Mixed-phase Stratocumulus Clouds Research Articles

Role of a key microphysical factor in mixed-phase stratocumulus clouds and their interactions with aerosols

Abstract. This study examines the ratio of ice crystal number concentration (ICNC) to cloud droplet number concentration (CDNC), that is ICNC / CDNC, in mixed-phase stratocumulus clouds. This examination is performed using a large-eddy simulation (LES) framework and is one of the efforts toward a more general understanding of mechanisms controlling cloud development and aerosol–cloud interactions, as well as the impacts of ice processes on them in mixed-phase stratocumulus clouds. For the examination, this study compares a case of polar mixed-phase stratocumulus clouds to one of midlatitude mixed-phase stratocumulus clouds with weak precipitation. It is found that ICNC / CDNC plays a critical role in causing differences in cloud development with respect to the relative proportion of liquid and ice mass between the cases by affecting in-cloud latent-heat processes. Note that this proportion has an important implication for cloud radiative properties and, thus, for climate. It is also found that ICNC / CDNC plays a critical role in causing differences in the interactions between clouds and aerosols and in the impacts of ice processes on clouds and their interactions with aerosols between the cases by affecting in-cloud latent-heat processes. Findings of this study suggest that ICNC / CDNC can be a simplified general factor that contributes to a more general understanding and parameterization of mixed-phase clouds, their interactions with aerosols and the roles of ice processes within them.

Midlatitude mixed-phase stratocumulus clouds and their interactions with aerosols: how ice processes affect microphysical, dynamic, and thermodynamic development in those clouds and interactions?

Abstract. Midlatitude mixed-phase stratocumulus clouds and their interactions with aerosols remain poorly understood. This study examines the roles of ice processes in those clouds and their interactions with aerosols using a large-eddy simulation (LES) framework. Cloud mass becomes much lower in the presence of ice processes and the Wegener–Bergeron–Findeisen (WBF) mechanism in the mixed-phase clouds compared to that in warm clouds. This is because while the WBF mechanism enhances the evaporation of droplets, the low concentration of aerosols acting as ice-nucleating particles (INPs) and cloud ice number concentration (CINC) prevent the efficient deposition of water vapor. Note that the INP concentration in this study is based on the observed spatiotemporal variability of aerosols. This results in the lower CINC compared to that with empirical dependence of the INP concentrations on temperature in a previous study. In the mixed-phase clouds, the increasing concentration of aerosols that act as cloud condensation nuclei (CCN) decreases cloud mass by increasing the evaporation of droplets through the WBF mechanism and decreasing the intensity of updrafts. In contrast to this, in the warm clouds, the absence of the WBF mechanism makes the increase in the evaporation of droplets inefficient, eventually enabling cloud mass to increase with the increasing concentration of aerosols acting as CCN. Here, the results show that when there is an increasing concentration of aerosols that act as INPs, the deposition of water vapor is more efficient than when there is the increasing concentration of aerosols acting as CCN, which in turn enables cloud mass to increase in the mixed-phase clouds.

Toward Improved Cloud-Phase Simulation with a Mineral Dust and Temperature-Dependent Parameterization for Ice Nucleation in Mixed-Phase Clouds

Abstract Mixed-phase clouds are frequently observed in the atmosphere. Here we present a parameterization for ice crystal concentration and ice nucleation rate based on parcel model simulations for mixed-phase stratocumulus clouds, as a complement to a previous parameterization for stratus clouds. The parcel model uses a singular (time independent) description for deposition nucleation and a time-dependent description for condensation nucleation and immersion freezing on mineral dust particles. The mineral dust and temperature-dependent parameterizations have been implemented in the Geophysical Fluid Dynamics Laboratory atmosphere model, version 4.0 (AM4.0) (new), while the standard AM4.0 (original) uses a temperature-dependent parameterization. Model simulations with the new and original AM4.0 show significant changes in cloud properties and radiative effects. In comparison to measurements, cloud-phase (i.e., liquid and ice partitioning) simulation appears to be improved in the new AM4.0. More supercooled liquid cloud is predicted in the new model, it is sustained even at temperatures lower than −25°C unlike in the original model. A more accurate accounting of ice nucleating particles and ice crystals is essential for improved cloud-phase simulation in the global atmosphere.

