Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and ...radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.
Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One ...likely reason is that this region has few continental and anthropogenic sources of cloud‐nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur‐based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea‐spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region's expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean‐cloud‐climate feedbacks in the region.
Plain Language Summary
Clouds over the Southern Ocean tend to have less droplets and ice crystals than similar clouds over northern oceans due to fewer sources of cloud‐nucleating aerosol particles in the region. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds. These measurements indicate that while sea‐spray‐derived salts do provide cloud nuclei, the majority of aerosol particles that influence summertime clouds in this region are biogenic—that is, derived from ocean microorganisms, with the ocean region near Antarctica being a large summertime source. These cloud‐nucleating particles may increase in a warming climate likely to have less sea ice and more phytoplankton activity near Antarctica. These additional particles could make low clouds reflect more light and offset a portion of the warming expected due to diminishing sea ice in a future climate.
Key Points
Biogenic sulfate dominates the number concentration of 0.1–0.5 microns diameter particles and cloud condensation nuclei (CCN) over the summertime Southern Ocean
Biogenic organics are a key component of ice nucleating particles over the Southern Ocean
As Antarctic climate changes, increased biological activity could partially offset warming effects of sea‐ice loss via influences on CCN
THE O₂/N₂ RATIO AND CO₂ AIRBORNE SOUTHERN OCEAN STUDY Stephens, Britton B.; Long, Matthew C.; Keeling, Ralph F. ...
Bulletin of the American Meteorological Society,
02/2018, Letnik:
99, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The Southern Ocean plays a critical role in the global climate system by mediating atmosphere–ocean partitioning of heat and carbon dioxide. However, Earth system models are demonstrably deficient in ...the Southern Ocean, leading to large uncertainties in future air–sea CO₂ flux projections under climate warming and incomplete interpretations of natural variability on interannual to geologic time scales. Here, we describe a recent aircraft observational campaign, the O₂/N₂ Ratio and CO₂ Airborne Southern Ocean (ORCAS) study, which collected measurements over the Southern Ocean during January and February 2016. The primary research objective of the ORCAS campaign was to improve observational constraints on the seasonal exchange of atmospheric carbon dioxide and oxygen with the Southern Ocean. The campaign also included measurements of anthropogenic and marine biogenic reactive gases; high-resolution, hyperspectral ocean color imaging of the ocean surface; and microphysical data relevant for understanding and modeling cloud processes. In each of these components of the ORCAS project, the campaign has significantly expanded the amount of observational data available for this remote region. Ongoing research based on these observations will contribute to advancing our understanding of this climatically important system across a range of topics including carbon cycling, atmospheric chemistry and transport, and cloud physics. This article presents an overview of the scientific and methodological aspects of the ORCAS project and highlights early findings.
Small cumulus clouds over the western United States were measured via airborne instruments during the wildfire season in summer of 2018. Statistics of the sampled clouds are presented and compared to ...smoke aerosol properties. Cloud droplet concentrations were enhanced in regions impacted by biomass burning smoke, at times exceeding 3,000 cm−3. Images and elemental composition of individual smoke particles and cloud droplet residuals are presented and show that most are dominantly organic, internally mixed with some inorganic elements. Despite their high organic content and relatively low hygroscopicity, on average about half of smoke aerosol particles >80 nm diameter formed cloud droplets. This reduced cloud droplet size in small, smoke‐impacted clouds. A number of complex and competing climatic impacts may result from wide‐spread reductions in cloud droplet size due to wildfires prevalent across the region during summer months.
Plain Language Summary
Wildfires over the western United States produce large quantities of smoke during the summer months. The smoke includes airborne particles that can act as nuclei for forming individual droplets in clouds. Particles and clouds in the region were sampled with a research aircraft to measure the properties of smoke particles and how they influenced the properties of small cumulus clouds. Clouds were strongly influenced by smoke across the western U.S. On average, sampled clouds had about 5x as many droplets, and droplets were about 1/2 the size, as in clouds not influenced by smoke. Because of their small droplet sizes, these smoky clouds are expected to reflect more light and produce less rain than clouds in clean air. Other complex effects are possible due to warming impacts of the smoke itself, and due to other potential impacts of smoke aerosols on larger, deeper clouds.
