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Microphysics of Aerodynamic Contrail Formation Processes

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Microphysics of Aerodynamic Contrail Formation Processes VOLUME 72 JOURNAL OF THE ATMOSPHERIC SCIENCES SEPTEMBER 2015 Microphysics of Aerodynamic Contrail Formation Processes JOACHIM JANSEN Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, Netherlands ANDREW J. HEYMSFIELD National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 2 December 2014, in final form 9 April 2015) ABSTRACT Aerodynamic condensation is a result of intense adiabatic cooling in the airflow over aircraft wings and behind propeller blades. Out of cloud, condensation appears as a burstlike fog (jet aircraft during takeoff and landing, propellers) or as an iridescent trail visible from the ground behind the trailing edge of the wing (jet aircraft in subsonic cruise flight) consisting of a monodisperse population of ice particles that grow to sizes comparable to the wavelength of light in ambient humidities above ice saturation. In this paper, the authors focus on aerodynamic contrail ice particle formation processes over jet aircraft wings. A 2D compressible flow model is used to evaluate two likely processes considered for the initial ice particle formation: homogeneous droplet nucleation (HDN) followed by homogeneous ice nucleation (HIN) and condensational growth of ambient condensation nuclei followed by their homogenous freezing. The model shows that the more numerous HDN particles outcompete frozen solution droplets for water vapor in a 0.5–1-m layer directly above the wing surface and are the only ice particles that become visible. Experi- mentally verified temperature and relative humidity–dependent parameterizations of rates of homogeneous droplet nucleation, growth, and freezing indicate that visible aerodynamic contrails form between T 52208 and 2508CandRH$ 80%. By contrast, combustion contrails require temperatures below 2388C and ice-saturated conditions to persist. Therefore, aerodynamic and combustion contrails can be observed simultaneously. 1. Introduction become an area of scientific interest because they can occur at much higher temperatures than combustion Combustion condensation trails, commonly associated contrails (Gierens et al. 2009; Kärcher et al. 2009). with ‘‘contrails,’’ are due to combustion of aircraft fuel and The phenomenon of aerodynamic condensation can be have been widely studied (e.g., the series of articles in the linked to the formation of aircraft produced ice particles April 2010 issue of the Bulletin of the American Meteoro- logical Society). These contrails generally occur at temper- (APIPs), first reported by Rangno and Hobbs (1983, 1984). atures colder than 2388C(Jensen et al. 1998) resulting Cooling behind the blades of propeller aircraft can produce from the offsetting effects of vapor and heat emitted visible aerodynamic condensation. During research flights during combustion (Schmidt 1941; Schumann 1996). In that measured cloud microphysical properties, Rangno and contrast, aerodynamic condensation is produced by Hobbs documented the production of ice crystals from the adiabatic expansion and the resulting cooling of moist passage of propeller aircraft through clouds at tempera- air over aircraft wings. These puffs of condensation are tures as warm as 288C. Ice particle concentrations were most readily seen by a passenger on an aircraft as con- more than 100 times greater than the expected concentra- densation over the wings during aircraft landing or tions of ambient ice nuclei at this temperature. Vonnegut takeoff (Fig. 1, left). Aerodynamic contrails have recently (1986), commenting on the APIP observations, suggested that adiabatic expansion in the flow over the propeller tips was sufficient to cool the cloud droplets to the temperature for homogeneous ice nucleation (HIN), ;2398C, if cloud Corresponding author address: Andrew Heymsfield, NCAR, 2 8 3450 Mitchell Lane, Boulder, CO 80301. temperatures were only a few degrees below 8 C, and E-mail: [email protected] that this could produce abundant numbers of ice crystals. DOI: 10.1175/JAS-D-14-0362.1 Ó 2015 American Meteorological Society 3293 3294 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 FIG. 1. Examples of overwing condensation. (left) A Boeing 777-F1B cargo aircraft landing at Schiphol Airport, Netherlands, on 4 Jun 2012 (the photograph was taken by J. Schäfer and is used with his permission). (right) An Embraer-190 two-engine jet aircraft flying over Milan, Italy, on 25 Jun 2012, heading southwest at ;10.6-km al- titude, showing both combustion and iridescent aerodynamic contrails [the photograph is from Santacroce (2012) and is used with the permission of M. Santacroce]. Following up on the Vonnegut suggestion, Foster was sufficient for HDN to occur. In unfiltered air, high 2 and Hallett (1993) carried out laboratory measure- concentrations of ice crystals (;104 cm 3) were gener- ments of HIN via rapid expansion of moist, cool air ated at temperatures a few degrees warmer than for in a cloud chamber. After the injection of droplets, ice clean air. The warmer onset of HIN may have resulted crystals were readily observed at temperatures colder from the presence of ice nuclei present in the unfiltered than 2408C, with concentrations, although not mea- laboratory air but could also be interpreted as due to sured, significantly greater than the concentration of condensation on larger CN, followed by HDN, and condensation nuclei (CN) present in the ambient air. finally by HIN. This is an important and relevant observation that we To gain a better understanding of the APIP for- will address later. mation process(es), Woodley et al. (1991, 2003) Foster and Hallett (1993) found the onset conditions studied APIP generation from the University of for ice nucleation to be consistent with HIN theory as Wyoming King Air research aircraft in supercooled long as the cloud droplets were exposed to the HIN fog at temperatures between about 258 and 2128C onset temperature with sufficient time to freeze. With- with almost no natural ice nuclei during the Mono out any cloud present initially, the onset temperature Lake Experiments (MOLAS). APIP generation from dropped below 2488C, dependent on the initial chamber nine different propeller aircraft, including the King temperature, rather than 2408C typically associated Air, was studied and interpreted. Adiabatic expan- with droplets freezing homogeneously. The homoge- sion at the propeller tips achieves a cooling of 2408C, neous freezing temperature is warmer as the droplet sufficient for HIN. Woodley et al. (2003) estimated volume increases (Pruppacher and Klett 1997 and ref- that the ice concentrations generated at the propeller 2 erences therein) and in this case the droplets were likely tips was .105 cm 3 and suggested from laboratory to be very small. Foster and Hallett interpreted the experiments that the HDN process is involved in process of APIP generation to be due to homogeneous APIP generation. droplet nucleation (HDN) from vapor, which typically Conditions conducive to APIP generation over pro- produces large numbers of submicron-size droplets— peller blades are analogous to those generating ice and ice crystals after HIN. particles during the cooling of air over aircraft wings at An earlier study by Maybank and Mason (1959) re- sufficiently low temperatures. Gierens et al. (2009) ported on expansions of a small volume of moist air from quantified adiabatic cooling over a generic, idealized temperatures of 2108 and 2208C to final temperatures airfoil at an ambient temperature of 2228C for a com- of 2458C and colder. It was concluded that ice crystals, mercial jet aircraft flying at subsonic speed and observed 2 in concentrations ; 106 cm 3, formed in clean air first by an overwing temperature drop exceeding 208C. In this HDN followed by HIN. This HIN pathway occurred only highly supersaturated environment, activation of ambi- when the temperature drop and thus the supersaturation ent CN and growth of the resulting droplets could be SEPTEMBER 2015 J A N S E N A N D H E Y M S F I E L D 3295 followed by their homogeneous freezing at ambient light. The color changes are due to particle growth temperatures colder than 2208C. (Sassen 1979; Kärcher et al. 2009). Using the wingspan 2 Using Gierens’s model, Kärcher et al. (2009) modeled (28.7 m) and the cruising speed (245 m s 1) to estimate the process of aerodynamic contrail formation from the distance of the contrail behind the wing trailing edge, ambient solution droplets at temperatures from 2388 we show in the next section that the particles grow to to 2688C and a pressure range of 150 to 300 hPa. They visible sizes in a time of 60–80 ms, depending on their initialized the particles with supercooled aqueous solu- point of origin at the wing root or tip. In the extremely tions of sulfuric acid (H2SO4) and other components low temperatures reached over the wing (T 2388C), that were positioned just upstream of the wing. These the homogeneous ice nucleation rate is sufficiently high solution droplets then swelled by condensation in the to freeze all but the most concentrated solution droplets supersaturated air in the flow over the wings, while some (Koop et al. 2000; Kärcher et al. 2009). It is therefore froze homogeneously, dependent on their size. Sub- almost certain that the particles in aerodynamic con- sequent growth of the crystals produced was driven by trails are ice and that they can occur simultaneously with the difference between ambient and ice saturation vapor combustion contrails. pressure, which, at water saturation, increases with de- Case studies can provide some insight into atmospheric creasing temperature (for temperatures of 2158C and conditions conducive of aerodynamic contrail formation. colder). Homogeneous droplet nucleation—the gener- Published case studies by Kärcher et al. (2009) and ation of droplets without the need for cloud condensa- Gierens et al. (2011) combine photos of aerodynamic tion nuclei—has not been considered previously in the contrail-generating aircraft and corresponding radio- study of aerodynamic contrails. sonde measurements and indicate formation tempera- This study aims to identify the process(es) responsible tures of approximately 2328 and 2348C, respectively. For for aerodynamic condensation and quantify the tem- Fig.
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