Olga Zolina

Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls

[1] Analysis of the duration of wet spells (consequent days with significant precipitation) in Europe and associated precipitation is performed over the period 1950–2008 using daily rain gauge data. During the last 60 years wet periods have become longer over most of Europe by about 15– 20%. The lengthening of wet periods was not caused by an increase of the total number of wet days. Becoming longer, wet periods in Europe are now characterized by more abundant precipitation. Heavy precipitation events during the last two decades have become much more frequently associated with longer wet spells and intensified in com‐ parison with 1950s and 1960s. The changes in the distri‐ bution of temporal characteristics of precipitation towards longer events and higher intensities should have a significant impact on the terrestrial hydrologic cycle including sub‐ surface hydrodynamics, surface runoff and European flooding. Citation: Zolina, O., C. Simmer, S. K. Gulev, and S. Kollet (2010), Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls, Geophys. Res. Lett., 37, L06704, doi:10.1029/2010GL042468.

Characteristics of the Recent Eastward Shift of Interannual NAO Variability

Recent observational studies have shown that the centers of action of interannual variability of the North Atlantic Oscillation (NAO) were located farther eastward during winters of the period 1978‐97 compared to previous decades of the twentieth century. In this study, which focuses on the winter season (December‐March), new diagnostics characterizing this shift are presented. Further, the importance of this shift for NAO-related interannual climate variability in the North Atlantic region is discussed. It is shown that an NAO-related eastward shift in variability can be found for a wide range of different parameters like the number of deep cyclones, near-surface air temperature, and turbulent surface heat flux throughout the North Atlantic region. By using a near-surface air temperature dataset that is homogenous with respect to the kind of observations used, it is shown that the eastward shift is not an artifact of changes in observational practices that took place around the late 1970s. Finally, an EOF-based Monte Carlo test is developed to quantify the probability of changes in the spatial structure of interannual NAO variability for a relatively short (20 yr) time series given multivariate ‘‘white noise.’’ It is estimated that the likelihood for differences in the spatial structure of the NAO between two independent 20-yr periods, which are similar (as measured by the angle and pattern correlation between two NAO patterns) to the observed differences, to occur just by chance is about 18%. From the above results it is argued that care has to be taken when conclusions about long-term properties of NAO-related climate variability are being drawn from relatively short recent observational data (e.g., 1978‐97).

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.

Changes in the Duration of European Wet and Dry Spells during the Last 60 Years

AbstractDaily rain gauge data over Europe for the period from 1950 to 2009 were used to analyze changes in the duration of wet and dry spells. The duration of wet spells exhibits a statistically significant growth over northern Europe and central European Russia, which is especially pronounced in winter when the mean duration of wet periods increased by 15%–20%. In summer wet spells become shorter over Scandinavia and northern Russia. The duration of dry spells decreases over Scandinavia and southern Europe in both winter and summer. For the discrimination between the roles of a changing number of wet days and of a regrouping of wet and dry days for the duration of the period, the authors suggest a fractional truncated geometric distribution. The changing numbers of wet days cannot explain the long-term variability in the duration of wet and dry periods. The observed changes are mainly due to the regrouping of wet and dry days. The tendencies in duration of wet and dry spells have been analyzed for a numb...

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...

Improving the accuracy of mapping cyclone numbers and frequencies

Abstract The uncertainties associated with the mapping of cyclone numbers and frequencies are analyzed using the 42-yr winter climatology of cyclone tracks derived from 6-hourly NCEP–NCAR reanalysis. Tracking is performed using an automated procedure, based on computer animation of the sea level pressure fields. Uncertainties in the mapping result from an incomplete catchment of cyclones by the grid cells: the coarse temporal resolution of data causes fast-moving storms to skip grid boxes. This introduces error into estimates of cyclone frequency, with cyclone counts systematically underestimated. To minimize these biases, it is possible to simulate higher temporal resolution of the storm tracks by linear interpolation applied to the original tracks. This simple procedure reduces bias in estimates of both cyclone frequencies and numbers and enables quantitative estimation of errors. Errors in cyclone counts are estimated over the Northern Hemisphere for different time resolutions and different grids. Stan...