A Case Study of Airmass Transformation and Cloud Formation at Summit, Greenland

AbstractThis study investigates cloud formation and transitions in cloud types at Summit, Greenland, during 16–22 September 2010, when a warm, moist air mass was advected to Greenland from lower latitudes. During this period there was a sharp transition between high ice clouds and the formation of a lower stratocumulus deck at Summit. A regional mesoscale model is used to investigate the air masses that form these cloud systems. It is found that the high ice clouds form in originally warm, moist air masses that radiatively cool while being transported to Summit. A sensitivity study removing high ice clouds demonstrates that the primary impact of these clouds at Summit is to reduce cloud liquid water embedded within the ice cloud and water vapor in the boundary layer due to vapor deposition on snow. The mixed-phase stratocumulus clouds form at the base of cold, dry air masses advected from the northwest above 4 km. The net surface radiative fluxes during the stratocumulus period are at least 20 W m−2 larger than during the ice cloud period, indicating that, in seasons other than summer, cold, dry air masses advected to Summit above the boundary layer may radiatively warm the top of the Greenland Ice Sheet more effectively than warm, moist air masses advected from lower latitudes.

The relative impact of cloud condensation nuclei and ice nucleating particle concentrations on phase partitioning in Arctic mixed-phase stratocumulus clouds

Abstract. This study investigates the interactions between cloud dynamics and aerosols in idealized large-eddy simulations (LES) of Arctic mixed-phase stratocumulus clouds (AMPS) observed at Oliktok Point, Alaska, in April 2015. This case was chosen because it allows the cloud to form in response to radiative cooling starting from a cloud-free state, rather than requiring the cloud ice and liquid to adjust to an initial cloudy state. Sensitivity studies are used to identify whether there are buffering feedbacks that limit the impact of aerosol perturbations. The results of this study indicate that perturbations in ice nucleating particles (INPs) dominate over cloud condensation nuclei (CCN) perturbations; i.e., an equivalent fractional decrease in CCN and INPs results in an increase in the cloud-top longwave cooling rate, even though the droplet effective radius increases and the cloud emissivity decreases. The dominant effect of ice in the simulated mixed-phase cloud is a thinning rather than a glaciation, causing the mixed-phase clouds to radiate as a grey body and the radiative properties of the cloud to be more sensitive to aerosol perturbations. It is demonstrated that allowing prognostic CCN and INPs causes a layering of the aerosols, with increased concentrations of CCN above cloud top and increased concentrations of INPs at the base of the cloud-driven mixed layer. This layering contributes to the maintenance of the cloud liquid, which drives the dynamics of the cloud system.

The Role of Precipitation in Controlling the Transition from Stratocumulus to Cumulus Clouds in a Northern Hemisphere Cold-Air Outbreak

Abstract Aircraft observations in a cold-air outbreak to the north of the United Kingdom are used to examine the boundary layer and cloud properties in an overcast mixed-phase stratocumulus cloud layer and across the transition to more broken open-cellular convection. The stratocumulus cloud is primarily composed of liquid drops with small concentrations of ice particles and there is a switch to more glaciated conditions in the shallow cumulus clouds downwind. The rapid change in cloud morphology is accompanied by enhanced precipitation with secondary ice processes becoming active and greater thermodynamic gradients in the subcloud layer. The measurements also show a removal of boundary layer accumulation mode aerosols via precipitation processes across the transition that are similar to those observed in the subtropics in pockets of open cells. Simulations using a convection-permitting (1.5-km grid spacing) regional version of the Met Office Unified Model were able to reproduce many of the salient features of the cloud field although the liquid water path in the stratiform region was too low. Sensitivity studies showed that ice was too active at removing supercooled liquid water from the cloud layer and that improvements could be made by limiting the overlap between the liquid water and ice phases. Precipitation appears to be the key mechanism responsible for initiating the transition from closed- to open-cellular convection by decoupling the boundary layer and depleting liquid water from the stratiform cloud.

