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Johannes Schmetz

Abstract

Previous work has discussed the existence of a linear relationship between the net solar radiative flux densities at the surface and at the top of the atmosphere (TOA) that can be exploited for inferring the net surface radiation directly from the satellite observed net radiation. In physical terms the net solar flux at the surface can be estimated from the difference between the satellite-inferred net flux at TOA and total solar absorption in the atmosphere.

This paper presents model calculations of the influence on solar absorption of water vapor, solar zenith angle, cloud-top altitude, and cloud optical thickness. The model results indicate a somewhat complex relation between the solar net fluxes at the surface and at the top of the atmosphere. It is pointed out that cloud altitude and optical depth have a large impact on solar atmospheric absorption; high clouds decrease solar absorption by the atmosphere whereas low clouds increase it. This difference between solar atmospheric absorption for low and high clouds increases with cloud optical depth. An intriguing result is that changes of total atmospheric absorption with cloud-top height are nearly completely compensated by corresponding changes in the net flux at the top of the atmosphere, thus leaving the surface solar net flux constant. Furthermore, this paper provides a very simple parameterization for estimating the clear-sky solar atmospheric absorption as a function of solar zenith angle and the vertically integrated water vapor content of the atmosphere.

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Olli M. Turpeinen and Johannes Schmetz

Abstract

The purpose of this note is to “validate” the upper tropospheric humidity (UTH) operationally extracted from the 6.3 μm channel data of METEOSAT. The validation is carded out by comparing the satellite data with observed humidifies from the conventional network of radiosondes. The validation is not an absolute error assessment of the UTH, but rather an intercomparison, because the upper-tropospheric radiosonde humidities may not be very accurate.

The results for three latitude belts indicate that the estimated humidity in the upper troposphere shows a fairly high linear correlation (coefficients 0.64–0.89) with the observed humidity and that the slope of the regression line ranges from 0.82–0.89. The rms-error of the UTH is generally less than 10%. The UTH tends to underestimate the observed humidity by about 4%. This relative bias is most pronounced in the midlatitude belts.

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Johannes Schmetz and W. Paul Menzel

Abstract

In this paper, the authors offer their observations from more than 30 years of involvement in the evolution of the space-based meteorological remote sensing systems. Successes and issues from the past are recalled that established meteorological satellites into their current pivotal role. Evolution of imaging and sounding satellite systems from user requirements to affordable realizations is noted; some examples from recent U.S. and European experiences in the area of operational meteorological satellites are presented. The authors discuss the importance of the balanced roles of the three partners in satellite development (government, research, and industry), the need to develop full utilization of new satellite programs quickly during their early life, and a vision for global cooperation early in the planning stages of meteorological satellite missions. The authors offer suggestions that could foster expanded international collaboration on science and applications as well as expedite more satellite observations being pursued in a sustained manner.

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Johannes Schmetz and Olli M. Turpeinen

Abstract

A retrieval method is described for estimating a mean column value of the upper tropospheric relative humidity (UTH) from radiance measurements in the 6.3 μm channel of the geostationary satellite METEOSAT. The physical retrieval method is based on an efficient radiative transfer scheme which uses the temperature forecast profiles from the European Centre for Medium Range Weather Forecasts (ECMWF) as ancillary data. Theoretical radiances for the given temperature profile and a set of fixed upper tropospheric humidities are employed to- relate the observed radiance to a mean humidity for a layer between 600 and 300 hPa. The retrieval is confined to areas with neither medium-nor high-level clouds.

A calibration procedure of the 6.3 μm channel is described which uses the radiative transfer scheme with measured radiosonde profiles of temperature and humidity and collocated satellite measurements. An example of the UTH product and a comparison with radiosondes is presented. An estimate of the error of the UTH is obtained from a sensitivity test of the radiation scheme to errors in the input profiles. Both the sensitivity and the comparison with radiosondes yield absolute error estimates for the UTH of 10%–15%.

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Byung-Ju Sohn and Johannes Schmetz

Abstract

Subdividing the Indian Ocean domain into three areas: (i) a moist cloudy area due to tropical deep convection, (ii) a moist clear area fed by the evaporation of hydrometeors from adjacent high clouds, and (iii) a dry area represented by descending air over the subtropics, the relationships between upper-tropospheric humidity over these three areas and tropical convections are examined using the European Geostationary Meteorological Satellite (Meteosat-5) observations. It is observed that the clear dry area shrinks and becomes drier in response to expansion of the cloudy area in the Tropics and vice versa. This change in upper-tropospheric humidity over the subtropics appears to mitigate the increase (decrease) in water vapor greenhouse effect caused by the expansion (contraction) of moist convective areas.

A simple sensitivity test shows that the strength of the water vapor feedback due to changes in the spatial extent of tropical convection is benign, though slightly negative, if the changes in subtropical dryness are considered.

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Johannes Schmetz, Donald Hinsman, and W. Paul Menzel

The Fourth International Winds Workshop (IWW4) was held in Saanenmoeser, Switzerland, from 20 to 23 October 1998. The workshop was organized by the European Organisation for the Exploitation of Meteorological Satellites, and the World Meteorological Organization was the local host. IWW4 followed previous meetings convened in Washington, D.C., in September 1991; Tokyo, Japan, in December 1993; and Ascona, Switzerland, in June 1996. The International Winds Workshop convenes the International Winds Working Group, which communicates with the Coordination Group for Meteorological Satellites on issues of importance regarding wind derivation from satellites. It provides a forum for data producers and users to share information on the characteristics of satellite-tracked winds and to optimize their use in several applications, especially numerical weather prediction. This report describes the proceedings of the Fourth International Winds Workshop and includes recommendations.

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Marianne Gube, Johannes Schmetz, and Ehrhard Raschke

Abstract

No abstract available.

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Johannes de Waard, W. Paul Menzel, and Johannes Schmetz

On 1 August 1991, a European geostationary satellite (METEOSAT-3) started operation from a new position at 50°W. This extends westward the European capability to monitor midlatitude storm tracks and assists the National Oceanic and Atmospheric Administration (NOAA) for surveying Atlantic basin and coastal areas. This note announces the new Atlantic Data Coverage with METEOSAT-3, summarizes the history of the METEOSAT program, describes the image data and its calibration, and indicates some prospective NOAA applications of the METEOSAT-3 data from 50°W.

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Andreas Ottenbacher, Maria Tomassini, Kenneth Holmlund, and Johannes Schmetz

Abstract

Low-level wind fields over the Atlantic have been derived from clouds in Meteosat high-resolution visible images experimentally with one production cycle per day over a period of more than 1 yr. The cloud motion winds from VIS imagery (VIS-CMW) use a template size of 32 × 32 VIS pixels, corresponding to about 80 km × 80 km at the subsatellite point, which is four times better than for the corresponding IR (infrared window) winds (160 km × 160 km). The yield is increased through the better spatial resolution of the VIS images and a better contrast between cloud and ocean surface, which effectively leads to an increase in wind vectors by a factor of 6. This implies a much better description of the low-level atmospheric flow by the VIS-CMW as compared to IR winds. The impact of the new VIS-CMW has been tested with a data assimilation experiment at the European Centre for Medium-Range Weather Forecasts, and small positive improvements have been found. The mean vector rms difference versus the verifying analysis shows improvement by up to 15% over some areas of the Atlantic Ocean. Comparisons of the short-term forecast using VIS cloud motion winds with independent scatterometer surface winds confirm the small improvements.

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Byung-Ju Sohn, Johannes Schmetz, Rolf Stuhlmann, and Joo-Young Lee

Abstract

In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.

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