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  • 51. Hedfors, J
    et al.
    Aldahan, A
    Kulan, A
    Possnert, G
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Vintersved, I
    Clouds and Be-7: Perusing connections between cosmic rays and climate2006In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 111, no D2, article id D02208Article in journal (Refereed)
    Abstract [en]

    [1] Time series data on Be-7, precipitation, temperature, and satellite imagery of cloud cover over Scandinavia, together with cosmic ray and sunspot activity, were used to elucidate the relationship between cosmic rays and clouds, and ultimately climate change. The results indicate a coherent negative correlation between total cloud cover and Be-7 on intraseasonal, seasonal, and decadal scales. Although the reasons behind this correlation are unclear, a full-scale implication of this feature is in the possible use of Be-7 and Be-10 records for proxy paleo-reconstruction of total cloud cover. This is a strongly needed, but generally difficult to quantify parameter in climate models.

  • 52. Heygster, Georg
    et al.
    Melsheimer, Christian
    Mathew, Nizy
    Toudal, Leif
    Saldo, Roberto
    Andersen, Soren
    Tonboe, Rasmus
    Schyberg, Harald
    Tveter, Frank Thomas
    Thyness, Vibeke
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Dahlgren, Per
    SMHI, Research Department, Meteorology.
    Integrated Observation and Modeling of the Arctic Sea Ice and Atmosphere2009In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 90, no 3, p. 293-297Article in journal (Refereed)
  • 53. Holl, G.
    et al.
    Eliasson, Salomon
    SMHI, Research Department, Atmospheric remote sensing.
    Mendrok, J.
    Buehler, S. A.
    SPARE-ICE: Synergistic ice water path from passive operational sensors2014In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 119, no 3, p. 1504-1523Article in journal (Refereed)
    Abstract [en]

    This article presents SPARE-ICE, the Synergistic Passive Atmospheric Retrieval Experiment-ICE. SPARE-ICE is the first Ice Water Path (IWP) product combining infrared and microwave radiances. By using only passive operational sensors, the SPARE-ICE retrieval can be used to process data from at least the NOAA 15 to 19 and MetOp satellites, obtaining time series from 1998 onward. The retrieval is developed using collocations between passive operational sensors (solar, terrestrial infrared, microwave), the CloudSat radar, and the CALIPSO lidar. The collocations form a retrieval database matching measurements from passive sensors against the existing active combined radar-lidar product 2C-ICE. With this retrieval database, we train a pair of artificial neural networks to detect clouds and retrieve IWP. When considering solar, terrestrial infrared, and microwave-based measurements, we show that any combination of two techniques performs better than either single-technique retrieval. We choose not to include solar reflectances in SPARE-ICE, because the improvement is small, and so that SPARE-ICE can be retrieved both daytime and nighttime. The median fractional error between SPARE-ICE and 2C-ICE is around a factor 2, a figure similar to the random error between 2C-ICE ice water content (IWC) and in situ measurements. A comparison of SPARE-ICE with Moderate Resolution Imaging Spectroradiometer (MODIS), Pathfinder Atmospheric Extended (PATMOS-X), and Microwave Surface and Precipitation Products System (MSPPS) indicates that SPARE-ICE appears to perform well even in difficult conditions. SPARE-ICE is available for public use.

  • 54. Hultgren, Pia
    et al.
    Dybbroe, Adam
    SMHI, Core Services.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    SCANDIA -its accuracy in classifying LOW CLOUD1999Report (Other academic)
  • 55. Huuskonen, Asko
    et al.
    Haase, Gunther
    SMHI, Research Department, Atmospheric remote sensing.
    Michelson, Daniel
    Leijnse, Hidde
    Holleman, Iwan
    Probert, Matt
    Gill, Rashpal
    Frech, Michael
    Kurri, Mikko
    Hohti, Harri
    Solar Monitoring as a Tool for improving the Homogeneity of Radar Networks2016Conference paper (Other academic)
  • 56. Huuskonen, Asko
    et al.
    Haase, Günther
    SMHI, Research Department, Atmospheric remote sensing.
    Michelson, Daniel
    SMHI, Research Department, Atmospheric remote sensing.
    Leijnse, Hidde
    Holleman, Iwan
    Probert, Matt
    Gill, Rashpal
    Frech, Michael
    Kurri, Mikko
    Hohti, Harri
    Operational Solar Monitoring for Improving theHomogeneity of the European Radar Network2018In: ERAD 2018 10TH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY & HYDROLOGY / [ed] Vos, Lotte de; Leijnse, Hidde; Uijlenhoet, Remko, 2018, p. 594-600Conference paper (Other academic)
  • 57. Hyvarinen, Otto
    et al.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Dybbroe, Adam
    SMHI, Core Services.
    Investigations of NOAA AVHRR/3 1.6 m m imagery for snow, cloud and sunglint discrimination (Nowcasting SAF)1999Report (Other academic)
  • 58.
    Håkansson, Nina
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Adok, Claudia
    Thoss, Anke
    SMHI, Research Department, Atmospheric remote sensing.
    Scheirer, Ronald
    SMHI, Research Department, Atmospheric remote sensing.
    Hörnquist, Sara
    SMHI, Research Department, Atmospheric remote sensing.
    Neural network cloud top pressure and height for MODIS2018In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 11, no 5, p. 3177-3196Article in journal (Refereed)
  • 59.
    Johansson, Barbro
    et al.
    SMHI, Professional Services.
    Lindström, Göran
    SMHI, Research Department, Hydrology.
    Olsson, Jonas
    SMHI, Research Department, Hydrology.
    Yacoub, Tahsin
    SMHI, Core Services.
    Haase, Günther
    SMHI, Research Department, Atmospheric remote sensing.
    Jacobsson, Karin
    SMHI, Professional Services.
    Sanner, Håkan
    SMHI, Core Services.
    Översvämningsprognoser i områden med ofullständiga data: Metodutveckling och utvärdering2007Report (Other academic)
    Abstract [sv]

