HR: 10:44h
AN: A22B-02    [Abstracts]
TI: MODIS Observations of Ship Tracks: The Response of Marine Stratus to Smoke Particles
AU: * Coakley, J A
EM: coakley@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, Corvallis, OR 97331-5503 United States
AU: Segrin, M S
EM: msegrin@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, Corvallis, OR 97331-5503 United States
AU: Tahnk, W R
EM: tahnk@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, Corvallis, OR 97331-5503 United States
AB: Ship tracks offer a unique opportunity for studying the response of marine stratus to increased particle concentrations. The study of ship tracks avoids the difficulties encountered in regional and global studies of cloud systems affected by haze in that in order to deduce the aerosol effects in the regional studies the response of clouds to the thermodynamics of the air mass in which the haze is imbedded must also be accounted for. With ship tracks, polluted clouds are compared with unpolluted clouds separated by only a few kilometers thereby insuring that the thermodynamics of the environments are on average the same and the only difference is the elevated particle concentrations in the polluted clouds. While marine stratus represent a specific cloud system having behavior that may not be representative of all low-level cloud types, such clouds nonetheless offer opportunities for testing the fidelity of at least a subset of cloud models. The use of MODIS observations to study ship tracks follows an earlier study based on AVHRR data. The earlier study demonstrated that polluted clouds lost on average approximately 20 percent of their liquid water. The finding contradicts the widely accepted theory that precipitation is suppressed in polluted clouds and thus should lead to increased cloud liquid water, cloud lifetimes, and cloud cover. The MODIS observations and the new analysis schemes developed for use with the MODIS data include several improvements on the earlier study with AVHRR data. First, MODIS radiances at 1.6 and 2.1 microns provide additional measures of cloud droplet radii thereby providing additional assessments of the changes in cloud liquid water which for the AVHRR observations were restricted to the use of 3.7-micron radiances to obtain droplet radii. Second, the new analysis procedures account for partial cloud cover in the 1-km fields of view and thus provide an objective means for estimating the growth in cloud liquid water due to increased cloud cover fractions for the polluted clouds. Third, a new automated scheme for comparing the polluted and nearby unpolluted clouds allows for comparison of clouds within 5 km, typically the autocorrelation length for cloud properties within marine stratus, thereby enhancing the contrast between the polluted and unpolluted clouds against the variability of the properties exhibited by marine stratus. Preliminary analyses are restricted to 1-km pixels found to be overcast by marine stratus. The results obtained with the MODIS radiances confirm the earlier findings that most polluted clouds lose liquid water. Based on recent LES model simulations of marine stratus, the loss is thought to result from the entrainment of the dry free tropospheric air above the clouds leading to a drying of the boundary layer where the clouds are polluted. Where the clouds remain unpolluted, the entrained dry air is less effective in drying the boundary layer. Analyses are underway to determine whether support for these model results can be found in the column water vapor amounts derived from the MODIS 0.905 and 0.935-micron radiances.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1640 Remote sensing
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution--urban and regional (0305)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting