R. Paul Lawson

The 2D-S (Stereo) Probe: Design and Preliminary Tests of a New Airborne, High-Speed, High-Resolution Particle Imaging Probe

The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-m pixel resolution at aircraft speeds up to 250 m s 1 . An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (100 m) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops 150 m when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to 150 m and collected at airspeeds 100 m s 1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.

In situ observations of the microphysical properties of wave, cirrus, and anvil clouds. Part II: Cirrus clouds

A Learjet research aircraft was used to collect microphysical data, including cloud particle imager (CPI) measurements of ice particle size and shape, in 22 midlatitude cirrus clouds. The dataset was collected while the aircraft flew 104 horizontal legs, totaling over 15 000 km in clouds. Cloud temperatures ranged from 28° to 61°C. The measurements show that cirrus particle size distributions are mostly bimodal, displaying a maximum in number concentration, area, and mass near 30 m and another smaller maximum near 200–300 m. CPI images show that particles with rosette shapes, which include mixed-habit rosettes and platelike polycrystals, constitute over 50% of the surface area and mass of ice particles 50 m in cirrus clouds. Approximately 40% of the remaining mass of ice particles 50 m are found in irregular shapes, with a few percent each in columns and spheroidal shapes. Plates account for 1% of the total mass. Particles 50 m account for 99% of the total number concentration, 69% of the shortwave extinction, and 40% of the mass in midlatitude cirrus. Plots and average equations for area versus particle size are shown for various particle habits, and can be used in studies involving radiative transfer. The average particle concentration in midlatitude cirrus is on the order o f1c m 3 with occasional 10-km averages exceeding 5 cm 3 . There is a strong similarity of microphysical properties of ice particles between wave clouds and cirrus clouds, suggesting that, like wave clouds, cirrus ice particles first experience conversion to liquid water and/or solution drops before freezing.

Ice nucleation and dehydration in the Tropical Tropopause Layer

Optically thin cirrus near the tropical tropopause regulate the humidity of air entering the stratosphere, which in turn has a strong influence on the Earth’s radiation budget and climate. Recent high-altitude, unmanned aircraft measurements provide evidence for two distinct classes of cirrus formed in the tropical tropopause region: (i) vertically extensive cirrus with low ice number concentrations, low extinctions, and large supersaturations (up to ∼70%) with respect to ice; and (ii) vertically thin cirrus layers with much higher ice concentrations that effectively deplete the vapor in excess of saturation. The persistent supersaturation in the former class of cirrus is consistent with the long time-scales (several hours or longer) for quenching of vapor in excess of saturation given the low ice concentrations and cold tropical tropopause temperatures. The low-concentration clouds are likely formed on a background population of insoluble particles with concentrations less than 100 L−1 (often less than 20 L−1), whereas the high ice concentration layers (with concentrations up to 10,000 L−1) can only be produced by homogeneous freezing of an abundant population of aqueous aerosols. These measurements, along with past high-altitude aircraft measurements, indicate that the low-concentration cirrus occur frequently in the tropical tropopause region, whereas the high-concentration cirrus occur infrequently. The predominance of the low-concentration clouds means cirrus near the tropical tropopause may typically allow entry of air into the stratosphere with as much as ∼1.7 times the ice saturation mixing ratio.

Improvement in Determination of Ice Water Content from Two-Dimensional Particle Imagery. Part I: Image-to-Mass Relationships

Ice water content in natural clouds is an important but difficult quantity to measure. The goal of a number of past studies was to find average relationships between the masses and lengths of ice particles to determine ice water content from in situ data, such as those routinely recorded with two-dimensional imaging probes. The general approach in these past studies was to measure maximum length L and mass M of a dataset of ice crystals collected at a ground site. Linear regression analysis was performed on the logarithms of the data to estimate an average mass-to-length relationship of the form M L. Relationships were determined for subsets of the dataset based on crystal habit (shape) as well as for the full dataset. In this study, alternative relationships for determining mass using the additional parameters of width W, area A, and perimeter P are explored. A 50% reduction in rms error in the determination of mass relative to using L alone is achieved using a single parameter that is a combination of L, W, A, and P. The new parameter is designed to take into account the shape of the ice particle without the need to classify the crystals first. An interesting result is that, when applied to the test dataset, the same reduction in rms error is also shown to be achievable using A alone. Using A alone facilitates the reanalysis and improvement of the determination of ice water content from large existing datasets of two-dimensional images, because A is simply the number of occulted pixels in the digital images. Possible sources of error in this study are investigated, as is the usefulness of first segregating the particles into crystal habits.