    Rapporten redovisar slutresultat från projektet ”Översvämningsprognoser Utveckling av metoder för ett rikstäckande system för vattenförings- och vattenståndsprognoser”. Projektet har i huvudsak varit finansierat av Räddningsverket, numera MSB, (Myndigheten för Samhällsskydd och Beredskap), men har även utnyttjat resultat från näraliggande projekt finansierade av Elforsk, SMHI och EU.I ett rikstäckande system måste prognoser göras för vattendrag där det saknas detaljerad information om avrinningsområdets och älvfårans egenskaper. Modeller kan inte tillämpas och verifieras på samma sätt som i områden med god datatillgång och tillgång till tidsserier med observerad vattenföring. Eftersom beräkningarna inte kan verifieras mot observationer blir det extra viktigt med bra nederbördsinformation och att kunna ge ett mått på osäkerheten i prognosen. Projektet har dels arbetat med metodutveckling, dels med en omfattande utvärdering av data och beräkningsresultat. En pilotstudie gjordes för flödet sommaren 2004 i Lagan/Ljungby. Fokus har varit på följande områden:- Utveckling och utvärdering av en metodik för att utnyttja meteorologiska och hydrologiska sannolikhetsprognoser- Utvärdering och minimering av osäkerheten i hydrologiska (vattenföring) och hydrauliska (vattenstånd) prognosmodeller.- Utveckling och utvärdering av metoder för att utnyttja radarobservationer av nederbörd.Projektet har visat att det är möjligt att göra vattenståndsprognoser med rimlig noggrannhet, utgående från data som finns tillgängliga i ett rikstäckande system. Tillgång till bra nederbördsinformation för dagarna före prognosen är viktig, speciellt i sjörika system med ett långsamt förlopp. För att kunna göra sannolikhetsprognoser räcker det inte att ta hänsyn till osäkerheten i den meteorologiska prognosen genom att direkt utnyttja meteorologiska ensembleprognoser. Spridningen i de meteorologiska nederbördsprognoserna är inte tillräcklig och osäkerheten i den hydrologiska modellen måste beaktas.De projektresultat som inom det närmaste året kommer att utnyttjas i ett rikstäckande system är de som är relaterade till hydrologisk modellering och sannolikhetsprognoser. Arbetet med att utveckla metoder för att utnyttja radarinformation i operationell skattning av arealnederbörd fortgår. Fallstudier har visat att vattenståndsprognoser kan göras med modeller baserad på översiktlig information om topografi och tvärsektioner i vattendragen. Däremot är det tidsödande att sätta upp en hydraulisk modell för en godtycklig älvsträcka. Tills vidare är det realistiskt att anta att vattenståndsprognoser främst kommer att göras i vattendrag som ingått i den översiktliga översvämningskarteringen.

  • 60.
    Johansson, Erik
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Ekman, Nnica M. L.
    Tjernstrom, Michael
    L'Ecuye, Ristan
    How Does Cloud Overlap Affect the Radiative Heating in the Tropical Upper Troposphere/Lower Stratosphere?2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 10, p. 5623-5631Article in journal (Refereed)
  • 61.
    Johansson, Erik
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    L'Ecuyer, T.
    Ekman, A. M. L.
    Tjernstrom, M.
    The vertical structure of cloud radiative heating over the Indian subcontinent during summer monsoon2015In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 20, p. 11557-11570Article in journal (Refereed)
    Abstract [en]

    Clouds forming during the summer monsoon over the Indian subcontinent affect its evolution through their radiative impact as well as the release of latent heat. While the latter is previously studied to some extent, comparatively little is known about the radiative impact of different cloud types and the vertical structure of their radiative heating/cooling effects. Therefore, the main aim of this study is to partly fill this knowledge gap by investigating and documenting the vertical distributions of the different cloud types associated with the Indian monsoon and their radiative heating/cooling using the active radar and lidar sensors on-board CloudSat and CALIPSO. The intraseasonal evolution of clouds from May to October is also investigated to understand pre-to-post monsoon transitioning of their radiative heating/cooling effects. The vertical structure of cloud radiative heating (CRH) follows the northward migration and retreat of the monsoon from May to October. Throughout this time period, stratiform clouds radiatively warm the middle troposphere and cool the upper troposphere by more than +/- 0.2 K day(-1) (after weighing by cloud fraction), with the largest impacts observed in June, July and August. During these months, the fraction of high thin cloud remains high in the tropical tropopause layer (TTL). Deep convective towers cause considerable radiative warming in the middle and upper troposphere, but strongly cool the base and inside of the TTL. This cooling is stronger during active (-1.23 K day(-1)) monsoon periods compared to break periods (-0.36 K day(-1)). The contrasting radiative warming effect of high clouds in the TTL is twice as largeduring active periods than in break periods. These results highlight the increasing importance of CRH with altitude, especially in the TTL. Stratiform (made up of alto- and nimbostratus clouds) and deep convection clouds radiatively cool the surface by approximately -100 and -400Wm(-2) respectively while warming the atmosphere radiatively by about 40 to 150Wm(-2). While the cooling at the surface induced by deep convection and stratiform clouds is largest during active periods of monsoon, the importance of stratiform clouds further increases during break periods. The contrasting CREs (cloud radiative effects) in the atmosphere and at surface, and during active and break periods, should have direct implications for the monsoonal circulation.

  • 62.
    Johansson, Erik
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Tjernstrom, Michael
    Ekman, Annica M. L.
    L'Ecuyer, Tristan
    Response of the lower troposphere to moisture intrusions into the Arctic2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, p. 2527-2536Article in journal (Refereed)
  • 63.
    Johnston, Sheldon
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    METEOSAT 8 SEVIRI and NOAA AVHRR Cloud Products: A Climate Monitoring SAF2007Report (Other academic)
    Abstract [en]

    The goals of this study are to compare the MSG SEVIRI and PPS AVHRR monthly mean cloud products of the CM-SAF. The study was done in two parts: first comparing the cloud mask products and then comparing the cloud top temperature and height products. This was done over a region from Greenland to eastern Russia and as far south as the Sahara. The study covered four seasonally-representative months. For the cloud mask using PPS version 1.0, the results showed large problems over the Sahara and parts of Spain during the summer months. This was primarily due to the high reflectances in channel 3a and mostprominent with NOAA 17.Much larger differences were found over water than over land surfaces, with the exception of Scandinavia where the differences were comparable to those found over water. The cloud-contaminated values were removed in one plot and this revealed that PPS had a larger number of cloud-contaminated pixels than MSG. This agrees with the concept that MSG reports increased cloudiness at higher viewing angles. This also explains why the differences over Scandinavia were so large and positive in value. The NOAA images at high latitudes have better spatial resolution and reports fewer cloudy and cloud-contaminated pixels than MSG.Sub-pixel and thin clouds greatly affected how well the two products converged. An attempt to use a weighted factor to adjust the effect of cloud-contaminated pixels on the total cloud cover failed to improve the convergence between the two cloud masks. The effect of the MSG viewing angle and the subsequent effects of reporting more cloudy pixels (or cloud-contaminated pixel – to include thin clouds) could be seen throughout all four months in the form of larger positive differences at latitudes approaching 80 degrees. Significant changes were seen with results from the PPS version 1.1. A significant decrease in the difference over the Sahara was the most discernable change. On the other hand, for NOAA 17, the agreement with MSG during twilight conditions was reduced by almost one half. The comparison of the cloud top temperature and height products revealed that MSG reported more low clouds during the summer months than PPS. This was mostly like due to the presence of convective clouds and the angle at which they are viewed (small cumulus clouds when viewed from nadir has a smaller diameter than when viewed slantwise).

