Aircrafttinduced Hole Punch and Canal Clouds

AIRCRaFT-INDUCED HOLE PUNCh aND CaNaL CLOUDS Inadvertent Cloud Seeding BY ANDREW J. HEYMSFIELD , Pa TRICK C. KENNEDY , STEVE Ma SSIE , Ca RL SC h MITT , Zh IEN Wa NG , Sa MUEL Ha IMOV , a ND ART Ra NGNO Passage of commercial aircraft through supercooled altocumulus can induce freezing of droplets by homogenous nucleation and induce holes and channels, increasing the previously accepted range of temperatures for aviation- induced cloud effects. he peculiar and striking for- mations called “hole punch” T and “canal” clouds, which are sometimes seen in midlevel altocumulus, appear as circular or elongated channels or voids from which streamers of precipitation descend (Fig. 1). Altocumulus clouds are midlevel clouds that form typically at temperatures of about −15°C and are composed of water droplets, especially near cloud top, sometimes mixed with a concentration of ice particles. The features of the altocumulus in which these holes form—visual shape, cloud-top temperatures, optical depths, and cloud particle effective radii (e.g., Figs. 2a–d)— confirm that the parent clouds of FIG . 1. (top) Image of hole punch and canal clouds captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite the holes and canals are liquid. at 1705 UTC 29 Jan 2007 (image courtesy of NASA Earth Observatory and Globally, midlevel liquid-layer specifically J. Schmaltz). State boundaries are shown. (bottom) Pictures topped stratiform clouds cover taken from near New Orleans, Louisiana, at 2110–2117 UTC 29 Jan 2007, 7.8% of Earth’s surface on aver- looking northwest toward Shreveport. (Photos courtesy of Jafvis.) AMERICAN METEOROLOGICAL SOCIETY JUNE 2010 | 753 Unauthenticated | Downloaded 09/26/21 08:04 AM UTC age (Zhang et al. 2010). Typically, there are modest few microns and larger occurs when the droplet tem- concentrations of ice in the middle atmosphere, and perature cools to about −37°C or below. Alternatively, virga may be seen to be streaming downward from the Ludlam suggested that the crystals could be initiated liquid water parent cloud. However, if high ice particle by nuclei emitted in the engine exhaust. concentrations are introduced by natural seeding In 1968, a photograph published in Weatherwise (e. g., cirrus fallstreaks) or deliberate or inadvertent generated considerable interest in the “Hole-in-cloud: seeding, the liquid droplets in that region may dis- A meteorological whodunit?” from the U. S. meteoro- appear. According to the Bergeron–Findeisen (B-F) logical community (Weatherwise, August 1967) and process, if ice is introduced into a liquid water cloud set off a round of speculation as to its cause. Most of at subfreezing temperatures, the liquid droplets will the proposed mechanisms focused on how the unusual be out of vapor pressure equilibrium relative to the ice pattern of evaporation might have occurred, including and the liquid will evaporate, with vapor depositing the subsiding of air in the remnants of a hurricane’s onto the ice crystals and causing them to grow larger. eye and local warming of the air in dissipation trails, This process, together with the addition of dynamical or distrails, caused by rocket exhaust, passing meteors, effects, can produce a visual hole or canal. or the heat produced by the passage of a jet aircraft The formation mechanism(s) of hole punch and (Weatherwise, February 1969, and following issues). canal clouds has been the subject of conjecture based The Weatherwise correspondents also suggested that primarily on visual or satellite-borne imagery. Ice jet aircraft contrails might be a source of ice particles streamers nested within or falling from circular that would grow at the expense of preexisting cloud holes or elongated channels etched out of midlevel, droplets. Another report specifically implicated jet air- supercooled clouds were first reported in the meteo- craft in the generation of observed hole punch clouds rological literature in the 1940s (Schumacher 1940). A embedded within altocumulus (Simon 1966). The large horizontal loop in an altocumulus, sketched in a author proposed that acoustic shock waves generated correspondence titled “Man-made cirrus?” (Weather by the aircraft produced the ice crystals. 