites) to probe subcloud phase for mixed‐phase clouds over the Southern Ocean. The results are compared with spaceborne active sensor data from CloudSat and CALIPSO. Ship‐based field experiment measurements are examined for selected cases to provide a more direct assessment of algorithm performance. The results show that applying the LTMP algorithm to geostationary satellites has potential to provide advanced time‐resolved observations for mixed‐phase clouds globally with improved sublayer cloud phase information that can support enhancement and validation of global models. A multispectral algorithm for daytime liquid‐top mixed‐phase cloud characterization is applied to Himawari‐8 AHISubcloud‐top phase probing results over the Southern Ocean are compared with spaceborne and shipborne observationsGlobal time‐resolved observations for mixed‐phase clouds with the advanced phase information are enabled from geostationary satellites

The vertical structure of clouds unresolved in large‐scale weather prediction and climate models is controlled by an overlap assumption. When a binary representation (cloud or no cloud) of subgrid horizontal variability is replaced by a probability density function (PDF) treatment of cloud‐related variables, a cloud occurrence overlap needs to be replaced by a PDF overlap. The PDF overlap can be quantified by a correlation length scale, z0, indicating how rapidly rank correlation of distributions at two levels diminishes with increasing level separation. In this study, we show that z0varies widely for different properties (e.g., number and mass mixing ratios) and different hydrometeor types (cloud liquid and ice, rain, snow, and graupel) and that corresponding fall speed, Vf, is the primary factor controlling the degree of their vertical alignment, with vertical shear of the horizontal wind playing a smaller role. Linear and power law parametric relationships between z0and Vfare derived using cloud‐resolving simulations of convection under midlatitude continental and tropical oceanic conditions, as well as observations from vertically pointing dual‐frequency radar profilers near Darwin, Australia. The functional form of z0‐Vfrelationship is further examined using simple conceptual models that link variability in horizontal and vertical directions and provide insights into the role of Vfand wind shear. Being based on a physical property (i.e., fall speed) of hydrometeors rather than artificially defined and model‐specific hydrometeor types, the proposed parameterization of vertical PDF overlap can be applied to a wide range of microphysics treatments in regional and global models. Rank correlations of cloud and precipitation distributions at adjacent levels are computed for model‐simulated and radar‐observed fieldsCorrelation length scale, or PDF overlap, is a function of mean fall speed of considered propertyWind shear reduces alignment of slow falling species, but has limited effect on faster precipitating hydrometeor categories

Shallow cloud decks residing in or near the boundary layer cover a large fraction of the Southern Ocean (SO) and play a major role in determining the amount of shortwave radiation reflected back to space from this region. In this article, we examine the macrophysical characteristics and thermodynamic phase of low clouds (tops <3 km) and precipitation using ground‐based ceilometer, depolarization lidar and vertically‐pointing W‐band radar measurements collected during the Macquarie Island Cloud and Radiation Experiment (MICRE) from April 2016 to March 2017. During MICRE, low clouds occurred ∼65% of the time on average (slightly more often in austral winter than summer). About 2/3 of low clouds were cold‐topped (temperatures ≤0°C). These were thicker and had higher bases on average than warm‐topped clouds. 83%–88% of cold‐topped low clouds were liquid phase at cloud base (depending on the season). The majority of low clouds had precipitation in the vertical range 150–250 m below cloud base, a significant fraction of which did not reach the surface. Phase characterization is limited to the period between April 2016 and November 2016. Small‐particle (low‐radar‐reflectivity) precipitation (which dominates precipitation occurrence) was mostly liquid below‐cloud, while large‐particle precipitation (which dominates total accumulation) was predominantly mixed/ambiguous or ice phase. Approximately 40% of cold‐topped clouds had mixed/ambiguous or ice phase precipitation below (with predominantly liquid phase cloud droplets at cloud base). Below‐cloud precipitation with radar reflectivity factors below about −10 dBZ were predominantly liquid, while reflectivity factors above about 0 dBZ were predominantly ice. The Southern Ocean is covered by low altitude cloud decks the majority of the time. Properties like cloud occurrence frequency, particle phase and precipitation habits determine how much solar radiation clouds reflect and how much infrared radiation they emit, which in turn affects the balance of the planet's incoming and outgoing radiation. In this paper, we examine low cloud properties observed from the ground at Macquarie Island, including how frequently they occur and at what temperatures. We study particle thermodynamic phase (liquid, ice or mixed) at cloud base and in precipitation below‐cloud. A majority of low clouds are predominantly composed of liquid phase droplets, although frozen precipitation is frequently found below cloud base. Low clouds form precipitation more often than not, much of which evaporates before reaching the ground. In below‐freezing low clouds, the majority of large raindrops & snowflakes that do reach the ground originate as frozen precipitation directly below cloud base. This indicates that ice formation is frequently active in clouds composed predominantly of liquid‐phase droplets. Lastly, we build upon an established radar‐lidar relationship that particles with radar reflectivity factors below −10 dBZ are generally liquid, whereas above 0 dBZ are most often ice phase. Ground observations at Macquarie Island indicate low clouds occur ∼65% of the time, with about 2/3 having cloud top temperatures below 0°CCold‐topped low clouds have liquid‐phase bases ∼85% of the time & precipitate ∼3/4 of the time; below‐cloud precip. is 40% ice/mixed phaseBelow‐cloud radar reflectivity factors <−10 dBZ are predominately due to liquid phase precipitation, while >0 dBZ are predominantly ice Ground observations at Macquarie Island indicate low clouds occur ∼65% of the time, with about 2/3 having cloud top temperatures below 0°C Cold‐topped low clouds have liquid‐phase bases ∼85% of the time & precipitate ∼3/4 of the time; below‐cloud precip. is 40% ice/mixed phase Below‐cloud radar reflectivity factors <−10 dBZ are predominately due to liquid phase precipitation, while >0 dBZ are predominantly ice

