Hans Alexandersson

HOMOGENEITY ADJUSTMENTS OF IN SITU ATMOSPHERIC CLIMATE DATA: A REVIEW

Long-term in situ observations are widely used in a variety of climate analyses. Unfortunately, most decade- to century-scale time series of atmospheric data have been adversely impacted by inhomogeneities caused by, for example, changes in instrumentation, station moves, changes in the local environment such as urbanization, or the introduction of different observing practices like a new formula for calculating mean daily temperature or different observation times. If these inhomogeneities are not accounted for properly, the results of climate analyses using these data can be erroneous. Over the last decade, many climatologists have put a great deal of effort into developing techniques to identify inhomogeneities and adjust climatic time series to compensate for the biases produced by the inhomogeneities. It is important for users of homogeneity-adjusted data to understand how the data were adjusted and what impacts these adjustments are likely to make on their analyses. And it is important for developers of homogeneity-adjusted data sets to compare readily the different techniques most commonly used today. Therefore, this paper reviews the methods and techniques developed for homogeneity adjustments and describes many different approaches and philosophies involved in adjusting in situ climate data.

Homogenization of swedish temperature data. Part I: Homogeneity test for linear trends

A new test for the detection of linear trends of arbitrary length in normally distributed time series is developed. With this test it is possible to detect and estimate gradual changes of the mean value in a candidate series compared with a homogeneous reference series. The test is intended for studies of artificial relative trends in climatological time series, e.g. an increasing urban heat island effect. The basic structure of the new test is similar to that of a widely used test for abrupt changes, the standard normal homogeneity test. The test for abrupt changes is found to remain unaltered after an important generalization.

Daily mean sea level pressure reconstructions for the European-North Atlantic region for the period 1850-2003

Abstract The development of a daily historical European–North Atlantic mean sea level pressure dataset (EMSLP) for 1850–2003 on a 5° latitude by longitude grid is described. This product was produced using 86 continental and island stations distributed over the region 25°–70°N, 70°W–50°E blended with marine data from the International Comprehensive Ocean–Atmosphere Data Set (ICOADS). The EMSLP fields for 1850–80 are based purely on the land station data and ship observations. From 1881, the blended land and marine fields are combined with already available daily Northern Hemisphere fields. Complete coverage is obtained by employing reduced space optimal interpolation. Squared correlations (r 2) indicate that EMSLP generally captures 80%–90% of daily variability represented in an existing historical mean sea level pressure product and over 90% in modern 40-yr European Centre for Medium-Range Weather Forecasts Re-Analyses (ERA-40) over most of the region. A lack of sufficient observations over Greenland and...

HOMOGENIZATION OF SWEDISH TEMPERATURE DATA. PART II: HOMOGENIZED GRIDDED AIR TEMPERATURE COMPARED WITH A SUBSET OF GLOBAL GRIDDED AIR TEMPERATURE SINCE 1861

Homogeneity tests of long seasonal temperature series from Sweden, Denmark, Finland, and Norway indicate that homogeneous series are rare and that an abrupt change of the relative mean level is a much more common type of nonhomogeneity than a gradual change. Furthermore, negative shifts were 20% more common than positive shifts. Homogenized temperature anomaly series that were constructed for six 5 degrees latitude x 5 degrees longitude grid boxes indicate that the temporal pattern of temperture changes has been similar in different parts of Sweden since 1861. The annual mean temperature over Sweden was found to have increased by 0.68 degrees C from the period 1861-1890 to 1965-1994. The corresponding changes for the seasons were: +0.18 degrees C (winter), +1.40 (spring), +0.42 (summer) and +0.60 (autumn). A direct comparson shows that non-homogeneities in the temperature series from individual grid boxes in a global data set can be as large as the total changes observed. We estimate that a 95 per cent confidence interval for the error, due to non-homogeneous long station records, in estimates of hemispheric temperature changes over land regions since the period 1861-1890 is +/-0.1 degrees C for the Northern Hemisphere and the globe and +/-0.25 degrees C for the Southern Hemisphere. For a region consisting of about five grid boxes, this error is +/-0.5 degrees C. The large non-homogeneities in individual grid-box series in the global data set is partly a consequence of the fact that homogeneous climate data are not always easily available for the open research community. We urge that efforts are made to improve this situation.

