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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 P. Wylie and W. Paul Menzel

Abstract

Over the last 8 yr frequency and location of cloud observations have been compiled using multispectral High Resolution Infrared Radiation Sounder (HIRS) data from the National Oceanic and Atmospheric Administration polar-orbiting satellites; this work is an extension of the 4-yr dataset reported by D. Wylie et al. The CO2 slicing algorithm applied to the HIRS data exhibits a higher sensitivity to semitransparent cirrus clouds than the cloud algorithm used by the International Satellite Cloud Climatology Project; the threshold for cloud detection appears to require visible optical depths (τ vis) greater than 0.1.

The geographical distributions of clouds in the 8-yr dataset are nearly the same as those reported from 4 yr of data. The detection of upper-tropospheric clouds occurs most often in the intertropical convergence zone and midlatitude storm belts with lower concentrations in subtropical deserts and oceanic subtropical highs. The areas of concentrated cloud cover exhibit latitudinal movement with the seasons as in other cloud datasets. HIRS finds clear sky in 25%, opaque cloud in 32%, and semitransparent cloud in 43% of all its observations. The effective emissivity of the all semitransparent clouds (τ vis < 6) ranges from 0.2 to 0.6 with an average value of about 0.5.

Time trends are reexamined in detail. A possible cirrus increase in 1991 reported by Wylie and coauthors in 1994 is found to be diminished upon reinspection. The revised 8-yr record has indications of an increase in high clouds in the northern midlatitudes (0.5% yr−1) but little change elsewhere. The seasonal cycle of cloud cover in the Southern Hemisphere becomes very noticeable in 1993.

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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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W. P. Menzel, W. L. Smith, and T. R. Stewart

Abstract

A CO2 cloud tracking technique to determine simultaneous heights and velocities of cloud motion winds is presented. Using animated CO2 channel imagery from VAS, multi-level cloud situations are separated into high, middle and low level cloud motion wind vectors by the CO2 slicing method. The VAS CO2 channel radiometric values are used in the CO2 absorption method to assign quantitative heights to the cloud vectors; cloud top pressures are determined from the ratio of the deviations in cloud produced radiances and the corresponding clear air values for three CO2 channels in a radiative transfer equation formulation. Two case studies are presented that show CO2 cloud-motion wind vectors to be in good agreement with radiosonde wind observations and CO2 cloud heights to be within a 50 mb rms deviation of radiosonde, bispectral and stereo height determinations.

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J. A. Zandlo, W. L. Smith, W. P. Menzel, and C. M. Hayden

Abstract

A method to depict quasi-continuous surface temperature features is presented. Half-hourly GOES window channel brightness temperature determinations are employed to monitor time changes in the surface temperature field. TIROS-N water vapor channel measurements, within 6 h of the GOES measurements, are used to generate water vapor absorption corrections to the window channel brightness temperatures. Two case studies are presented that show the resulting surface radiating temperature estimates to be accurate close to 1 K. In regions where conventional ground based measurements are sparse, this method is demonstrated to be most useful. The potential for using time sequences of these surface temperature fields as a diagnostic aid for forecasting severe weather is exhibited in one of the case studies.

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Tom H. Zapotocny, W. Paul Menzel, James P. Nelson III, and James A. Jung

