scholarly journals Relaxation times of Arctic mixed-phase clouds to short-term aerosol perturbations under different surface forcings

2018 ◽  
Author(s):  
Gesa K. Eirund ◽  
Anna Possner ◽  
Ulrike Lohmann
Keyword(s):  
Sea Ice ◽  
Cloud Condensation Nuclei ◽  
Relaxation Times ◽  
Ocean Surface ◽  
Open Ocean ◽  
Mixed Phase ◽  
The Arctic ◽  
Cloud Base ◽  
Surface Conditions ◽  
Mixed Phase Clouds

Abstract. The formation and persistence of low lying mixed-phase clouds (MPCs) in the Arctic depends on a multitude of processes, such as surface conditions, the environmental state, air mass advection and the ambient aerosol concentration. In this study, we focus on the relative importance of different aerosol perturbations (cloud condensation nuclei and ice nucleating particles; CCN and INP, respectively) on MPC properties in the central Arctic. To address this topic, we performed high resolution large eddy simulations (LES) using the COSMO model and designed a case study for the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign in March 2013. Motivated by ongoing sea ice retreat, we additionally contrast the simulated MPC that formed over an open ocean surface and a sea ice surface. We find that surface conditions highly impact cloud dynamics: over sea ice, a rather homogeneous, optically thin, mixed-phase stratus cloud forms. In contrast, the MPC over the open ocean has a stratocumulus-like cloud structure. With cumuli feeding moisture into the stratus layer, the cloud features a higher liquid (LWC) and ice water content (IWC) and has a lifted cloud base compared to the cloud over sea ice. Furthermore, we analyzed the aerosol impact on these two dynamically different regimes. Perturbations in the INP concentration increase the IWC and decrease the LWC consistently in both regimes. The cloud microphysical response to potential CCN perturbations occurs faster in the stratocumulus regime over the ocean, where the increased moisture flux favors rapid cloud droplet formation and growth, leading to an increase in LWC following the aerosol injection. In addition, the IWC increases through increased immersion freezing and subsequent growth by deposition. Over sea ice, the maximum response is delayed by a factor of 2.5 compared to open ocean surface. However, independent of the cloud regime and aerosol perturbation, the cloud regains its original state after at most 12 h for an aerosol perturbation of 1000 cm−3. Cloud microphysical and macrophysical peoperties relax to their unperturbed range, and any aerosol perturbation is efficiently buffered. A substantial fraction of the aerosol is transported out of the boundary layer into the capping inversion, where the supersaturation is insufficient for aerosol activation. Our results are robust across different temperature ranges and insensitive to the aerosol injection period. Based on these results we postulate an efficient aerosol processing and transport mechanism that appears to inhibit any long-term aerosol impact on Arctic MPC properties.

scholarly journals Response of Arctic mixed-phase clouds to aerosol perturbations under different surface forcings

2019 ◽  
Vol 19 (15) ◽  
pp. 9847-9864 ◽  
Author(s):  
Gesa K. Eirund ◽  
Anna Possner ◽  
Ulrike Lohmann
Keyword(s):  
Sea Ice ◽  
Open Ocean ◽  
Mixed Phase ◽  
The Arctic ◽  
Cloud Base ◽  
Small Scale ◽  
Surface Conditions ◽  
Aerosol Cloud ◽  
Cloud Regime ◽  
Mixed Phase Clouds

