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Earth and Environmental Science	100%

Selected Abstracts

Discriminating raining from non-raining cloud areas at mid-latitudes using meteosat second generation SEVIRI night-time data

METEOROLOGICAL APPLICATIONS, Issue 2 2008
B. Thies
Abstract A new method for the delineation of precipitation during night-time using multispectral satellite data is proposed. The approach is not only applicable to the detection of mainly convective precipitation by means of the commonly used relation between infrared cloud-top temperature and rainfall probability but enables also the detection of stratiform precipitation (e.g. in connection with mid-latitude frontal systems). The presented scheme is based on the conceptual model that precipitating clouds are characterized by a combination of particles large enough to fall, an adequate vertical extension [both represented by the cloud water path (CWP)], and the existence of ice particles in the upper part of the cloud. As no operational retrieval exists for Meteosat Second Generation (MSG) to compute the CWP during night-time, suitable combinations of brightness temperature differences (,T) between the thermal bands of Meteosat Second Generation-Spinning Enhanced Visible and InfraRed Imager (MSG SEVIRI, ,T3.9,10.8, ,T3.9,7.3, ,T8.7,10.8, ,T10.8,12.1) are used to infer implicit information about the CWP and to compute a rainfall confidence level. ,T8.7,10.8 and ,T10.8,12.1 are particularly considered to supply information about the cloud phase. Rain area delineation is realized by using a minimum threshold of the rainfall confidence. To obtain a statistical transfer function between the rainfall confidence and the channel differences, the value combination of the channel differences is compared with ground-based radar data. The retrieval is validated against independent radar data not used for deriving the transfer function and shows an encouraging performance as well as clear improvements compared to existing optical retrieval techniques using only IR thresholds for cloud-top temperature. Copyright © 2008 Royal Meteorological Society [source]

Assimilation of SEVIRI infrared radiances with HIRLAM 4D-Var

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 645 2009
M. Stengel
Abstract Four-dimensional variational data assimilation (4D-Var) systems are ideally suited to obtain the best possible initial model state by utilizing information about the dynamical evolution of the atmospheric state from observations, such as satellite measurements, distributed over a certain period of time. In recent years, 4D-Var systems have been developed for several global and limited-area models. At the same time, spatially and temporally highly resolved satellite observations, as for example performed by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board the Meteosat Second Generation satellites, have become available. Here we demonstrate the benefit of a regional NWP model's analyses and forecasts gained by the assimilation of those radiances. The 4D-Var system of the HIgh Resolution Limited Area Model (HIRLAM) has been adjusted to utilize three of SEVIRI's infrared channels (located around 6.2 µm, 7.3 µm, and 13.4 µm, respectively) under clear-sky and low-level cloud conditions. Extended assimilation and forecast experiments show that the main direct impact of assimilated SEVIRI radiances on the atmospheric analysis were additional tropospheric humidity and wind increments. Forecast verification reveals a positive impact for almost all upper-air variables throughout the troposphere. Largest improvements are found for humidity and geopotential height in the middle troposphere. The observations in regions of low-level clouds provide especially beneficial information to the NWP system, which highlights the importance of satellite observations in cloudy areas for further improvements in the accuracy of weather forecasts. Copyright © 2009 Royal Meteorological Society [source]

The potential of variational retrieval of temperature and humidity profiles from Meteosat Second Generation observations

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 638 2009
F. Di Giuseppe
Abstract The quality of temperature and humidity retrievals from the infrared Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensors on the geostationary Meteosat Second Generation (MSG) satellites is assessed by means of a one-dimensional variational algorithm. The study is performed with the aim of improving the spatial and temporal resolution of available observations to feed analysis systems designed for high-resolution regional-scale numerical weather prediction (NWP) models. The non-hydrostatic forecast model COSMO in the ARPA-SIMC operational configuration is used to provide background fields. Only clear-sky observations over sea are processed. An optimized one-dimensional variational set-up comprised of two water-vapour and three window channels is selected. It maximizes the reduction of errors in the model backgrounds while ensuring ease of operational implementation through accurate bias correction procedures and correct radiative transfer simulations. The 1Dvar retrieval quality is first quantified in relative terms, employing statistics to estimate the reduction in the background model errors. Additionally the absolute retrieval accuracy is assessed by comparing the analysis with independent radiosonde observations. The inclusion of satellite data brings a substantial reduction in the warm and dry biases present in the forecast model. Moreover it is shown that the use of the retrieved profiles generated by the 1Dvar in the COSMO nudging scheme can locally reduce forecast errors. Copyright © 2009 Royal Meteorological Society [source]

Estimating the top-of-atmosphere longwave radiative forcing due to Saharan dust from satellite observations over a west African surface site

ATMOSPHERIC SCIENCE LETTERS, Issue 3 2007
H. E. Brindley
Abstract This paper presents a methodology for estimating the longwave top-of-atmosphere direct radiative forcing due to Saharan dust aerosol from satellite observations made by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) and Geostationary Earth Radiation Budget (GERB) instruments. Narrow-band observations from SEVIRI are used to detect dust presence and quantify its loading, while GERB provides an estimate of the dust impact on the total outgoing longwave radiation. Applying the technique to observations made over the Banizoumbou surface station in Niger through March,June 2006 indicates a midday longwave forcing efficiency of 17 ± 5 W m,2 per unit aerosol optical depth. Copyright © 2007 Royal Meteorological Society [source]