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.

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%.

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.

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.

Abstract

Outgoing longwave radiative fluxes (OLR) and the longwave cloud-radiative forcing at the atmosphere are retrieved from METEOSAT radiance observations in the thermal infrared window (IR: 10.5–12.5 μm) and water vapor (WV: 5.7–7.1 μm) channels for April 1985. The analysis exploits an operationally preprocessed radiance dataset that includes a scene identification of clear sky, low level, medium level and high level clouds. Monthly means of the OLR and the longwave cloud-radiative forcing are inferred for areas of about 200 km × 200 km. Extended regions with a forcing larger than 60 W m⁻² are found within the intertropical convergence zone (ITCZ) over southern Sudan and around 5°S over Brazil and the adjacent Atlantic Ocean.

The contribution of three levels of cloud to the longwave radiative forcing is estimated: high level coulds (≤400 hPa) contribute about 80% to the total longwave forcing in regions with strong convective activity (ITCZ). Medium level coulds (700 ≤ cloud top < 400 hPa) induce a maximum forcing of 15–20 W m⁻² over the Ethiopian highland, while low level cloud forcing reaches values of 5–10 W m⁻² over the marine stratocumulus regions and within the midlatitude westerlies.

Systematic errors in the longwave cloud-radiative forcing due to calibration errors, cloud contamination of clear sky radiances and a dry bias in the humidity of the upper troposphere, which may occur as a result of minimizing the cloud contamination, are discussed; it is concluded that the present study underestimates maximum values of the longwave cloud-radiative forcing by about 10 W m⁻².

Abstract

Satellite-derived cloud-motion vector (CMV) production has been troubled by inaccurate height assignment of cloud tracers, especially in thin semitransparent clouds. This paper presents the results of an intercomparison of current operational height assignment techniques. Currently, heights are assigned by one of three techniques when the appropriate spectral radiance measurements are available. The infrared window (IRW) technique compares measured brightness temperatures to forecast temperature profiles and thus infers opaque cloud levels. In semitransparent or small subpixel clouds, the carbon dioxide (CO₂) technique uses the ratio of radiances from different layers of the atmosphere to infer the correct cloud height. In the water vapor (H₂O) technique, radiances influenced by upper-tropospheric moisture and IRW radiances are measured for several pixels viewing different cloud amounts, and their linear relationship is used to extrapolate the correct cloud height. The results presented in this paper suggest that the H₂O technique is a viable alternative to the CO₂ technique for inferring the heights of semitransparent cloud elements. This is important since future National Environmental Satellite, Data, and Information Service (NESDIS) operations will have to rely on H₂0-derived cloud-height assignments in the wind field determinations with the next operational geostationary satellite. On a given day, the heights from the two approaches compare to within 60–110 hPa rms; drier atmospheric conditions tend to reduce the effectiveness of the H₂O technique. By inference one can conclude that the present height algorithms used operationally at NESDIS (with the C0₂ technique) and at the European Satellite Operations Center (ESOC) (with their version of the H₂0 technique) are providing similar results. Sample wind fields produced with the ESOC and NESDIS algorithms using Meteosat-4 data show good agreement.

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.

Abstract

A method and a passive microwave retrieval algorithm have been developed to retrieve upper-tropospheric water vapor (UTW) from Special Sensor Microwave Water Vapor Profiler (SSM/T-2) measurements taken at three discrete frequencies near the 183-GHz water vapor line. The algorithm is based on physical relaxation utilizing statistical covariance information to provide initial-guess profiles and to constrain the updating step in the relaxation process. The scheme incorporates a method to remove SSM/T-2 brightness temperature bias in comparison with collocated simulated brightness temperatures. Correction functions are designed for the three SSM/T-2 183-GHz channels. The algorithm is validated against radiosonde observations and collocated SSM/T-2 brightness temperatures. Under clear-sky and nonprecipitating-cloud conditions, the UTW retrievals exhibit an rms error of 0.68 kg m⁻² with integrated water vapor biases below 5% for the upper-tropospheric layers of 700–500 and 500–200 hPa. The retrieval provides an independent source of satellite-derived water vapor information in the upper troposphere, distinct from upper-tropospheric humidity information retrieved from thermal infrared (IR) measurements around the 6.3-μm water vapor absorption band. The microwave retrievals can then be used to cross-check IR retrievals and/or to augment IR retrievals, dependent upon the problem at hand.