This study uses shipborne cloud radar and surface radiation measurements collected over the Southern Ocean to characterize the cloud frequency, cloud fraction, and cloud radiative effects on the ocean surface. These cloud and radiative properties are also used to evaluate a regional forecast model. Low‐level clouds, either alone or cooccurring with cloud layers aloft, are present ~ 77% of the time in this data set. These clouds either had a very low or a very high cloud fraction at 12 km horizontal resolution, with about half of the clouds characterized by a cloud fraction higher than 80%. Overall, shortwave surface cooling effect dominates longwave heating, with an estimate net radiative cooling of −22 W m−2, resulting from a −71 W m−2shortwave cooling and a +49 W m−2longwave heating. A strong relationship between daily surface cloud radiative effect and daily low‐level cloud fraction is found, which, if confirmed with a larger data set, could be exploited in satellite retrievals or model parameterizations for the Southern Ocean. The regional model underestimates the frequency of low‐level clouds but largely overestimates the frequency of multilayer situations. The associated radiative errors are large and complex, including reduced surface radiative cooling due to low‐level clouds compensated by enhanced surface cooling in multilayer situations. Climate model simulations exhibit large biases in the amount of radiation reaching the ocean surface of the Southern Ocean. This paper uses state‐of‐the‐art ship observations of clouds and radiation over the Southern Ocean to investigate the reasons behind such biases. Compensating errors are found in the Australian regional forecast model between low‐level cloud situations where the model tends to underestimate the ocean surface cooling by radiation and multi‐layer cloud situations where the model overestimates this ocean surface cooling. Shortwave cooling dominates longwave heating in the surface radiative balance in our Southern Ocean shipborne data setThe frequency of low‐level clouds and multilayer situations is inaccurately reproduced in our regional model ACCESSErrors in simulated cloud cover result in compensating errors on the surface cloud radiative effect in the model