Abstract. The formation and persistence of low-lying mixed-phase clouds (MPCs) in the Arctic depends on a multitude of processes, such as surface conditions, the environmental state, air mass advection, and the ambient aerosol concentration. In this study, we focus on the relative importance of different instantaneous aerosol perturbations (cloud condensation nuclei and ice-nucleating particles; CCN and INPs, respectively) on MPC properties in the European Arctic. To address this topic, we performed high-resolution large-eddy simulation (LES) experiments using the Consortium for Small-scale Modeling (COSMO) model and designed a case study for the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign in March 2013. Motivated by ongoing sea ice retreat, we performed all sensitivity studies over open ocean and sea ice to investigate the effect of changing surface conditions. We find that surface conditions highly impact cloud dynamics, consistent with the ACCACIA observations: over sea ice, a rather homogeneous, optically thin, mixed-phase stratus cloud forms. In contrast, the MPC over the open ocean has a stratocumulus-like cloud structure. With cumuli feeding moisture into the stratus layer, the cloud over the open ocean features a higher liquid (LWP) and ice water path (IWP) and has a lifted cloud base and cloud top compared to the cloud over sea ice. Furthermore, we analyzed the aerosol impact on the sea ice and open ocean cloud regime. Perturbation aerosol concentrations relevant for CCN activation were increased to a range between 100 and 1000 cm−3 and ice-nucleating particle perturbations were increased by 100 % and 300 % compared to the background concentration (at every grid point and at all levels). The perturbations are prognostic to allow for fully interactive aerosol–cloud interactions. Perturbations in the INP concentration increase IWP and decrease LWP consistently in both regimes. The cloud microphysical response to potential CCN perturbations occurs faster in the stratocumulus regime over the ocean, where the increased moisture flux favors rapid cloud droplet formation and growth, leading to an increase in LWP following the aerosol injection. In addition, IWP increases through new ice crystal formation by increased immersion freezing, cloud top rise, and subsequent growth by deposition. Over sea ice, the maximum response in LWP and IWP is delayed and weakened compared to the response over the open ocean surface. Additionally, we find the long-term response to aerosol perturbations to be highly dependent on the cloud regime. Over the open ocean, LWP perturbations are efficiently buffered after 18 h simulation time. Increased ice and precipitation formation relax the LWP back to its unperturbed range. On the contrary, over sea ice the cloud evolution remains substantially perturbed with CCN perturbations ranging from 200 to 1000 CCN cm−3.

scholarly journals Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus

2013 ◽  
Vol 13 (11) ◽  
pp. 31079-31125 ◽  
Author(s):  
J. Sedlar ◽  
M. D. Shupe
Keyword(s):  
Vertical Velocity ◽  
Vertical Mixing ◽  
Vertical Motion ◽  
Temperature Inversion ◽  
Arctic Sea Ice ◽  
Cloud Layer ◽  
Mixed Phase ◽  
The Arctic ◽  
Cloud Base ◽  
Mixed Phase Clouds

Abstract. Over the Arctic Ocean, little is known, observationally, on cloud-generated buoyant overturning vertical motions within mixed-phase stratocumulus clouds. Characteristics of such motions are important for understanding the diabatic processes associated with the vertical motions, the lifetime of the cloud layer and its micro- and macrophysical characteristics. In this study, we exploit a suite of surface-based remote sensors over the high Arctic sea ice during a week-long period of persistent stratocumulus in August 2008 to derive the in-cloud vertical motion characteristics. In-cloud vertical velocity skewness and variance profiles are found to be strikingly different from observations within lower-latiatude stratocumulus, suggesting these Arctic mixed-phase clouds interact differently with the atmospheric thermodynamics (cloud tops extending above a stable temperature inversion base) and with a different coupling state between surface and cloud. We find evidence of cloud-generated vertical mixing below cloud base, regardless of surface-cloud coupling state, although a decoupled surface-cloud state occurred most frequently. Detailed case studies are examined focusing on 3 levels within the cloud layer, where wavelet and power spectral analyses are applied to characterize the dominant temporal and horizontal scales associated with cloud-generated vertical motions. In general, we find a positively-correlated vertical motion signal across the full cloud layer depth. The coherency is dependent upon other non-cloud controlled factors, such as larger, mesoscale weather passages and radiative shielding of low-level stratocumulus by multiple cloud layers above. Despite the coherency in vertical velocity across the cloud, the velocity variances were always weaker near cloud top, relative to cloud mid and base. Taken in combination with the skewness, variance and thermodynamic profile characteristics, we observe vertical motions near cloud-top that behave differently than those from lower within the cloud layer. Spectral analysis indicates peak cloud-generated w variance timescales slowed only modestly during decoupled cases relative to coupled; horizontal wavelengths only slightly increased when transitioning from coupling to decoupling. The similarities in scales suggests that perhaps the dominant forcing for all cases is generated from the cloud layer, and it is not the surface forcing that characterizes the time and space scales of in-cloud vertical velocity variance. This points toward the resilient nature of Arctic mixed-phase clouds to persist when characterized by thermodynamic regimes unique to the Arctic.

