Heikki Tuomenvirta

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.

Indices for daily temperature and precipitation extremes in Europe analyzed for the period 1901–2000

[1]xa0We analyze century-long daily temperature and precipitation records for stations in Europe west of 60°E. A set of climatic indices derived from the daily series, mainly focusing on extremes, is defined. Linear trends in these indices are assessed over the period 1901–2000. Average trends, for 75 stations mostly representing Europe west of 20°E, show a warming for all temperature indices. Winter has, on average, warmed more (∼1.0°C/100 yr) than summer (∼0.8°C), both for daily maximum (TX) and minimum (TN) temperatures. Overall, the warming of TX in winter was stronger in the warm tail than in the cold tail (1.6 and 1.5°C for 98th and 95th, but ∼1.0°C for 2nd, 5th and 10th percentiles). There are, however, large regional differences in temperature trend patterns. For summer, there is a tendency for stronger warming, both for TX and TN, in the warm than in the cold tail only in parts of central Europe. Winter precipitation totals, averaged over 121 European stations north of 40°N, have increased significantly by ∼12% per 100 years. Trends in 90th, 95th and 98th percentiles of daily winter precipitation have been similar. No overall long-term trend occurred in summer precipitation totals, but there is an overall weak (statistically insignificant and regionally dependent) tendency for summer precipitation to have become slightly more intense but less common. Data inhomogeneities and relative sparseness of station density in many parts of Europe preclude more robust conclusions. It is of importance that new methods are developed for homogenizing daily data.

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 (ru200a2) 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...

Multicentennial megadrought in northern Europe coincided with a global El Niño–Southern Oscillation drought pattern during the Medieval Climate Anomaly

The El Nino-Southern Oscillation (ENSO) is a pacemaker of global climate, and the accu- rate prediction of future climate change requires an understanding of the ENSO variability. Recently, much-debated aspects of the ENSO have included its long-term past and future changes and its associations with the North Atlantic and European sectors, potentially in inter- action with the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation. Here we present the fi rst European dendroclimatic precipitation reconstruction that extends through the alternating climate phases of the Medieval Climate Anomaly and the Little Ice Age. We show that northern Europe underwent a severe precipitation defi cit during the Medieval Climate Anomaly, which was synchronous with droughts in various ENSO-sensitive regions worldwide, while the subsequent centuries during the Little Ice Age were markedly wetter. We attribute this drought primarily to an interaction between the ENSO and the North Atlantic Oscillation, and to a lesser (or negligible) degree to an interaction between the ENSO and the Atlantic Multidecadal Oscillation.

Oak decline in Helsinki portrayed by tree-rings, climate and soil data

Oak decline has recently been observed in and around Helsinki. Tree-ring widths of pedunculate oak were used to assess pre-mortem growth patterns, their dependence on climatic factors and linkages to soil thickness. Tree-ring chronologies were constructed in three tree vigour classes (healthy, declining and dying oaks). Characteristic “summer response” was found as a positive influence of summer precipitation was the most important climatic factor limiting the radial growth in all vigour classes. On the other hand, a differing “winter response” was found for tree-rings of dying and other classes of oaks. The growth of dying oaks was more sensitive to variations in mid-winter temperatures, due presumably to higher risk of frost damage to their roots. Recent summer droughts, which may have increased the potential for bark necrosis due to reinforcing effects from defoliation in decreasing the capability of oaks to acclimatize to winter frost, may thus have played a role in this decline. Amplified water stress was indicated by dendrochronological parameters on sites with shallow soils.

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