Shapes, sizes and light scattering properties of ice crystals in cirrus and a persistent contrail during SUCCESS

A persistent contrail in the shape of a racetrack was generated by the NASA DC-8 research aircraft during the SUCCESS project. The contrail was visible on GOES imagery for six hours. Microphysical measurements collected by the DC-8 show that after 40 min the core of the contrail contained mostly small (1 to 20 µm) ice particles in concentrations >1000 L−1, with larger (>300 µm) ice crystals in concentrations 300 µm. The larger ice crystals in the periphery were mostly columns and bullet rosettes, similar to habits of larger ice crystals found in ambient cirrus in the area. Measurements suggest that the shape of phase functions of randomly-oriented columns and rosettes are mostly featureless. The measured phase functions are closest in shape to those predicted by ray-tracing theory for random-fractal and spatial-dendrite ice crystals.

Growth of ice crystals in a precipitating contrail

This study examines how jet aircraft contrails develop precipitation trails, using data collected on 12 May, 1996 during SUCCESS. The DC-8 sampled the precontrail conditions, produced a contrail largely in clear air at −52°C, and sampled the contrail and developing trails for over an hour. The environment was highly ice-supersaturated, reaching nearly water saturation in some locations. Inside the contrail core, almost all ice particles remained small (∼1 to 10 µm) due to high crystal concentrations (∼10¹ to 10² cm−3) which reduced the vapor density to saturation. Mixing of moist environmental air and vapor-depleted contrail air produced localized regions of supersaturation along the contrail periphery, where crystals grew to several hundred microns at about 0.1 µm s−1 These particles could then fall from the contrail into the vapor-rich, undepleted, supersaturated environment below. As heavier crystals left the contrail, others moved into the regions of ice supersaturation. Precipitation trails developed as this process continued over time.

RACORO EXTENDED-TERM AIRCRAFT OBSERVATIONS OF BOUNDARY LAYER CLOUDS

A first-of-a-kind, extended-term cloud aircraft campaign was conducted to obtain an in situ statistical characterization of continental boundary layer clouds needed to investigate cloud processes and refine retrieval algorithms. Coordinated by the Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF), the Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign operated over the ARM Southern Great Plains (SGP) site from 22 January to 30 June 2009, collecting 260 h of data during 59 research flights. A comprehensive payload aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft measured cloud microphysics, solar and thermal radiation, physical aerosol properties, and atmospheric state parameters. Proximity to the SGPs extensive complement of surface measurements provides ancillary data that support modeling studies and facilitates evaluation of a variety of surface retrieval algorithms. The five-...

The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds

AbstractThe rapid glaciation of tropical cumulus clouds has been an enigma and has been debated in the literature for over 60 years. Possible mechanisms responsible for the rapid freezing have been postulated, but until now direct evidence has been lacking. Recent high-speed photography of electrostatically suspended supercooled drops in the laboratory has shown that freezing events produce small secondary ice particles. Aircraft observations from the Ice in Clouds Experiment–Tropical (ICE-T), strongly suggest that the drop-freezing secondary ice production mechanism is operating in strong, tropical cumulus updraft cores. The result is the production of small ice particles colliding with large supercooled drops (hundreds of microns up to millimeters in diameter), producing a cascading process that results in rapid glaciation of water drops in the updraft. The process was analyzed from data collected using state-of-the-art cloud particle probes during 54 Learjet penetrations of strong cumulus updraft cores...

Cloud particle measurements in thunderstorm anvils and possible weather threat to aviation

Since 1990, there have been at least 10 known incidents where jet aircraft have experienced loss of thrust in one or more turbofan engines while maneuvering in the anvil region near the central core of a thunderstorm. The exact cause of the uncommanded thrust reduction, commonly called engine rollback, is still under investigation. It appears that the rollback incidents may be associated with ingestion of high mass concentrations of ice particles, snow, and possibly small concentrations of supercooled liquid water in the anvil region. The characteristics of cloud particles in thunderstorm anvils have not been extensively studied. Results from analysis of aircraft observations in the anvils of midlatitude and tropical thunderstorms are discussed. Aircraft and limited radar observations show that most anvils associated with small, garden-variety thunderstorms contain low ( < ∼ 0.4 g m -3 ) mass concentrations of ice particles. In larger, more intense midlatitude storms, anvils may contain ice water contents from 1 to 3 g m -3 . The mean of the maximum particle dimension in the anvil region of the more intense storms showed a strong modal size of about 2 mm. The particles themselves appear to be ice crystals and aggregates of ice crystals, i.e., snowflakes

Observations and Numerical Simulations of the Origin and Development of Very Large Snowflakes

Abstract The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the co...