Arctic low-level boundary layer clouds: in-situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the cloud top layer

Abstract. Aircraft borne optical in-situ size distribution measurements were performed within Arctic boundary layer clouds, with a special emphasis on the cloud top layer, during the VERtical Distribution of Ice in Arctic Clouds (VERDI) campaign. The observations were carried out within a joint research activity of seven German institutes to investigate Arctic boundary layer-, mixed-phase clouds in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 μm to 20 μm. In the upper transition zone (i.e. adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating two droplet populations from different air volumes. The formation of a second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol was detected directly above the stratus cloud might form a second mode of small cloud droplets. However, theoretical considerations and a model simulation revealed that, instead, turbulent mixing and evaporation of larger droplets most likely are the main reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

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Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-617-2015 ◽

2015 ◽

Vol 15 (2) ◽

pp. 617-631 ◽

Cited By ~ 31

Author(s):

M. Klingebiel ◽

A. de Lozar ◽

S. Molleker ◽

R. Weigel ◽

A. Roth ◽

...

Keyword(s):

Boundary Layer ◽

Liquid Phase ◽

Droplet Size ◽

Size Distributions ◽

Cloud Droplets ◽

Model Calculations ◽

Boundary Layer Clouds ◽

Droplet Size Distributions ◽

Size Mode

Abstract. Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

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Investigation of Droplet Size Distributions and Drizzle Formation Using A New Trajectory Ensemble Model. Part II: Lucky Parcels

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2789.1 ◽

2009 ◽

Vol 66 (4) ◽

pp. 781-805 ◽

Cited By ~ 31

Author(s):

L. Magaritz ◽

M. Pinsky ◽

O. Krasnov ◽

A. Khain

Keyword(s):

Boundary Layer ◽

Droplet Size ◽

Threshold Value ◽

Ensemble Model ◽

Size Distributions ◽

Cloud Droplets ◽

Stratocumulus Clouds ◽

Water Vapor Mixing Ratio ◽

Large Eddies ◽

Droplet Size Distributions

Abstract A novel trajectory ensemble model of the cloud-topped boundary layer containing 1340 Lagrangian parcels moving with a turbulent-like flow with the observed statistical properties was applied to investigate the formation of the microphysical structure of stratocumulus clouds (Sc) in a nonmixing limit (when turbulent mixing between the parcels is not taken into account). The Sc observed in two research flights during the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II)—RF01 (no drizzle) and RF07 (weak drizzle)—are simulated. The mechanisms leading to a high variability of droplet size distributions (DSDs) with different spectrum width and dispersion are discussed. Drizzle formation was investigated using the radar reflectivity–LWC and LWC–effective drop radius diagrams simulated by the model in the nondrizzle and drizzle cases. It is shown that in the RF07 case large cloud droplets that trigger drop collisions and drizzle formation form only in a small fraction (about 1%) of the parcels (which will be referred to as lucky parcels) in which LWC exceeds ∼1.5 g m−3. This value exceeds the horizontally averaged LWC maximum value of 0.9 g m−3 by two to three standard deviations, indicating a small amount of lucky parcels. In a nondrizzling cloud simulation this threshold is exceeded extremely rarely. The dependence of the threshold value of LWC on aerosol concentration is discussed. The lucky parcels (at least in the nonmixing limit) start their updraft in the vicinity of the surface, where the water vapor mixing ratio is maximum, and ascend to the highest levels close to the cloud top. It is shown that the lucky parcel tracks are related to the large eddies in the boundary layer, which indicates the substantial role of large eddies in drizzle formation.

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Large Graupel Produced by Thin Clouds in the Arctic

10.5194/egusphere-egu2020-6138 ◽

2020 ◽

Author(s):

Kyle Fitch ◽

Tim Garrett ◽

Ahmad Talaei

Keyword(s):

