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Philip E. Ardanuy, Larry L. Stowe, Arnold Gruber, Mitchell Weiss, and Craig S. Long

Abstract

Collocated and coincident cloud and outgoing longwave radiation observations taken by experiments on board the Nimbus-7 satellite have been used to infer the daytime longwave cloud-radiative forcing. Through the specification of a time-series of daily values of cloud amount, cloud-top temperature, surface temperature, and outgoing longwave radiation, the clear-sky flux is obtained for both the summer (June, July, and August 1979) and winter (December 1979. January and February 1980) seasons. The longwave component of the cloud-radiative forcing is then computed by subtracting the observed outgoing longwave flux from the inferred clear-sky longwave flux. The results are compared to independent cloud forcing estimates produced using high spatial resolution radiometers and found to agree closely.

The resultant cloud forcing is analyzed regionally, zonally, and globally for each season to quantify, through observation, the role that clouds play in modulating the outgoing longwave radiation. The largest cloud forcing is found over regions of tropical convection, and reaches peak values of about 80 W m−2 in the vicinity of the summer and winter monsoon. Cloud forcing values of less than 10 W m−2 are evident over the deserts the subtropical oceans, and in the polar latitudes, Zonally, the cloud forcing reaches maxima over the Intertropical Convergence Zone (40 to 50 W m−2) and over the polar frontal zones of both hemispheres (25 to 30 W m−2), and minima in the subtropical belts and at the poles. Globally, the cloud forcing is found to be 24 W m−2. The globally averaged cloud cover for the same period is 50%.

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Larry L. Stowe, Herbert Jacobowitz, George Ohring, Kenneth R. Knapp, and Nicholas R. Nalli

Abstract

As part of the joint National Oceanic and Atmospheric Administration–National Aeronautics and Space Administration (NOAA–NASA) Pathfinder program, the NOAA/National Environmental Satellite, Data and Information Service (NESDIS) has created a research-quality atmospheric, climate-scale dataset through the reprocessing of archived Advanced Very High Resolution Radiometer (AVHRR) observations from four afternoon satellites, in orbit since 1981. The raw observations were recalibrated using a vicarious calibration technique for the AVHRR reflectance channels and an improved treatment of the nonlinearity of the three infrared emittance channels. State-of-the-art algorithms are used in the Pathfinder Atmosphere (PATMOS) project to process global AVHRR datasets into statistics of channel radiances, total cloud amount, components of the earth's radiation budget, and aerosol optical thickness over oceans. The radiances and earth radiation budget components are determined for clear-sky and all-sky conditions. The output products are generated on a quasi-equal-area grid with a spatial resolution of approximately 110 km, with twice-a-day temporal resolution, and averaged over 5-day (pentad) and monthly time periods. The quality of the products is assessed relative to independent surface or satellite observations of these parameters. This analysis shows that the PATMOS data are sufficiently accurate for studies of the interaction of clouds and aerosol with solar and terrestrial radiation, and of climatic phenomena with large signals, for example, the annual cycle, monsoons, and the four ENSOs and two major volcanic eruptions that occurred during the 19-yr PATMOS period. Analysis also indicates that smaller climate signals, such as those associated with longer-term trends in surface temperature, may be difficult to detect due to the presence of artifacts in the time series that result from the drift of each satellite's observation time over its mission. However, a simple statistical method is employed to remove much of the effect caused by orbital drift. The uncorrected PATMOS dataset is accessible electronically.

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Larry L. Stowe, H. Y. Michael Yeh, Thomas F. Eck, Charlie G. Wellemeyer, H. Lee Kyle, and The Nimbus-7 Cloud Data Processing Team

Abstract

Regional and seasonal variations in global cloud cover observed by the Nimbus-7 satellite over 1 year are analyzed by examining the 4 midseason months—April, July and October 1979 and January 1980. The Nimbus-7 data set is generated from the Temperature Humidity Infrared Radiometer (THIR) 11.5 micron radiances together with Total Ozone Mapping Spectometer (TOMS)-derived UV reflectivities, climatological atmospheric temperature lapse rates, and concurrent surface temperature and snow/ice information from the Air Force three-dimensional-nephanalysis (3DN) archive. The analysis presented here includes total cloud amount, cloud amounts at high, middle and low altitudes, cirrus and deep convective clouds and cloud and cloud-sky 11.5 micron-derived radiances. Also, noon versus midnight cloud amounts are examined and the Nimbus-7 data are compared to three previously published cloud climatologies.

The Nimbus-7 bispectral algorithm gives a monthly mean global noontime cloud cover of 51%, averaged over the 4 months. When only the IR is used, this cloud cover is 49% at noontime and 56% at midnight, indicating that the Earth's cloud cover has a substantial diurnal cycle. Each hemisphere shows a cloud cover maximum in its summer and a minimum in its winter. The Southern Hemisphere shows more clouds than the Northern Hemisphere except for the month of July.

The difference between the cloud-top and clear-scene radiance has maxima in the equatorial cloud belt and minima in the polar regions. Because of thew polar minima and the frequent presence of snow, Nimbus-7 cloud traction estimates are less reliable in the polar regions. In the tropics the data show more clouds at midnight than at noon. Over the tropical ocean, overcast regions show lower cloud top radiation temperatures at noon than at midnight, but over land the reverse occurs.

In July, cloud amounts in the intertropical convergence zone (ITCZ) peak at about 10°N latitude with local maxima greater than 70% around the west coasts of Africa and Central America, and from India east to the dateline. Cloud-top radiances indicate that mid- and high-level clouds predominate in the ITCZ, with 5% to 15% each of cirrus and deep convective clouds, respectively. In January, the peak of the ITCZ shifts to 10°S with local cloud maxima greater than 90% over Brazil and to the north and northwest of Australia. Comparison is made with several other cloud data sets, including a look at the new preliminary International Satellite Cloud Climatology Project (ISCCP) results. There are considerable differences among the several data sets examined.

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