  • 64.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Computed global radiation using interpolated, gridded cloudiness from the MESA-BETA analysis compared to measured global radiation1989Report (Other academic)
  • 65.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Five years of solar UV-radiation monitoring in Sweden1996Report (Other academic)
  • 66.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    FOCUSED SUN OBSERVATIONS USING A BREWER OZONE SPECTROPHOTOMETER1992In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 97, no D14, p. 15813-15817Article in journal (Refereed)
    Abstract [en]

    Accurate measurements of total ozone at high latitudes during winter have become increasingly important for studies of polar ozone depletion. The direct Sun measurements are the most direct and physically well defined type of measurement of total ozone with Dobson and Brewer spectrophotometers along with the focused Sun measurements. The direct Sun and focused Sun method are influenced by many sources of error at low solar elevations. A large error arises from the addition of radiation scattered from air along the path of the solar beam. A method to correct for this error using the Brewer instrument is described. The method gives reasonably accurate values down to solar elevations of 5-degrees compared to 10-degrees-20-degrees without corrections. This is especially important for high-latitude stations, where the low solar elevations are a limiting factor for accurate measurements of the total ozone particularly during the winter.

  • 67.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Measurements of total ozone 1997 - 19992000Report (Other academic)
    Abstract [en]

    A summary of the quality control, quality assurance and measurements of total ozone at Norrköping and Vindeln during the period 1997-1999 is made. The Brewer #006 and Brewer #128 were compared to the travelling reference Brewer #017 at Vindeln in 1999. Major changes in the measurements and instruments are discussed. The reprocessing of all Brewer data since 1996 and the introduction of the new format recommended by WOUDC at the end of 1999 are documented.

    At the end of this three-year period the ozone layer has shown an increase after many years with several periods of considerable depletion. Therefore, the previous downward trend that was as large as -8% per decade has disappeared. The data set consists of 12 years of almost uninterrupted data at Norrköping. Most of the gaps have been possible to fill with satellite data to have monthly values based on a complete set of daily values. An intercomparison between ground based and TOMS Version 7 satellite based data shows an astonishing agreement in most cases.

    Data recorded at Abisko in 1926 and 1927 on the initiative of G.M.B. Dobson was found in the archive. They were recomputed to modem scales and the result shows that the thickness of the ozone layer at that time is similar to the present thickness. The long-term data sets now available from Vindeln and Norrköping give the opportunity to study the temporal and in some respect also the spatial characteristics of the total ozone. Both the spatial correlation and  he auto-correlation vary considerably over time.

    The Web-site for total ozone (under http://www.smhi.se) has made measurements available fora lot of people in almost real-time. Ozone data can be viewed as graphs or transferred by ftp. There are also some pages with general information as well as some links to related sites. In particular, this method for distribution of information has been found to be useful for the public, media and decision-makers.

  • 68.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Normalvärden för perioden 1961-90 av globalstrålning och solskenstid i Sverige1993Report (Other academic)
  • 69.
    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.

  • 70.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Solar ultraviolet radiation in Sweden1986Report (Other academic)
  • 71.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Solar UV-radiation monitoring 19961996Report (Other academic)
  • 72.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    UV-radiation 1983-2003 measured at Norrkoping, Sweden2006In: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 83, no 1-4, p. 59-76Article in journal (Refereed)
    Abstract [en]

    Total ozone has been monitored with high accuracy at many sites for a relative long period of time. This is not the case for UV. The main reason is that UV is difficult to measure accurately. Extensive monitoring started first during the 1990s. However, there are some data series still in operation that were initiated at earlier dates. This paper will discuss one such series from Norrkoping, Sweden that began in March 1983. The instruments used to measure UV radiation have some characteristics producing an output signal that is not directly related to the UV. Here, these problems are discussed and corrections are applied. The goal is to produce a homogenous and an accurate set of UV data as is possible. An analysis of the various sources of uncertainty is done and summarised as total uncertainty. This was found to be about 6% for the yearly values of CIE weighted UV. However, the precision is much better at about 0.6%. Finally, some results are shown for yearly and seasonal UV. The trends for the full period 1983-2003 are tested for significance. The spring, summer, autumn and yearly data showed a significant increase in UV of respectively 0.8, 0.3, 0.8 and 0.5% per year. To get some indication of the cause of the variation of global radiation (indicator of cloudiness) and total ozone measured at the same site were also processed. A small, but insignificant, decrease of total ozone was found, whilst global radiation showed an increase. It seems that cloudiness is the major factor for the observed variation in UV for this period. The data are available on the internet.

  • 73.
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    UV-radiation measured in Norrköping 1983-20032004Report (Other academic)
    Abstract [en]

    The purpose of this work is to homogenise a number of different data sets of daily values of UV-radiation to enable studies of the temporal variation of UV-radiation and atmospheric processes. Efforts are concentrated on the longest series measured using a Robertson-Berger and a Solar Light Model 501 radiometer. The lack of practical and reliable calibration standards have made measurements of UV uncertain. There are also several sources of uncertainty inherited in the designs of the instruments as well as uncertainties in the input data to the models used for corrections and for filling gaps of missing data.The primary goal is to achieve a homogenous data set. The second goal is to mimic a true CIE-weighted irradiation, McKinley and Diffey (1987). The third goal is to be as close as possible to an absolute irradiance scale.There are some factors that will prevent the achievement of these goals. These factors will introduce uncertainty in various fashions and affect the data differently. The applied corrections are assumed to remove systematic differences in a statistical sense. However, as will be discussed below, large differences may still remain in the hourly data.For those not familiar with measurements in the UV spectral range it may seem surprising that UV data recorded by different instruments differ so much. This is partly due to the very large change (several orders of magnitude) in the solar spectrum combined with small differences in the spectral responsivity of the instruments in this range. Apparently small differences in spectral responsivity will produce relatively large variable differences in the output only by changing the solar elevation during the day or during the year. The yearly effect will be seen in the results below.

