Alliance Icing Research Study Research Articles

Airborne Measurement of Liquid and Total Water Content

Abstract Two identical liquid water content (LWC) King probes—one total water content/liquid water content (TWC/LWC) Nevzorov probe and two constant-temperature T probes that are different in size to distinguish particles of different densities and diameters (section 2c)—were flown during the Alliance Icing Research Study (AIRS) II field campaign in the fall of 2003. This paper assesses measurements performed during several flights in mostly stratiform clouds. The two LWC King probes tracked well; however, discrepancies of up to 0.1 g m−3 for 1-s LWC measurements of 0.3 g m−3 were observed. Agreement between probes of different geometry and size was generally favorable, while levels of disagreement between the probes changed during numerous cloud penetrations from less than 20% up to a factor of 2, varying with flight conditions and microphysical structure of the cloud. Disagreement between probes was even larger when detecting ice water content (IWC). Measurement differences were attributed to different collection efficiencies resulting from preferred particle size, shape, and density and local aerodynamic effects around the aircraft. Measurements from a single probe are subject to uncertainty at a single point in time beyond the noise and drift level of the instrument. This uncertainty is evaluated considering particle habit, diameter, and density, and probe geometry and size, in addition to particle impact, breakup/splash, and bounce. From a working point of view, the intercomparison of several probes is subject to real but unknown spatial differences because of different locations between air samples. Comparison of identical probes is not appropriate because each measurement in itself is unique by definition. Thus, instead of duplication of instruments, subject to these levels of agreement, the use of a single probe is a practical approach while remaining aware of its limitations and capabilities.

A Comprehensive Habit Diagram for Atmospheric Ice Crystals: Confirmation from the Laboratory, AIRS II, and Other Field Studies

Abstract Recent laboratory experiments and in situ observations have produced results in broad agreement with respect to ice crystal habits in the atmosphere. These studies reveal that the ice crystal habit at −20°C is platelike, extending to −40°C, and not columnar as indicated in many habit diagrams found in atmospheric science journals and texts. These diagrams were typically derived decades ago from laboratory studies, some with inherent habit bias, or from combinations of laboratory and in situ observations at the ground, observations that often did not account for habit modification by precipitation from overlying clouds of varying temperatures. Habit predictions from these diagrams often disagreed with in situ observations at temperatures below −20°C. More recent laboratory and in situ studies have achieved a consensus on atmospheric ice crystal habits that differs from the traditional habit diagrams. These newer results can now be combined to give a comprehensive description of ice crystal habits for the atmosphere as a function of temperature and ice supersaturation for temperatures from 0° to −70°C, a description dominated by irregular and imperfect crystals. Cloud particle imager (CPI) habit observations made during the Second Alliance Icing Research Study (AIRS II) and elsewhere corroborate this comprehensive habit description, and a new habit diagram is derived from these results.

Ice and water content of stratiform mixed-phase cloud

Spatial characterization and probability distribution of particles in stratiform mixed-phase clouds in the temperature range 5 to −45 °C were investigated based on 81 hours of flight on the NCAR C-130 during the Alliance Icing Research Study II. Cloud particles were video-recorded impacting on the optical window of a cloudscope, and Liquid Water Content (LWC) and Ice Water Content (IWC) derived from measurements of the power to evaporate particles of different size, shape and density collected by the T probe constant temperature sensors. Data were recorded at 1 Hz for a resolution of 130 m. Transitions between liquid, glaciated and mixed-phase regions were sharp; frequent supercooled liquid regions lasted less than 2 seconds ( 0.9); mixed-phase corresponded to about 40% of the measurements. The large T probe showed mixed-phase clouds with ice fraction less than half at temperatures below −20 °C, while the small T probe measured higher ice proportion, suggesting that ice was present in the form of preferentially collected small particles. At high temperatures (>−15 °C) ice and water were measured in concentrations up to 0.2 and 0.6 g m−3 respectively; at temperatures below −20 °C they were both measured in concentrations up to 0.3 g m−3. Copyright © 2009 Royal Meteorological Society

