DAILY AIR TEMPERATURE AND PRESSURE SERIES ... - BALTEX

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204 ANDERS MOBERG ET AL. 5.6. DISCUSSION OF THE SEA LEVEL PRESSURE SERIES We have reconstructed a series of daily mean sea level air pressures for Stockholm from the original observational data. Performing the reductions to sea level before 1859 was the most difficult part of the work, because little is known about the properties of the used barometers and their positions. Furthermore, no barometer temperatures were observed before 1785, so these values had to be estimated from outdoor temperatures. Comparisons with data from other European stations reveal that the air pressure values for Stockholm, as calculated directly from the observations, exhibit several sub-periods with various systematic biases. A homogenization was therefore done by adding certain corrections obtained from homogeneity tests. The homogenization is expected to correct for most of the systematic errors in the original data, and also for possible systematic errors introduced by us whenever we made wrong assumptions concering the properties of the early barometers and conditions around the observations. A plot (Figure 11b) of the homogenized annual mean pressures back to 1756 for both Stockholm and Uppsala (Bergström and Moberg, 2002) shows that the year-to-year variability is very similar in these two series. The similarity is even more striking in plots of daily air pressures. Two examples of time-series plots of daily pressures in selected years (1788 and 1819) are shown in Figure 13. The plots illustrate that the day-to-day pressure changes is almost identical in the two series. This is to be expected for two such nearby stations. Other selections of years (not shown) reveal the same high degree of similarity. This is a very good indicator that both records are very reliable reconstructions of daily air pressures. In particular, the data can be used with a high degree of credibility as input for drawing pressure charts over Europe for any particular day. Regarding long-term trends (multi-decadal scales), the reliability of the data must be considered as lower than for the conditions in individual days. This is rather obvious because the pressure difference between various multi-decade long periods is small in comparison with many of the errors in the observed data. Furthermore, the homogenization of the long-term trends was largely based on comparison with data for surrounding stations, which means that the reliability of trends is dependent on the quality of the reference data. The reliability of trends is regarded as very high back to 1890, where the homogenization was based on comparisons with the NACD data set of Frich et al. (1996). This data set consists of a large number of series which have been homogeneity tested and maintained by the national meteorological services in several countries. Although we used homogenized reference data back to 1780, the reliability of pressure trends before 1890 must be expected to decrease further back in time. This is because the reference data we used (from Jones et al., 1999) had, in their turn, been homogenized based on comparisons with fewer and fewer surrounding stations further and further back in time. Finally, the Stockholm pressure series can not be used at all for studies of changes in pressure trends before 1780, as the
DAILY AIR TEMPERATURE AND PRESSURE SERIES FOR STOCKHOLM (1756–1998) 205 Figure 13. Daily mean air temperatures and sea level pressures for Stockholm (black) and Uppsala (grey) for two selected years, 1788 and 1819. oldest part was homogenized by assuming that the mean value before 1780 should be the same as that afterwards. Nevertheless, these earliest data still seem to be reliable for reconstructions of weather patterns, drawing of synoptic charts, etc. for individual days. 6. Concluding Remarks Reconstruction of a 250-year-long daily meteorological record is a time-consuming work. In our case, it included not only the pure calculations leading to the final daily values, but also digitizing of the original data, detective work for identification and correction of various kinds of errors and making reasonable assumptions when basic information about important conditions was missing. We found it extremely valuable to have access to parallell data for the neighbouring station in Uppsala (65 km north of Stockholm), described by Bergström and Moberg (2002). Because of the possibility to compare the two series, many errors in individual temperature
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DAILY AIR TEMPERATURE AND PRESSURE SERIES ... - BALTEX

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