scholarly journals Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus

2014 ◽  
Vol 14 (7) ◽  
pp. 3461-3478 ◽  
Author(s):  
J. Sedlar ◽  
M. D. Shupe
Keyword(s):  
Vertical Velocity ◽  
Vertical Motion ◽  
Temperature Inversion ◽  
Arctic Sea Ice ◽  
Cloud Layer ◽  
Mixed Phase ◽  
The Arctic ◽  
Lower Latitude ◽  
Cloud Base ◽  
Mixed Phase Clouds

Abstract. Over the Arctic Ocean, little is known on cloud-generated buoyant overturning vertical motions within mixed-phase stratocumulus clouds. Characteristics of such motions are important for understanding the diabatic processes associated with the vertical motions, the lifetime of the cloud layer and its micro- and macrophysical characteristics. In this study, we exploit a suite of surface-based remote sensors over the high-Arctic sea ice during a weeklong period of persistent stratocumulus in August 2008 to derive the in-cloud vertical motion characteristics. In-cloud vertical velocity skewness and variance profiles are found to be strikingly different from observations within lower-latitude stratocumulus, suggesting these Arctic mixed-phase clouds interact differently with the atmospheric thermodynamics (cloud tops extending above a stable temperature inversion base) and with a different coupling state between surface and cloud. We find evidence of cloud-generated vertical mixing below cloud base, regardless of surface–cloud coupling state, although a decoupled surface–cloud state occurred most frequently. Detailed case studies are examined, focusing on three levels within the cloud layer, where wavelet and power spectral analyses are applied to characterize the dominant temporal and horizontal scales associated with cloud-generated vertical motions. In general, we find a positively correlated vertical motion signal amongst vertical levels within the cloud and across the full cloud layer depth. The coherency is dependent upon other non-cloud controlled factors, such as larger, mesoscale weather passages and radiative shielding of low-level stratocumulus by one or more cloud layers above. Despite the coherency in vertical velocity across the cloud, the velocity variances were always weaker near cloud top, relative to cloud middle and base. Taken in combination with the skewness, variance and thermodynamic profile characteristics, we observe vertical motions near cloud top that behave differently than those from lower within the cloud layer. Spectral analysis indicates peak cloud-generated w variance timescales slowed only modestly during decoupled cases relative to coupled; horizontal wavelengths only slightly increased when transitioning from coupling to decoupling. The similarities in scales suggests that perhaps the dominant forcing for all cases is generated from the cloud layer, and it is not the surface forcing that characterizes the time- and space scales of in-cloud vertical velocity variance. This points toward the resilient nature of Arctic mixed-phase clouds to persist when characterized by thermodynamic regimes unique to the Arctic.

scholarly journals Aerosol indirect effects on the nighttime Arctic Ocean surface from thin, predominantly liquid clouds

2017 ◽  
Vol 17 (12) ◽  
pp. 7311-7332 ◽  
Author(s):  
Lauren M. Zamora ◽  
Ralph A. Kahn ◽  
Sabine Eckhardt ◽  
Allison McComiskey ◽  
Patricia Sawamura ◽  
...  
Keyword(s):  
Sea Ice ◽  
Arctic Ocean ◽  
Indirect Effects ◽  
Regional Scale ◽  
Ocean Surface ◽  
Open Ocean ◽  
The Arctic ◽  
Meteorological Forcing ◽  
Aerosol Indirect Effects ◽  
The Arctic Ocean

