| Jet condensation trails (contrails) are suspected to be an important contributor to the increase in high clouds that has been recognized since the middle of this century. Contrails can persist and spread in large groups, or "outbreaks", when occurring in a unique range of atmospheric conditions. During these situations they can significantly modify the Earth-atmosphere radiation balance.; High resolution satellite imagery is utilized to study outbreaks of contrails occurring in the mid-season months of 1987 for the United States. To better understand the atmospheric environments in which contrails persist and spread, near simultaneous observations of satellite water vapor and rawinsonde temperature data are used. To identify contrail radiative effects, comparisons are made between the contrail influence on outgoing longwave radiation (i.e. the "greenhouse" effect) and on reflected visible radiation (i.e. the albedo effect).; Results of the radiative analyses indicate that contrails significantly modulate the Earth-atmosphere radiation balance, especially when located within "natural" cirrus. The contrail effect varies with latitude, contrail morphology, and the sky situation (i.e. cloudy, partly cloudy, or clear). The net radiative effect, or contrail "forcing", is slightly positive when contrails occur within otherwise clear sky and slightly negative when contrails occur within naturally cloudy areas (mainly cirrus). Since contrails occur more commonly, and have a greater areal coverage when existing within natural clouds, it is concluded that, overall, contrails produce a slight decrease in available energy below the cloud. A predictive model for contrail occurrence is developed from the identification of a unique range of "favorable" conditions for contrail persistence and spreading. The model is evaluated for a separate set of observations as well as a case day (April 17, 1987). The latter demonstrates how the predictive model can be applied on a "real time" basis and potentially utilized to reduce the climatic impact of contrails. |
