The grounding of all commercial aircraft in U.S. airspace for the 3-day period of September 11-14, 2001, following the terrorist attacks on New York and Washington, D.C., provides a unique opportunity to study the potential role of jet aircraft contrails in climate. The unique nature of contrails, having the ability to reduce the transfer of both incoming solar and outgoing infrared radiation, allows for a suppression of diurnal temperature range (DTR) to potentially occur when contrail coverage is both widespread and long lasting over a specific region (i.e. across an adjacent day and night). During the Sept. 11-14th, 2001 period contrails were noticeably absent on high resolution satellite imagery, other than the few produced by military aircraft. Preliminary analysis of U.S. station DTR data for the period September 8-16, 2001, shows a distinct increase for the 11-14th. Comparisons of 3-day averaged U.S. DTR data for the corresponding periods in 1971-2000 show no consistent increase for September 11-14, arguing against the possibility of a climatic singularity. This suggests that the lack of aircraft flying in the upper troposphere helped reduce the cirrus cover over the U.S. during the time of aircraft groundings and thus, allowed a temporary increase in DTR to occur until flights resumed during the afternoon of the 14th. To further verify the importance of the decrease in contrails during the Sept. 11-14th period, an analysis of Advanced Very High Resolution Radiometer (AVHRR) infrared imagery (1.1km resolution) was completed to identify regions where contrails had occurred most frequently during the corresponding periods for 1995-2000 in the U.S. and adjacent coastal waters. A spatial analysis was then completed to determine associations between these regions and locations where DTR increase was the greatest compared to the 1971-2000 period. The results support the contention that contrail decreases contribute to DTR increases.

To further demonstrate the importance of jet contrails in the U.S. cloud cover we pose the question: Where would contrails likely have formed on September 11-14, 2001, had commercial air traffic not been suspended? For the contrail areas identified during 1995-2000 (a total of 48 cases) we obtained the 6-hourly NCEP/NCAR Reanalyses of mid to upper tropospheric meteorological conditions (temperature, moisture, geopotential, omega, wind). The central year (1998) was retained for testing the meteorological “rules” for contrail production derived for the calibration period. Contrail areas are distinguished from climatology (1971-2000) by steeper values of the upper tropospheric (300-200mb) lapse rate (i.e., a higher, colder tropopause), and a much stronger spatial gradient of relative humidity and specific humidity at 300mb (i.e., moisture advection). Moreover, contrail outbreak (cirrus) areas are distinguishable by having significantly lower (higher) mean values of the mid- to upper troposphere moisture. Following a successful verification of the meteorological conditions for contrails occurring in 1998, the "rules" were applied to the Sept. 11-14, 2001 period. The results confirm that multiple contrail outbreaks would have occurred in specific regions of the U.S. and adjacent coastal waters during the Sept. 11-14th, 2001 period had commercial aircraft not been grounded.