Supercooled Drizzle Research Articles

Ground-Based Remote Sensing of Aerosol, Clouds, Dynamics, and Precipitation in Antarctica: First Results from the 1-Year COALA Campaign at Neumayer Station III in 2023

Abstract Novel observations of aerosol and clouds by means of ground-based remote sensing have been performed in Antarctica over the Ekström Ice Shelf on the coast of Dronning Maud Land at Neumayer Station III (70.67°S, 8.27°W) from January to December 2023. The deployment of the OCEANET-Atmosphere remote sensing observatory in the framework of the Continuous Observations of Aerosol-Cloud Interaction (COALA) campaign has brought Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) aerosol and cloud profiling capabilities next to meteorological and air chemistry in situ observations at the Antarctic station. We present an overview of the site, the instrumental setup, and data analysis strategy and introduce 3 scientific highlights from austral fall and winter, namely, 1) observations of a persistent mixed-phase cloud embedded in a plume of marine aerosol. Remote sensing–based retrievals of cloud-relevant aerosol properties and cloud microphysical parameters confirm that the free-tropospheric mixed-phase cloud layer formed in an aerosol-limited environment. 2) Two extraordinary warm air intrusions: one with intense snowfall produced the equivalent of 10% of the yearly snow accumulation and a second one with record-breaking maximum temperatures and heavy icing due to supercooled drizzle. 3) Omnipresent aerosol layers in the stratosphere. Our profiling capabilities could show that 50% of the 500-nm aerosol optical depth of 0.06 was caused by stratospheric aerosol, while the troposphere was usually pristine. As demonstrated by these highlights, the 1-yr COALA observations will serve as a reference dataset for the vertical structure of aerosol and clouds above the region, enabling future observational and modeling studies to advance understanding of atmospheric processes in Antarctica.

Microphysical Characteristics and Evolution of Seeded Orographic Clouds

AbstractThe spatial distribution and magnitude of snowfall resulting from cloud seeding with silver iodide (AgI) is closely linked to atmospheric conditions, seeding operations, and dynamical, thermodynamical, and microphysical processes. Here, microphysical processes leading to ice and snow production are analyzed in orographic clouds for three cloud-seeding events, each with light or no natural precipitation and well-defined, traceable seeding lines. Airborne and ground-based radar observations are linked to in situ cloud and precipitation measurements to determine the spatiotemporal evolution of ice initiation, particle growth, and snow fallout in seeded clouds. These processes and surface snow amounts are explored as particle plumes evolve from varying amounts of AgI released, and within changing environmental conditions, including changes in liquid water content (LWC) along and downwind of the seeding track, wind speed, and shear. More AgI did not necessarily produce more liquid equivalent snowfall (LESnow). The greatest amount of LESnow, largest area covered by snowfall, and highest peak snowfall produced through seeding occurred on the day with the largest and most widespread occurrence of supercooled drizzle, highest wind shear, and greater LWC along and downwind of the seeding track. The day with the least supercooled drizzle and the lowest LWC downwind of the seeding track produced the smallest amount of LESnow through seeding. The stronger the wind was, the farther away the snowfall occurred from the seeding track.

Mixed‐Phase Clouds and Precipitation in Southern Ocean Cyclones and Cloud Systems Observed Poleward of 64°S by Ship‐Based Cloud Radar and Lidar

AbstractMixed‐phase clouds (MPCs), composed of both liquid and ice, are prevalent in Southern Ocean cyclones. A characterization of these clouds on fine vertical scales is required in order to understand the microphysical processes within these clouds, and for model and satellite evaluations over this region. We investigated three examples of cloud systems collected by ship‐mounted remote‐sensing instruments adjacent to East Antarctica at latitudes between 64°S and 69°S. These cases allow us to examine the properties of midlevel MPCs, with cloud tops between 2 and 6 km. Midlevel MPCs contain multiple layers of supercooled liquid water (SLW) embedded within ice during the passage of cyclones. SLW layers are capped by strong temperature inversions and are observed at temperatures as low as −31°C. Convective generating cells (GCs) are present inside supercooled liquid‐topped midlevel MPCs. The horizontal extent, vertical extent, and maximum upward Doppler velocity of these GCs were 0.6–3.6 km, 0.7–1.0 km, and 0.5–1.0 m s−1, respectively, and are consistent with observations from previous lower‐latitude studies. Ice precipitation is nearly ubiquitous, except in the thinnest clouds at the trailing end of the observed systems. Seeding of lower SLW layers from above leads to periods with either larger ice particles or greater ice precipitation rates. Periods of supercooled drizzle lasting up to 2 h were observed toward the end of two of the three cyclone systems. This supercooled drizzle turns into predominantly ice precipitation as the result of seeding by ice clouds located above the precipitating SLW layer.