Trends in Nordic and Arctic Temperature Extremes and Ranges

The national meteorological institutes in the Nordic countries have produced a comprehensive dataset of climatic extreme temperatures (monthly mean daily maximum and minimum, and monthly absolute highest and lowest temperatures) comprising stations from Fenno-Scandia, the Nordic Seas, and Greenland. Mean maximum and minimum temperatures show statistically significant negative trends in western coastal Greenland during the period 1950-95, while over the Nordic Seas and Fenno-Scandia the trends are generally positive. The diurnal temperature range (DTR) is decreasing significantly throughout the study area and is unrelated to regional temperature trends, which show both warming and cooling. The opposite temperature trends between western coastal Greenland and Fenno-Scandia since the 1950s are in accordance with a strengthening of the North Atlantic Oscillation (NAO). However, the simple NAO index fails to explain the decrease of DTR. In Fenno-Scandia, the reliable long-term mean maximum and minimum temperatures show cooling in winter and warming in spring and summer during the period 1910-95. Simultaneously, DTR has been decreasing in all seasons except winter. Most of the decrease has occurred since the 1940s. Atmospheric circulation indices defined by zonal and meridional sea level pressure differences, along with sea level pressure and cloud cover anomalies were used to build a multiple linear regression model for the Fenno-Scandian DTR. During the period 1910-95 the model explains from 53% (winter) to 80% (summer) of the variation in DTR and reproduces the statistically significant decreasing trend on annual level. Cloud cover is the dominant predictor, while circulation provides substantial improvement in explanation.

Turbulence characteristics in a near neutrally stratified urban atmosphere

Turbulence measurements from the city of Uppsala, Sweden, are analysed. Measurements were taken at two sites: one in the central area, ca. 6 m above roof level, the average building height being ca. 15 m; the other at ca. 8 and 50 m above the ground on a tower situated 100 m downwind of a sharp discontinuity between the densely built-up urban area and flat grass-covered land. The average stability was close to neutral, the range being -0.2 < z/L < 0.2. The main emphasis of the study is on the non-dimensional standard deviations of the velocity components Σi/u*t and on the corresponding non-dimensional energy spectra, u*t being a local velocity scale defined as √Τi/ρ(Τl is the local momentum flux). Comparison with results obtained from surface-layer measurements at ‘ideal sites’ (with u*, being the ordinary friction velocity) shows good general agreement. The most complete agreement is found for the tower 50 m measurements, a result which is notable as this measurement point is found to be within a distinctly transitional zone between the urban and post-urban boundary layers. The results from the central city measurement point are also fairly close to the ‘ideal’ results, the deviations found being small in view of the fact that the site is probably inside the layer in which the roughness elements (the buildings) have direct influence. The measurements at the tower 8 m level show certain distinct deviations from ideal results: all three Σi/u*l, are higher by ca. 10%, the excessive energy being found at the low frequency end of the spectrum. Arguments are presented for this feature to be due to a spectral lag effect.

