Aircraft Groundings: Evidence of Jet Contrail Influence on Climate

The grounding of all commercial aircraft within U.S. airspace for the 3-day period following the 11 September 2001 terrorist attacks provides a unique opportunity to study the potential role of jet aircraft contrails in climate. Contrails are most similar to natural cirrus clouds due to their high altitude and strong ability to efficiently reduce outgoing infrared radiation. However, they typically have a higher albedo than cirrus; thus, they are better at reducing the surface receipt of incoming solar radiation. These contrail characteristics potentially suppress the diurnal temperature range (DTR) when contrail coverage is both widespread and relatively long lasting over a specific region. During the 11–14 September 2001 grounding period natural clouds and contrails were noticeably absent on high-resolution satellite imagery across the regions that typically receive abundant contrail coverage. A previous analysis of temperature data for the grounding period reported an anomalous increase in the U.S.-averaged, 3-day DTR value. Here, the spatial variation of the DTR anomalies as well as the separate contributions from the maximum and minimum temperature departures are analyzed. These analyses are undertaken to better evaluate the role of jet contrail absence and synoptic weather patterns during the grounding period on the DTR anomalies. It is shown that the largest DTR increases occurred in regions where contrail coverage is typically most prevalent during the fall season (from satellite-based contrail observations for the 1977–79 and 2000–01 periods). These DTR increases occurred even in those areas reporting positive departures of tropospheric humidity, which may reduce DTR, during the grounding period. Also, there was an asymmetric departure from the normal maximum and minimum temperatures suggesting that daytime temperatures responded more to contrail absence than did nighttime temperatures, which responded more to synoptic conditions. The application of a statistical model that ‘‘retro-predicts’’ contrail-favored areas (CFAs) on the basis of upper-tropospheric meteorological conditions existing during the grounding period, supports the role of contrail absence in the surface temperature anomalies; especially for the western United States. Along with previous studies comparing surface climate data at stations beneath major flight paths with those farther away, the regionalization of the DTR anomalies during the September 2001 ‘‘control’’ period implies that contrails have been helping to decrease DTR in areas where they are most abundant, at least during the early fall season.