Meteosat Water Vapor Research Articles

An improved fast radiative transfer model for assimilation of satellite radiance observations

To assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium-Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line-by-line model computed radiances, that the fast model can reproduce the line-by-line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent-linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non-linear response.

An improved fast radiative transfer model for assimilation of satellite radiance observations

AbstractTo assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium‐Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line‐by‐line model computed radiances, that the fast model can reproduce the line‐by‐line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent‐linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non‐linear response.

Direct comparison of meteosat water vapor channel data and general circulation model results

Following a model to satellite approach, this study points out the ability of the general circulation model (GCM) of the Laboratoire de Météorologie Dynamique to reproduce the observed relationship between tropical convection and subtropical moisture in the upper troposphere. Those parameters are characterized from Meteosat water vapor equivalent brightness temperatures (WVEBT) over a monthly scale. The simulated WVEBT field closely resembles to the observed distribution. The pure water vapor features and the convective areas are well located and their seasonal variations are captured by the model. A dry (moist) bias is found over convective (subsiding) areas, whereas the model globally best acts over Atlantic ocean than over Africa. The observed and simulated seasonal variations show that an extension of the ITCZ is correlated to a moistening of the upper troposphere in subtropical areas. Those results imply a positive large scale relationship between convective and subsiding areas in both observation and simulation, and suggest the relevance of our approach for further climatic studies.

A mesoscale vortex diagnosed from combined satellite and model data

AbstractMesoscale vortices originating as tropopause folds can lead to cyclogenesis, cloud formation and/or convective overturning. The presence of such mesoscale vortices can often be inferred from the nature and behaviour of dry zones in Meteosat water vapour imagery. Diagnostics from an operational limited‐area forecast model are presented for one such dry zone, which appeared to rotate in the water vapour imagery. Model and imagery show that a vortex‐induced combination of potential instability and ascent led to the development of a small band of cloud and thundery rain.

A functional analytic method to derive displacement vector fields from satellite image sequences

Over the past 20 years the maximum cross-correlation method has been employed both operationally and for research in geophysical sciences. Some weaknesses of the well-understood and established technique have remained, which suggest that a non statistical approach is required to overcome these deficiencies. Presently a new functional analytic approach is adopted from computer vision to derive displacement vector fields from satellite image sequences. Case studies of artificially created images with defined motion patterns as well as displacement vector fields derived from Meteosat water vapour pictures and NOAA-AVHRR sea surface temperature pictures reveal the applicability of the method. Due to the basic constraints, the method is particularly suitable for fuzzy greyvalue structures. Nevertheless, modifications concerning the atmospheric and oceanographic kinematics are still required to allow also strong shear flow to be accounted for.

On the calibration of the Meteosat water vapor channel

The calibration of the Meteosat water vapor channel image data is of prime importance for a quantitative use. The vicarious calibration technique employs the use of data from radiosonde ascents. A screening technique, removing spurious radiosonde information, has been improved and used operationally since February 4, 1994. It involves new quality control checks on satellite zenith angle and the upper tropospheric relative humidity measured by the radiosonde, leading to a lower (about 8%) and more stable calibration. The operational calibration technique uses averages for the clear‐sky (above 700 hPa) satellite observations over large areas and relates them to expected radiances at the top of the atmosphere, determined from radiosonde data. The approach, valid owing to the small gradients in the water vapor field in such areas, restricts the number of usable radiosonde observations severely. An alternative calibration method is outlined, which uses a smaller clear‐sky area, thus accepting more radiosonde ascents. This method further decreases the variability of the calibration coefficient by about 20%.