Abstract. Aerosol indirect effects have potentially large impacts on the Arctic Ocean surface energy budget, but model estimates of regional-scale aerosol indirect effects are highly uncertain and poorly validated by observations. Here we demonstrate a new way to quantitatively estimate aerosol indirect effects on a regional scale from remote sensing observations. In this study, we focus on nighttime, optically thin, predominantly liquid clouds. The method is based on differences in cloud physical and microphysical characteristics in carefully selected clean, average, and aerosol-impacted conditions. The cloud subset of focus covers just ∼ 5 % of cloudy Arctic Ocean regions, warming the Arctic Ocean surface by ∼ 1–1.4 W m−2 regionally during polar night. However, within this cloud subset, aerosol and cloud conditions can be determined with high confidence using CALIPSO and CloudSat data and model output. This cloud subset is generally susceptible to aerosols, with a polar nighttime estimated maximum regionally integrated indirect cooling effect of ∼ −0.11 W m−2 at the Arctic sea ice surface (∼ 8 % of the clean background cloud effect), excluding cloud fraction changes. Aerosol presence is related to reduced precipitation, cloud thickness, and radar reflectivity, and in some cases, an increased likelihood of cloud presence in the liquid phase. These observations are inconsistent with a glaciation indirect effect and are consistent with either a deactivation effect or less-efficient secondary ice formation related to smaller liquid cloud droplets. However, this cloud subset shows large differences in surface and meteorological forcing in shallow and higher-altitude clouds and between sea ice and open-ocean regions. For example, optically thin, predominantly liquid clouds are much more likely to overlay another cloud over the open ocean, which may reduce aerosol indirect effects on the surface. Also, shallow clouds over open ocean do not appear to respond to aerosols as strongly as clouds over stratified sea ice environments, indicating a larger influence of meteorological forcing over aerosol microphysics in these types of clouds over the rapidly changing Arctic Ocean.

Impacts of Aerosol Concentration Variability on Turbulence in Convective Low Level Mixed-phase Clouds in the Arctic

2020 ◽  
Author(s):  
Jan Chylik ◽  
Stephan Mertes ◽  
Roel Neggers
Keyword(s):  
Boundary Layer ◽  
Climate Models ◽  
Cloud Condensation Nuclei ◽  
Mixed Phase ◽  
The Arctic ◽  
Microphysics Scheme ◽  
Low Level ◽  
Large Eddy ◽  
Mixed Phase Clouds ◽  
The Impact

<p>Arctic mixed-phase clouds are still not properly represented in weather forecast and climate models. Recent field campaigns in the Arctic have successfully probed low level mixed-phase clouds, however it remains difficult to gain understanding of this complex system from observational datasets alone. Complementary high-resolution simulations, properly constrained by relevant measurements, can serve as a virtual laboratory that provides a deeper insight into a developing boundary layer in the Arctic.</p><p><br>Our study focus on the impact of variability in cloud condensation nuclei (CCN) concentrations on the turbulence in Arctic mixed-phase clouds. Large-Eddy Simulations of convective mixed-phase clouds over open water were performed as observed during the ACLOUD campaign, which took place in Fram Strait west of Svalbard in May and June 2017. The Dutch Atmospheric Large Eddy Simulation (DALES) is used including a well-established double-moment mixed-phase microphysics scheme of Seifert & Beheng.</p><p><br>The results highlight various impact mechanisms of CCN on the boundary layer thermodynamic state, turbulence, and clouds. Lower CCN concentrations generally lead to decreased turbulence near the cloud top. However, they can also enhance the turbulence in the lower part of the boundary layer due to increased amount of sublimation of ice hydrometeors. Further implications for the role of mixed-phase clouds in the Arctic Amplification will be discussed.</p>

scholarly journals The Impact of Warm and Moist Airmass Perturbations on Arctic Mixed-Phase Stratocumulus

2020 ◽  
Vol 33 (22) ◽  
pp. 9615-9628
Author(s):  
Gesa K. Eirund ◽  
Anna Possner ◽  
Ulrike Lohmann
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Lower Troposphere ◽  
Temperature Inversion ◽  
Open Ocean ◽  
Mixed Phase ◽  
Specific Humidity ◽  
The Arctic ◽  
Free Troposphere ◽  
The Impact

