Spreading and growth of contrails in a sheared environment

A case study of persistent contrail evolution in a sheared environment is s imulated over time-scales of 15-180 min using a large-eddy simulation model with detailed microphysics. Model results are compared to satellite and in situ measurements of persistent contrails from the Subsonic Aircraft: Cont rail and Cloud Effects Special Study (SUCCESS) experiment. In simulations w ith large ambient supersaturations and moderate wind shear, crystals with l engths >200 mu m are generated within 45 min by depositional growth. These crystals fall rapidly, and the contrail horizontal extent increases due to the wind shear. Strong radiative heating (with rates up to 10 K d(-1)) driv es a local updraft and lofts the contrail core several hundred meters. The observed rate of contrail spreading and maintenance of optical depths large r than 0.05 can be approximately explained simply by growth and precipitati on of ice crystals nucleated during the initial contrail formation if the e nvironmental humidity is very high (relative humidity with respect to ice > 125%). This result is consistent with observed high humidities in regions w here persistent contrails formed during SUCCESS.