Supercooled drizzle development in response to semi-coherent vertical velocity fluctuations within an orographic-layer cloud

Abstract. Observations of supercooled liquid water are nearly ubiquitous within wintertime orographic-layer clouds over the Intermountain West; however, observations of regions containing supercooled drizzle drops (SCDDs) are much rarer and the factors controlling SCDD development and location less well understood. As part of the Seeded and Natural Orographic Wintertime clouds – the Idaho Experiment (SNOWIE) and its goal of improving understanding of natural cloud structure, this study examines the role of fine-scale (sub-kilometer) vertical velocity fluctuations on the microphysical evolution and location of SCDDs within the observed mixed-phase, wintertime orographic clouds from one research flight in SNOWIE. For the case examined, SCDDs developed in an elevated, postfrontal-layer cloud with cold cloud tops (T<-30 ∘C) and low number concentrations of both ice (less than 0.5 L−1) and cloud droplets (less than 30 cm−3). Regions of supercooled drizzle at flight level extended more than a kilometer along the mean wind direction and were first located at and below layers of semi-coherent vertical velocity fluctuations (SCVVFs) embedded within the cloud and subsequently below cloud top. The microphysical development of SCDDs in this environment is catalogued using size and mass distributions derived from in situ probe measurements. Regions corresponding to hydrometeor growth are determined from radar reflectivity profiles retrieved from an airborne W-band cloud radar. Analysis suggests that SCVVF layers are associated with local SCDD development in response to the kinematic perturbation pattern. This drizzle development and subsequent growth by collision–coalescence is inferred from vertical reflectivity enhancements (−20 dBZ km−1), with drizzle production confirmed by in situ measurements within one of these SCVVF layers. The SCDD production and growth occurs embedded within cloud over shallow (km or less) layers before transitioning to drizzle production at cloud top further downwind, indicating that wind shear and resultant vertical velocity fluctuations may act to enhance or speed up SCDD development compared to classic cloud top broadening mechanisms in orographic (or similarly sheared) cloud environment(s).

Persistent Supercooled Drizzle at Temperatures below -25°C Observed at McMurdo Station, Antarctica.

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 hours. 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 ~1000 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 (IFN) 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.

Observations of the Origin and Distribution of Ice in Cold, Warm, and Occluded Frontal Systems during the DIAMET Campaign

Abstract Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L−1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L−1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m−3, but lower in the cold front case.

The common occurrence of highly supercooled drizzle and rain near the coastal regions of the western United States

AbstractHighly supercooled rain and drizzle from cloud tops at −12 to −21°C down to the 0° isotherm was documented by aircraft observations in clouds over a wide range of meteorological situations under relatively pristine marine aerosol conditions. The Gulfstream‐1 aircraft during the CalWater campaign in February and early March 2011 measured clouds over the coastal waters of northern California, orographically triggered convective clouds over the foothills of the Sierra Nevada, and orographic layer clouds over Yosemite National Park. Supercooled drizzle in layer clouds near Juneau, Alaska, was measured by the Wyoming King Air as part of a FAA project to study aircraft icing in this region. Low concentrations of cloud condensation nuclei (CCN) were commonly observed in all of these clouds, allowing for the formation of clouds with small concentrations of mostly large drops that coalesced into supercooled drizzle and raindrops. Another common observation was the absence of ice nuclei (IN) and/or ice crystals in measurable concentrations, associated with persistent supercooled drizzle and rain. Average ice crystal concentrations were 0.007 l−1 at the top of convective clouds at −12°C and 0.03 l−1 in the case of layer clouds at −21°C. In combination, these two conditions of low concentrations of CCN and very few IN provide ideal conditions for the formation of highly supercooled drizzle and rain. These results help explain the anomalously high incidences of aircraft icing at cold temperatures in U.S. west coast clouds and highlight the need to include aerosol effects when simulating aircraft icing with cloud models.