Synoptic Variability of Ocean–Atmosphere Turbulent Fluxes Associated with Atmospheric Cyclones

Synoptic-scale variability in the air‐sea turbulent fluxes in the areas of midlatitudinal western boundary currents is analyzed. In the Gulf Stream area, ocean‐atmosphere fluxes on synoptic time- and space scales are clearly coordinated with the propagating synoptic-scale atmospheric transients. The statistical analysis of 6-hourly resolution sea level pressure and surface turbulent fluxes from the NCEP‐NCAR reanalysis for the period from 1948 to 2000 in the area of strong sea surface temperature gradients in the Gulf Stream gives strong proof for the association between the propagating cyclones and synoptic patterns of surface turbulent fluxes. It is shown that sea‐air interaction in this area is controlled by the sharpness of surface temperature gradients in the ocean and by the intensity of the advection of the air masses in different parts of cyclones during the cold-air and warm-air outbreaks. A simple parameter based on the joint consideration of the characteristics of sea surface temperature and sea level pressure fields is used to characterize the synoptic variability of air‐sea turbulent fluxes. The effectiveness of the relationship between surface temperature and surface pressure on one side and air‐sea flux anomalies on the other vary from year to year in phase with variability in the frequencies of deep atmospheric cyclones in the Gulf Stream area. The limits of applicability of the approach, its sensitivity to higher-resolution sea surface temperature data, and the possibility of its further applications are discussed.

Atmospheric Moisture Transport to the Arctic: Assessment of Reanalyses and Analysis of Transport Components

AbstractThe atmospheric water cycle of the Arctic is evaluated via seven global reanalyses and in radiosonde observations covering the 1979–2013 period. In the regional moisture budget, evaporation and precipitation are the least consistent terms among different datasets. Despite the assimilation of radiosoundings, the reanalyses present a tendency to overestimate the moisture transport. Aside from this overestimation, the reanalyses exhibit a remarkable agreement with the radiosondes in terms of spatial and temporal patterns. The northern North Atlantic, subpolar North Pacific, and Labrador Sea stand out as the main gateways for moisture to the Arctic in all reanalyses. Because these regions correspond to the end of the storm tracks, the link between moisture transports and extratropical cyclones is further investigated by decomposing the moisture fluxes in the mean flow and transient eddy parts. In all reanalyses, the former term tends to cancel out when averaged over a latitude circle, leaving the latt...

Multidecadal trends in the duration of wet spells and associated intensity of precipitation as revealed by a very dense observational German network

Precipitation durations and intensities over the period 1950‐2008 are analysed using daily rain gauge data from the Deutsche Wetterdienst raingauge network—one of the densest and most properly maintained precipitation observational networks in Europe. Truncated geometric distribution of the family of discrete distributions was applied for quantifying probability distribution of the durations of wet spells. Further intensities of wet spells of different durations were analysed along with wet spell lengths. During the cold season (October‐March) wet periods over the whole of Germany demonstrate a robust pattern of lengthening by about 2‐3% for the mean durations of wet spells and up to 6% for extremely long wet periods. This tendency is clearly associated with growing (up to 10% per decade in Eastern Germany) intensity of precipitation during long wet periods (more than 5 days) and the weakening of precipitation events associated with short and moderately long wet periods with both signals being statistically significant. Trends are superimposed with interdecadal variability, which is the strongest in Northern and Central Germany. In the warm season (April‐September) there is no robust pan-German trend pattern in the wet spell durations and associated precipitation intensities. Strong structural changes in winter precipitation over Germany potentially imply growing rates of winter ground water recharge over Germany and increasing probability of winter flash and river flooding.

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...