Alice Kapala

Behaviour of the centres of action above the Atlantic since 1881. Part I: Characteristics of seasonal and interannual variability

Based on generated time series of the central pressure, latitudinal and longitudinal position of the Iceland Low, Azores High, Inter Tropical Convergence Zone and South Atlantic Subtropical High, the behaviour of these atmospheric centres of action since 1881 is examined with regard to the intra- and interannual variations, persistence of anomalies, linear trends and interactions. The critical analysis of these time series as well as those of the pressure gradients between the neighbouring centres suggests that the existing non-periodic fluctuations of the surface atmospheric circulation on the time-scale of one or more decades suppress the long-term changes. These fluctuations are similar to the ‘internal’ variations of the climate system, recognized in the climate simulations of coupled ocean–atmosphere models.

Seasonally dependent changes of precipitation extremes over Germany since 1950 from a very dense observational network

[1] The newly updated collection of daily precipitation measurements over Western Germany (more than 2000 stations in total) is used to analyze linear trends in extreme and heavy precipitation for different seasons over the period 1950–2004. Heavy and extreme precipitation has been quantified using the 95% and 99% percentiles with respect to the Gamma distribution fitted to daily precipitation data. The significance of linear trends was quantified using several statistical tests including estimates of field significance. Positive linear tendencies in heavy precipitation for the winter, spring and autumn seasons were found for the whole domain with the largest increase of 13% per decade in Central and Southern Germany. For the summer season, however, heavy precipitation exhibits mostly negative trends of up to 8% per decade e.g., for the Central and Southwestern parts of Germany. Trends derived from the estimates of heavy precipitation without seasonal breakdown, however, do not show any clear spatial pattern. Estimates of field significance show that the conclusions concerning the seasonal diversity in trend sign hold for most of Western Germany. The results are insensitive to changes of the beginning and the end of the records by several years; thus the seasonal linear trend patterns are not influenced by interdecadal variability. Seasonality is also identified in the linear trends of mean precipitation characteristics. Analysis performed for different classes of precipitation intensity shows that during winter the linear increase of heavy and extreme precipitation is associated with downward linear tendencies for weak precipitation. In summer statistically significant negative linear trends were identified for all classes of precipitation intensities. Our results also imply that the amplitude of the annual cycle of heavy and extreme precipitation underwent a considerable decrease during the last 55 years between 30% to 60% per decade.

Improving Estimates of Heavy and Extreme Precipitation Using Daily Records from European Rain Gauges

Abstract The long-term variability in heavy precipitation characteristics over Europe for the period 1950–2000 is analyzed using high-quality daily records of rain gauge measurements from the European Climate Assessment (ECA) dataset. To improve the accuracy of heavy precipitation estimates, the authors suggest estimating the fractional contribution of very wet days to total precipitation from the probability distribution of daily precipitation than from the raw data, as it is adopted for the widely used R95tot precipitation index. This is feasible under the assumption that daily precipitation follows an analytical distribution like the gamma probability density function (PDF). The extended index R95tt based on the gamma PDF is compared to the classical R95tot index. The authors find that R95tt is more stable, especially when precipitation extremes are estimated from the limited number of wet days of seasonal and monthly time series. When annual daily time series are analyzed, linear trends in R95tt and R...

BEHAVIOUR OF THE CENTRES OF ACTION ABOVE THE ATLANTIC SINCE 1881. PART II: ASSOCIATIONS WITH REGIONAL CLIMATE ANOMALIES

This paper constitutes the second part of the analysis of the behaviour of centres of action above the Atlantic since 1881. The present study examines the applicability of the generated time series of the central pressure and geographical location of the Iceland Low (IL), Azores High (AH), Inter Tropical Convergence Zone (ITCZ) and South Atlantic Subtropical High (SSH) to the description of the surface circulation and its influence on the climate anomalies above the Atlantic and adjacent land areas in the period 1950—1989. Using the normalized anomalies of the pressure gradient between the cores of the AH and IL (AH—IL gradient) as a NAG (North Atlantic Oscillation) index, it can be shown that t his index is a practicable tool for the detection of characteristic spatial anomaly patterns in different climate variables above the North Atlantic and surrounding land areas. The ‘strong’ NAG (the AH-IL gradient above ‘normal’) in winter leads to a warming above northern and central Europe as well as to the development of the dipole-like anomaly patterns in the wind speed, sea-surface temperature, surface and tropospheric temperature and other variables above the western and eastern North Atlantic. The composite analysis for the ‘weak’ NAG reveals inverse patterns. The application of the characteristic parameters of the ITCZ and of the two subtropical highs as predictors of the rainfall variability in northeast Brazil and the Sahel for the period 1950-1983 shows that for the most part, these variations can be explained by the anomalies of the latitude position and intensity of the ITCZ.

Precipitation Variability and Extremes in Central Europe: New View from STAMMEX Results

The STAMMEX (Spatial and Temporal Scales and Mechanisms of Extreme Precipitation Events over Central Europe) project has developed a high-resolution gridded long-term precipitation dataset based on the daily-observing precipitation network of the German Weather Service DWD, which runs one of the worlds densest rain gauge networks, comprising more than 7,500 stations. Several quality-controlled daily gridded products with homogenized sampling were developed covering the periods 1931–onward (with 0.5° resolution), 1951–onward (0.5° and 0.25°), and 1971–2000 (0.5°, 0.25°, and 0.1°). Different methods were tested to select the best gridding methodology that minimizes errors of integral grid estimates over hilly terrain. Besides daily precipitation values with uncertainty estimates, the STAMMEX datasets include a variety of statistics that characterize temporal and spatial dynamics of the precipitation distribution (quantiles, extremes, wet/ dry spells, etc.). Comparisons with existing continental-scale daily...

Sensitivity of summer precipitation simulated by the CLM with respect to initial and boundary conditions

The knowledge of the uncertainty resulting from the sensitivity of a regional climate model to physical parameterisation schemes, horizontal resolution, domain size and location, as well as to initial and boundary conditions is crucial for the interpretation of model results, e.g. for dynamical downscaling of global climate predictions. In this study, we assess some of these aspects by investigating daily precipitation statistics simulated by the regional climate model CLM (Climate version of the Local Model). Different sensitivity experiments related to initialisation date, domain size and location and to the lateral boundary forcing have been performed for the summer of 1974. The evaluation domain is Germany, located approximately in the centre of the model domain. Comparisons to the control run show that the spatially averaged precipitation statistics can be significantly affected by this modification, not only in specific regions, but also in entire Germany. The results also indicate that the strength of the lateral boundary forcing has a crucial influence on the simulated characteristics.