16 research outputs found
The Appearance and Disappearance of Ship Tracks on Large Spatial Scales
The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon,
NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes,
visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between
the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to
deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate
that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely
appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and
cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for
clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the
appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why
the majority of ship tracks observed from satellites off the coast of California are found south of 358N. A small
rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a
particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally
less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel
clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery
pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains
why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in
satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top
altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.This work was supported in part by the Office of Naval Research and by the National Science Foundation through the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography, an NSF Science and Technology Center
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Altitude differentiated aerosol extinction over tenerife (North Atlantic Ocean) during ACE-2 by means of ground and airborne photometry and lidar measurements
Effects of Aerosol and SST Gradients on Marine Stratocumulus Albedo
Geophysical Research Letters, Vol. 31, No. 6, L06113, doi:10.1029/2003GL018909.The article of record as published may be located at http://dx.doi.org/10.1029/2003GL018909Airborne data are reported on the effect of sea surface
temperature and aerosol gradients on the albedo of marine
stratocumulus clouds off the coast of central California.
Both types of gradients, at the magnitudes and spatial scales
observed, produce significant and comparable trends in the
cloud albedo ( 15% changes over 40 km)
Factors influencing the mesoscale variations in marine stratocumulus albedo
Tellus, 59B, 66-76, 2007.The article of record as published may be found at http://dx.doi.org/10.1111/j.1600-0889.2006.00231.xMeasurements of both horizontal gradients and vertical profiles of aerosols, cloud droplets and thermodynamic parameters
in the cloud topped marine boundary layer off of central California are presented. They suggest that, while aerosols
can indeed modulate cloud albedo, other parameters such as sea surface temperature may similarly affect cloud albedo.
Additionally, the impact of aerosols, through sedimentation and precipitation, on cloud optical depths and thus albedo
is not always in accord with conventional expectations and can either enhance or decrease the albedo, depending on
ambient conditions. Taken together, these results suggest that current estimates of indirect forcing by aerosols could be
significantly in error
Shipboard sunphotometer measurements of aerosol optical depth spectra during ACE-2
During the North Atlantic Regional Aerosol Characterization Experiment (ACE-2) held
in summer 1997, the NASA Ames six-channel airborne tracking sunphotometer (AATS-6)
was operated aboard the R/V Vodyanitskiy in the eastern Atlantic Ocean off the coast of
Portugal and North Africa. This instrument measures the direct beam solar transmission
through the earth’s atmosphere in six narrow bandpass regions at wavelengths centered
between 380.7 and 1020.7 mn. Aerosol optical .depth spectra derived from ACE-2
measurements acquired during a variety of lower tropospheric aerosol loading conditions will
be discussed. Comparisons with coincident optical depth data derived from airborne and/or
satellite radiometer measurements will also be shown
International Global Atmospheric Chemistry (IGAC) Project's First Aerosol Characterization Experiment (ACE 1)- Overview
The southern hemisphere marine Aerosol Characterization Experiment (ACE 1) was the first of a series of experiments that will quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. The goals of this series of process studies are to reduce the overall uncertainty in the calculation of climate forcing by aerosols and to understand the multiphase atmospheric chemical system sufficiently to be able to provide a prognostic analysis of future radiative forcing and climate response. ACE 1, which was conducted from November 15 to December 14, 1995, over the southwest Pacific Ocean, south of Australia, quantified the chemical, physical, radiative, and cloud nucleating properties and furthered our understanding of the processes controlling the aerosol properties in this minimally polluted marine atmosphere. The experiment involved the efforts of scientists from 45 research institutes in 11 countries
The Role of Background Cloud Microphysics in the Radiative Formation of Ship Tracks
The authors investigate the extent to which the contrast brightness of ship tracks, that is, the relative change
in observed solar reflectance, in visible and near-infrared imagery can be explained by the microphysics of the
background cloud in which they form. The sensitivity of visible and near-infrared wavelengths for detecting
reflectance changes in ship tracks is discussed, including the use of a modified cloud susceptibility parameter,
termed the ‘‘contrast susceptibility,’’ for assessing the sensitivity of background cloud microphysics on potential
track development. It is shown that the relative change in cloud reflectance for ship tracks is expected to be
larger in the near-infrared than in the visible and that 3.7-mm channels, widely known to be useful for detecting
tracks, have the greatest sensitivity. The usefulness of contrast susceptibility as a predictor of ship track contrast
is tested with airborne and satellite remote sensing retrievals of background cloud parameters and track contrast.
Retrievals are made with the high spatial resolution Moderate Resolution Imaging Spectroradiometer Airborne
Simulator flown on the National Aeronautics and Space Administration’s high-altitude ER-2 aircraft, and with
the larger-scale perspective of the advanced very high resolution radiometer. Observed modifications in cloud
droplet effective radius, optical thickness, liquid water path, contrast susceptibility, and reflectance contrast are
presented for several ship tracks formed in background clouds with both small and large droplet sizes. The
remote sensing results are augmented with in situ measurements of cloud microphysics that provide data at the
smaller spatial scales
Influence of humidity on the aerosol scattering coefficient and its effect on the upwelling radiance during ACE-2
Tellus (ACE-2 Special Issue), 52B, 546-567
Aerosol optical depth retrieval using ATSR-2 and AVHRR data during TARFOX
Satellite retrieved aerosol optical properties are compared to aircraft measurements for a case study during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Two satellite instruments are used: the Along Track Scanning Radiometer 2 (ATSR-2) and the advanced very high resolution radiometer (AVHRR). The aerosol optical depth in the mid-visible (0.555 μm) retrieved from the ATSR-2 data agrees within 0.03 with colocated sunphotometer measurements. Also, the spectral behavior of the aerosol optical depth is retrieved accurately. Good correlation is found between aerosol optical depths for AVHRR channel 1 (0.64 μm) and sunphotometer derived values, but the satellite retrieved values are 0.05 to 0.15 lower. The Ångström wavelength exponent is determined both from the ATSR-2 and the AVHRR data. The ATSR-2 derived Ångström exponents are in good agreement with the values computed from the sunphotometer data. The Ångström exponents determined from AVHRR data show very large variations. Both the ATSR-2 and the AVHRR aerosol optical depth images show a large gradient. Vertical profile data of temperature, relative humidity, and particle scattering indicate that this gradient is probably caused by changes in the dry aerosol properties, rather than a change in the relative humidity. Copyright 1999 by the American Geophysical Union