P8.13 CONTRAIL STUDIES AND FORECASTS IN THE SUBARCTIC ATMOSPHERE ABOVE FAIRBANKS, ALASKA Martin Stuefer* and Gerd Wendler Geophysical Institute, University of Alaska, Fairbanks 1. INTRODUCTION Contrails are of interest for scientists investigating atmospheric radiation transfer processes, the chemical state of the atmosphere, and their potential for climatic change (IPCC 1999). Due to the composition of ice crystals, the radiative characteristics of contrails are similar to those of thin layers of naturally occurring cirrus clouds. Meerkoetter et al. (1999) pointed out the effect of contrails to reduce daily amplitudes of temperatures in the lower atmosphere by reducing the net radiation to the surface during the day and reducing the infrared losses from the surface during the night. For quantification efforts of the radiative forcing it is necessary to estimate the persistence of contrails, which may range from a few seconds to several hours. Due to uncertainties in different remote sensing techniques for contrail detection, Minnis et al. (2003) point out the need of ground based observations of contrails. Ground based observations are also one of the main tools for tuning and validation of theoretical contrail- coverage models (Duda et. al 2002). Schmidt (1941) and Appleman (1953) described originally contrail formation criteria. By considering the entrainment of the heated and moist exhaust gas to the ambient air, a 'critical' temperature is calculated as the threshold temperature to determine if saturation occurs; contrails are expected to form for critical temperatures higher than the wake- ambient air- temperatures. To detect a contrail visibly, a minimum ice crystal content is necessary. Contrail registration and highly accurate measurements of temperature and humidity were carried out within the framework of the ‘contrail and cloud effects special study (SUCCESS) experiment (Jensen et al. 1998); in situ measurements in the plume of a DC-8 aircraft confirmed previous assumptions that contrail formation requires saturation with respect to water. In visible contrails super-saturation with respect to water was observed, a phase change from water droplets to ice crystals might have occurred immediately. For describing the thermodynamics of an air parcel that is influenced by the entrainment of moist and warm exhaust gases, an isobaric mixing process is assumed. We have used the approach of Goff and Gratch (1946) for the temperature dependence of the saturation vapor *Corresponding author address: Martin Stuefer, Geophysical Institute, University of Alaska, Fairbanks, AK 99775; e-mail: stuefer@gi.alaska.edu pressure, de S dT . For the derivation of threshold temperatures for contrail formation a previously saturated environment was considered first. The stipulation for the threshold temperature T crit,100 is defined as: dr S dT (T crit ,100 ) = dr f dT = CF (1) where r s denotes the saturation mixing ratio (g/kg) and dr f accounts for the change of mixing ratio due to the water vapor produced by the engine combustion. Schumann (1996) summarized the Schmidt/Appleman theory; he considered also the propulsion efficiency of an aircraft (Busen and Schumann, 1995) in order to derive accurate contrail factors. Critical temperatures T crit,h for non-saturated conditions (relative humidity h <100 %) were derived according to: T crit , h = T crit ,100 − ( r s T crit ,100 − h 100 r s T crit , h ) CF (2) Equations 1 and 2 were solved iteratively in order to obtain the critical temperatures T crit,100 (p) and T crit,h (p, h) for a previously estimated contrail factor CF. 2. DATA AND SELECTED METHODOLOGY We have collected a comprehensive dataset for contrail formation for the subarctic atmosphere overhead Fairbanks. The observational methods included direct visual observations and continuous all sky digital camera imagery (Wendler and Stuefer, 2002). Visual observations were especially required for a definite classification of ‘no-contrail’ cases and over-flights, when an aircraft formed a contrail, which dissolved within a few seconds. As contrails often dissipate into faint contrail- patches until they reach an invisible state, persistence interpretation from different observations may vary. We had 4 observers, who did visual sky inspection at times of aircraft passages for this study; the lead author of this paper conducted 90 % of the observations ensuring homogeneity of the data. The analysis of the persistence of longer lasting visible contrails was supported by a digital camera, which was directed to the zenith and equipped with a 180 degrees fish-eye lens. We used a commercial Canon Powershot G2 camera situated below a highly transparent dome on the roof of the Geophysical Institute of the University of