  • 74.
    Josefsson, Weine
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Karlsson, Jan-Erik
    Measurements of total ozone 1994-19961997Report (Other academic)
  • 75.
    Josefsson, Weine
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Effect of clouds on UV irradiance: As estimated from cloud amount, cloud type, precipitation, global radiation and sunshine duration2000In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 105, no D4, p. 4927-4935Article in journal (Refereed)
    Abstract [en]

    Ten years of measurements of UV irradiance, monitored by the Robertson-Berger (RB) meter in Norrkoping, 58.58 degrees N, 16.15 degrees E, Sweden, have been combined with concurrent synoptic cloud observations, measurements of sunshine duration, and global radiation to establish the relative influence of clouds on UV irradiance. It is shown that the cloud effect for UV wavelengths is less than for the whole solar spectrum (global radiation). Relations retrieved for global radiation may be used by correcting for the differences. High-level clouds are more transparent than low- and medium-level clouds. As expected, it was found that precipitating clouds in general are more opaque than nonprecipitating clouds. If there is any solar elevation dependency in the effect of clouds, it is small. Using only total cloud amount as parameter to model, the cloud effect on UV irradiance will give a substantial uncertainty, which can be decreased considerably using cloud type and/or information on precipitation conditions. It has also been shown that sunshine duration can be used in a similar way as cloud covet.

  • 76.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    A 10 year cloud climatology over Scandinavia derived from NOAA advanced very high resolution radiometer imagery2003In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 23, no 9, p. 1023-1044Article in journal (Refereed)
    Abstract [en]

    Results from a satellite-based method to compile regional cloud climatologies covering the Scandinavian region are presented. Systematic processing of multispectral image data from the NOAA Advanced Very High Resolution Radiometer (AVHRR) instrument has been utilized to provide monthly cloud climatologies covering the period 1991-2000. Considerable local-scale variation of cloud amounts was found in the region. The inland Baltic Sea and adjacent land areas exhibited a large-amplitude annual cycle in cloudiness (high cloud amounts in winter, low cloud amounts in summer) whereas a weak-amplitude reversed annual cycle (high cloud amounts with a weak maximum in summer) was found for the Scandinavian mountain range. As a contrast, conditions over the Norwegian Sea showed high and almost unchanged cloud amounts during the course of the year. Some interesting exceptions to these patterns were also seen locally. The quality of the satellite-derived cloud climatology was examined through comparisons with climatologies derived from surface cloud observations, from the International Satellite Cloud Climatology Project (ISCCP) and from the European Centre for Medium-range Weather Forecasts ERA-40 data set. In general, cloud amount deviations from surface observations were smaller than 10% except for some individual winter months, when the separability between clouds and snow-covered cold land surfaces is often poor. The ISCCP data set showed a weaker annual cycle in cloudiness, generally caused by higher summer-time cloud amounts in the region. Very good agreement was found with the ERA-40 data set, especially for the summer season. However, ERA-40 showed higher cloud amounts than SCANDIA and ISCCP during the winter season. The derived cloud climatology is affected by errors due to temporal AVHRR sensor degradation, but they appear to be small for this particular study. The data set is proposed as a valuable data set for validation of cloud description in numerical weather prediction and regional climate simulation models. Copyright (C) 2003 Royal Meteorological Society.

  • 77.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    A NOAA AVHRR cloud climatology over Scandinavia covering the period 1991-20002001Report (Other academic)
    Abstract [en]

    A ten-year NOAA A VHRR cloud climatology with a horizontal resolution of four km has been compiled over the Scandinavian region based on results from near real-lime cloud classifications of the SMHl SCANDlA mode!. The frequency and geographic distribution ofthe cloud groups Low-, Medium- and High-level clouds, water and ice clouds and deep convective clouds have been studied in addition to the ten-ycar monthly means of total fractional cloud cover in the region. Furthennore, attempts to estimatc the diurnal cycle of cloudiness and typieal cloud patterns in various weather rcgimes (e.g., North Atlantic Oscillation phases) have been made.

    The cloud climate in the region was found ta be significantly affected by the distribution of land and sea. In particular. the Baltic Sea was shown to suppress summertime cloudiness substantially and this effect was shown to influence cloud conditions in major parts of the Scandinavian region. Huwever, interesting deviations from this cloudiness pattern were found in the Scandinavian mountain range, in the northern part af Scandinavia and over the Norwegian Sea.

    The quality af the satellite-based cloud information was examined by comparing with corresponding surface-observations given by SYNOP-based cloud climatologies for the same period. Results showed good agreement but specific problems were found in winter. In addition, some effects of the degradation of visible AVHRR channels were notieed. Comparisons have also been rnade with internationally used global cloud climate data sets, namely the  SYNOP-based CRU data set and the cloud climatologics from the ISCCP D2 series.

  • 78.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Cloud classifications with the SCANDIA model1996Report (Other academic)
  • 79.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Cloud climate investigations in the Nordic region using NOAA AVHRR data1997In: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 57, no 3-4, p. 181-195Article in journal (Refereed)
    Abstract [en]

    A method to estimate monthly cloud conditions (monthly cloud frequencies) from multispectral satellite imagery is described. The operational cloud classification scheme SCANDIA (the SMHI Cloud ANalysis model using DIgital AVHRR data), based on high resolution imagery from the polar orbiting NOAA-satellites, has been used to produce monthly cloud frequencies for the entire year of 1993 and some additional months in 1991, 1992, 1994 and 1995. Cloud analyses were made for an area covering the Nordic countries with a horizontal resolution of four km. Examples of seasonal, monthly and diurnal variation in cloud conditions are given and an annual mean for 1993 is presented. Comparisons with existing surface observations showed very good agreement for horizontal cloud distributions but approximately 5% smaller cloud amounts were found in the satellite estimations. The most evident problems were encountered in the winter season due to difficulties in identifying low-level cloudiness at very low sun elevations. The underestimation in the summer season was partly fictious and caused by the overestimation of convective cloud cover by surface observers. SCANDIA results were compared to ISCCP (International Satellite Cloud Climatology Project) cloud climatologies for two selected months in 1991 and 1992. ISCCP cloudiness was indicated to be higher, especially during the month with anticyclonic conditions where a cloudiness excess of more than 10% were found. The regional variation of cloud conditions in the area was found to be inadequately described by ISCCP cloud climatologies. An improvement of the horizontal resolution of ISCCP data seems necessary to enable use for regional applications. The SCANDIA model is proposed as a valuable tool for local and regional monitoring of the cloud climatology at high latitudes. More extensive comparisons with ISCCP cloud climatologies are suggested as well as comparisons with modelled cloudiness from atmospheric general circulation models and climate models. Special studies of cloud conditions in the Polar areas are also proposed.