Sensitivities in Large-Eddy Simulations of Mixed-Phase Arctic Stratocumulus Clouds Using a Simple Microphysics Approach*

Abstract Arctic mixed-phase stratocumulus clouds are maintained by feedbacks between microphysical and dynamical phenomena, but the details of these interactions are incompletely understood. Although large-eddy simulations are a promising means of elucidating microphysics–turbulence relationships, the use of sophisticated microphysical schemes complicates analysis of their results. Here, the ability of a simplified one-moment scheme to capture basic features of this cloud type is investigated through simulations based on Mixed-Phase Arctic Cloud Experiment (MPACE), SHEBA/FIRE-ACE, and Indirect and Semi-Direct Aerosol Campaign (ISDAC) intercomparison studies. The results of the simple scheme show reasonable agreement with liquid and ice water path predictions reported by models using schemes of similar or greater complexity. Additional tests are performed to evaluate the sensitivity of the results to three main parameters of the scheme: the snow and ice size distribution intercept parameters and the exponent appearing in the temperature-dependent phase-partition function, which is used to diagnose cloud condensate amounts. Sensitivities of the SHEBA and ISDAC cases, both of which have low surface heat fluxes and low precipitation rates, tend to be similar, while the MPACE case, with higher surface fluxes and precipitation rates, shows somewhat different trends. Results of all three cases are found to be sensitive to the snow size distribution intercept parameter, but this quantity can be adequately estimated using a recently developed diagnostic expression based on observations of Arctic clouds.

Dynamical and Microphysical Evolution during Mixed-Phase Cloud Glaciation Simulated Using the Bulk Adaptive Habit Prediction Model

Abstract A bulk microphysics scheme predicting ice particle habit evolution has been implemented in the Weather Research and Forecasting Model. Large-eddy simulations are analyzed to study the effects of ice habit and number concentration on the bulk ice and liquid masses, dynamics, and lifetime of Arctic mixed-phase boundary layer clouds. The microphysical and dynamical evolution simulated using the adaptive habit scheme is compared with that assuming spherical particles with a density of bulk ice or a reduced density and with mass–dimensional parameterizations. It is found that the adaptive habit method returns an increased (decreased) ice (liquid) mass as compared to spheres and provides a more accurate simulation as compared to dendrite mass–size relations. Using the adaptive habit method, simulations are then completed to understand the microphysical and dynamical interactions within a single-layer mixed-phase stratocumulus cloud observed during flight 31 of the Indirect and Semi-Direct Aerosol Campaign. With cloud-top longwave radiative cooling as a function of liquid mass acting as the primary dynamic driver of turbulent eddies within these clouds, the consumption of liquid at the expense of ice growth and subsequent sedimentation holds a strong control on the cloud lifetime. Ice concentrations ≥ 4 L−1 collapse the liquid layer without any external maintaining sources. Layer maintenance is possible at 4 L−1 when a constant cloud-top cooling rate or the water mass lost due to sedimentation is supplied. Larger concentrations require a more substantial source of latent or sensible heat for mixed-phase persistence.

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

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.

Microphysical and optical properties of Arctic mixed-phase clouds. The 9 April 2007 case study.

Abstract. Airborne measurements in Arctic boundary-layer stratocumulus were carried out near Spitsbergen on 9 April 2007 during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign. A unique set of co-located observations is used to describe the cloud properties, including detailed in situ cloud microphysical and radiation measurements along with airborne and co-located spaceborne remote sensing data (CALIPSO lidar and CloudSat radar). CALIPSO profiles indicate cloud top levels at temperature between −24°C and −21°C. In situ measurements confirm that the cloud-top lidar attenuated backscatter signal along the aircraft trajectory is linked with the presence of liquid water, a common feature observed in Arctic mixed-phase stratocumulus clouds. A low concentration of large ice crystals is also observed up to the cloud top resulting in significant CloudSat radar echoes. Since the ratio of the extinction of liquid water droplets to ice crystals is high, broadband radiative effects near the cloud top are mostly dominated by water droplets. CloudSat observations and in situ measurements reveal high reflectivity factors (up to 15 dBZ) and precipitation rates (1 mm h−1). This feature results from efficient ice growth processes. About 25% of the theoretically available liquid water is converted into ice water with large precipitating ice crystals. Using an estimate of mean cloud cover, a considerable value of 106 m3 h−1 of fresh water could be settled over the Greenland sea pool. European Centre for Medium-Range Weather Forecast (ECMWF) operational analyses reproduces the boundary layer height variation along the flight track. However, small-scale features in the observed cloud field cannot be resolved by ECMWF analysis. Furthermore, ECMWF's diagnostic partitioning of the condensed water into ice and liquid reveals serious shortcomings for Arctic mixed-phased clouds. Too much ice is modelled.