AbstractThe Arctic is known to be particularly sensitive to climate change. This Arctic amplification has partially been attributed to poleward atmospheric heat transport in the form of airmass intrusions. Locally, such airmass intrusions can introduce moisture and temperature perturbations. The effect of airmass perturbations on boundary layer and cloud changes and their impact on the surface radiative balance has received increased attention, especially over sea ice with regard to sea ice melt. Utilizing cloud-resolving model simulations, this study addresses the impact of airmass perturbations occurring at different altitudes on stratocumulus clouds for open-ocean conditions. It is shown that warm and moist airmass perturbations substantially affect the boundary layer and cloud properties, even for the relatively moist environmental conditions over the open ocean. The cloud response is driven by temperature inversion adjustments and strongly depends on the perturbation height. Boundary layer perturbations weaken and raise the inversion, which destabilizes the lower troposphere and involves a transition from stratocumulus to cumulus clouds. In contrast, perturbations occurring in the lower free troposphere lead to a lowering but strengthening of the temperature inversion, with no impact on cloud fraction. In simulations where free-tropospheric specific humidity is further increased, multilayer mixed-phase clouds form. Regarding energy balance changes, substantial surface longwave cooling arises out of the stratocumulus break-up simulated for boundary layer perturbations. Meanwhile, the net surface longwave warming increases resulting from thicker clouds for airmass perturbations occurring in the lower free troposphere.

scholarly journals Microphysical sensitivity of coupled springtime Arctic stratocumulus to modelled primary ice over the ice pack, marginal ice, and ocean

2017 ◽  
Vol 17 (6) ◽  
pp. 4209-4227 ◽  
Author(s):  
Gillian Young ◽  
Paul J. Connolly ◽  
Hazel M. Jones ◽  
Thomas W. Choularton
Keyword(s):  
Sea Ice ◽  
Liquid Water ◽  
Cloud Microphysics ◽  
Mixed Phase ◽  
The Arctic ◽  
Ice Formation ◽  
Large Eddy ◽  
Break Up ◽  
Mixed Phase Clouds ◽  
Ice Pack

Abstract. This study uses large eddy simulations to test the sensitivity of single-layer mixed-phase stratocumulus to primary ice number concentrations in the European Arctic. Observations from the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign are considered for comparison with cloud microphysics modelled using the Large Eddy Model (LEM, UK Met. Office). We find that cloud structure is very sensitive to ice number concentrations, Nice, and small increases can cause persisting mixed-phase clouds to glaciate and break up.Three key dependencies on Nice are identified from sensitivity simulations and comparisons with observations made over the sea ice pack, marginal ice zone (MIZ), and ocean. Over sea ice, we find deposition–condensation ice formation rates are overestimated, leading to cloud glaciation. When ice formation is limited to water-saturated conditions, we find microphysics comparable to aircraft observations over all surfaces considered. We show that warm supercooled (−13 °C) mixed-phase clouds over the MIZ are simulated to reasonable accuracy when using both the DeMott et al.(2010) and Cooper(1986) primary ice nucleation parameterisations. Over the ocean, we find a strong sensitivity of Arctic stratus to Nice. The Cooper(1986) parameterisation performs poorly at the lower ambient temperatures, leading to a comparatively higher Nice (2.43 L−1 at the cloud-top temperature, approximately −20 °C) and cloud glaciation. A small decrease in the predicted Nice (2.07 L−1 at −20 °C), using the DeMott et al.(2010) parameterisation, causes mixed-phase conditions to persist for 24 h over the ocean. However, this representation leads to the formation of convective structures which reduce the cloud liquid water through snow precipitation, promoting cloud break-up through a depleted liquid phase. Decreasing the Nice further (0.54 L−1, using a relationship derived from ACCACIA observations) allows mixed-phase conditions to be maintained for at least 24 h with more stability in the liquid and ice water paths. Sensitivity to Nice is also evident at low number concentrations, where 0.1  ×  Nice predicted by the DeMott et al.(2010) parameterisation results in the formation of rainbands within the model; rainbands which also act to deplete the liquid water in the cloud and promote break-up.

scholarly journals Microphysical sensitivity of coupled springtime Arctic stratocumulus to modelled primary ice over the ice pack, marginal ice, and ocean

2016 ◽  
Author(s):  
Gillian Young ◽  
Paul J. Connolly ◽  
Hazel M. Jones ◽  
Thomas W. Choularton
Keyword(s):  
Sea Ice ◽  
Liquid Water ◽  
Cloud Microphysics ◽  
Mixed Phase ◽  
The Arctic ◽  
Ice Formation ◽  
Large Eddy ◽  
Break Up ◽  
Mixed Phase Clouds ◽  
Ice Pack

Abstract. This study uses large eddy simulations to test the sensitivity of single-layer mixed-phase stratocumulus to primary ice number concentrations in the European Arctic. Observations from the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign are considered for comparison with cloud microphysics modelled using the Large Eddy Model (LEM, UK Met. Office). We find that cloud structure is very sensitive to ice number concentrations, N_ice , and small increases can cause persisting mixed-phase clouds to glaciate and break up. Three key sensitivities are identified with comparison to in situ cloud observations over the sea ice pack, marginal ice zone (MIZ), and ocean. Over sea ice, we find deposition-condensation ice formation rates are overestimated, leading to cloud glaciation. When ice formation is limited to water-saturated conditions, we find microphysics comparable to the aircraft observations over all surfaces considered. We show that warm supercooled (−13 °C) mixed-phase clouds over the MIZ are simulated to reasonable accuracy when using both the DeMott et al. (2010) and Cooper (1986) parameterisations. Over the ocean, we find a strong sensitivity of Arctic stratus to ice number concentrations. Cooper (1986) performs poorly at the lower ambient temperatures, leading to comparatively higher ice number concentrations (2.43 L−1 at the cloud top temperature, approximately −20 °C) and cloud glaciation. A small decrease in the predicted Nice (2.07 L−1 at −20 °C), using the DeMott et al. (2010) parameterisation, causes mixed-phase conditions to persist for 24 h over the ocean. However, this representation leads to the formation of convective structures which reduce the cloud liquid water through snow precipitation, promoting cloud break up. Decreasing the ice crystal number concentration further (0.54 L−1, using a relationship derived from ACCACIA observations) allows mixed-phase conditions to be maintained for at least 24 h with more stability in the liquid and ice water paths. Sensitivity to Nice is also evident at low number concentrations, where 0.1×Nice predicted by the DeMott et al. (2010) parameterisation results in the formation of rainbands within the model; rainbands which also act to deplete the liquid water in the cloud and promote break up.

scholarly journals The Vertical Profile of Liquid and Ice Water Content in Midlatitude Mixed-Phase Altocumulus Clouds

2008 ◽  
Vol 47 (9) ◽  
pp. 2487-2495 ◽  
Author(s):  
Lawrence D. Carey ◽  
Jianguo Niu ◽  
Ping Yang ◽  
J. Adam Kankiewicz ◽  
Vincent E. Larson ◽  
...  
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Cloud Microphysics ◽  
Mixed Phase ◽  
The Arctic ◽  
Cloud Base ◽  
Ice Water Content ◽  
Mixed Phase Clouds ◽  
Ice Water ◽  
Water Contents

Abstract The microphysical properties of mixed-phase altocumulus clouds are investigated using in situ airborne measurements acquired during the ninth Cloud Layer Experiment (CLEX-9) over a midlatitude location. Approximately ⅔ of the sampled profiles are supercooled liquid–topped altocumulus clouds characterized by mixed-phase conditions. The coexistence of measurable liquid water droplets and ice crystals begins at or within tens of meters of cloud top and extends down to cloud base. Ice virga is found below cloud base. Peak liquid water contents occur at or near cloud top while peak ice water contents occur in the lower half of the cloud or in virga. The estimation of ice water content from particle size data requires that an assumption be made regarding the particle mass–dimensional relation, resulting in potential error on the order of tens of percent. The highest proportion of liquid is typically found in the coldest (top) part of the cloud profile. This feature of the microphysical structure for the midlatitude mixed-phase altocumulus clouds is similar to that reported for mixed-phase clouds over the Arctic region. The results obtained for limited cases of midlatitude mixed-phase clouds observed during CLEX-9 may have an implication for the study of mixed-phase cloud microphysics, satellite remote sensing applications, and the parameterization of mixed-phase cloud radiative properties in climate models.

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