African and Atlantic short-term climatic variations described from Meteosat water vapor channel

Abstract. Pluriannual series of Meteosat-2 water vapor (WV) images are used to build average maps of decadal and monthly brightness temperatures in the 6.3 µm channel. This processing is applied to all the 3-hourly scenes, clear or cloudy, for July 1983 to July 1987. The ISCCP cloudiness analyses confirm that the warmest spots in the monthly WV images correspond to scenes either clear or covered with low clouds, whereas the coldest areas correspond to scenes where cloud tops above 440 hPa frequently occur. The WV statistics are then used to characterize seasonal and interannual variations of both the ITCZ (InterTropical Convergence Zone) and the warm (dry) areas, corresponding to subtropical subsidence. Thanks mainly to the seasonal variations, relationships between the variations in the ITCZ and in dry subtropical areas can be studied. It is shown that, for the Meteosat sector, a wetter subtropical high troposphere is associated with an enhanced activity of the ITCZ, and vice versa. For this area where the north-south assymetry is large, the negative water vapor feedback previously proposed seems not to occur.

African and Atlantic short-term climatic variations described from Meteosat water vapor channel

<strong class="journal-contentHeaderColor">Abstract.</strong> Pluriannual series of Meteosat-2 water vapor (WV) images are used to build average maps of decadal and monthly brightness temperatures in the 6.3 µm channel. This processing is applied to all the 3-hourly scenes, clear or cloudy, for July 1983 to July 1987. The ISCCP cloudiness analyses confirm that the warmest spots in the monthly WV images correspond to scenes either clear or covered with low clouds, whereas the coldest areas correspond to scenes where cloud tops above 440 hPa frequently occur. The WV statistics are then used to characterize seasonal and interannual variations of both the ITCZ (InterTropical Convergence Zone) and the warm (dry) areas, corresponding to subtropical subsidence. Thanks mainly to the seasonal variations, relationships between the variations in the ITCZ and in dry subtropical areas can be studied. It is shown that, for the Meteosat sector, a wetter subtropical high troposphere is associated with an enhanced activity of the ITCZ, and vice versa. For this area where the north-south assymetry is large, the negative water vapor feedback previously proposed seems not to occur.

Half hourly wind data from satellite derived water vapour measurements

Vector fields derived from successive METEOSAT water vapour (WV) images (5.7 – 7.1 μm) have been produced operationally at ESOC (European Space Operations Centre) four times per day since autumn 1993. The development of a fast Transputer Augmented Workstation (TAW) has enabled a real time derivation of these vector fields on a half hourly basis. The WV vector fields describe accurately the large scale high level atmospheric motion, but the existence of some bad vectors reduce the overall quality of the fields. In order to identify these odd winds, an automatic quality control, which assigns a quality indicator to each vector, is performed. These indicators can then be utilised in e.g. Numerical Weather Prediction (NWP) models during data assimilation. The development of the NWP assimilation schemes have now enabled the usage of data at the frequency produced by the TAW and the TAW WV vector fields provide an excellent opportunity to validate the new assimilation schemes. Simultaneously performance of the TAW quality control can be assessed. The TAW is also producing information related to the Upper Tropospheric Humidity (UTH) and Total Column Water (TCW). In conjunction with the WV vector fields the complicated interaction between atmospheric humidity and dynamics can be better understood. In order to address these question a Water Vapour Wind Vector (WVWV) validation campaign was performed was performed during August and September 1994. After a brief introduction to the TAW and the quality of the WVWV fields, this paper will present the first results derived with the TAW, the critical aspects addressed during the WVWV campaign as well as outlining the main areas of future development and research.

Determination of displacement vector fields from satellite image sequences

Presently a new functional analytic approach is adopted from computer vision to derive displacement vector fields from satellite image sequences. Case studies of artificially created images with defined motion patterns as well as displacement vector fields derived from METEOSAT water vapour pictures and NOAA-AVHRR sea surface temperature pictures reveal the applicability of the method. Due to the basic constraints, the method is particularly suitable for fuzzy greyvalue structures.

