HR: 11:49h
AN: A22D-06    [Abstracts]
TI: Effects of Solar Heating on the Indirect Effect of Aerosols as Deduced from Observations of Ship Tracks
AU: * Christensen, M W
EM: chrismat@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, 104 COAS Admin Bldg., Corvallis, OR 97331-5503, United States
AU: Coakley, J A
EM: coakley@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, 104 COAS Admin Bldg., Corvallis, OR 97331-5503, United States
AU: Segrin, M S
EM: mk.segrin@gmail.com
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, 104 COAS Admin Bldg., 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, 104 COAS Admin Bldg., Corvallis, OR 97331-5503, United States
AB: Inferring the indirect radiative effect of aerosols from observations is confounded by the response of clouds to many competing thermodynamic processes. These processes often overwhelm the effects of the particles. Observations of ship tracks, on the other hand, provide differences in cloud properties between polluted and nearby unpolluted clouds that are many times the differences among the unpolluted clouds on opposite sides of the track, thereby revealing the changes in the clouds are due solely to the haze. Marine stratocumulus affected by haze generally exhibit larger optical depths, and thus higher albedos, and smaller droplet radii than nearby unpolluted clouds. But this response is governed by the environment in which the clouds are embedded and also by the period over which the clouds are subjected to solar heating. One kilometer Moderate Resolution Imaging Spectroradiometer (MODIS) observations for Terra (morning) and Aqua (afternoon) were used to follow the morning to afternoon evolution of marine stratocumulus off the west coast of the U.S. that were affected by ship stack exhaust. The observations covered the summer months of 2002 and 2003. Low-level winds from NCEP re-analyses were used to identify the clouds common to both the Terra and Aqua observations. The 2002 and 2003 data contained several hundred ship track pairs in which the polluted clouds observed by the Terra MODIS were also observed by the Aqua MODIS. For overcast conditions, morning clouds had higher optical depths but the same droplet effective radius as the afternoon clouds. Consistent with the greater optical depths, liquid water amount was also higher (~10%) for the morning clouds. Under broken conditions, polluted clouds had more liquid water than nearby unpolluted clouds, whereas for overcast conditions polluted clouds had less liquid water. While work is underway to increase the ensemble of cases studied, the comparison of morning and afternoon clouds is revealing that while the changes in optical depth were the same for the morning and afternoon clouds, the decreases in droplet radius were greater for the afternoon clouds. This response appeared to be consistent with the afternoon breakup of the marine cloud layer. The unpolluted afternoon clouds had larger droplet radii than their morning counterparts, presumably from the growth of the droplets and formation of drizzle that promoted the breakup of the unpolluted clouds.
DE: 0321 Cloud/radiation interaction
DE: 3310 Clouds and cloud feedbacks
DE: 3311 Clouds and aerosols
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting