Search

The cirrus-level “condensation trails” (contrails) produced by jet aircraft are considered to influence surface climate and its recent changes. To reveal the synoptic atmospheric environments typically associated with multiple co-occurrences of contrails occurring in otherwise clear or partly cloudy skies (outbreaks) for the United States, and ultimately to assist in forecasting these events, a composite (i.e., multicase average) “synoptic climatology” at regional scales is developed for the midseason months (January, April, July, October) of 2000–02. The NCEP–NCAR reanalysis data that emphasize upper-troposphere (UT) variables are allied with manually identified outbreaks appearing on satellite Advanced Very High Resolution Radiometer digital data, using a geographic information system. The highest frequencies of outbreaks by far occur in the Midwest (32.6% of all-U.S. total), followed by the Northeast (17.6%) and Southeast (17.2%). In these regions, all of which have a high density of jet air traffic, an additional 2% cirrus cloud coverage from outbreak-related contrails is inferred. Large interannual and interseasonal variations in contrail outbreak frequencies support the role of meteorological variations. For most regions, the outbreak-associated synoptic circulation composite conditions involve UT ridging and a higher and colder tropopause than the climatological average; meridionally enhanced gradients of the UT vertical motion, located between sinking air to the east (in the ridge) and rising air to the west, in advance of a trough; similarly strong gradients of mid–upper-troposphere humidity, comprising dry air located to the east and moist air to the west; and horizontal speed shear ahead of an advancing jet stream. Notwithstanding, there is a geography (i.e., areal differentiation) to contrail outbreak environments: composites for the Northeast suggest an influence of land–sea contrasts on synoptic systems and, therefore, on contrail outbreaks. For the Northwest, there is evident a greater impact of horizontal wind shear contrasted with other regions. The synoptic climatology results are supported by the all-U.S. averages of contrail outbreak UT conditions [climate diagnostics (CDNs)] previously determined for early–mid-September periods of 1995–2001. Moreover, a comparison of these CDNs with those derived for nearby thick natural clouds, including cirrus, helps to clarify their different synoptic associations: the UT conditions typical of thick clouds represent an intensification of those associated with contrail outbreaks and include the greater upward vertical motion, moister air, and stronger westerly winds characteristic of a trough. Given the location of most contrail outbreaks downstream of multilayered cloud systems, contrails may help to extend the “natural” cirrus and cirrostratus spatial coverage. [ABSTRACT FROM AUTHOR]

The potential of condensation trails (contrails) from jet aircraft to affect regional-scale surface temperatures has been debated for years, but was difficult to verify until an opportunity arose as a result of the three-day grounding of all commercial aircraft in the United States in the aftermath of the terrorist attacks on 11 September 2001. Here we show that there was an anomalous increase in the average diurnal temperature range (that is, the difference between the daytime maximum and night-time minimum temperatures) for the period 11-14 September 2001. Because persisting contrails can reduce the transfer of both incoming solar and outgoing infrared radiation and so reduce the daily temperature range, we attribute at least a portion of this anomaly to the absence of contrails over this period.