A Pattern Recognition Technique for Retrieving Humidity Profiles from Meteosat or GOES Imagery

Abstract A retrieval technique based on cloud classification is designed to derive humidity profiles from Meteosat visible (VIS), infrared window (IR), and water vapor (WV) channels, or equivalent sensors available on other satellites. Dewpoint depression (DPD) is the variable retrieved at six standard levels: 1000, 850, 700, 500, 400, and 300 mb. Collocation of soundings and Meteosal-2 imagery was obtained over Europe for March, June, and July 1988. Results are derived from over 2000 dependent and 1000 independent samples. It is found that a classification in seven (IR only) or nine (VIS-IR) classes contains the essential information on cloud type for the application sought. Measures were extracted from approximately 8-km pixel resolution images on 80-km × 80-km and 160-km × 160-km arm, little dependency on horizontal scale was found for the mean humidity profiles associated with each cloud class. The WV channel proved very useful in improving DPDs at higher levels while the VIS channel improved inferenc...

Wind Extraction from Meteosat Water Vapor Channel Image Data

Abstract Wind extraction from successive Meteostat water vapor channel (WVC) images has been performed in an operational environment at the European Space Operation Centre for several months. Motions are computed with a cross-correlation algorithm; height assignment is deduced from the observed WVC brightness temperature. The scheme is fully automated. No distinction is made between clouds and pure water vapor structures. The accuracy of the produced winds is assessed by statistical comparisons with forecast winds and radiosonde winds. Above 400 hPa, WVC derived winds are mainly produced in cloudy areas. They are more numerous and generally as reliable as cloud-motion winds extracted from thermal Infrared channel images, showing a great potential to improve observation of high-troposphere circulations. Below 400 hPa, WVC-derived winds are sparse and usually do not fit with conventional wind observations. WVC-derived winds are very sensitive to signal-to-noise ratios in the imagery.

Wind vectors evaluated from structures within consecutive meteosat water vapour images

Abstract In cloud free areas information on wind vectors can be derived from structures within consecutive water vapour images. Our method of evaluation includes image restoration by Fast Fourier Transformation and filtering as well as a systematic selection of parameter values. The cross correlation technique is applied not only to the images but also to matrices generated from an analysis of their first and second order derivatives in kind of the main curvatures of the grey value functions. The resulting field of displacement vectors is found to match the field of wind vectors measured by radiosondes at the 500 hPa level.

Relation between METEOSAT Water Vapor Radiance Fields and Large Scale Tropical Circulation Features

Mean monthly images from the water vapor channel of METEOSAT characteristically contain large-scale spatial structures, especially in tropical regions. The aim of this paper is to establish connections between these structures and large-scale circulation features. For this purpose, statistical relationships between radiances and some meteorological parameters provided by ECMWF analyses are studied. Temporal correlations are computed for two sizes of regions, in order to compare temporal changes associated with both large-scale circulations and smaller scale systems. The correlations obtained are poor, suggesting that the chosen parameters are not well related at short time scales. Temporal averages appear more suitable for these comparisons. As expected, the mean relative humidity yields the best correlation with the mean water vapor radiances. A (weaker) relationship exists also with mean dynamic fields: large water vapor radiances are almost always related to subsidence in the middle troposphere, divergence near the surface, and convergence in the upper troposphere. However, there is regional variability in the results., one explanation may be different contributions from horizontal advecion and vertical motions to the humidity of the middle troposphere.

Large-scale transport of a CO-enhanced air mass from Europe to the Middle East

On 14 November 1981, the shuttle-borne Measurement of Air Pollution from Satellites (MAPS) experiment observed a carbon monoxide (CO) enhanced air mass in the middle troposphere over the Middle East. The primary source of this polluted air was estimated by constructing adiabatic isentropic trajectories backwards from the MAPS measurement location over a 36 h period. The isentropic diagnostics indicate that CO-enhanced air was transported southeastward over the Mediterranean from an organized synoptic-scale weather regime, albeit of moderate intensity, influencing central Europe on 12 November. Examination of the evolving synoptic scale vertical velocity and precipitation patterns during this period, in conjunction with METEOSAT visible, infrared, and water vapor imagery, suggests that the presence of this disturbed weather system over Europe may have created upward transport of CO-enhanced air between the boundary-layer and midtropospheric levels, and subsequent entrainment in the large-scale northwesterly jet stream flow over Europe and the Mediterranean.