THE EFFECT OF RADIATION SCREENS ON NORDIC TIME SERIES OF MEAN TEMPERATURE

A short survey of the historical development of temperature radiation screens is given based upon research in the archives of the Nordic meteorological institutes. In the middle of the nineteenth century most thermometer stands were open shelters, free-standing or fastened to a window or wall. Most of these were soon replaced by wall or window screens, i.e. small wooden or metal cages. Large free-standing screens were also introduced in the nineteenth century, but it took to the 1980s before they had replaced the wall screens completely in all Nordic countries. During recent years, small cylindrical screens suitable for automatic weather stations have been introduced. At some stations they have replaced the ordinary free-standing screen as part of a gradual move towards automation. The first free-standing screens used in the Nordic countries were single louvred. They were later improved by double louvres. Compared with observations from ventilated thermometers the monthly mean temperatures in the single louvred screens were 02‐04C higher during May‐August, whereas in the double louvred screens the temperatures were unbiased. Unless the series are adjusted, this improvement may lead to inhomogeneities in long climatic time series. The change from wall screen to free-standing screen also involved a relocation from the microclimatic influence of a house to a location free from obstacles. Tests to evaluate the effect of relocation by parallel measurements yielded variable results. However, the bulk of the tests showed no effect of the relocation in winter, whereas in summer the wall screen tended to be slightly warmer (00‐03C) than the double louvred screen. At two Norwegian sites situated on steep valley slopes, the wall screen was ca .0 5C colder in midwinter. The free-standing Swedish shelter, which was used at some stations up to 1960, seems to have been overheated in spring and summer (maximum overheating of about 04C in early summer). The new screen for automatic sensors appears to be unbiased compared with the ordinary free-standing screen concerning monthly mean temperature. # 1997 Royal Meteorological Society. Int. J. Climatol., 17, 1667‐1681

Past and current climate change

This section describes long-term observed climatic changes in atmospheric parameters. The focus is on surface climate conditions, but changes in atmospheric circulation are discussed as they often are behind climatic variability seen on regional and local scales. For a summary introduction on mean atmospheric states and conditions in the Baltic Sea Basin see Annex 1.2 with sections on the general atmospheric circulation (A.1.2.1), surface air temperature (A.1.2.2), precipitation (A.1.2.3), clouds (A.1.2.4), and global radiation (A.1.2.5).

Recent Mild and Wet Years in Relation to Long Observation Records and Future Climate Change in Sweden

Recent mild and wet years in Sweden were compared with long observation series of temperature, precipitation and runoff. Spatial average series for northern and southern Sweden were constructed and analyzed for the period 1901-2002. Precipitation increased considerably during the period, whereas temperature and runoff increases were weaker. On average, for the whole country, the differences between the period 1991-2002 and 1901-1990 were +0.7 degrees C for temperature, +11% in precipitation and +7% in runoff. The differences in temperature and precipitation, but not runoff, were significant at the 5% level. However, the 1930s were equally mild, and the runoff was almost as high in the 1920s. The characteristic feature of the past decade is the combination of high temperature, precipitation and runoff. The deviation between the most recent decade and the preceding years is consistent with climate scenario projections for Sweden, but there are also differences in the seasonal pattern.

The Lund instrumental record of meteorological observations: reconstruction of monthly sea-level pressure 1780–1997

The reconstructed surface air pressure series from Lund, southern Sweden, covers the period 1780-1997 and comprises mon than 234000 valid observations (three observations per day), i.e. > 98% of all possible observation occasions. For the Early Instrumental Period (EIP; 1780-1860) data were digitised from the original records. For most of the Modern Instrumental Period (MIP; 1861-) a series was compiled from various databases containing instrument corrected data. During EIP, the series of raw monthly means show several substantial inhomogeneities. With the aid of a detailed reconstruction of the station history, it was possible to remove almost all inhomogeneities during EIP by applying the correct instrument corrections (for barometer temperature, to standard gravity and to mean sea-level pressure) to the series of original observations. In particular, corrections for the temperature and altitude of the barometer eliminated several inhomogeneities. A prerequisite for applying these corrections is the availability of high-resolution data (actual raw observations or daily averages). Further homogenisation was attained by intercomparison of the monthly mean pressure with acknowledged homogeneous series (mainly the UKMO monthly grid, station records from Copenhagen and Edinburgh). Statistical tests of homogeneity showed that no substantial inhomogeneities remain in the final version. The modern part of the final monthly pressure series largely follows that of the southern Baltic Sea region. Furthermore, it shows relatively high pressure during spring (MAM) in the period 1780-1820, which was paralleled by severe wind erosion in southern Scandinavia during this time. Relatively high pressure throughout the year is also notable during a period of precipitation deficit in 1970s. Copyright (C) 1999 Royal Meteorological Society.