Contrail studies and forecasts in the subarctic atmosphere above Fairbanks, Alaska

Aircraft exhaust condensation trail (contrail) formation in the subarctic setting of Fairbanks, Alaska is being studied using continuous observational methods. Since March of 2000 a contrail database has been established, which includes contrail characteristics, FAA flight data, and atmospheric measurements derived from radiosonde ascents at Fairbanks International Airport. The contrail analysis is based on all sky digital camera imagery and direct observations of aircrafts. Depending on the sky visibility and the time of the over-flight within 2 hours of the radiosonde ascents, we could record 322 aircrafts, which formed contrails. The contrails have been divided into classes based on duration, ranging from a few seconds to several hours. Due to the low subarctic tropopause altitude of typically 10,000 m overhead Fairbanks, about 35% of the aircraft passages occured in the lower stratosphere with reduced vertical atmospheric- mixing processes when compared to the troposphere. Taking into account uncertainties in atmospheric measurements with radiosondes, contrail threshold temperatures are discussed with homogenous and long- term observational methods.

Using our database we verified a conventional theory of mixing clouds; a hit rate for correctly predicting the occurrence and non-occurrence of contrails of 91% was obtained. The mixing of water vapor included in aircraft exhaust gases with the ambient air was parameterized with mean contrail factors of 0.037 g (kg K)-1. Layers in the subarctic atmosphere above Fairbanks, where contrails are likely to form, are during the winter months about 3300 m thick; in contrast we found a mean layer thickness of 1870 m during the summer months.

The observational results and Mesoscale Model, Version 5 (MM5) were used for forecasting contrail layers over Alaska. Contour maps of layers and atmospheric cross-sections showing contrail layers were derived operationally. A local verification of the forecast quality was accomplished. A possible bias might occur in humidity measurements at cold temperature levels, which might also affect forecasts of humidity of the upper troposphere or lower stratosphere. Despite this fact, our contrail verification study shows hit rates higher than 82% within forecast periods up to 36 hours.