1948), was the first of such features to be attributed Aircraft continue to be the focus of attention with to an aircraft. Early on, Ludlam (1956) likened the respect to the generation of unusual holes in clouds. features to supercooled stratocumulus clouds seeded Distrails can create linear voids in clouds similar in by dry-ice pellets or silver iodide smoke released from appearance to the canals shown in Fig. 1. According an aircraft. He suggested that the ice crystals observed to Glickman (2000), distrails are “the opposite of a within the holes could be produced by spontaneous condensation trail.” Glickman (2000) thus describes freezing of the droplets from adiabatic cooling pro- their production: “Heat of combustion of the aircraft duced in the vortices shed by a propeller or by the fuel, released into the swept path by the exhaust rapid expansion of air at the wing tips. Spontaneous stacks of the aircraft, can, under certain conditions, freezing or “homogeneous ice nucleation” (HN; see evaporate existing clouds (if not too dense) and yield Bigg 1953) is a process by which droplets freeze with- a distrail.” Corfidi and Brandli (1986) observed from out the need for ice nuclei, which for cloud droplets a Geostationary Operational Environmental Satellite (GOES) imagery what they referred to as distrails in altocumulus. More recently, Duda and Minnis (2002) AFFILIATIONS: HEYMSFIELD , Ma SSIE , a ND SC h MITT —National documented the properties of the altocumulus and Center for Atmospheric Research, Boulder, Colorado; KENNEDY — ice fall streaks in two distrails observed in GOES Colorado State University, Fort Collins, Colorado; Wa NG a ND imagery with cloud-top temperatures of −28° and Ha IMOV —University of Wyoming, Laramie, Wyoming; Ra NGNO *— −31°C. Pedgley (2008) supported earlier views that Catalina, Arizona aircraft induce freezing of cloud droplets by sudden *Retired expansion and cooling to below –40°C within vortices CORRESPONDING AUTHOR: Andrew Heymsfield, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO trailing from tips of propeller blades and wings. 80307 Currently, ice production by aircraft is convention- E-mail: [email protected] ally considered to occur as contrails, which typically form behind jet aircraft as a result of engine combus- The abstract for this article can be found in this issue, following the table of contents. tion when the air temperature is −40°C and below, DOI:10.1175/2009BAMS2905.1 characteristic of cirrus-forming (ice cloud) conditions (Appleman 1953; Scorer and Davenport 1970). Jet In final form 4 September 2009 ©2010 American Meteorological Society aircraft fly predominantly at these temperature levels. Water vapor exhausted from an engine during com- 754 | JUNE 2010 Unauthenticated | Downloaded 09/26/21 08:04 AM UTC FIG . 2. Hole punch and canal clouds captured by the MODIS on NASA’s Terra satellite on 29 Jan 2007: (a) high-resolution imagery, (b) cloud-top temperature, (c) cloud optical depth, and (d) cloud effective radius. State boundaries are shown. bustion is rapidly condensed to droplets that almost tures can potentially generate ice crystals. Adiabatic immediately freeze to ice crystals (Appleman 1953; expansion that occurs over the upper surfaces of the Brasseur and Gupta 2010). Contrail crystals falling wings of jet aircraft from lift can produce local cool- into a supercooled cloud could dissipate droplets ing by some 20°C, which, under conditions of high but not ice crystals. Although extensive contrail cir- humidity and air temperatures of −20°C and below, rus can form when the ambient relative humidity is can produce condensation, droplet production, and in excess of saturation with respect to ice, contrail then HN (Gierens et al. 2009). Jet aircraft do not typi- crystals usually sublimate in dry layers in the upper cally fly in the −20° to −40°C temperature range, so the troposphere and thus do not generally fall into lower importance of this process may be restricted to climb altocumulus clouds. Hole punch clouds are therefore and descent or regional jet aircraft operations. not likely the result of contrails. Propeller-driven aircraft, which routinely oper- New calculations, supported by an observed in- ate at temperature levels from about −10° to below stance, suggest that jet aircraft operating at tempera- −30°C, have been shown to generate ice crystals. In the tures warmer than known contrail-forming tempera- early 1980s, Rangno and Hobbs directed attention to AMERICAN METEOROLOGICAL SOCIETY JUNE 2010 | 755 Unauthenticated | Downloaded 09/26/21 08:04 AM UTC the power setting. The motivation for these carefully designed studies of APIP production was to ascertain the conditions when research and operational weather modification aircraft were influencing the clouds under study. For example, Rangno and Hobbs (1983) and Hobbs and Rangno (1984) suggested that the unexpectedly high concentrations of ice particles that were observed to develop in a maritime cloud topped at a temperature of −4°C (Mossop et al. 1968) were the result of APIPs produced by repeated penetrations by two propeller aircraft. This sug- gestion was vigorously defended (Mossop 1984). The following section presents de- FIG . 3. Two B-17 Flying Fortresses’ vapor trails light up the night sky over

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