  • 80.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    DEVELOPMENT OF AN OPERATIONAL CLOUD CLASSIFICATION MODEL1989In: International Journal of Remote Sensing, ISSN 0143-1161, E-ISSN 1366-5901, Vol. 10, no 4-5, p. 687-693Article in journal (Refereed)
  • 81.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    ESTIMATION OF CLOUDINESS AT HIGH-LATITUDES FROM MULTISPECTRAL SATELLITE MEASUREMENTS1995In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 24, no 1, p. 33-40Article in journal (Refereed)
    Abstract [en]

    Clouds play an important role in the climate system, and strongly modify radiation conditions in the atmosphere and at the earth's surface. Present estimations show that the net impact of clouds in the atmosphere results in a cooling several times larger than the expected warming that would result from a doubling of the CO2-concentration in the atmosphere. Regional and global monitoring of cloud conditions is therefore necessary for studying the role of clouds in possible climate feedback mechanisms. This paper presents a method to estimate cloud conditions (cloud cover) in the Nordic region, from multispectral satellite data. A cloud classification scheme, based on high-resolution imagery data from polar orbiting NOAA-satellites, was used to produce monthly cloud frequencies for the summer of 1993. Comparisons with existing surface observations have shown very good agreement. Cloud conditions are shown to be highly sensitive to characteristics of the earth's surface. Large differences between land and sea areas were found, especially in the beginning of summer. Cloud frequencies were significantly lower over the Baltic Sea compared to surrounding land areas. Mountainous areas showed, on the contrary, much higher cloud frequencies than surrounding areas.

  • 82.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Information från Meteosat - forskningsrön och operationell tillämpning1985Report (Other academic)
  • 83.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Satellite sensing techniques and applications for the purpose of BALTEX2000In: Meteorologische Zeitschrift, ISSN 0941-2948, E-ISSN 1610-1227, Vol. 9, no 2, p. 111-116Article in journal (Refereed)
    Abstract [en]

    Various satellite sensing techniques and their corresponding applications suitable for use in validation and modelling activities in BALTEX are presented and discussed. Special emphasis is given to data and mature applications available during the main BALTEX BRIDGE experiment. For atmospheric simulations and studies, sensors measuring radiation budget quantities, cloud properties, moisture content and precipitation are considered as most important. Sensors measuring ice conditions and sea state parameters are identified for oceanographical applications and sensors measuring snow conditions and surface conditions are listed for hydrological studies. One example of an application based on extracted cloud information from NOAA AVHRR imagery is demonstrated. Estimations of mean cloud conditions in summer for the period 1991-1998 are shown for the total cloud cover, Cirrus cloudiness and low-level cloudiness over the Nordic region. It is shown that the Baltic Sea and other sea surfaces in the region has a large impact in suppressing summertime cloudiness, in particular low-level cloudiness. As a contrast, cloud patterns for high-level clouds show low correlation with the spatial distribution of sea surfaces. The influence of topographic features (i.e., the Scandinavian mountain range) seems more important here. Cirrus cloudiness peak on the lee side (to the east) of mountains suggesting a frequent presence of lee-wave cirrus clouds. As a summary, the following satellites and sensors will be the main satellite data sources for BALTEX: the ScaRaB instruments on the Ressurs and METEOR satellites, the CERES instrument on the EOS-AMI satellite, the AVHRR and ATOVS sensors on the NOAA satellites. the MVIRI and SEVIRI sensors on the METEOSAT satellites, the SAR instruments on the ERS, Radarsat and ENVISAT satellites and the SSM/I instrument on the DMSP satellites. Of particular interest is also radio occultation measurements of the radio signals from the GPS satellites. The need for a central BALTEX coordination facility (a satellite data function) with the objective to compile and transfer satellite data from various processing centres to BALTEX research groups is particularly stressed.

  • 84.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Satellite-estimated cloudiness from NOAA AVHRR data in the Nordic are during 19931994Report (Other academic)
  • 85.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Validation of modelled cloudiness using satellite-estimated cloud climatologies1996In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 48, no 5, p. 767-785Article in journal (Refereed)
    Abstract [en]

    A method to evaluate forecasts of total fractional cloud cover using satellite measurements is demonstrated. Cloud analyses in the form of monthly cloud climatologies are extracted from NOAA. AVHRR data which are compared to corresponding cloud forecast information from the HIRLAM and ECMWF numerical weather prediction models. The satellite-based cloud information is extracted for a summer month in 1994 and a winter month in 1995 by use of the SMHI cloud classification model SCANDIA. Cloud analyses are conducted for an area covering a substantial part of northern Europe. Deficiencies in forecasted cloud amounts are found for both models, especially the underestimation of cloudiness for short forecast lengths with the HIRLAM model. Forecast improvements using the HIRLAM model are indicated when introducing a cloud initialisation technique using cloud fields from initial 6-hour forecasts (first-guess fields). Future systematic validations using this technique are, however, needed to make firm conclusions on the general model behaviour. SCANDIA-derived cloud information is proposed as a valuable complement to other datasets used for cloud forecast validation (e.g., the SSM/I- and ISCCP data sets).