Abstract

The impact of 10 data types used in the Eta Data Assimilation/Forecast System (EDAS) is studied for extended-length time periods during three seasons. Five of the data types are remotely sensed satellite data, and the other five are in situ. The satellite data types include three-layer and vertically integrated precipitable water, temperature data down to cloud top, infrared cloud-drift winds, and water vapor cloud-top winds. The five in situ data types consist of two rawinsonde and two aircraft observation types along with surface land observations. The work described in this paper is relevant for Eta Model users trying to identify the impact of remotely sensed, largely maritime data types and in situ, largely land-based data types. The case studies chosen consist of 11-day periods during December 1998, April 1999, and July 1999. During these periods, 11 EDAS runs were executed twice daily. The 11 runs include a control run, which utilizes all data types used in the EDAS, and 10 experimental runs in which one of the data types is denied. Differences between the experimental and control runs are then accumulated and analyzed to demonstrate the 0-h sensitivity and 24-h forecast impact of these data types in the EDAS. Conventional meteorological terms evaluated include temperature, u component of the wind, and relative humidity on five pressure levels. These diagnostics are computed over the entire model domain and within a subsection centered on the continental United States (CONUS). The entire domain results show that a modest positive forecast impact is achieved from all 10 data types during all three time periods. Rawinsonde temperature and moisture observations and infrared cloud-drift wind observations have the largest positive impact season to season; however, both precipitable water data types provide significant positive forecast impact during the summer and transition seasons. Rawinsonde temperature and moisture, rawinsonde winds, aircraft winds, and infrared cloud-drift winds have the largest positive impact season to season over CONUS. The three-layer precipitable water data type produces large positive forecast impact over CONUS during July. In general, the forecast impacts are smaller for nearly all data types over CONUS than over the entire model domain. There are also more negative forecast impacts for both the in situ and remotely sensed data types over CONUS than over the entire domain.

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Jun Li, Christopher C. Schmidt, James P. Nelson III, Timothy J. Schmit, and W. Paul Menzel

Abstract

The potential for using Geostationary Operational Environmental Satellite (GOES) Sounder radiance measurements to monitor total atmospheric ozone is examined. A statistical regression using GOES Sounder spectral bands 1–15 radiances allows estimation of total atmospheric ozone. Hourly GOES ozone products have been generated since May 1998. GOES ozone estimates are compared with Total Ozone Mapping Spectrometer (TOMS) ozone measurements from the Earth Probe satellite and ground-based Dobson spectrometer ozone observations. Results show that the percentage root-mean-square (rms) difference between instantaneous TOMS and GOES ozone estimates ranges from 4% to 7%. Also, daily comparisons for 1998 between GOES ozone values and ground-based observations at Bismarck, North Dakota; Wallops Island, Virginia; and Nashville, Tennessee, show that the rms difference is approximately 21 Dobson units. Given the hourly measurements and high-spatial density provided by the GOES Sounder, GOES ozone estimates and associated products show promise.

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Tom H. Zapotocny, W. Paul Menzel, James A. Jung, and James P. Nelson III

Abstract

The impact of in situ rawinsonde (raob) data, remotely sensed Geostationary Operational Environmental Satellite (GOES), and Polar Operational Environmental Satellite (POES) data routinely used in NCEP’s Eta Data Assimilation/Forecast System (EDAS) is studied for extended-length time periods during four seasons. The work described in this paper is relevant for users of the Eta Model trying to compare and contrast the overall forecast impact of traditional, mostly land-based rawinsonde data with remotely sensed data that are available domainwide.

The case studies chosen consist of 15-day periods during fall 2001, winter 2001/02, spring 2002, and summer 2002. During these periods, a 32-km/60-layer November 2001 version of the EDAS is run four times at both 0000 and 1200 UTC. The four runs include a control run, which utilizes all data types routinely used in the EDAS, and three experimental runs in which either all rawinsonde, GOES, or POES data are denied. Differences between the experimental and control runs are then accumulated over the 15-day periods and analyzed to demonstrate the 24- and 48-h forecast impact of these data types in the EDAS. Conventional meteorological terms evaluated include mean sea level pressure as well as temperature, both components of the wind, and relative humidity. Comparisons are made on seven pressure levels extending from near the earth’s surface to the lower stratosphere. The diagnostics are computed over both the entire horizontal model domain, and within a subsection covering the continental United States and adjacent coastal waters (extended CONUS).

The 24-h domainwide results show that a positive forecast impact is achieved from all three data sources during all four seasons. Cumulatively, the rawinsonde data have the largest positive impact over both the entire model domain and extended CONUS. However, GOES data have the largest contribution for several fields, especially moisture during summer and fall 2001. In general, GOES data also provide larger forecast impacts than POES data, especially for the wind components. All three data types demonstrate comparable forecast impact in terms of relative humidity. Finally, raob and POES data display a “spike” in positive forecast impact in the lower stratosphere during three of the four seasons.