Observations of Freezing Drizzle in Extratropical Cyclonic Storms during IMPROVE-2

Abstract Observations of supercooled drizzle aloft within two storms impacting the Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project are presented. The storms were characterized by a structure and evolution similar to the split-front model of synoptic storms. Both storms were also characterized by strong cross-barrier flow. An analysis of aircraft and radar data indicated the presence of supercooled drizzle during two distinct storm periods: 1) the intrafrontal period immediately following the passage of an upper cold front and 2) the postfrontal period. The conditions associated with these regions of supercooled drizzle included 1) temperatures between −3° and −19°C, 2) ice crystal concentrations between 1 and 2 L−1, and 3) bimodal cloud droplet distributions of low concentration [cloud condensation nuclei (CCN) concentration between 20 and 30 cm−3 and cloud drop concentration <35 cm−3]. Unique to this study was the relatively cold cloud top (<−15°C) and relatively high ice crystal concentrations in the drizzle region. These conditions typically hinder drizzle formation and survival; however, the strong flow over the mountain barrier amplified vertical motions (up to 2 m s−1) above local ridges, the mountain crest, and updrafts in embedded convection. These vertical motions produced high condensate supply rates that were able to overcome the depletion by the higher ice crystal concentrations. Additionally, the relatively high vertical motions resulted in a near balance of ice crystal fall speed (0.5–1.0 m s−1), leading to nearly terrain-parallel trajectories of the ice particles and a reduction of the flux of ice crystals from the higher levels into the low-level moisture-rich cloud, allowing the low-level cloud water and drizzle to be relatively undepleted. One of the key observations in the current storms was the persistence of drizzle drops in the presence of significant amounts of ice crystals over the steepest portion of the mountain crest. Despite the high radar reflectivity produced by the ice crystals (>15 dBZ) in this region, the relatively high condensate supply rate led to hazardous icing conditions. The current study reveals that vertical motions generated by local topographic features are critical in precipitation processes such as drizzle formation and thus it is essential that microphysical models predict these motions.

Observations of supercooled water and secondary ice generation by a vertically pointing X-band Doppler radar

When a vertically pointing Doppler radar (VPDR) observes two groups of hydrometeors having distinct fall velocities, the observed Doppler spectra are bimodal. This bimodality has been repeatedly observed in snow. At altitudes higher than the level at which the bimodality appears, the reflectivity of the snow rapidly increases at the same time as its fall velocity reaches values beyond those expected from the aggregation of snow crystals. This is indicative of riming by supercooled cloud droplets. We show here that the spectral bimodality observed in the snow after indication of riming may be associated with secondary ice generation or with supercooled drizzle. Higher snow content favors the formation of secondary ice, while in situations of low snow content, the presence of the supercooled cloud may lead to the formation of drizzle. In both cases, the bimodality indicates the significant content of the supercooled cloud. This type of observation can be used operationally to detect the supercooled water coexisting with the snow.

Supercooled drizzle formed by condensation–coalescence in the mid-winter season of the Canadian Arctic

Supercooled drizzle (freezing drizzle) was observed at Inuvik, N.W.T., Canada (68°22′N, 133°42′W) on December 20, 21 and 27, 1995. Meteorological conditions in which the supercooled drizzle could form under low temperatures (colder than −20°C) in the mid-winter season of the Canadian Arctic were examined from the sounding data and data measured by a passive microwave radiometer at ground level. The following results were obtained. (1) Supercooled drizzle fell to the ground with ice pellets and frozen drops on snow crystals. (2) The maximum size of supercooled drizzle particles increased as the depth of cloud layer saturated with respect to water increased. (3) Because a layer of air temperature higher than 0°C was not detected from the sounding data at Inuvik, melting of snow particles was impossible. It was concluded, therefore, that supercooled drizzle was formed by the condensation–coalescence process below freezing temperature.

A Wind Tunnel Investigation of Interactions between Supercooled Precipitation-Size Water Drops

Abstract Laboratory experiments were performed to investigate interactions between small (400–900 μm) precipitation-size drops at temperatures colder than 0°C. The investigation was accomplished by creating a light shower of supercooled drizzle drops in a vertical wind tunnel. The experiments were performed to test the hypothesis that ice may nucleate by mechanical shock when supercooled precipitation-size drops collide. Experimental results did not strongly support collision–freezing as a viable mechanism that would fully explain the large discrepancy between observed concentrations of “first” ice and ice nuclei in clouds characterized by an active coalescence mechanism. Also reported is the result of one trial in which a subject drop prematurely froze and subsequently interacted with a supercooled drop in the tunnel rain shower to form a dumbbell-shaped ice particle. Two-dimensional optical array probe data obtained at the −10°C level in warm-based Midwestern rain clouds are presented that also show dum...

Characterizing the Supercooled Large Droplet Environment with Corresponding Turboprop Aircraft Response

This continuing study evaluated turboprop aircraft performance response to various environmental conditions. These conditions included clear air, warm rain, ice only, mixed phase, and supercooled drops encountered during 19 separate e ights. Supercooled droplets consisting of cloud, drizzle, and rain sizes were the main focus of this study. Aircraft response was quantie ed by rates of change in aircraft rateof-climb capability, lift and drag coefe cients and, lift over drag ratio. The aircraft performance parameters were compared to environmental hydrometeor parameters, such as 80% volumetric diameter (80VD)*liquid water content (LWC), quantifying the environmental conditions. Results show that encounters with supercooled drizzle drops, or ZL, resulted in maximum rates of performance degradation. These high rates of degradation forced the pilot to take evasive action within 5 min of entering these hazardous conditions. The Wyoming King Air experienced substantial increases in drag and stall speed, substantial decreases in climb capability, and signie cant lift degradations while encountering ZL. Encounters with supercooled cloud and rain-sized drops resulted in minor to low rates of performance degradation, whereas encounters with supercooled drops in low ice particle concentrations resulted in only minor rates of degradation. In addition, aircraft response to high ice particle concentrations and low liquid water, following a ZL encounter, resulted in rapid performance recovery, possibly because of erosion of ice on the airframe. Aggregates of needles and hexagonal plates led to the highest rates of recovery, whereas cold, clear air resulted in the lowest rates of recovery. Furthermore, e ight analyses facilitated quantifying ZL horizontal extents and encounter frequencies. For example, ZL measured by a one-dimensional cloud probe during atmospheric research e ights exceeded 0.05 g /m 3 twice per 10 e ight hours over a horizontal length of 29 km. The results presented herein show a strong relationship between aircraft response and environmental parameters utilizing the largest drops in the hydrometeor distribution (80VD*LWC and ZL LWC). The results suggest that the most severe icing is actually caused by drizzle-sized drops as opposed to freezing rain. In addition, the results suggest that activating the de-icing boots (with typical chordwise coverage) after a ZL encounter may have little effect on aircraft performance recovery.

Turboprop Aircraft Performance Response to Various Environmental Conditions

This study evaluated aircraft performance response to various environmental conditions encountered during flight. These conditions included clear air, ice only, mixed phase, and supercooled drops. Supercooled drops consisting of cloud, drizzle and rain sizes were the main focus of this study. Aircraft response was quantified by rates of change in aircraft climb capability, drag coefficient, and lift over drag ratio. The aircraft performance parameters were compared to an environmental hydrometeor parameter quantifying the environmental conditions. Results show that encounters with supercooled drizzle drops, or SCDD, resulted in maximum rates of performance degradation. Encounters with supercooled cloud and rain sized drops resulted in minor to low rates of performance degradation while encounters with supercooled drops in low ice particle concentrations resulted in only minor rates of degradation. In addition, aircraft response to high ice particle concentrations, in low liquid water, following a SCDD encounter resulted in rapid performance recovery. The results presented herein show a strong relationship between aircraft response and an environmental parameter utilizing a weighted volume diameter and liquid water content. The results suggest that the most severe icing is actually caused by SCDD as opposed to freezing rain. (Author)

Meteorological Conditions Associated with the ATR72 Aircraft Accident near Roselawn, Indiana, on 31 October 1994

An ATR72 commuter aircraft crashed near Roselawn, Indiana, on 31 October 1994 killing all 68 people on board. Available weather data, including those from a Next Generation Radar, a radar wind profiler, a Geostationary Operational Environmental Satellite, and pilot reports of icing have been examined in combination with analysis fields from the Rapid Update Cycle model and forecast fields from the Pennsylvania State University/National Center for Atmospheric Research MM5 numerical model. Synthesis of this information provides a relatively complete and consistent picture of the ambient meteorological conditions in the region of the ATR72 holding pattern at ~3.1 km above mean sea level. Of particular interest is the evidence that these conditions favored the development of supercooled drizzle drops within a strong frontal zone, as indicated by cloud-top temperatures of −10° to −15°C, weak radar reflectivity, and strong, vertical wind shear within the cloud and warm front.