This study focuses on methods to estimate dry marine aerosol surface area (SA) from bulk optical measurements. Aerosol SA is used in many models' ice nucleating particle (INP) parameterizations, as well as influencing particle light scattering, hygroscopic growth, and reactivity, but direct observations are scarce in the Southern Ocean (SO). Two campaigns jointly conducted in austral summer 2018 provided co‐located measurements of aerosol SA from particle size distributions and lidar to evaluate SA estimation methods in this region. Mie theory calculations based on measured size distributions were used to test a proposed approximation for dry aerosol SA, which relies on estimating effective scattering efficiency (Q) as a function of Ångström exponent (å). For distributions with dry å< 1, Q= 2 was found to be a good approximation within ±50%, but for distributions with dry å> 1, an assumption of Q= 3 as in some prior studies underestimates dry aerosol SA by a factor of 2 or more. We propose a new relationship between dry åand Q, which can be used for −0.2 < å< 2, and suggest å= 0.8 as the cutoff between primary and secondary marine aerosol‐dominated distributions. Application of a published methodology to retrieve dry marine aerosol SA from lidar extinction profiles overestimated aerosol SA by a factor of 3–5 during these campaigns. Using Microtops aerosol optical thickness measurements, we derive alternative lidar conversion parameters from our observations, applicable to marine aerosol over the SO. The Southern Ocean (SO) surrounding Antarctica is one of the few places where aerosol concentrations and composition are similar to pre‐industrial values. This makes data collected in this region important for improving and understanding climate model simulations. However, direct observations of aerosols are rare because of the remoteness, frequent storms, and high winds and waves common to the SO. In this study, we use some of these rare aerosol observations to test methods for estimating important aerosol quantities using other measurements that are easier to collect. The improvements presented here may increase the availability of key data for improving climate models by replacing rare measurements with ones that can be collected continuously and autonomously. Methods to estimate dry marine aerosol surface area (SA) from bulk optical measurements were tested for the Southern Ocean regionA new relationship between effective scattering efficiency and dry Ångström exponent is proposed for nephelometer measurementsOverestimation of aerosol SA from previous methods is reduced by derivation of new lidar backscatter conversion parameters Methods to estimate dry marine aerosol surface area (SA) from bulk optical measurements were tested for the Southern Ocean region A new relationship between effective scattering efficiency and dry Ångström exponent is proposed for nephelometer measurements Overestimation of aerosol SA from previous methods is reduced by derivation of new lidar backscatter conversion parameters

We report on observations of corona discharges at the uppermost region of clouds characterized by emissions in a blue band of nitrogen molecules at 337 nm, with little activity in the red band of lightning leaders at 777.4 nm. Past work suggests that they are generated in cloud tops reaching the tropopause and above. Here we explore their occurrence in two convective environments of the same storm: one is developing with clouds reaching above the tropopause, and one is collapsing with lower cloud tops. We focus on those discharges that form a distinct category with rise times below 20 μs, implying that they are at the very top of the clouds. The discharges are observed in both environments. The observations suggest that a range of storm environments may generate corona discharges and that they may be common in convective surges. Discharges in thunderstorm clouds with little or no lightning leader emissions are from cold streamers and are called corona discharges. They are thought to be generated in the upper regions of clouds that may reach into the stratosphere. Here we explore if storms with lower cloud tops may also generate these by comparing measurements of a collapsing and a developing thunderstorm environment that are part of a larger storm system. We find that the discharges are created in both environments and, therefore, may be quite common. Corona discharges at cloud tops form a distinct type of electric discharges with rise times <20 µs in 337 nm and little activity in 777.4 nmThey are generated both in developing storm cells overshooting the tropopause height and collapsing cells with remaining convective activityMost are detected by ground‐based lightning detection systems implying they are fast break‐down discharges with high currents Corona discharges at cloud tops form a distinct type of electric discharges with rise times <20 µs in 337 nm and little activity in 777.4 nm They are generated both in developing storm cells overshooting the tropopause height and collapsing cells with remaining convective activity Most are detected by ground‐based lightning detection systems implying they are fast break‐down discharges with high currents