Two additional findings from this study are also important. The first is that the forecast impact of all data types drops by at least a factor of 2 during all seasons between 24 and 48 h. The second is that GOES data show a preference for providing nearly equal improvement to the 0000 and 1200 UTC forecast cycles, while rawinsonde and especially POES data provide consistently larger forecast impacts at 1200 than at 0000 UTC.

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Tom H. Zapotocny, W. Paul Menzel, James A. Jung, and James P. Nelson III

Abstract

The impact of in situ rawinsonde observations (raob), remotely sensed Geostationary Operational Environmental Satellite (GOES), and Polar-Orbiting Operational Environmental Satellite (POES) observations routinely used in NCEP’s Eta Data Assimilation/Forecast System (EDAS) is studied for extended-length time periods during four seasons. This work examines the contribution of nine individual components of the total observing system. The nine data types examined include rawinsonde mass and wind observations, GOES mass and wind observations, POES observations from the Microwave Sounding Unit (MSU), the Advanced Microwave Sounding Unit (AMSU-A and AMSU-B), the High Resolution Infrared Radiation Sounder (HIRS), and column total precipitable water and low-level wind observations from the Special Sensor Microwave Imager (SSM/I). The results are relevant for users of the Eta Model trying to compare/contrast the overall forecast impact of traditional, largely land-based rawinsonde observations against remotely sensed satellite observations, which are available domainwide.

The case studies chosen consist of 15-day periods during fall 2001, winter 2001/02, spring 2002, and summer 2002. Throughout these periods, a November 2001 32-km version of the EDAS is run 10 times at both 0000 and 1200 UTC. The 10 runs include a control run, which utilizes all data types routinely used in the EDAS, and 9 experimental runs in which one of the component data types noted above is denied. Differences between the experimental and control runs are then accumulated over the 15-day periods and analyzed to demonstrate the 00-h sensitivity and 24-h forecast impact of these individual data types in the EDAS. The diagnostics are computed over the entire horizontal model domain and a subsection covering the continental United States (CONUS) and adjacent coastal waters on isobaric surfaces extending into the lower stratosphere.

The 24-h forecast impact results show that a positive forecast impact is achieved from most of the nine component data sources during all four time periods. HIRS, MSU, and SSM/I wind observations yield only a slight positive forecast impact to all fields. Rawinsonde and GOES wind observations have the largest positive forecast impact for temperature over both the entire model domain and the extended CONUS. The same data types also provide the largest forecast impact to the u component of the wind, while GOES wind observations provide the largest forecast impact to moisture.

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Donald P. Wylie, W. Paul Menzel, Harold M. Woolf, and Kathleen I. Strabala

Abstract

Trends in global upper-tropospheric transmissive cirrus cloud cover are beginning to emerge from a four-year cloud climatology using NOAA polar-orbiting High-Resolution Infrared Radiation Sounder (HIRS) multispectral data. Cloud occurrence, height, and effective emissivity am determined with the C02 slicing technique on the four years of data (June 1989–May 1993). There is a global preponderance of transmissive high clouds, 42% on the average; about three-fourths of these are above 500 hPa and presumed to be cirrus. In the ITCZ, a high frequency of cirrus (greater than 50%) is found at all times; a modest seasonal movement tracks the sun. Large seasonal changes in cloud cover occur over the oceans in the storm belts at midlatitudes; the concentrations of these clouds migrate north and south with the seasons following the progressions of the subtropical highs (anticyclones). More cirrus is found in the summer than in the winter in each hemisphere.

A significant change in cirrus cloud cover occurs in 1991, the third year of the study. Citrus observations increase from 35% to 43% of the data, a change of eight percentage points. Other cloud forms, opaque to terrestrial radiation, decrease by nearly the same amount. Most of the increase is thinner cirrus with infrared optical depths below 0.7. The increase in cirrus happens at the same time as the 1991–92 El Niño/Southern Oscillation (ENSO) and the eruption of Mt. Pinatubo. The cirrus changes occur at the start of the ENSO and persist into 1993 in contrast to other climatic indicators that return to near pre-ENSO and volcanic levels in 1993.

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