This paper introduces the new generation of European geostationary meteorological satellites, Meteosat Second Generation (MSG), scheduled for launch in summer 2002. MSG is spin stabilized, as is the current Meteosat series, however, with greatly enhanced capabilities. The 12-channel imager, called the Spinning Enhanced Visible and Infrared Imager (SEVIRI), observes the full disk of the earth with an unprecedented repeat cycle of 15 min. SEVIRI has eight channels in the thermal infrared (IR) at 3.9,6.2,7.3, 8.7, 9.7, 10.8, 12.0, and 13.4 μum; three channels in the solar spectrum at 0.6, 0.8, and 1.6 μm; and a broadband high-resolution visible channel. The high-resolution visible channel has a spatial resolution of 1.67 km at nadir; pixels are oversampled with a factor of 1.67 corresponding to a sampling distance of 1 km at nadir. The corresponding values for the eight thermal IR and the other three solar channels are 4.8-km spatial resolution at nadir and an oversampling factor of 1.6, which corresponds to a sampling distance of 3 km at nadir.

Radiometric performance of all channels exceeds specifications. Thermal IR channels have an onboard calibration with an accuracy better than 1 K. Solar channels are calibrated with an operational vicarious procedure aiming at an accuracy of 5%. Meteorological products are derived in the so-called Satellite Application Facilities (SAF) and in the central Meteorological Product Extraction Facility (MPEF) at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) in Darmstadt, Germany. The products support nowcasting, numerical weather prediction (NWP), and climatological applications. The most important product for NWP, the atmospheric motion vectors, are derived from different channels to improve data coverage and quality. Novel products are, among others, indices describing the instability of the clear atmosphere and total column ozone. The paper also discusses the use of MSG for future applications, in particular, observations of the rapid cloud development, cloud microphysics, and land applications are considered as areas of high potential. As an additional scientific payload, MSG carries a Geostationary Earth Radiation Budget (GERB) instrument observing the broadband thermal infrared and solar radiances exiting the earth-atmosphere system.

Abstract

The displacement of clouds in successive satellite images reflects the atmospheric circulation at various scales. The main application of the satellite-derived cloud-motion vectors is their use as winds in the data analysis for numerical weather prediction. At low latitudes in particular they constitute an indispensible data source for numerical weather prediction.

This paper describes the operational method of deriving cloud-motion winds (CMW) from the IR image (10.5–12.5 µm) of the European geostationary Meteostat satellites. The method is automatic, that is, the cloud tracking uses cross correlation and the height assignment is based on satellite observed brightness temperature and a forecast temperature profile. Semitransparent clouds undergo a height correction based on radiative forward calculations and simultaneous radiance observations in both the IR and water vapor (5.7–7.1 µm) channel. Cloud-motion winds are subject to various quality checks that include manual quality control as the last step. Typically about 3000 wind vectors are produced per day over four production cycles.

This paper documents algorithm changes and improvements made to the operational CMWs over the last five years. The improvements are shown by long-term comparisons with both collocated radiosondes and the first guess of the forecast model of the European Centre for Medium-Range Weather Forecasts. In particular, the height assignment of a wind vector and radiance filtering techniques preceding the cloud tracking have ameliorated the errors in Meteostat winds. The slow speed bias of high-level CMWs (<400 hPa) in comparison to radiosonde winds have been reduced from about 4 to 1.3 m s⁻¹ for a mean wind speed of 24 m s⁻¹. Correspondingly, the rms vectors error of Meteosat high-level CMWs decreased from about 7.8 to 5 m s⁻¹. Medium- and low-level CMWs were also significantly improved.