Eirik J. Førland

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.

Changes in the Probability of Heavy Precipitation: Important Indicators of Climatic Change

A simple statistical model of daily precipitation based on the gamma distribution is applied to summer (JJA in Northern Hemisphere, DJF in Southern Hemisphere) data from eight countries: Canada, the United States, Mexico, the former Soviet Union, China, Australia, Norway, and Poland. These constitute more than 40% of the global land mass, and more than 80% of the extratropical land area. It is shown that the shape parameter of this distribution remains relatively stable, while the scale parameter is most variable spatially and temporally. This implies that the changes in mean monthly precipitation totals tend to have the most influence on the heavy precipitation rates in these countries. Observations show that in each country under consideration (except China), mean summer precipitation has increased by at least 5% in the past century. In the USA, Norway, and Australia the frequency of summer precipitation events has also increased, but there is little evidence of such increases in any of the countries considered during the past fifty years. A scenario is considered, whereby mean summer precipitation increases by 5% with no change in the number of days with precipitation or the shape parameter. When applied in the statistical model, the probability of daily precipitation exceeding 25.4 mm (1 inch) in northern countries (Canada, Norway, Russia, and Poland) or 50.8 mm (2 inches) in mid-latitude countries (the USA, Mexico, China, and Australia) increases by about 20% (nearly four times the increase in mean). The contribution of heavy rains (above these thresholds) to the total 5% increase of precipitation is disproportionally high (up to 50%), while heavy rain usually constitutes a significantly smaller fraction of the precipitation events and totals in extratropical regions (but up to 40% in the tropics, e.g., in southern Mexico). Scenarios with moderate changes in the number of days with precipitation coupled with changes in the scale parameter were also investigated and found to produce smaller increases in heavy rainfall but still support the above conclusions. These scenarios give changes in heavy rainfall which are comparable to those observed and are consistent with the greenhouse-gas-induced increases in heavy precipitation simulated by some climate models for the next century. In regions with adequate data coverage such as the eastern two-thirds of contiguous United States, Norway, eastern Australia, and the European part of the former USSR, the statistical model helps to explain the disproportionate high changes in heavy precipitation which have been observed.

Progress in the study of climatic extremes in northern and central Europe

A study of the long-term changes of various climatic extremes was made jointly by a number of European countries. It was found that the changes in maximum and minimum temperatures follow, in broad terms, the corresponding well-documented mean temperature changes. Minimum temperatures, however, have increased slightly more than maximum temperatures, although both have increased. As a result, the study confirms that the diurnal temperature range has mostly decreased during the present century in Northern and Central Europe. Frost has become less frequent. Two extreme-related precipitation characteristics, the annual maximum daily precipitation and the number of days with precipitation ≥ 10 mm, show no major trends or changes in their interannual variability. An analysis of return periods indicated that in the Nordic countries there were high frequencies of ‘extraordinary’ 1-day rainfalls both in the 1930s and since the 1980s. There have been no long-term changes in the number of high wind speeds in the German Bight. Occurrences of thunderstorms and hails show a decreasing tendency in the Czech Republic during the last 50 years. Finally, using proxy data sources, a 500-year temperature and precipitation event graph for the Swiss Mittelland is presented. It shows large interdecadal variations as well as the exceptionality of the latest decade 1986-1995.

Temperature and Precipitation Development at Svalbard 1900–2100

Substantial variations in temperature and precipitation have been observed since the first permanent weather station was established in the Svalbard region in 1911. Temperature and precipitation development are analysed for the longest observational series, and periods with positive and negative trends are identified. For all temperature series, positive linear trends are found for annual values as well as spring, summer, and autumn series. A very strong winter warming is identified for the latest decades. Evaluation of temperature trends downscaled from global climate models forced with observed greenhouse gas emissions suggests that the downscaled results do span the observation-based trends at Svalbard Airport 1912–2010. Novel projections focussing on the Svalbard region indicate a future warming rate up to year 2100 three times stronger than observed during the latest 100 years. The average winter temperature in the Longyearbyen area at the end of this century is projected to be around 10°C higher than in present climate. Also for precipitation, the long-term observational series indicate an increase and the projections indicate a further increase up to year 2100.