Use of X-band radars to support the detection of in-flight icing hazards

The NASA Icing Remote Sensing System was operated for the Alliance Icing Research Study II field program during the winter of 2003 around Montreal, Canada and around Cleveland, Ohio during the winter of 2005. Icing research aircraft flights from these field programs provided verification data on liquid water content, air temperature and also cloud particle imagery and distributions. The purpose of this work is to show that the NASA Icing Remote Sensing System X-band radar reflectivity profiles could be used beyond merely defining vertical cloud boundaries, by operationally deriving a qualitative small drop icing hazard warning flag. Several case studies are presented which depict a variety of synoptic weather scenarios. These cases demonstrate that X-band reflectivities below -10 dBZ and above the minimum detectable are uniquely indicative of a particle population dominated by small, liquid droplets. A discussion is included for each case on how an in-flight icing hazard flag from the radar reflectivity profile would improve the operational hazard detection system. Comparison of the NASA Icing Remote Sensing System's X-band radar data to a nearby similar X-band from McGill University is done to ensure data quality and consistency.

An Investigation of the Influence of Droplet Number Concentration and Giant Aerosol Particles upon Supercooled Large Drop Formation in Wintertime Stratiform Clouds

Abstract Supercooled large drops (SLD) can be a significant hazard for aviation. Past studies have shown that warm-rain processes are prevalent, or even dominant, in stratiform clouds containing SLD, but the primary factors that control SLD production are still not well understood. Giant aerosol particles have been shown to accelerate the formation of the first drizzle drops in some clouds and thus are a viable source of SLD, but observational support for testing their effectiveness in supercooled stratiform clouds has been lacking. In this study, new observations collected during six research flights from the Alliance Icing Research Study II (AIRS II) are analyzed to assess the factors that may be relevant to SLD formation, with a particular emphasis on the importance of giant aerosol particles. An initial comparison of observed giant aerosol particle number concentrations with the observed SLD suggests that they were present in sufficient numbers to be the source of the SLD. However, microphysical calculations within an adiabatic parcel model, initialized with the observed aerosol distributions and cloud properties, suggest that the giant aerosol particles were only a limited source of SLD. More SLD was produced in the modeled clouds with low droplet concentrations, simply by an efficient warm-rain process acting at temperatures below 0°C. For cases in which the warm-rain process is limited by a higher droplet concentration and small cloud depth/liquid water content, the giant aerosol particles were then the only source of SLD. The modeling results are consistent with the observed trends in SLD across the six AIRS II cases.

Giant and Ultragiant Aerosol Particle Variability over the Eastern Great Lakes Region

Abstract Numerous studies have indicated the potential for giant and ultragiant aerosol particles to expedite the warm-rain process as a result of their extreme sizes. The central question regarding their importance is, Are they present in large enough numbers to influence the microphysics of the clouds significantly? Thus, quantification of these particles and their variability is paramount. New observations collected during the second Alliance Icing Research Study (AIRS II) are presented as evidence of the presence and variability of giant and ultragiant aerosol particles over a continental region—in this case, within the eastern Great Lakes region and parts of the midwestern United States and Canada during one month in winter 2003. Sources and factors contributing to the amount of these particles observed in the lower atmosphere were difficult to identify separately; future studies incorporating high-resolution weather modeling are likely needed.

Cloud fraction parameterization as a function of mean cloud water content and its variance using in‐situ observations

The main objective of the present work is to use in‐situ data to parameterize cloud fraction (Cs) as a function of both cloud condensed water content and its variability over scales of 10 km and 100 km. In‐situ data were obtained during the Alliance Icing Research Study (AIRS) field project and represent mid‐latitude winter stratiform clouds. The main observations used in the analysis were condensed total water content (qc) and liquid water content. It is shown that Cs, defined as the length of cloudy segments divided by the clear and cloudy segments along a flight leg, can change up to 5%–25% if the averaging scale is changed from 10 km to 100 km. A new parameterization of Cs versus the ratio of qc to its variance is suggested. The sub‐grid scale variability is usually not known directly from the models and it needs to be obtained by prognostic or diagnostic equations. It is found that cloud sub‐grid variability, represented by the standard deviation (σ) of qc, increases with increasing qc for both 10 and 100 km scales, and σqc for 10 km scales is larger by about 50% as compared to 100 km scales for a given qc value. These conclusions suggest that both averaging scales and sub‐grid scale variability should be considered for cloud cover parameterizations.