A 1‐year blended surface precipitation data set using Parsivel disdrometer, surface W‐band radar, and tipping bucket measurements is produced for the Macquarie Island Cloud and Radiation Experiment (MICRE) and compared with retrievals from CloudSat (spaceborne 94 GHz radar). Surface precipitation was observed 44% ± 4% of the time between April 2016 and March 2017. Precipitation composed primarily of small particles (diameter <1 mm) occurred about 36% ± 2% of the time, constituting 10% of total accumulation. Remaining precipitation contained enough large particles such that the disdrometer could be used to identify the precipitation type as rain, ice, snow or wet snow. Seasonal and annual statistics on frequency of occurrence and accumulation for each precipitation type observed during MICRE are presented. Most ice and mixed phase precipitation was shallow, originating at a height of 3 km or lower, and occurred most often when Macquarie Island was to the northwest of the nearest cyclonic low‐pressure center. In contrast, rain was more often deep and occurred most frequently when the island was to the southeast of cyclonic lows. A weak diurnal cycle in frequency and mean rate was present with a minimum between 12:00 and 14:00 local time and maximum between 03:00 and 06:00 local time. The CloudSat 2C‐Precip‐Column product missed the lightest precipitation (because the near‐surface reflectivity is <−15 dBZ) and overestimated total liquid precipitation and occurrence of mixed phase precipitation, but captured reasonably well the distribution of rain rates for rates >0.5 mm/hr. Understanding the nature of precipitation over the Southern Ocean (SO) is crucial to understanding cloud properties, which climate models struggle to simulate correctly with significant impacts on global climate sensitivity (how much the Earth is likely to warm due to increases in greenhouse gases). Studies assessing the performance of climate simulations rely primarily on satellite measurements and reanalysis of precipitation in remote regions like the SO due to the lack of available surface measurements. However, there are large disagreements between and among satellite and reanalysis products regarding SO precipitation characteristics. This study examines precipitation characteristics obtained from a recent field campaign located at Macquarie Island, in the middle of the SO. The analysis presented here focuses on SO precipitation type (liquid, frozen or mixed) and accumulation, as well as seasonal and storm‐related variability. Ground observations from Macquarie Island are also compared to CloudSat satellite precipitation estimates that are a crucial data source for precipitation in the region. Macquarie Island Cloud and Radiation Experiment data are used to study seasonal, diurnal, and synoptic variation in surface precipitationRain, ice/mixed, and small particle precipitation occurred year‐round, comprising 74%, 16%, and 10% of total accumulation, respectivelyCloudSat 2C‐Precip‐Column overestimated liquid accumulation and mixed phase occurrence but underestimated light precipitation occurrence Macquarie Island Cloud and Radiation Experiment data are used to study seasonal, diurnal, and synoptic variation in surface precipitation Rain, ice/mixed, and small particle precipitation occurred year‐round, comprising 74%, 16%, and 10% of total accumulation, respectively CloudSat 2C‐Precip‐Column overestimated liquid accumulation and mixed phase occurrence but underestimated light precipitation occurrence

Intense snowfall sublimation was observed during a precipitation event over Davis in the Vestfold Hills, East Antarctica, from 08 to 10 January 2019. Radar observations and simulations from the Weather Research and Forecasting model revealed that orographic gravity waves (OGWs), generated by a north‐easterly flow impinging on the ice ridge upstream of Davis, were responsible for snowfall sublimation through a foehn effect. Despite the strong meridional moisture advection associated with an atmospheric river (AR) during this event, almost no precipitation reached the ground at Davis. We found that the direction of the synoptic flow with respect to the orography determined the intensity of OGWs over Davis, which in turn directly influenced the snowfall microphysics. We hypothesize that turbulence induced by the OGWs likely enhanced the aggregation process, as identified thanks to dual‐polarization and dual‐frequency radar observations. This study suggests that despite the intense AR, the precipitation distribution was determined by local processes tied to the orography. The mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main Antarctic oases. A case study of a snowfall event over Davis, Antarctica is presented. Despite the strong precipitation, snowfall did not reach the ground due to intense sublimation (transition from solid to gas state). Meteorological radar observations and atmospheric model simulations revealed that a dry downslope wind was responsible for the sublimation of snowfall below the cloud base. Despite the intense transport of moisture associated with the low pressure system during this event, almost no precipitation reached the ground at Davis. We found that the wind direction with respect to the main ridge upstream of Davis determined the intensity of the sublimation. This study suggests that despite favorable large‐scale conditions for intense snowfall at Davis, local processes related to the topography determined how much precipitation reached the ground. The mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main ice‐free regions of Antarctica. Despite intense moisture advection by an atmospheric river, foehn winds tied to orographic gravity waves (OGWs) led to snowfall sublimationThe flow direction determined the intensity of the foehn and hence the temporal and spatial precipitation variabilityThe event can be divided into three phases during which the features of the OGWs influenced the observed microphysics in distinct ways Despite intense moisture advection by an atmospheric river, foehn winds tied to orographic gravity waves (OGWs) led to snowfall sublimation The flow direction determined the intensity of the foehn and hence the temporal and spatial precipitation variability The event can be divided into three phases during which the features of the OGWs influenced the observed microphysics in distinct ways

