Steven P. Palm

Antarctica cloud cover for October 2003 from GLAS satellite lidar profiling

Seeing clouds in polar regions has been a problem for the imagers used on satellites. Both clouds and snow and ice are white, which makes clouds over snow hard to see. And for thermal infrared imaging both the surface and the clouds cold. The Geoscience Laser Altimeter System (GLAS) launched in 2003 gives an entirely new way to see clouds from space. Pulses of laser light scatter from clouds giving a signal that is separated in time from the signal from the surface. The scattering from clouds is thus a sensitive and direct measure of the presence and height of clouds. The GLAS instrument orbits over Antarctica 16 times a day. All of the cloud observations for October 2003 were summarized and compared to the results from the MODIS imager for the same month. There are two basic cloud types that are observed, low stratus with tops below 3 km and high cirrus form clouds with cloud top altitude and thickness tending at 12 km and 1.3 km respectively. The average cloud cover varies from over 93 % for ocean and coastal regions to an average of 40% over the East Antarctic plateau and 60-90% over West Antarctica. When the GLAS monthly average cloud fractions are compared to the MODIS cloud fraction data product, differences in the amount of cloud cover are as much as 40% over the continent. The results will be used to improve the way clouds are detected from the imager observations. These measurements give a much improved understanding of distribution of clouds over Antarctica and may show how they are changing as a result of global warming.

Height distribution between cloud and aerosol layers from the GLAS spaceborne lidar in the Indian Ocean region

[1] The Geoscience Laser Altimeter System (GLAS), a nadir pointing lidar on the Ice Cloud and land Elevation Satellite (ICESat) launched in 2003, now provides important new global measurements of the relationship between the height distribution of cloud and aerosol layers. GLAS data have the capability to detect, locate, and distinguish between cloud and aerosol layers in the atmosphere up to 40 km altitude. The data product algorithm tests the product of the maximum attenuated backscatter coefficient b 0 (r) and the vertical gradient of b 0 (r) within a layer against a predetermined threshold. An initial case result for the critical Indian Ocean region is presented. From the results the relative height distribution between collocated aerosol and cloud shows extensive regions where cloud formation is well within dense aerosol scattering layersatthesurface. Citation: Hart, W. D., J. D. Spinhirne, S. P. Palm, and D. L. Hlavka (2005), Height distribution between cloud and aerosol layers from the GLAS spaceborne lidar in the Indian Ocean region, Geophys. Res. Lett., 32, L22S06, doi:10.1029/ 2005GL023671.

A Comparison of Cloud Cover Statistics from the GLAS Lidar with HIRS

The cloud dataset from the Geoscience Laser Altimeter System (GLAS) lidar on the Ice, Cloud, and Land Elevation Satellite (ICESat) spacecraft is compared to the cloud analysis of the Wisconsin NOAA High Resolution Infrared Radiation Sounder (HIRS) Pathfinder. This is the first global lidar dataset from a spacecraft of extended duration that can be compared to the HIRS climatology. It provides an excellent source of cloud information because it is more sensitive to clouds that are difficult to detect, namely, thin cirrus and small boundary layer clouds. The second GLAS data collection period from 1 October to 16 November 2003 was used for this comparison, and a companion dataset of the same days were analyzed with HIRS. GLAS reported cloud cover of 0.70 while HIRS reported slightly higher cloud cover of 0.75 for this period. The locations where HIRS overreported cloud cover were mainly in the Arctic and Antarctic Oceans and parts of the Tropics. GLAS also confirms that upper-tropospheric clouds (above 6.6 km) cover about 0.33 of the earth, similar to the reports from HIRS data. Generally, the altitude of the cloud tops reported by GLAS is, on average, higher than HIRS by 0.4 to 4.5 km. The largest differences were found in the Tropics, over 4 km, while in midlatitudes average differences ranged from 0.4 to 2 km. Part of this difference in averaged cloud heights comes from GLAS finding more high cloud coverage in the Tropics, 5% on average but 13% in some areas, which weights its cloud top average more toward the high clouds than the HIRS. The diffuse character of the upper parts of high clouds over tropical oceans is also a cause for the difference in reported cloud heights. Statistics on cloud sizes also were computed from GLAS data to estimate the errors in cloud cover reported by HIRS from its 20-km field-of-view (FOV) size. Smaller clouds are very common with one-half of all clouds being 41 km in horizontal size. But, clouds 41 km cover only 5% of the earth. Cloud coverage is dominated by larger clouds with one-half of the coverage coming from clouds 1000 km. GLAS cloud size statistics also show that HIRS possibly overreports some cloud forms by 2%–3%. Looking at groups of GLAS data 21 km long to simulate the HIRS FOV, the authors found that 5% are partially filled with cloud. Since HIRS does not account for the part of the FOV without cloud, it will overreport the coverage of these clouds. However, low-altitude and optically thin clouds will not be reported by HIRS if they are so small that they do not affect the upwelling radiation in the HIRS FOV enough to trigger the threshold for cloud detection. These errors are partially offing.

Geoscience laser altimeter system (GLAS) on the ICESat mission: pre-launch and on-orbit measurement performance

GLAS is a space lidar on NASAs ICESat Mission. GLAS was qualified and delivered and ICEsat is scheduled for launch in December 2002. This talk summarizes the as-built characteristics of GLAS and its predicted measurement performance.GLAS is a space lidar on NASA’s ICESat Mission, which was was launched into a 590 km altitude circular polar orbit on January 12, 2003. This talk will summarize the as-built characteristics of GLAS and its pre-launch and onorbit scientific measurement performance.

Global aerosol distribution from the GLAS polar orbiting lidar instrument

The Geoscience Laser Altimeter System (GLAS) launched in 2003 has provided the first global aerosol profiling from space. GLAS is a two wavelength nadir viewing instrument. The measurement requirement to profile all radiatively significant aerosol layers has been exceeded. Data processing algorithms have provided aerosol scattering cross section profiles, boundary detection and height for all aerosol layers, aerosol optical depth and extinction cross section for data from 2003. The data products are openly available to the global science community.

Atmospheric measurements by the geoscience laser altimeter system: initial results

The Geoscience Laser Altimeter System launched in early 2003 is the first satellite instrument in space to globally observe the distribution of clouds and aerosol through laser remote sensing. The instrument is a basic backscatter lidar that operates at two wavelengths, 532 and 1064 nm. The mission data products for atmospheric observations include the cali- brated, observed, attenuated backscatter cross section for cloud and aerosol; height detection for multiple cloud layers; plane- tary boundary layer height; cirrus and aerosol optical depth and the height distribution of aerosol and cloud scattering cross sec- tion profiles. The data will enhance knowledge in several areas of atmospheric science, in particular the distribution, transport and influence of atmospheric aerosol. Measurements of the coverage and height of polar and cirrus cloud should be signifi- cantly more accurate than previous global measurement. Initial results from the first several months of operation show the de- tailed height structure of clouds and aerosol on a global basis as expected.

Verification of data and atmospheric measurements for the 2002 GLAS orbital lidar mission.

The Geoscience Laser Altimeter System (GLAS) is a multi-disciplinary earth science laser remote sensing instrument to be launched as part of the NASA Earth Observing System in early 2002.