Key Points
Western wildfires produce organic particles that readily act as cloud condensation nuclei due to their large size and partial hygroscopicity
Wildfire smoke strongly impacts the microphysics of small cumulus clouds, which have high droplet concentrations and small droplet sizes
Diverse impacts on radiative forcing and precipitation are possible over the western U.S. and downwind due to wildfire smoke
This work addresses the need for improved measurements of condensed water in clouds from research aircraft. First, new spectral line fitting codes are shown to improve retrievals of (H216O) in ...evaporated clouds by traditional near-infrared tunable diode laser spectroscopy. These codes are then applied to measurements of cloud water contents (CWC) with the University of Colorado Closed-Path Laser Hygrometer (CLH-2) during a series of flights in 2018 into clouds of varying types over the Southern Ocean. By comparing with observations of CWCs from cloud probes ranging from hot wires to droplet imagers, it is found that measurements of CWC using bulk water are generally accurate, but they can also be adversely affected by inlet artifacts such as icing, especially in the presence of mixed phases of water. Because the same can be true of other methods, there is currently no one instrument that accurately measures CWC under all conditions.Second, using knowledge gained from improvements in optics and electronics for CWC measurements, alongside improved retrievals of (H216O) abundances using sophisticated line-fitting methods developed for those CWC measurements, it is demonstrated through a series of laboratory studies that traditional hygrometer measurements of multiple isotopologues of water may be possible on research aircraft with distributed feedback lasers operating in the near-infrared (1.4 μm and 2.6 μm). It is proposed to conduct these measurements as the ratio of absorbances of pairs of adjacent absorption lines of two isotopologues (e.g., (H218O) and (H216O)) to characterize fractionation that can be used to elucidate important issues related to properties of clouds, such as liquid and ice-water contents and precipitation, and the microphysics of cloud particle formation. Through such measurements it should be possible to reduce outstanding uncertainties in cloud processes and the hydrological cycle, such as precipitation efficiency related to the aerosol indirect effect, mixed-phase cloud microphysics, and atmospheric transport and mixing in the vicinity of clouds. In addition, augmenting existing TDLAS instruments with the capability to detect isotopologue pairs will allow for characterization of inlet artifacts, such as condensation on sample lines and icing on inlet surfaces.
Abstract
Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even to temperatures below −20°C, and comprise a large fraction of the marine boundary layer (MBL) clouds. Earth system ...models and reanalysis products have struggled to reproduce the observed cloud phase distribution and occurrence of cloud ice in the region. Recent simulations found the microphysical representation of ice nucleation and growth has a large impact on these properties, however, measurements of SO ice nucleating particles (INPs) to validate simulations are sparse. This study presents measurements of INPs from simultaneous aircraft and ship campaigns conducted over the SO in austral summer 2018, which include the first in situ observations in and above cloud in the region. Our results confirm recent observations that INP concentrations are uniformly lower than measurements made in the late 1960s. While INP concentrations below and above cloud are similar, higher ice nucleation efficiency above cloud supports model simulations that the dominant INP composition varies with height. Model parameterizations based solely on aerosol properties capture the mean relationship between INP concentration and temperature but not the observed variability, which is likely related to the only modest correlations observed between INPs and environmental or aerosol metrics. Including wind speed in addition to activation temperature in a marine INP parameterization reduces bias but does not explain the large range of observed INP concentrations. Direct and indirect inference of marine INP size suggests MBL INPs, at least during Austral summer, are dominated by particles with diameters smaller than 500 nm.
The bulk microphysical properties and number distribution functions (N(D)) of supercooled liquid water (SLW) and ice inside and between ubiquitous generating cells (GCs) observed over the Southern ...Ocean (SO) during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) measured by in situ cloud probes onboard the NCAR/NSF G‐V aircraft are compared. SLW was detected inside all GCs with an average liquid water content of 0.31 ± 0.19 g m−3, 11% larger than values between GCs. The N(D) of droplets (maximum dimension D < 50 μm) inside and between GCs had only slight differences. For ice particles, on the other hand, the mean concentration (median mass diameter) with D > 200 μm inside GCs was 2.0 ± 3.3 L−1 (323 ± 263 μm), 65% (37%) larger than values outside GCs. As D increases, the percentage differences became larger (up to ~500%). The more and larger ice particles inside GCs suggest the GC updrafts provide a favorable environment for particle growth by deposition and riming and that mixing processes are less efficient at redistributing larger particles. The horizontal scale of observed GCs ranged from 200 to 600 m with a mean of 395 ± 162 m, smaller than GC widths observed in previous studies. This study expands knowledge of the microphysical properties and processes acting in GCs over a wider range of conditions than previously available.
Key Points
Supercooled liquid water was detected in all generating cells (GCs) characterized by small horizontal widths over the Southern Ocean
The liquid water content and number concentration of droplets inside GCs were slightly larger than values between the GCs
The ice particle sizes, number concentrations, and dispersions inside GCs were larger than those between GCs
Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even to temperatures below −20°C, and comprise a large fraction of the marine boundary layer (MBL) clouds. Earth system models ...and reanalysis products have struggled to reproduce the observed cloud phase distribution and occurrence of cloud ice in the region. Recent simulations found the microphysical representation of ice nucleation and growth has a large impact on these properties, however, measurements of SO ice nucleating particles (INPs) to validate simulations are sparse. This study presents measurements of INPs from simultaneous aircraft and ship campaigns conducted over the SO in austral summer 2018, which include the first in situ observations in and above cloud in the region. Our results confirm recent observations that INP concentrations are uniformly lower than measurements made in the late 1960s. While INP concentrations below and above cloud are similar, higher ice nucleation efficiency above cloud supports model simulations that the dominant INP composition varies with height. Model parameterizations based solely on aerosol properties capture the mean relationship between INP concentration and temperature but not the observed variability, which is likely related to the only modest correlations observed between INPs and environmental or aerosol metrics. Including wind speed in addition to activation temperature in a marine INP parameterization reduces bias but does not explain the large range of observed INP concentrations. Direct and indirect inference of marine INP size suggests MBL INPs, at least during Austral summer, are dominated by particles with diameters smaller than 500 nm.