  • 86.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Anttila, Kati
    Trentmann, Jorg
    Stengel, Martin
    Meirink, Jan Fokke
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Hanschmann, Timo
    Kothe, Steffen
    Jaaskelainen, Emmihenna
    Sedlar, Joseph
    SMHI, Research Department, Atmospheric remote sensing.
    Benas, Nikos
    van Zadelhoff, Gerd-Jan
    Schlundt, Cornelia
    Stein, Diana
    Finkensieper, Stefan
    Håkansson, Nina
    SMHI, Research Department, Atmospheric remote sensing.
    Hollmann, Rainer
    CLARA-A2: the second edition of the CM SAF cloud and radiation data record from 34 years of global AVHRR data2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 9, p. 5809-5828Article in journal (Refereed)
  • 87.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Inter-Comparison and Evaluation of the Four Longest Satellite-Derived Cloud Climate Data Records: CLARA-A2, ESA Cloud CCI V3, ISCCP-HGM, and PATMOS-x2018In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 10, no 10, article id 1567Article in journal (Refereed)
  • 88.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Dybbroe, Adam
    SMHI, Research Department, Atmospheric remote sensing.
    Evaluation of Arctic cloud products from the EUMETSAT Climate Monitoring Satellite Application Facility based on CALIPSO-CALIOP observations2010In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, no 4, p. 1789-1807Article in journal (Refereed)
    Abstract [en]

    The performance of the three cloud products cloud fractional cover, cloud type and cloud top height, derived from NOAA AVHRR data and produced by the EUMETSAT Climate Monitoring Satellite Application Facility, has been evaluated in detail over the Arctic region for four months in 2007 using CALIPSO-CALIOP observations. The evaluation was based on 142 selected NOAA/Metop overpasses allowing almost 400 000 individual matchups between AVHRR pixels and CALIOP measurements distributed approximately equally over the studied months (June, July, August and December 2007). Results suggest that estimations of cloud amounts are very accurate during the polar summer season while a substantial loss of detected clouds occurs in the polar winter. Evaluation results for cloud type and cloud top products point at specific problems related to the existence of near isothermal conditions in the lower troposphere in the polar summer and the use of reference vertical temperature profiles from Numerical Weather Prediction model analyses. The latter are currently not detailed enough in describing true conditions relevant on the pixel scale. This concerns especially the description of near-surface temperature inversions which are often too weak leading to large errors in interpreted cloud top heights.

  • 89.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Håkansson, Nina
    SMHI, Research Department, Atmospheric remote sensing.
    Characterization of AVHRR global cloud detection sensitivity based on CALIPSO-CALIOP cloud optical thickness information: demonstration of results based on the CM SAF CLARA-A2 climate data record2018In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 11, no 1, p. 633-649Article in journal (Refereed)
    Abstract [en]

    The sensitivity in detecting thin clouds of the cloud screening method being used in the CM SAF cloud, albedo and surface radiation data set from AVHRR data (CLARA-A2) cloud climate data record (CDR) has been evaluated using cloud information from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the CALIPSO satellite. The sensitivity, including its global variation, has been studied based on collocations of Advanced Very High Resolution Radiometer (AVHRR) and CALIOP measurements over a 10-year period (2006-2015). The cloud detection sensitivity has been defined as the minimum cloud optical thickness for which 50% of clouds could be detected, with the global average sensitivity estimated to be 0.225. After using this value to reduce the CALIOP cloud mask (i.e. clouds with optical thickness below this threshold were interpreted as cloud-free cases), cloudiness results were found to be basically unbiased over most of the globe except over the polar regions where a considerable underestimation of cloudiness could be seen during the polar winter. The overall probability of detecting clouds in the polar winter could be as low as 50% over the highest and coldest parts of Greenland and Antarctica, showing that a large fraction of optically thick clouds also remains undetected here. The study included an in-depth analysis of the probability of detecting a cloud as a function of the vertically integrated cloud optical thickness as well as of the cloud's geographical position. Best results were achieved over oceanic surfaces at mid-to high latitudes where at least 50% of all clouds with an optical thickness down to a value of 0.075 were detected. Corresponding cloud detection sensitivities over land surfaces outside of the polar regions were generally larger than 0.2 with maximum values of approximately 0.5 over the Sahara and the Arabian Peninsula. For polar land surfaces the values were close to 1 or higher with maximum values of 4.5 for the parts with the highest altitudes over Greenland and Antarctica. It is suggested to quantify the detection performance of other CDRs in terms of a sensitivity threshold of cloud optical thickness, which can be estimated using active lidar observations. Validation results are proposed to be used in Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulation Package (COSP) simulators for cloud detection characterization of various cloud CDRs from passive imagery.

  • 90.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Håkansson, Nina
    SMHI, Research Department, Atmospheric remote sensing.
    Mittaz, Jonathan P. D.
    Hanschmann, Timo
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Impact of AVHRR Channel 3b Noise on Climate Data Records: Filtering Method Applied to the CM SAF CLARA-A2 Data Record2017In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 9, no 6, article id 568Article in journal (Refereed)
  • 91.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Johansson, Erik
    SMHI, Research Department, Atmospheric remote sensing.
    Multi-Sensor Calibration Studies of AVHRR-Heritage Channel Radiances Using the Simultaneous Nadir Observation Approach2014In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 6, no 3, p. 1845-1862Article in journal (Refereed)
    Abstract [en]

    The European Space Agency project for studies of cloud properties in the Climate Change Initiative programme (ESA-CLOUD-CCI) aims at compiling the longest possible time series of cloud products from one single multispectral sensor-The five-channel Advanced Very High Resolution Radiometer (AVHRR) instrument. A particular aspect here is to include corresponding products based on other existing (Moderate Resolution Imaging Spectroradiometer (MODIS), Advanced Along-Track Scanning Radiometer (AATSR), MEdium Resolution Imaging Spectrometer (MERIS), Visible and Infrared Radiometer Suite (VIIRS)) and future Sea and Land Surface Temperature Radiometer (SLSTR) sensors measuring in similar (AVHRR-heritage) spectral channels. Initial inter-comparisons of the involved AVHRR-heritage channel radiances over a short demonstration period (2007-2009) were performed. Using Aqua-MODIS as reference, AVHRR (NOAA-18), AATSR, and MERIS channel radiances were evaluated using the simultaneous nadir (SNO) approach. Results show generally agreeing radiances within approximately 3% for channels at 0.6 mu m and 0.8 mu m. Larger deviations (+5%) were found for the corresponding AATSR channel at 0.6 mu m. Excessive deviations but with opposite sign were also indicated for AATSR 1.6 mu m and MERIS 0.8 mu m radiances. Observed differences may largely be attributed to residual temporal and spatial matching differences while excessive AATSR and MERIS deviations are likely partly attributed to incomplete compensation for spectrally varying surface and atmospheric conditions. However, very good agreement was found for all infrared channels among all the studied sensors. Here, deviations were generally less than 0.2% for the measured brightness temperatures with the exception of some remaining non-linear deviations at extreme low and high temperatures.

