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D. P. Wylie
and
W. P. Menzel

Abstract

Statistics of cloud characteristics over North America have been accumulated for the past 2 yr. The frequency of cloud cover with the associated heights and infrared attenuation were charted using the C02 channel radiometric data from the VISSR Atmospheric Sounder (VAS). Cloud top pressures were determined from the ratio of VAS CO2 channel radiances in a radiative transfer equation formulation. Cloud emissivities were then calculated from infrared window channel observations The VAS C02 derived cloud top height and emissivity assignments have been found to be reliable in most cloud type, including thin cirrus clouds where other techniques have been inconsistent. Observations since 1985 reveal that 20%–30% of the United States was covered with thin semitransparent clouds (radiative attenuation was less than 95%), 45% was covered with thick opaque clouds, and 25%–35% had clear sky conditions. It is likely that 5% of the opaque cloud should have been identified as semitransparent cirrus. The geographical distribution of cloud cover shows a latitudinal dependence mainly over the Pacific Ocean. Moderate seasonal and diurnal changes were also found which agree with other published cloud studies.

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Donald Wylie
,
Edwin Eloranta
,
James D. Spinhirne
, and
Steven P. Palm

Abstract

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.

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W. P. Menzel
,
D. P. Wylie
, and
K. I. Strabala

Abstract

GOES VAS multispectral observations in the carbon dioxide absorption band at 15 μm have been used to compile cloud-cover statistics over the continental United States for the past 4 years. The CO2 technique calculates both cloud-top pressures and effective emissivities and reliably distinguishes semitransparent cirrus from opaque clouds. The frequency of semitransparent cirrus clouds exhibits small seasonal variation; they are generally present 25%–30% of the time in all seasons. Diurnal variations of semitransparent cirrus are found only in the summer months and correspond to diurnal variations in convection in the Rocky Mountains and southeastern United States, increases of 20% in cirrus are noted subsequent to the convective cloud activity. In the winter months, no diurnal change in semitransparent cirrus is detected. Attempts to correlate cirrus with some common atmospheric features reveal that a majority of cirrus occurred where dynamic parameters indicate rising vertical motion but that considerable cirrus were also found where the dynamics was weak. Intercomparison with ground reports of cloud cover reveals that the satellite observations are corroborating or complementary 80% of the time; many of the disagreements come from the satellite identifying cold ground as low cloud or ground observations missing high thin clouds.

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J. A. Curry
,
P. V. Hobbs
,
M. D. King
,
D. A. Randall
,
P. Minnis
,
G. A. Isaac
,
J. O. Pinto
,
T. Uttal
,
A. Bucholtz
,
D. G. Cripe
,
H. Gerber
,
C. W. Fairall
,
T. J. Garrett
,
J. Hudson
,
J. M. Intrieri
,
C. Jakob
,
T. Jensen
,
P. Lawson
,
D. Marcotte
,
L. Nguyen
,
P. Pilewskie
,
A. Rangno
,
D. C. Rogers
,
K. B. Strawbridge
,
F. P. J. Valero
,
A. G. Williams
, and
D. Wylie

An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted during April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmatic and scientific objectives of the project, the experimental design (including research platforms and instrumentation), the conditions that were encountered during the field experiment, and some highlights of preliminary observations, modeling, and satellite remote sensing studies.

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Taneil Uttal
,
Judith A. Curry
,
Miles G. McPhee
,
Donald K. Perovich
,
Richard E. Moritz
,
James A. Maslanik
,
Peter S. Guest
,
Harry L. Stern
,
James A. Moore
,
Rene Turenne
,
Andreas Heiberg
,
Mark. C. Serreze
,
Donald P. Wylie
,
Ola G. Persson
,
Clayton A. Paulson
,
Christopher Halle
,
James H. Morison
,
Patricia A. Wheeler
,
Alexander Makshtas
,
Harold Welch
,
Matthew D. Shupe
,
Janet M. Intrieri
,
Knut Stamnes
,
Ronald W. Lindsey
,
Robert Pinkel
,
W. Scott Pegau
,
Timothy P. Stanton
, and
Thomas C. Grenfeld

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goals, experimental design, instrumentation, and the resulting datasets. Examples of various data products available from the SHEBA project are presented.

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