New dual‐frequency microwave technique for retrieving liquid water path over land

We present and demonstrate a new methodology for retrieving liquid water path over land using satellite‐based microwave observations. As input, the technique exploits Advanced Microwave Scanning Radiometer for EOS (AMSR‐E) brightness temperature polarization‐difference signals at 37 and 89 GHz. Regression analysis performed on model simulations indicates that over variable atmospheric and surface conditions these polarization‐difference signals can be simply parameterized in terms of the surface emissivity polarization‐difference (Δε), surface temperature, liquid water path (LWP), and precipitable water vapor (PWV). By exploiting the weak frequency dependence of Δε, a simple expression is obtained which enables fast and direct (noniterative) retrievals of LWP. The new methodology is demonstrated and validated using several months of AMSR‐E observations over (1) the Southern Great Plains (SGP) of the United States and (2) an area near Montreal, Canada, instrumented during the Alliance Icing Research Study II (AIRS II) field campaign. Comparisons are also made with MODIS LWP retrieval results for one scene over the SGP region. Retrieval results in clear‐sky conditions indicate an uncertainty on the order of 0.06 mm, in agreement with theoretical estimates. In cloudy conditions, results using the new method are systematically smaller than results for both ground‐based microwave radiometers and MODIS but are well correlated.

Ice water content and precipitation rate as a function of equivalent radar reflectivity and temperature based on in situ observations

Using ice particle spectra measured in stratiform ice clouds in midlatitude and Arctic regions, ice water content (IWC) and precipitation rate (Fm) retrieval algorithms as a function of temperature and radar reflectivity factor (Z) have been developed. These parameterizations were compared with (1) direct measurements of IWC using a Nevzorov probe, (2) precipitation retrieved using an X‐band Doppler scanning radar and a Precipitation Occurrence Sensor System (POSS), (3) the Canadian Global Environmental Multiscale (GEM) and High‐Resolution Model Application Project (HIMAP) models, and (4) derived IWC and precipitation from measured ice spectra during four field projects. The derived IWC and Fm from measured spectra have a correlation coefficient (r) better than 0.8. The IWC retrieved using the X‐band scanning radar during the First Alliance Icing Research Studies (AIRS I) agreed with IWC measured using the Nevzorov Probe much better than conventional IWC retrieval schemes. The retrieved precipitation rate based on the new algorithm using the POSS reflectivity measurements during the Second Alliance Icing Research Study (AIRS II) project agreed reasonably well with the precipitation rates predicted by the GEM and HIMAP models. This study clearly demonstrated that conventional IWC‐Ze relationships fail to replicate the IWC measured with Nevzorov probe. Furthermore, the Fm –Z relationship currently in use in the Canadian operational radar network appears to underestimate the ice precipitation rate. However, since there are some uncertainties in the direct measurements of IWC and no reliable direct measurements of precipitation, further studies are required to validate these parameterizations.

Cloud microphysical characteristics versus temperature for three Canadian field projects

Abstract. The purpose of this study is to better understand how cloud microphysical characteristics such as liquid water content (LWC) and droplet number concentration (Nd) change with temperature (T). The in situ observations were collected during three research projects including: the Radiation, Aerosol, and Cloud Experiment (RACE) which took place over the Bay of Fundy and Central Ontario during August 1995, the First International Regional Arctic Cloud Experiment (FIRE.ACE) which took place in the Arctic Ocean during April 1998, and the Alliance Icing Research Study (AIRS) which took place in the Ontario region during the winter of 1999–2000. The RACE, FIRE.ACE, and AIRS projects represent summer mid-latitude clouds, Arctic clouds, and mid-latitude winter clouds, respectively. A LWC threshold of 0.005 g m-3 was used for this study. Similar to other studies, LWC was observed to decrease with decreasing T. The LWC-T relationship was similar for all projects, although the range of T conditions for each project was substantially different, and the variability of LWC within each project was considerable. Nd also decreased with decreasing T, and a parameterization for Nd versus T is suggested that may be useful for modeling studies.Key words. Atmospheric composition and structure (cloud physics and chemistry) – Meteorology and atmospheric dynamics (climatology; general circulation)