  • 92.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Johansson, Erik
    SMHI, Research Department, Atmospheric remote sensing.
    On the optimal method for evaluating cloud products from passive satellite imagery using CALIPSO-CALIOP data: example investigating the CM SAF CLARA-A1 dataset2013In: ATMOSPHERIC MEASUREMENT TECHNIQUES, ISSN 1867-1381, Vol. 6, no 5, p. 1271-1286Article in journal (Refereed)
    Abstract [en]

    A method for detailed evaluation of a new satellite-derived global 28 yr cloud and radiation climatology (Climate Monitoring SAF Clouds, Albedo and Radiation from AVHRR data, named CLARA-A1) from polar-orbiting NOAA and Metop satellites is presented. The method combines 1 km and 5 km resolution cloud datasets from the CALIPSO-CALIOP (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite - Observation Cloud-Aerosol Lidar with Orthogonal Polarization) cloud lidar for estimating cloud detection limitations and the accuracy of cloud top height estimations. Cloud detection is shown to work efficiently for clouds with optical thicknesses above 0.30 except for at twilight conditions when this value increases to 0.45. Some misclassifications of cloud-free surfaces during daytime were revealed for semi-arid land areas in the sub-tropical and tropical regions leading to up to 20 % overestimated cloud amounts. In addition, a substantial fraction (at least 20-30 %) of all clouds remains undetected in the polar regions during the polar winter season due to the lack of or an inverted temperature contrast between Earth surfaces and clouds. Subsequent cloud top height evaluation took into account the derived information about the cloud detection limits. It was shown that this has fundamental importance for the achieved results. An overall bias of -274m was achieved compared to a bias of -2762m when no measures were taken to compensate for cloud detection limitations. Despite this improvement it was concluded that high-level clouds still suffer from substantial height underestimations, while the opposite is true for low-level (boundary layer) clouds. The validation method and the specifically collected satellite dataset with optimal matching in time and space are suggested for a wider use in the future for evaluation of other cloud retrieval methods based on passive satellite imagery.

  • 93.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Johansson, Erik
    SMHI, Research Department, Atmospheric remote sensing.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Advancing the uncertainty characterisation of cloud masking in passive satellite imagery: Probabilistic formulations for NOAA AVHRR data2015In: Remote Sensing of Environment, ISSN 0034-4257, E-ISSN 1879-0704, Vol. 158, p. 126-139Article in journal (Refereed)
    Abstract [en]

    Two alternative methods for probabilistic cloud masking of images from the Advanced Very High Resolution Radiometer (AVHRR) sensor have been examined. Both methods are based on Bayesian theory and were trained using data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. Results were evaluated by comparing to independent CALIPSO-CALIOP observations and to a one-year ground-based cloud dataset composed from five different remote sensing systems over the observation site in Cabauw in the Netherlands. In addition, results were compared to two different cloud masks; one derived from the geostationary Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensor and one from the Climate Monitoring Satellite Application Facility Clouds (CMSAF), Albedo and Radiation dataset from AVHRR data (CLARA-A1). It was demonstrated that the probabilistic methods compare well with the referenced satellite datasets and for daytime conditions they provide even better performance than the reference methods. Among the two probabilistic approaches, it was found that the formulation based on a Naive Bayesian formulation (denoted PPS-Prob Naive) performed clearly superior to the formulation based on a linear summation of conditional cloud probabilities (denoted PPS-Prob SPARC) for daytime conditions. For the study based on the observations over the Cabauw site, the overall daytime Kuipers Skill Score for PPS-Prob Naive was 0.84, for PPS-Prob SPARC 0.79, for CLARA-A1 0.74 and for SEVIRI 0.66. Corresponding results for night-time conditions were less favourable for the probabilistic formulations (Kuipers Skill Score 0.74 for PPS-Prob Naive, 0.68 for PPS-Prob SPARC, 0.80 for CLARA-A1 and 0.79 for SEVIRI) but still relatively close to the reference dataset The Cabauw distribution of cloudiness occurrences in different octa categories was reproduced very closely by all methods, including the probabilistic formulations. Results based on Cabauw observations were also largely in good agreement with results deduced from comparisons with the CALIPSO-CALIOP cloud mask. The PPS-Prob Naive approach will be implemented in an upcoming version of the Polar Platform System (PPS) cloud software issued by the EUMETSAT Nowcasting Satellite Application Facility (NWC SAF). It will also be used in the second release of the CMSAF CLARA cloud climate data record based on historic AVHRR GAC data (to be denoted CIARA-A2). (C) 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-SA license

  • 94.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Willén, Ulrika
    SMHI, Research Department, Climate research - Rossby Centre.
    Wyser, Klaus
    SMHI, Research Department, Climate research - Rossby Centre.
    USE OF A HIGH-RESOLUTION CLOUD CLIMATE DATA SET FOR VALIDATION OF ROSSBY CENTRE CLIMATE SIMULATIONS2004In: 2004 EUMETSAT METEOROLOGICAL SATELLITE CONFERENCE: Ocean and Climate Observations, EUMETSAT , 2004, p. 465-473Conference paper (Other academic)
  • 95.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Riihela, A.
    Mueller, R.
    Meirink, J. F.
    Sedlar, Joseph
    SMHI, Research Department, Atmospheric remote sensing.
    Stengel, M.
    Lockhoff, M.
    Trentmann, J.
    Kaspar, F.
    Hollmann, R.
    Wolters, E.
    CLARA-A1: a cloud, albedo, and radiation dataset from 28 yr of global AVHRR data2013In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, no 10, p. 5351-5367Article in journal (Refereed)
    Abstract [en]

    A new satellite-derived climate dataset - denoted CLARA-A1 ("The CM SAF cLoud, Albedo and RAdiation dataset from AVHRR data") - is described. The dataset covers the 28 yr period from 1982 until 2009 and consists of cloud, surface albedo, and radiation budget products derived from the AVHRR (Advanced Very High Resolution Radiometer) sensor carried by polar-orbiting operational meteorological satellites. Its content, anticipated accuracies, limitations, and potential applications are described. The dataset is produced by the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF) project. The dataset has its strengths in the long duration, its foundation upon a homogenized AVHRR radiance data record, and in some unique features, e. g. the availability of 28 yr of summer surface albedo and cloudiness parameters over the polar regions. Quality characteristics are also well investigated and particularly useful results can be found over the tropics, mid to high latitudes and over nearly all oceanic areas. Being the first CM SAF dataset of its kind, an intensive evaluation of the quality of the datasets was performed and major findings with regard to merits and shortcomings of the datasets are reported. However, the CM SAF's long-term commitment to perform two additional reprocessing events within the time frame 2013-2018 will allow proper handling of limitations as well as upgrading the dataset with new features (e. g. uncertainty estimates) and extension of the temporal coverage.

