Abstract:Abstract.From November 2015 to December 2016, the ARM West Antarctic Radiation Experiment (AWARE) measured submicron 15 aerosol properties near McMurdo Station at the southern tip of the Ross Island. Submicron organic mass (OM), particle number, and cloud condensation nuclei concentrations were higher in summer than other seasons. The measurements included a range of compositions and concentrations that likely reflected both local anthropogenic emissions and natural background sources. We isolated the natural … Show more
“…With the absence of aerosol concentration measurements aloft, and based on the model comparison with the observations, we suggest that the activated N IFN and N A values during the drizzle event should have been on the order of 0.2 L −1 and 20 cm −3 , respectively. These values are within the typical range of McMurdo wintertime N IFN (below −20 °C) and N A from surface‐based reports (Belosi et al, ; Liu et al, ). It is important to note that these number concentrations are rough estimates, because of numerous unexplored degrees of freedom in the model configuration as well as the many uncertainties associated with the comparison of the model results with the observations.…”
The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below −20 °C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below −25 °C and the drizzle persisted for a period exceeding 7.5 hr. Using ground‐based, satellite, and reanalysis data, we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1,000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large eddy simulations constrained by the observations suggest that activated ice freezing nuclei and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L−1 and 20 cm−3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres.
“…With the absence of aerosol concentration measurements aloft, and based on the model comparison with the observations, we suggest that the activated N IFN and N A values during the drizzle event should have been on the order of 0.2 L −1 and 20 cm −3 , respectively. These values are within the typical range of McMurdo wintertime N IFN (below −20 °C) and N A from surface‐based reports (Belosi et al, ; Liu et al, ). It is important to note that these number concentrations are rough estimates, because of numerous unexplored degrees of freedom in the model configuration as well as the many uncertainties associated with the comparison of the model results with the observations.…”
The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below −20 °C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below −25 °C and the drizzle persisted for a period exceeding 7.5 hr. Using ground‐based, satellite, and reanalysis data, we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1,000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large eddy simulations constrained by the observations suggest that activated ice freezing nuclei and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L−1 and 20 cm−3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres.
“…The latitudinal and seasonal patterns of MODIS N d are supported by the multiyear records of CCN from Antarctic ground sites at King Sejong Station (62°S) ( 47 ) and McMurdo Station (77°S) ( 48 ) ( Fig. 2 and SI Appendix , Fig.…”
Section: Resultsmentioning
confidence: 61%
“…Seasonal mean sea ice contours from OSTIA fractional sea ice are shown as dashed (1%) and solid blue lines (50%). Locations are shown for McMurdo Station ( 48 ) (solid square) and King Sejong Station ( 47 ) (empty square). The position of the DJF lower tropospheric storm track ( 74 ) is shown with a gray line.…”
Section: Resultsmentioning
confidence: 99%
“…Satellite-derived N d values in the SO are evaluated using observations from a variety of campaigns and ground stations. In situ CCN observations from McMurdo Station ( 48 ) and King Sejong Station ( 47 ) are drawn from the reported monthly and seasonal mean values in the literature. MODIS N d for these regions is shown averaged across a 4° box centered at the respective stations ( SI Appendix , Fig.…”
The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm−3 and 24 cm−3. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m−2 to −0.6 W⋅m−2. The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.
“…The seasonal cycle of aerosols is different in the Antarctic region than over the Arctic, as the winter Arctic haze does not have an analog in the Southern Hemisphere, and Antarctic nuclei number concentrations are an order of magnitude larger during summer than winter (Lachlan‐Cope et al., 2020; Liu et al., 2018). An adequate amount of cloud condensation nuclei (CCN) appears to be present to produce common liquid cloud droplet concentrations of 100 cm −3 or greater (e.g., Liu et al., 2018). The persistence of the supercooled liquid at McMurdo as observed by Silber et al.…”
Supercooled water is common in the clouds near coastal Antarctica and occasionally occurs at temperatures at or below −30°C. Yet the ice physics in most regional and global numerical models will glaciate out these clouds. This presents a challenge for the simulation of highly supercooled clouds that were observed at McMurdo, Antarctica during the Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE) project during 2015–2017. The polar optimized version of the Weather Research and Forecasting model (Polar WRF) with the recently developed two‐moment P3 microphysics scheme was used to simulate observed supercooled liquid water cases during March and November 2016. Nudging of the simulations to observed rawinsonde profiles and Antarctic automatic weather station observations provided increased realism and much greater cloud water amounts. Sensitivity tests that adjust the ice physics for extremely low ice nucleating particle (INP) concentrations decrease cloud ice and increases the cloud liquid water closer to observed amounts. In these tests, a liquid layer near cloud top is simulated, in agreement with observations. Accurate representation of INP concentrations appears to be critical for the simulation of coastal Antarctic clouds.
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