Operational calibration of the METEOSAT water vapor channel by calculated radiances

A method is presented for calibrating the water vapor channel (5.7-7.1 microm) of the geostationary meteorological satellite METEOSAT by radiative transfer calculations. Radiances are calculated from the temperature and moisture profiles of conventional radiosondes and linearly related to collocated satellite measured digital counts. Collocations are considered only for areas with neither medium nor high level cloud. Radiosonde data are routinely received twice per day (1200 and 2400 UT). Radiosonde profiles from an 8-day period in May 1988, and simultaneous Meteosat-2 water vapor measurements are analyzed. The total of 340 collocations provides a calibration coefficient with a precision of 2% assuming a 95% confidence. A preliminary analysis of calibration coefficients of the recently launched METEOSAT-3 shows a significant increase of 6% over a period of 48 days in Sept./Oct. 1988. The calibrated water vapor radiances are operationally used to estimate the upper tropospheric humidity field and to correct the height assignment of semitransparent clouds.

Observation of mean dynamic fields from Meteosat large scale water vapor structure motions

With the objective characterizing atmospheric dynamics for particular months, structures appearing at large scales on Meteosat water vapor images are tracked with similar methods as those used for cloud tracking in other channels at smaller scales. For this purpose sampled ISCCP images at resolution 30 km with time intervals of 3 hours are used. Mean monthly “motion fields” are computed. These fields are very consistent with the general circulation of the upper troposphere. They are shown to represent the motions of synoptic systems rather than the wind at a particular level. Nevertheless, significant interannual variations of the large scale circulations can be observed.

Reduction of Noise Interference from METEOSAT Water Vapor Image Data by Means of Fourier Transform and Frequency Domain Filtering

Images provided by the water vapor channel data of meteorological satellites are suitable for the determination of wind vectors in the midtroposphere. Preprocessing the image data affects the quality and quantity of the derived wind vectors. Standard space domain filtering of image pixels with suitable algorithms gives rise to water vapor images which are usable for wind determination. However, if the superimposed noise shows obvious spatial frequency components, the image spectra make it much easier to attenuate those spatial frequency regions than is possible with the image itself. Then, interactive spatial frequency domain filtering after a Fourier transform of the original images allows better results for less effort. A space domain filter algorithm to tackle such a problem is, of course, conceivable. However, on the interactive system we use, constructing a space domain filter is much more difficult than constructing a spatial frequency filter.

METEOSAT Water Vapor Channel and a Low-Latitude Vortex

METEOSAT water-vapor (5.7–7.1 μm) images are used to illustrate the formation of a low-latitude, upper-level vortex over NE Brazil in February 1979. Winds derived from cloud motions over three consecutive half-hourly METEOSAT infrared (10.5–12.5 μm) images show that the basic cause of the development is the disruption of an upper trough penetrating into low latitudes from the north. Cross-equatorial flow is set up and a cyclonic circulation appears in the southern hemisphere.

The Dynamics of Polar Jet Streams as Depicted by the METEOSAT WV Channel Radiance Field

Abstract A polar jet stream flowing over northern Europe is investigated using both the radiosounding network and the METEOSAT water vapor (WV) channel imagery. A maximum radiance band associated with the polar jet stream is shown to accurately delineate the tropopause break; a major difference in the subtropical jet stream radiative signature is noted, however. An analytic approximation of the radiative transfer equation is established for the METEOSAT WV channel and leads to the conclusion that the radiance field, away from the polar frontal zones, is representative of the temperature field on a water vapor isosteric surface.