A satellite-based climatic description of jet aircraft contrails and associations with atmospheric conditions, 1977-79

The possible contribution of jet aircraft condensation trails (contrails) t o recent observed increases in high cloudiness constitutes a potentially im portant human effect on climate that has received relatively little attenti on. Very high resolution (0.6 km) thermal-infrared imagery from the Defense Meteorological Satellite Program polar orbiters, concentrated in the night time and morning hours, is interpreted to derive a climatic description of contrails over the United States and adjacent areas for the midseason month s (April, July, October, and January) of 1977-79. A manual technique of ide ntifying contrails on the imagery is validated by comparison with more rece nt ground-based observations. Contrail spatial distributions are mapped at a 1 degrees Int X 1 degrees long resolution for monthly and multimonth time periods. Contrail incidence is widespread over the United States and adjacent areas, with highest frequencies occurring over the following regions: the extreme Southwest (particularly southern California), the Southeast (especially so utheast Georgia and northeast Florida), the west coast of British Columbia and Vancouver Island, and the eastern Midwest centered on southeast Indiana and western Kentucky. Contrails are most frequent during the transition-se ason months (April and October), and are least frequent in July. Latitudina lly. contrail incidence peaks over the northern (southern) regions in July (January), suggesting a first-order association with the seasonal variation of upper-tropospheric westerly winds. Analysis of synoptic-scale midtropos pheric circulation patterns confirms that the highest contrail frequencies occur in association with baroclinic phenomena, particularly cyclone waves and jet streams. Moreover, contrails tend frequently to occur in conjunctio n with other clouds, including the cirrus associated with jet-stream and fr ontal systems. Analyses of rawinsonde data for three representative contrail "outbreak" (m ultiple occurrence) events during the study months confirm some earlier stu dies that suggest contrails form below a cold, elevated tropopause (i.e.. a round ridgelines in the geopotential height held), in contrast with noncont rail days. Accordingly, the temperature advection in the troposphere accomp anying the contrail outbreaks is positive, or warm. and relatively weak. Th is contrail climatic description provides a context within which recent sur face climate changes at regional and subregional scales may be cast.