Factors controlling contrail cirrus optical depth 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
2National Aeronautics and Space Administration (NASA), Langley Research Center, Hampton, VA, USA
Received: 13 Mar 2009 – Published in Atmos. Chem. Phys. Discuss.: 11 May 2009Abstract. Aircraft contrails develop into contrail cirrus by depositional growth and
sedimentation of ice particles and horizontal spreading due to wind shear. Factors
controlling this development include temperature, ice supersaturation, thickness of
ice-supersaturated layers, and vertical gradients in the horizontal wind field. An
analytical microphysical cloud model is presented and validated that captures these
processes. Many individual contrail cirrus are simulated that develop differently
owing to the variability in the controlling factors, resulting in large samples of
cloud properties that are statistically analyzed. Contrail cirrus development is
studied over the first four hours past formation, similar to the ages of line-shaped
contrails that were tracked in satellite imagery on regional scales. On these time
scales, contrail cirrus optical depth and microphysical variables exhibit a marked
variability, expressed in terms of broad and skewed probability distribution functions.
Simulated mean optical depths at a wavelength of 0.55 μm range from 0.05-0.5
and a substantial fraction 20-50% of contrail cirrus stay subvisible (optical
depth <0.02), depending on meteorological conditions.
Revised: 03 Aug 2009 – Accepted: 10 Aug 2009 – Published: 31 Aug 2009
A detailed analysis based on an observational case study over the continental USA
suggests that previous satellite measurements of line-shaped persistent contrails have
missed about 89%, 50%, and 11% of contrails with optical depths 0-0.05,
0.05-0.1, and 0.1-0.2, respectively, amounting to 65% of contrail coverage
of all optical depths. When comparing observations with simulations and when estimating
the contrail cirrus climate impact, not only mean values but also the variability in
optical depth and microphysical properties need to be considered.
Citation: Kärcher, B., Burkhardt, U., Unterstrasser, S., and Minnis, P.: Factors controlling contrail cirrus optical depth, Atmos. Chem. Phys., 9, 6229-6254, doi:10.5194/acp-9-6229-2009, 2009.