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  • 1.
    Andersson, Tage
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Ivarsson, Karl-Ivar
    SMHI.
    A MODEL FOR PROBABILITY NOWCASTS OF ACCUMULATED PRECIPITATION USING RADAR1991In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 30, no 1, p. 135-141Article in journal (Refereed)
    Abstract [en]

    A new model for making probability forecasts of accumulated spot precipitation from weather radar data is presented. The model selects a source region upwind of the forecast spot. All pixels (horizontal size 2 x 2 km2) within the source region are considered, having the same probability of hitting the forecast spot. A pixel hitting the forecast spot is supposed to precipitate there a short time (about 10 min.). A drawing is performed, and a frequency distribution of accumulated precipitation during the first time step of the forecast is obtained. A second drawing gives the frequency distribution of accumulated precipitation during the first to second time step, a third one during the first to third, and so on until the end of the forecast period is reached. A number of forecasts for 1-h accumulated precipitation, with lead times of 0, 1, and 2 h, have been performed and verified. The forecasts for 0-h lead time got the highest Brier skill scores, +50% to 60% relative to climatological forecasts for accumulated precipitation below 1 mm.

  • 2.
    Carlund, Thomas
    et al.
    SMHI, Core Services.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Comparison and uncertainty of aerosol optical depth estimates derived from spectral and broadband measurements2003In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 42, no 11, p. 1598-1610Article in journal (Refereed)
    Abstract [en]

    An experimental comparison of spectral aerosol optical depth tau(a,lambda) derived from measurements by two spectral radiometers [a LI-COR, Inc., LI-1800 spectroradiometer and a Centre Suisse d'Electronique et de Microtechnique (CSEM) SPM2000 sun photometer] and a broadband field pyrheliometer has been made. The study was limited to three wavelengths ( 368, 500, and 778 nm), using operational calibration and optical depth calculation procedures. For measurements taken on 32 days spread over 1 yr, the rms difference in tau(a,lambda) derived from the two spectral radiometers was less than 0.01 at 500 and 778 nm. For wavelengths shorter than 500 nm and longer than 950 nm, the performance of the LI-1800 in its current configuration did not permit accurate determinations of tau(a,lambda). Estimates of spectral aerosol optical depth from broadband pyrheliometer measurements using two models of the Angstromngstrom turbidity coefficient were examined. For the broadband method that was closest to the sun photometer results, the mean (rms) differences in tau(a,lambda) were 0.014 (0.028), 0.014 (0.019), and 0.013 ( 0.014) at 368, 500, and 778 nm. The mean differences are just above the average uncertainties of the sun photometer tau(a,lambda) values (0.012, 0.011, and 0.011) for the same wavelengths, as determined through a detailed uncertainty analysis. The amount of atmospheric water vapor is a necessary input to the broadband methods. If upper-air sounding data are not available, water vapor from a meteorological forecast model yields significantly better turbidity results than does using estimates from surface measurements of air temperature and relative humidity.

  • 3.
    Dybbroe, Adam
    et al.
    SMHI, Core Services.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Thoss, Anke
    SMHI, Research Department, Atmospheric remote sensing.
    NWCSAF AVHRR cloud detection and analysis using dynamic thresholds and radiative transfer modeling. Part I: Algorithm description2005In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 44, no 1, p. 39-54Article in journal (Refereed)
    Abstract [en]

    New methods and software for cloud detection and classification at high and midlatitudes using Advanced Very High Resolution Radiometer (AVHRR) data are developed for use in a wide range of meteorological, climatological, land surface, and oceanic applications within the Satellite Application Facilities (SAFs) of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), including the SAF for Nowcasting and Very Short Range Forecasting Applications (NWCSAF) project. The cloud mask employs smoothly varying (dynamic) thresholds that separate fully cloudy or cloud-contaminated fields of view from cloud-free conditions. Thresholds are adapted to the actual state of the atmosphere and surface and the sun-satellite viewing geometry using cloud-free radiative transfer model simulations. Both the cloud masking and the cloud-type classification are done using sequences of grouped threshold tests that employ both spectral and textural features. The cloud-type classification divides the cloudy pixels into 10 different categories: 5 opaque cloud types, 4 semitransparent clouds, and 1 subpixel cloud category. The threshold method is fuzzy in the sense that the distances in feature space to the thresholds are stored and are used to determine whether to stop or to continue testing. They are also used as a quality indicator of the final output. The atmospheric state should preferably be taken from a short-range NWP model, but the algorithms can also run with climatological fields as input.