  • 96.
    Karlsson, Karl-Göran
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Willen, Ulrika
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Wyser, Klaus
    SMHI, Research Department, Climate research - Rossby Centre.
    Evaluation of regional cloud climate simulations over Scandinavia using a 10-year NOAA advanced very high resolution radiometer cloud climatology2008In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 113, no D1, article id D01203Article in journal (Refereed)
    Abstract [en]

    A satellite-derived (NOAA Advanced Very High Resolution Radiometer) cloud climatology over the Scandinavian region covering the period 1991 - 2001 has been used to evaluate the performance of cloud simulations of the Swedish Meteorological and Hydrological Institute Rossby Centre regional climate model (RCA3). Several methods of adapting the satellite and model data sets to allow a meaningful comparison were applied. RCA3-simulated total cloud cover was shown to agree within a few percent of the satellite-retrieved cloud amounts on seasonal and annual timescales. However, a substantial imbalance between the respective RCA3 contributions from low-, medium- and high-level clouds was seen. The differences from satellite-derived contributions were +2.4% for high-level clouds, -5.2% for medium-level clouds and +4.0% for low- level clouds. In addition, an overrepresentation of cloud categories with high optical thicknesses was seen for all vertical cloud groups, particularly during the summer season. Some specific features of the geographical distribution of cloudiness were also noticed. Most pronounced were the excess of cloud amounts over the Scandinavian mountain range and a deficit leeward of the mountains. The overall results imply problems with the RCA3-modeled surface radiation budget components by causing reduced incoming solar radiation and increased downwelling longwave radiation.

  • 97. Kaurola, J
    et al.
    Taalas, P
    Koskela, T
    Borkowski, J
    Josefsson, Weine
    SMHI, Research Department, Atmospheric remote sensing.
    Long-term variations of UV-B doses at three stations in northern Europe2000In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 105, no D16, p. 20813-20820Article in journal (Refereed)
    Abstract [en]

    Recent analysis of the total ozone observations indicate a negative trend of about 4%/decade in the Northern Hemisphere midlatitudes during the last two decades [WMO, 1999]. The effect of this decline on surface UV levels is of interest to a variety of applications. In this work the long-term variation of UV radiation at three stations located in northern Europe (Belsk, Norrkoping, and Jokioinen) has been studied using data from (1) ground-based observations, (2) surface UV doses determined using TOMS satellite measurements, and (3) reconstructed UV doses using observations of global radiation, total ozone, and radiative transfer modeling. For each station the estimates of daily UV doses from various sources have been intercompared, and a trend analysis has been performed to reveal long-term changes in the UV radiation. Data sets, which start in the late 1970s or early 1980s, show a general positive trend in annual doses of UV radiation. Some of these upward trends are statistically significant. For Belsk the increases are in the range of 5-15% per decade during spring and summer. The largest increases, about 20%/decade, has been observed in Norrkoping during spring. At Jokioinen there has been a slight upward trend in UV throughout the year. The analysis of reconstructed Belsk data from 1966 onward shows that the positive trend since late 1970s was preceeded by a negative trend. The reason for such changes is probably not only related to the changes in the total ozone but also to changes in aerosol content and cloudiness. The agreement of the UV series based on different data sources is good. This was studied using a subset of data in which it was required that data from all possible sources were available. The different trend estimates were in very close agreement with each other. However, there were often differences in absolute values, which is probably related to problems in calibration and limitations of the models.

  • 98. Koehler, Birgit
    et al.
    Barsotti, Francesco
    Minella, Marco
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Minero, Claudio
    Tranvik, Lars J.
    Vione, Davide
    Simulation of photoreactive transients and of photochemical transformation of organic pollutants in sunlit boreal lakes across 14 degrees of latitude: A photochemical mapping of Sweden2018In: Water Research, ISSN 0043-1354, E-ISSN 1879-2448, Vol. 129, p. 94-104Article in journal (Refereed)
  • 99. Koehler, Birgit
    et al.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Weyhenmeyer, Gesa A.
    Machida, Nanako
    Tranvik, Lars J.
    Sunlight-induced carbon dioxide emissions from inland waters2014In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 28, no 7, p. 696-711Article in journal (Refereed)
    Abstract [en]

    The emissions of carbon dioxide (CO2) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55 degrees N to 69 degrees N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 +/- 0.04 g C m(-2) yr(-1), which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 +/- 1 kt C yr(-1) was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO2 emissions from Swedish lakes. With a median depth of 3.2m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength-and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr(-1) under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO2 emissions contribute up to about one tenth to the global CO2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.

  • 100.
    Koenigk, Torben
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Devasthale, Abhay
    SMHI, Research Department, Atmospheric remote sensing.
    Karlsson, Karl-Göran
    SMHI, Research Department, Atmospheric remote sensing.
    Summer Arctic sea ice albedo in CMIP5 models2014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 4, p. 1987-1998Article in journal (Refereed)
    Abstract [en]

    Spatial and temporal variations of summer sea ice albedo over the Arctic are analyzed using an ensemble of historical CMIP5 model simulations. The results are compared to the CLARA-SAL product that is based on long-term satellite observations. The summer sea ice albedo varies substantially among CMIP5 models, and many models show large biases compared to the CLARA-SAL product. Single summer months show an extreme spread of ice albedo among models; July values vary between 0.3 and 0.7 for individual models. The CMIP5 ensemble mean, however, agrees relatively well in the central Arctic but shows too high ice albedo near the ice edges and coasts. In most models, the ice albedo is spatially too uniformly distributed. The summer-to-summer variations seem to be underestimated in many global models, and almost no model is able to reproduce the temporal evolution of ice albedo throughout the summer fully. While the satellite observations indicate the lowest ice albedos during August, the models show minimum values in July and substantially higher values in August. Instead, the June values are often lower in the models than in the satellite observations. This is probably due to too high surface temperatures in June, leading to an early start of the melt season and too cold temperatures in August causing an earlier refreezing in the models. The summer sea ice albedo in the CMIP5 models is strongly governed by surface temperature and snow conditions, particularly during the period of melt onset in early summer and refreezing in late summer. The summer surface net solar radiation of the ice-covered Arctic areas is highly related to the ice albedo in the CMIP5 models. However, the impact of the ice albedo on the sea ice conditions in the CMIP5 models is not clearly visible. This indicates the importance of other Arctic and large-scale processes for the sea ice conditions.

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