Estimates of cloud radiative forcing in contrail clusters using GOES imagery

Using data from the Geostationary Operational Environmental Satellite (GOES ), the evolution of solar and longwave radiative forcing in contrail cluste rs is presented in several case studies. The first study examines contrails developing over the midwestern United States in a region of upper troposph eric moisture enhanced by the remnants of Hurricane Nora on September 26, 1 997. Two other cases involve contrail clusters that formed over the Chesape ake Bay and the Atlantic Ocean on February 11 and March 5, 1999, respective ly. The last study includes contrails forming over the tropical Pacific nea r Hawaii. Observations of tropical contrails near Hawaii show that the cont rail optical properties are similar to those measured from satellite in the midlatitudes, with visible optical depths between 0.3 and 0.5 and particle sizes between 30 and 60 mum as the contrails mature into diffuse cloudines s. Radiative transfer model simulations of the tropical contrail case sugge st that ice crystal shape may have an important effect on radiative forcing in contrails. The magnitudes of the observed solar and longwave radiative forcings were 5.6 and 3.2 W m(-2) less than those from the corresponding mo del simulations, and these differences are attributed to the subpixel scale low clouds and uncertainties in the anisotropic reflectance and limb-darke ning models used to estimate the observed forcing. Since the broadband radi ative forcing in contrails often changes rapidly, contrail forcing estimate s based only on the polar orbiting advanced very high resolution radiometer (AVHRR) data could be inaccurate due to the lack of sufficient temporal sa mpling.