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Hans Alexandersson

Abstract

A simple and rather general model of the precipitation process is reviewed and some applications and comparisons are made using data from Sweden. This model has been used by several authors so the article is partly a survey of earlier works but also adds some new aspects, comparisons and practical techniques. The model used is a compound Poisson–exponential (Cpe) process. This is a continuous stochastic process, which is compounded from a Poisson process with a number parameter (the mean number of independent precipitation events) and an exponential distribution with an amount parameter (the mean amount at each event). The meaning and the limitations of these physical interpretations are discussed briefly. The basic process can be applied on, e.g., integrated precipitation amounts, the Cpe distribution, and on maximum amounts, the max Cpe distribution. The first application has been discussed in many works, the latter was developed recently and independently by Revfeim (1983a) and Alexandersson (1983). One advantage of this model is that it does not need to be extended or modified to handle periods with zero precipitation. Another advantage is that the parameters can be estimated from the series of monthly precipitation totals. It is important that these techniques do not involve too lengthy calculations which would considerably hamper the practical use. Thus a very fast way of deriving percentiles from a single table for a Cpe distribution is developed here.

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Heikki Tuomenvirta, Hans Alexandersson, Achim Drebs, Povl Frich, and Per Oyvind Nordli

Abstract

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.

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