Temperature and precipitation variations in Norway 1900–1994 and their links to atmospheric circulation

The main aim of the present study was to identify to what degree decadal scale variability and long-term trends in temperature and precipitation in Norway can be attributed to variations in the dominating atmospheric circulation patterns. Empirical models were developed and tested on monthly series of temperature and precipitation in different regions in Norway. The monthly mean sea level pressure (SLP) field over the northern North Atlantic and northern Europe was used as a predictor. Principal components (PCs) deduced from this field were used as a basis for stepwise multiple regression analysis. The downscaling models were developed using 1925–1969 as a training period, while 1900–1924 and 1970–1994 were used as validation periods. Model testing revealed that the temperature variability during 1970–1994 in most cases was better simulated than the variability during 1900–1924. The models reproduced most of the observed trends and decadal scale variability from 1940 to present. They also reproduced the precipitation trends in western Norway before 1940. However, the temperature increase observed over all the country in 1900–1940 was not reproduced. Nor was the increased winter precipitation in southeastern Norway during the same period. It is concluded that the temperature and precipitation changes observed in Norway during the last 40 years can mainly be attributed to variations in the SLP field. Variations in the precipitation conditions in the eastern parts of the country, and in temperature all over the country, during 1900–1940 are probably connected to changes in external forcings and/or atmosphere–ocean interactions. Copyright

Past and future climate variations in the Norwegian Arctic: overview and novel analyses

Sparse stations and serious measuring problems hamper analyses of climatic conditions in the Arctic. This paper presents a discussion of measuring problems in the Arctic and gives an overview of observed past and projected future climate variations in Svalbard and Jan Mayen. Novel analyses of temperature conditions during precipitation and trends in fractions of solid/liquid precipitation at the Arctic weather stations are also outlined. Analyses based on combined and homogenized series from the regular weather stations in the region indicate that the measured annual precipitation has increased by more than 2.5% per decade since the measurements started in the beginning of the 20th century. The annual temperature has increased in Svalbard and Jan Mayen during the latest decades, but the present level is still lower than in the 1930s. Downscaled scenarios for Svalbard Airport indicate a further increase in temperature and precipitation. Analyses based on observations of precipitation types at the regular weather stations demonstrate that the annual fraction of solid precipitation has decreased at all stations during the latest decades. The reduced fraction of solid precipitation implies that the undercatch of the precipitation gauges is reduced. Consequently, part of the observed increase in the annual precipitation is fictitious and is due to a larger part of the “true“ precipitation being caught by the gauges. With continued warming in the region, this virtual increase will be measured in addition to an eventual real increase.

Homogenizing Long Norwegian Precipitation Series

Abstract The standard normal homogeneity test has been applied to 165 Norwegian precipitation series of 75 years or more. Of these series, 50 were found to be homogeneous, while 79 became homogeneous after being adjusted for a single inhomogeneity. Almost 50% of the inhomogeneities were caused by relocation of the precipitation gauge. Other reasons for inhomogeneities were changes in the immediate environment (trees, buildings) and changes in instruments (windshield, new gauge type). About 80% of the inhomogeneities could be traced to information in the station history flies. Most of the inhomogeneities caused by changes in instrumentation led to increased gauge catch. There was a similar tendency for inhomogeneities caused by changes in environment. Consequently, trend studies based on groups of untested series may give dubious trends. Homogeneity testing and station history archives are essential tools for finding the real trends and fluctuations in precipitation series.

Mapping temperatures in Norway applying terrain information, geostatistics and GIS

The spatial variation in monthly temperature normals (1961-90) from southern Norway was studied. Linear regression as well as an approach combining a deterministic and a geostatistical model were applied (residual kriging). The deterministic component describes the large-scale trend in the temperature, and in this study is defined as the vertical temperature gradient. The spatial variability caused by differences in elevation is removed by reducing the temperatures to sea level. The resulting temperature fields are more suited for spatial statistical analysis. These temperatures are closer to fulfilling the assumptions most statistical interpolation methods require, i.e. stationarity and isotropy. The reduced temperatures were interpolated applying kriging. It is shown that residual kriging gives better estimates than kriging on station level temperatures. Residual kriging is also more credible than the linear regression approach, which regionally gives large estimation errors.

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).