An Example of Supercooled Drizzle Drops Formed through a Collision-Coalescence Process

Abstract The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of −7.5° and −12°C, respectively. The cloud was characterized by a low droplet concentration of 21 cm−3 and a large droplet median volume diameter of 29 µm, with a concentration of interstitial aerosol particles of less than 15 cm−3 (larger than 0. 13 µm in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 µm observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 L−1 for droplets larger than 40 µm, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0...

Airfoil performance degradation by supercooled cloud, drizzle, and rain drop icing

A wind-tunnel test was conducted to determine the performance degradation of a scaled two-dimensional NAC A 23012 airfoil (outboard wing section of the Wyoming King Air 200T) resulting from various liquid hydrometeor sizes. The hydrometeor sizes consisted of cloud, drizzle, and rain drops. The NASA Lewis Research Center's LEWICE 1.6 computer code was used to predict the ice shapes based on natural distributions obtained by the Wyoming King Air. The tests were conducted for angles of attack between —2 and +18 deg. Test results showed a moderate reduction in CLmax for the cloud, drizzle, and rain drop ice shapes; however, the rain drop (or freezing rain) ice shape delayed flow separation when compared to the cloud and drizzle shapes, resulting in a higher angle of attack at stall. Profile drag increased by 5 35% for the cloud drop ice shape, 48-56% for the drizzle ice shape, and 10-42% for the rain ice shape. After a simulated de-icing boot activation, test results showed a 50% reduction in CLmax for the residual drizzle ice shape and 34% reduction for the residual rain ice shape. Profile drag increased by up to 790% for the residual drizzle ice shape and up to 590% for the residual rain ice shape. The residual cloud ice was assumed to be cleared by the de-icing boot activation. These results do not support the current aircraft community belief that freezing rain causes the most severe icing conditions. On the contrary, aircraft encounters with supercooled drizzle, or freezing drizzle, may result in the most severe icing.

A Simple Objective Method Used to Forecast Convective Activity during the 1989 PACE Cloud-seeding Experiment

Abstract A simple objective procedure used exploratively to forecast the occurrence, height, and coalescence activity of summertime convective clouds in Illinois during the cloud-seeding trials of the 1989 Precipitation Augmentation for Crops Experiment is described. The method used the temperature of the convective condensation level (TCCL) and potential buoyancy (PB) at 500 mb, easily determined from morning National Weather Service sounding data, to forecast afternoon convection. Maximum echo top heights were found to group according to TCCL and PB. The physical basis of TCCL and PB to implicitly represent a period of time for coalescence to produce supercooled drizzle and raindrops is discussed. The technique performed well at forecasting the occurrence and height of afternoon convective clouds. Aircraft measurements of supercooled raindrop concentrations showed that a discriminator function, dependent only on TCCL and PB, gave a good indication of the presence or absence of supercooled drizzle and ra...

Illinois Precipitation Research: A Focus on Cloud and Precipitation Modification

At the heart of the 40-year atmospheric research endeavors of the Illinois State Water Survey have been studies to understand precipitation processes in order to learn how precipitation is modified purposefully and accidentally, and to measure the physical and socioeconomic consequences of cloud and precipitation modification. Major field and laboratory activities of past years are briefly treated as a basis for describing the key findings of the past ten years. Recent studies of inadvertent and purposeful cloud and rain modification and their effects are emphasized, including a 1989 field project conducted in Illinois and key findings from an on-going exploratory experiment addressing cloud and rain modification. Results are encouraging for the use of dynamic seeding on summer cumuliform clouds of the Midwest. Typical in-cloud results at −10°C reveal multiple updrafts that tend to be filled with large amounts of supercooled drizzle and raindrops. Natural ice production is vigorous, and initial concentrat...