This study investigates the occurrence of mixed‐phase clouds (MPC, i.e., cloud layers containing both liquid and ice water at sub‐freezing temperatures) over the Southern Ocean (SO) using space‐ and surface‐based lidar and radar observations. The occurrence of supercooled clouds is dominated by geometrically thin (<1 km) layers that rarely contain ice. We diagnose layers that are geometrically thicker than 1 km to contain ice ~65% and ~4% of the time from below by surface remote sensors and from above by orbiting remote sensors, respectively. We examine the discrepancy in MPC occurrence statistics as diagnosed from below and above the cloud layer. From above, we find that MPC occurrence has a gradient associated with the Antarctic Polar Front near 55°S with a rare occurrence of satellite‐derived MPC south of that latitude. In contrast, surface sensors find ice in 33% of supercooled liquid water layers. We infer using observing system simulation experiments and data analysis that space‐based lidar cannot identify the occurrence of MPC except when secondary ice‐forming processes operate in convection that is, sufficiently strong to loft ice crystals to cloud tops. We conclude that the CALIPSO phase statistics of MPC have a severe low bias in MPC occurrence. Based on surface‐based statistics in the SO, we present a parameterization of the frequency of MPC as a function of cloud top temperature that differs substantially from that used in recent climate model simulations. The existence of snow in predominantly liquid clouds has important implications for the amount of sunlight absorbed mostly at the sea surface over the high latitude oceans. Particularly over the Southern Ocean, where satellite measurements suggest that ice concentrations are low, knowledge of how often clouds are snowing has critical climate implications. Observations from the surface have high fidelity in identifying snow below cold clouds. We use new measurements collected from Australian research vessels to establish an accurate survey of snow occurrence. We find that the occurrence of snow below cold clouds is much higher from ship observations than inferred from satellite. We explore reasons for this discrepancy and settle on an explanation that the low concentrations of ice‐nucleating aerosol particles result in low concentrations of ice particles except where convective motions are strong enough to create ice particles spontaneously by freezing large drops. We provide a simple temperature‐based parameterization of snow occurrence using surface‐based measurements for atmospheric models to use. Lidar observations from above cloud layers significantly undercount the occurrence of mixed phase clouds over the Southern Ocean (SO)A latitudinal gradient in mixed‐phase clouds is found associated with the Antarctic Polar Front of the Antarctic Circumpolar CurrentCurrent parameterizations that modify the phase detrainment temperature of shallow convection are not supported by observations. An alternate parameterization developed from SO surface data is suggested Lidar observations from above cloud layers significantly undercount the occurrence of mixed phase clouds over the Southern Ocean (SO) A latitudinal gradient in mixed‐phase clouds is found associated with the Antarctic Polar Front of the Antarctic Circumpolar Current Current parameterizations that modify the phase detrainment temperature of shallow convection are not supported by observations. An alternate parameterization developed from SO surface data is suggested

The properties of Southern Ocean (SO) liquid phase non precipitating clouds (hereafter clouds) are examined using shipborne data collected during the Measurements of Aerosols, Radiation and Clouds over the Southern Ocean and the Clouds Aerosols Precipitation Radiation and atmospheric Composition Over the SoutheRN ocean I and II campaigns that took place south of Australia during Autumn 2016 and Summer 2017–2018. Cloud properties are derived using data from W‐band radars, lidars, and microwave radiometers using an optimal estimation algorithm. The SO clouds tended to have larger liquid water paths (LWP, 115 ± 117 g m−2), smaller effective radii (re, 8.7 ± 3 μm), and higher number concentrations (Nd, 90 ± 107 cm−3) than typical values of eastern ocean basin stratocumulus. The clouds demonstrated a tendency for the LWP to increase with Ndpresumably due to precipitation suppression up to Ndof approximately 100 cm−3when mean LWP decreased with increasing Nd. Due to higher optical depth, cloud albedos were less susceptible to changes in Ndcompared to subtropical stratocumulus. The highest latitude clouds of the datasets, observed along and near the Antarctic coast, presented a distinctly bimodal character. One mode had the properties of marine clouds further north. The other mode occurred in an aerosol environment characterized by high cloud condensation nuclei concentrations and elevated sulfate aerosol without obvious continental aerosol markers. These regions of higher cloud condensation nuclei tended to have higher Nd, smaller reand higher LWP suggesting sensitivity of cloud properties to seasonal biogenic aerosol production in the high latitude SO. The properties of liquid Southern Ocean (SO) clouds vary with latitudeSO low clouds are susceptible to aerosol concentrationsSO clouds near Antarctica have high droplet numbers compared to lower latitude SO clouds The properties of liquid Southern Ocean (SO) clouds vary with latitude SO low clouds are susceptible to aerosol concentrations SO clouds near Antarctica have high droplet numbers compared to lower latitude SO clouds