Boundary Layer ◽

Analytical Solutions ◽

Microwave Remote Sensing ◽

The Arctic ◽

Homogeneous Field ◽

Size Distributions ◽

Arctic Boundary Layer ◽

Radiative Balance ◽

Boundary Layer Clouds ◽

Thin Cloud

<div>Riming is a critical process for both numerical modeling and microwave remote sensing because of the significant changes to hydrometeor shape and density that occur. Basic continuous collection theory invokes a simple model that assumes a relatively large, massive graupel falls through a homogeneous field of much smaller, suspended supercooled droplets in still air. However, numerous studies have shown that turbulence enhances the rate of collisions between liquid droplets, and even more so for graupel-droplet collisions. Modeling and observational studies of turbulence-induced riming enhancement have all focused on convective clouds with relatively large dissipation rates. Here we combine theoretical work on the analytical solutions of precipitation size distributions with observations and simulations of Arctic graupel to show that this enhancement is common in &#8220;thin&#8221; Arctic boundary layer clouds with LWP<50gm-2. The median enhancement of rime mass over that expected from continuous collection ranges from 6.4 to 10.7 (for collection efficiencies ranging from 1 to 0.6) for 1,628 carefully selected graupel falling from thin clouds. Analytical solutions for precipitation size distributions imply that average updraft speed must be approximately one-third of particle settling speed to explain the enhancement quantitatively using bulk cloud and precipitation measurements. Analysis of an 8-day thin cloud case revealed that the two days of thin-cloud graupel occurred in conjunction with boundary layer capping inversions that were significantly weaker than on other days of the period. These graupel days were also preceded by boundary layer profiles indicating two days of very strong cloud top radiative cooling &#8211; implying that this generated a mixed layer that eroded the capping inversion. Finally, 1-D Lagrangian simulations of graupel settling in turbulent flow show that the particles spend more time in strong updrafts where riming time increases to a significant degree. These findings challenge the current understanding of riming growth and extent turbulence-induced collision enhancement to thin mixed-phase boundary layer clouds in the Arctic. Such enhanced riming leads to increased bulk density of precipitation particles and is therefore expected have strong implications for cloud lifecycles and corresponding radiative balance in the Arctic.</div>

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Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry

Atmospheric Measurement Techniques ◽

10.5194/amt-6-1189-2013 ◽

2013 ◽

Vol 6 (5) ◽

pp. 1189-1200 ◽

Cited By ~ 19

Author(s):

E. Bierwirth ◽

A. Ehrlich ◽

M. Wendisch ◽

J.-F. Gayet ◽

C. Gourbeyre ◽

...

Keyword(s):

Remote Sensing ◽

Boundary Layer ◽

Optical Thickness ◽

Effective Radius ◽

Spectral Radiance ◽

Arctic Boundary Layer ◽

Boundary Layer Clouds ◽

In Situ Data ◽

Cloud Optical Thickness

Abstract. Arctic boundary-layer clouds in the vicinity of Svalbard (78° N, 15° E) were observed with airborne remote sensing and in situ methods. The cloud optical thickness and the droplet effective radius are retrieved from spectral radiance data from the nadir spot (1.5°, 350–2100 nm) and from a nadir-centred image (40°, 400–1000 nm). Two approaches are used for the nadir retrieval, combining the signal from either two or five wavelengths. Two wavelengths are found to be sufficient for an accurate retrieval of the cloud optical thickness, while the retrieval of droplet effective radius is more sensitive to the number of wavelengths. Even with the comparison to in-situ data, it is not possible to definitely answer the question which method is better. This is due to unavoidable time delays between the in-situ measurements and the remote-sensing observations, and to the scarcity of vertical in-situ profiles within the cloud.

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Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: test of three approaches

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-7493-2008 ◽

2008 ◽

Vol 8 (24) ◽

pp. 7493-7505 ◽

Cited By ~ 57

Author(s):

A. Ehrlich ◽

E. Bierwirth ◽

M. Wendisch ◽

J.-F. Gayet ◽

G. Mioche ◽

...

Keyword(s):

Boundary Layer ◽

Liquid Water ◽

Mixed Phase ◽

Pure Liquid ◽

Spectral Slope ◽

Arctic Boundary Layer ◽

Ice Clouds ◽

Boundary Layer Clouds ◽

Mixed Phase Clouds

Abstract. Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) spectral range which is characterized by ice and water absorption. While IS analyzes the spectral slope of the reflectance in this wavelength range, IS utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that IS, IP and IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index IS and the PCA ice index IP are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. The reflectance-albedo ice index IA is mainly dominated by the uppermost cloud layer (τ<1.5). Therefore, typical boundary-layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements shows the ability of the three approaches to identify the ice phase in Arctic boundary-layer clouds.

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Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne spectral and hyperspectral radiance measurements

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-5-7753-2012 ◽

2012 ◽

Vol 5 (5) ◽

pp. 7753-7781 ◽

Cited By ~ 1

Author(s):

E. Bierwirth ◽

A. Ehrlich ◽

M. Wendisch ◽

J.-F. Gayet ◽

C. Gourbeyre ◽

...

Keyword(s):

Remote Sensing ◽

Boundary Layer ◽

Optical Thickness ◽

Effective Radius ◽

Spectral Radiance ◽

Arctic Boundary Layer ◽

Boundary Layer Clouds ◽

In Situ Data ◽

Cloud Optical Thickness

Abstract. Arctic boundary-layer clouds in the vicinity of Svalbard (78° N, 15° E) were observed with airborne remote sensing and in situ methods. The cloud optical thickness and the droplet effective radius are retrieved from spectral radiance data in nadir and and from hyperspectral radiances in a 40° field of view. Two approaches are used for the spectral retrieval, combining the signal from either two or five wavelengths. Two wavelengths are found to be sufficient for an accurate retrieval of the cloud optical thickness, while the retrieval of droplet effective radius is more sensitive to the method applied. The comparison to in situ data cannot give a definite answer as to which method is better because of unavoidable time delays between the in situ measurements and the remote-sensing observations.

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