Plain Language Summary
Although Antarctica is remote, the continent and the Southern Ocean (SO) that surrounds it play a fundamental role in shaping regional and global climate. The clouds in this region are unique, with less ice and more liquid water present at low temperatures than in other areas. This is likely related to very low concentrations of rare aerosol particles called ice nucleating particles (INPs), which cause liquid water droplets in clouds to freeze. Largely due to a lack of observations, SO clouds are poorly represented in global models, and the interactions between aerosol particles and clouds are one of the largest remaining uncertainties. This study presents results of INP measurements from several recent field campaigns over the SO, including the first observations within and above clouds in the region. Our results suggest different types of particles are present below and above clouds, which have varying ability to nucleate ice. They also highlight the need for additional measurements of INP composition and size, which are key variables needed to improve model simulations.
Key Points
First vertically resolved measurements of ice nucleating particles (INPs) over the Southern Ocean, including in‐cloud observations
Correlation between normalized INP concentrations and wind speed suggests marine active site density is variable
Higher ice nucleation efficiency observed above cloud, consistent with an increasing influence of mineral dust with height
Abstract
Southern Ocean (SO) low‐level mixed phase clouds have been a long‐standing challenge for Earth system models to accurately represent. While improvements to the Community Earth System Model ...version 2 (CESM2) resulted in increased supercooled liquid in SO clouds and improved model radiative biases, simulated SO clouds in CESM2 now contain too little ice. Previous observational studies have indicated that marine particles are major contributor to SO low‐level cloud heterogeneous ice nucleation, a process that initiates a number of cloud processes that govern cloud radiative properties. In this study, we utilize detailed aerosol and ice nucleating particle (INP) measurements from two recent measurement campaigns to assess simulated aerosol abundance, number size distributions, and composition and INP parameterizations for use in CESM2. Our results indicate that CESM2 has a positive bias in simulated surface‐level total aerosol surface area at latitudes north of 58°S. Measured INP populations were dominated by marine INPs and we present evidence of refractory INPs present over the SO assumed here to be mineral dust INPs. Results highlight a critical need to assess simulated mineral dust number and size distributions in CESM2 in order to adequately represent SO INP populations and their response to long‐term changes in atmospheric transport patterns and land use change. We also discuss important cautions and limitations in applying a commonly used mineral dust INP parameterization to remote regions like the pristine SO.
Southern Ocean (SO) low‐level mixed phase clouds have been a long‐standing challenge for Earth system models to accurately represent. While improvements to the Community Earth System Model version 2 ...(CESM2) resulted in increased supercooled liquid in SO clouds and improved model radiative biases, simulated SO clouds in CESM2 now contain too little ice. Previous observational studies have indicated that marine particles are major contributor to SO low‐level cloud heterogeneous ice nucleation, a process that initiates a number of cloud processes that govern cloud radiative properties. In this study, we utilize detailed aerosol and ice nucleating particle (INP) measurements from two recent measurement campaigns to assess simulated aerosol abundance, number size distributions, and composition and INP parameterizations for use in CESM2. Our results indicate that CESM2 has a positive bias in simulated surface‐level total aerosol surface area at latitudes north of 58°S. Measured INP populations were dominated by marine INPs and we present evidence of refractory INPs present over the SO assumed here to be mineral dust INPs. Results highlight a critical need to assess simulated mineral dust number and size distributions in CESM2 in order to adequately represent SO INP populations and their response to long‐term changes in atmospheric transport patterns and land use change. We also discuss important cautions and limitations in applying a commonly used mineral dust INP parameterization to remote regions like the pristine SO.
Plain Language Summary
Clouds over the Southern Ocean play an important role in our climate by reflecting significant amounts of solar radiation that would otherwise be absorbed by the ocean. Earth system models used to simulate climate struggle to accurately represent Southern Ocean clouds, largely because there have been limited observations to evaluate and improve models. One specific process that may be important for modeling Southern Ocean clouds is ice nucleation, where ice nucleation active particles serve as “seeds” for ice formation in clouds. In this study, we use measurements from two recent field campaigns to test a state‐of‐the‐art Earth system model's representation of atmospheric particles. We also test three different methods for representing the concentrations of available ice nucleating particles. The results from this work highlight a need for increased knowledge of the quantities, sizes and altitudes of mineral dust particles transported from distant land sources to the Southern Ocean and also emphasizes that Earth system models need to include ice nucleation from marine particles in order to accurately represent aerosol‐cloud‐climate interactions in these remote regions.
Key Points
Marine and mineral dust aerosol contributed to ice nucleating particle populations measured from ship and aircraft over the Southern Ocean
The model predicts observed latitudinal variability in aerosol surface area concentrations at high southern latitudes
Model‐predicted mineral dust ice nucleating particle number concentrations vary by 4 orders of magnitude over the Southern Ocean
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