Focusing on conditions of subsidence when low clouds are present, ground‐based observations in both the North Atlantic and the Southern Ocean reveal strong relationships between cloud boundary (base and top heights) and different measures of lower tropospheric instability. The difference in potential temperature between the surface and 800 hPa (a metric called M) provides a stronger relationship than measures of inversion strength such as the lower tropospheric stability and estimated inversion strength. This is because (1) inversion strength itself does not correlate well with cloud boundaries, and (2) M contains information that appears important for cloud boundaries. These include the surface forcing through the use of sea surface rather than near‐surface air temperature and an upper level close to the real cloud top. These results expand upon previous work on the importance of M as a predictor of cloud morphology. However, important differences are found in low‐cloud conditions for the North Atlantic as compared to the Southern Ocean (for a given value of M): stronger inversions, deeper boundary layers, and much larger sea level pressures. Therefore, the relationship between cloud boundaries and M differs between the two regions. A general circulation model provides similar relationships as observed between M and both cloud top height and temperature but tends to place clouds higher and at colder temperatures than observed for a given M. This might cause issues with the representation of precipitation, cloud cover and radiation in the Southern Ocean. Marine cold air outbreak parameter M correlates better with low‐level cloud boundaries than other stability measures in midlatitudesDifferences in large‐scale conditions at Northern and Southern Hemisphere sites explain differences in sensitivities to MA general circulation model reproduces similar relationships, but sensitivities differ causing cooler clouds than observed for a given M

A radar wind profiler data set collected during the 2?year Department of Energy Atmospheric Radiation Measurement Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign is used to estimate convective cloud vertical velocity, area fraction, and mass flux profiles. Vertical velocity observations are presented using cumulative frequency histograms and weighted mean profiles to provide insights in a manner suitable for global climate model scale comparisons (spatial domains from 20?km to 60?km). Convective profile sensitivity to changes in environmental conditions and seasonal regime controls is also considered. Aggregate and ensemble average vertical velocity, convective area fraction, and mass flux profiles, as well as magnitudes and relative profile behaviors, are found consistent with previous studies. Updrafts and downdrafts increase in magnitude with height to midlevels (6 to 10?km), with updraft area also increasing with height. Updraft mass flux profiles similarly increase with height, showing a peak in magnitude near 8?km. Downdrafts are observed to be most frequent below the freezing level, with downdraft area monotonically decreasing with height. Updraft and downdraft profile behaviors are further stratified according to environmental controls. These results indicate stronger vertical velocity profile behaviors under higher convective available potential energy and lower low-level moisture conditions. Sharp contrasts in convective area fraction and mass flux profiles are most pronounced when retrievals are segregated according to Amazonian wet and dry season conditions. During this deployment, wet season regimes favored higher domain mass flux profiles, attributed to more frequent convection that offsets weaker average convective cell vertical velocities. Radar observations collected during The Department of Energy's ARM Program Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign are used to measure thunderstorm air motions, area coverage and relative impact. Although there is no one-size-fits-all solution for evaluating current Global Climate Model cloud simulation capabilities, new observations including those found in this study may provide the necessary information to improve climate models to the scales they attempt to represent. Our findings on the vertical structure of Amazon convective clouds, and the sensitivity of cloud intensity to the changes in the atmospheric conditions that trigger those clouds, complement previous thunderstorm observations. Contrasts in the observed cloud properties are most pronounced when separated according to Amazonian “wet” and “dry” season conditions. For example, while deeper clouds were more frequent during the wet season, the more intense vertical air motions were observed within dry season storms. Characterization of vertical velocity, area fraction, and mass flux profiles for Amazon convectionProfiles aligned to be statistically representative of ensemble and aggregate convective properties to GCM domain scalesConvective profiles indicate significant sensitivity to environmental forcing, wet and dry season regime controls