Effects of the Vertical Profiles of Cloud Droplets and Ice Particles on the Visible and Near-Infrared Radiative Properties of Mixed-Phase Stratocumulus Clouds

Such cloud radiative propenies as reflectance, transmittance, and absorptance of mixed-phase clouds have been simulated for the stratocumulus cloud observed on 30 January 1999 within the Japanese Cloud and Climate Study (JACCS) program, in which simultaneous observations of the cloud microphysical and radiative properties were conducted. The stratocumulus cloud was in mixed-phased condition, and highly heterogeneous vertically and horizontally with different mixing ratios of liquid water droplets and ice particles. A vertically-homogeneous, plane-parallel mixed-phase cloud model could reproduce the observed visible (VIS; wavelength < 0.7 μm) band radiative properties, but it could not reproduce the near-infrared (NIR; > 0.7 μm) band radiative properties. A multi-layered, plane-parallel mixed-phase cloud model could consistently reproduce the observed VIS-band and NIR-band radiative properties within the measurement accuracy. It is found that the venical profiles of water droplets and ice particles are an important factor to determine the radiative properties of mixed-phase clouds. The simulated results suggested that the NIR-band reflectance and absorptance could vary by more than 0.1 due to different vertical distributions of the cloud microphysical properties even the model clouds had constant liquid-water-path and ice-water-path. The visible and near-infrared solar reflection can be also affected by the vertical profiles of cloud microphysical properties; it is suggested that the conventional passive remote sensing under the assumption of vertical-homogeneity, may bring large errors in estimation of the microphysical propenies of mixed-phase clouds.

On the potential influence of ice nuclei on surface‐forced marine stratocumulus cloud dynamics

The mixed phase cloudy boundary layer that occurs during off‐ice flow in the marine Arctic was simulated in an environment with a strong surface heat flux (nearly 800 W m−2). A two‐dimensional, eddy‐resolving model coupled to a detailed cloud microphysical model was used to study both liquid phase and mixed phase stratocumulus clouds and boundary layer (BL) dynamics in this environment. Since ice precipitation may be important to BL dynamics, and ice nuclei (IN) concentrations modulate ice precipitation rates, the role of IN in cloud and BL development was explored. The results of several simulations illustrate how mixed phase microphysical processes affect the evolution of the cloudy BL in this environment. In agreement with past studies, BLs with mixed phase clouds had weaker convection, shallower BL depths, and smaller cloud fractions than BLs with clouds restricted to the liquid phase only. It is shown that the weaker BL convection is due to strong ice precipitation. Ice precipitation reduces convective strength directly by stabilizing downdrafts and more indirectly by sensibly heating the BL and inhibiting vertical mixing of momentum thereby reducing surface heat fluxes by as much as 80 W m−2. This feedback between precipitation and surface fluxes was found to have a significant impact on cloud/BL morphology, producing oscillations in convective strength and cloud fraction that did not occur if surface fluxes were fixed at constant values. Increases in IN concentrations in mixed phase clouds caused a more rapid Bergeron‐Findeisen process leading to larger precipitation fluxes, reduced convection and lower cloud fraction. When IN were removed from the BL through precipitation, fewer crystals were nucleated at later simulation times leading to progressively weaker precipitation rates, greater cloud fraction, and stronger convective BL eddies.