  • 4.
    Dybbroe, Adam
    et al.
    SMHI, Core Services.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Thoss, Anke
    SMHI, Research Department, Atmospheric remote sensing.
    NWCSAF AVHRR cloud detection and analysis using dynamic thresholds and radiative transfer modeling. Part II: Tuning and validation2005In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 44, no 1, p. 55-71Article in journal (Refereed)
    Abstract [en]

    Algorithms for cloud detection (cloud mask) and classification (cloud type) at high and midlatitudes using data from the Advanced Very High Resolution Radiometer (AVHRR) on board the current NOAA satellites and future polar Meteorological and Operational Weather Satellites (METOP) of the European Organisation for the Exploitation of Meteorological Satellites have been extensively validated over northern Europe and the adjacent seas. The algorithms have been described in detail in Part I and are based on a multispectral grouped threshold approach, making use of cloud-free radiative transfer model simulations. The thresholds applied in the algorithms have been validated and tuned using a database interactively built up over more than 1 yr of data from NOAA-12, -14, and -15 by experienced nephanalysts. The database contains almost 4000 rectangular (in the image data)-sized targets (typically with sides around 10 pixels), with satellite data collocated in time and space with atmospheric data from a short-range NWP forecast model, land cover characterization, elevation data, and a label identifying the given cloud or surface type as interpreted by the nephanalyst. For independent and objective validation, a large dataset of nearly 3 yr of collocated surface synoptic observation (Synop) reports, AVHRR data, and NWP model output over northern and central Europe have been collected. Furthermore, weather radar data were used to check the consistency of the cloud type. The cloud mask performs best over daytime sea and worst at twilight and night over land. As compared with Synop, the cloud cover is overestimated during night (except for completely overcast situations) and is underestimated at twilight. The algorithms have been compared with the more empirically based Swedish Meteorological and Hydrological Institute (SMHI) Cloud Analysis Model Using Digital AVHRR Data (SCANDIA), operationally run at SMHI since 1989, and results show that performance has improved significantly.

  • 5.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Quality of total ozone measured by the focused sun method using a Brewer spectrophotometer2003In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 42, no 1, p. 74-82Article in journal (Refereed)
    Abstract [en]

    Strong ozone depletions and large natural variations in total ozone have been observed at high latitudes. Accurate measurements of total ozone are important so as not to misinterpret the involved processes and to track correctly the variations. The primary basis for ground-based monitoring of total ozone is the network of Dobson and Brewer ozone spectrophotometers. However, these instruments have limitations. At high latitudes, the fundamental direct sun observation used by these instruments is not possible during large parts of the year. In particular, the low sun and the resulting weak signals present a challenge. The focused sun observation method can extend the possible range of measurements using the Brewer instrument. Here, this method is discussed from the point of accuracy. Direct (synchronized) validation against the fundamental direct sun method is not possible with the current instrument configuration. Alternative methods to overcome the obstacle of nonsynchronous observations are applied. An estimate of the uncertainty of the validation is provided. The results show that the focused sun method gives data that are in line with the estimated uncertainty of the validation. No major additional uncertainty is needed to explain the observed scatter. The main conclusion is that the focused sun observation method can have an uncertainty close to the fundamental direct sun method and thus can be used to extend the possible range of observation for the Brewer ozone spectrophotometer.

  • 6.
    Robertson, Lennart
    et al.
    SMHI, Research Department, Air quality.
    Langner, Joakim
    SMHI, Research Department, Air quality.
    Engardt, Magnuz
    SMHI, Research Department, Air quality.
    An Eulerian limited-area atmospheric transport model1999In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 38, no 2, p. 190-210Article in journal (Refereed)
    Abstract [en]

    A limited-area, offline, Eulerian atmospheric transport model has been developed. The model is based on a terrain-following vertical coordinate and a mass-conserving, positive definite advection scheme with small phase and amplitude errors. The objective has been to develop a flexible, all purpose offline model. The model includes modules for emission input, vertical turbulent diffusion, and deposition processes. The model can handle an arbitrary number of chemical components and provides a framework for inclusion of modules describing physical and chemical transformation processes between different components. Idealized test cases, as well as simulations of the atmospheric distribution of Rn-222, demonstrate the ability of the model to meet the requirements of mass conservation and positiveness and to produce realistic simulations of a simple atmospheric tracer.

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