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We employ JRA-55 (Japanese 55-year Reanalysis), a recent second-generation global reanalysis providing data of high quality in the stratosphere, to examine whether a distinguishable effect of geomagnetic activity on Northern Hemisphere stratospheric temperatures can be detected. We focus on how the statistical significance of stratospheric temperature differences may be robustly assessed during years with high and low geomagnetic activity. Two problems must be overcome. The first is the temporal autocorrelation of the data, which is addressed with a correction of the t statistics by means of the estimate of the number of independent values in the series of correlated values. The second is the problem of multiplicity due to strong spatial autocorrelations, which is addressed by means of a false discovery rate (FDR) procedure. We find that the statistical tests fail to formally reject the null hypothesis, i.e. no significant response to geomagnetic activity can be found in the seasonal-mean Northern Hemisphere stratospheric temperature record.

We employ JRA-55, a recent second-generation global reanalysis providing data of high-quality in the stratosphere, to examine whether a distinguishable effect of geomagnetic activity on northern hemisphere stratospheric temperatures can be detected. We focus on how the statistical significance of stratospheric temperature differences may be robustly assessed during years with high and low geomagnetic activity. Two problems must be overcome. The first is the temporal autocorrelation of the data, which is addressed with a correction of the t-statistics by means of the estimate of the number of independent values in the series of correlated values. The second is the problem of multiplicity due to strong spatial autocorrelations, which is addressed by means of a false discovery rate (FDR) procedure. We find that the statistical tests fail to formally reject the null hypothesis, i.e. no significant response to geomagnetic activity can be found in the seasonal-mean northern-hemisphere stratospheric temperature record.

Discharge time series of major large-catchment European rivers are known to display significant decadal and interdecadal fluctuations. However, the hydroclimate variability causing such fluctuations remains poorly understood, particularly due to a lack of a spatio-temporal integrated assessment. Here, we demonstrate for the first time that European hydroclimate variability is dominated by a meridional delayed oscillation characterized by a lag of approximately 5 years in interdecadal discharge fluctuations of continental (northern) European rivers with respect to those of Euro-Mediterranean (southern) rivers. We demonstrate a connection of this coherent signal with the large-scale atmospheric circulation over the North Atlantic, and suggest a hitherto unexplored multiannual atmosphere-ocean mechanism in the subpolar North Atlantic at its root.

This article statistically analyses the location errors of the precipitation patterns forecast by three limited area models, namely the Fifth-Generation NCAR/Penn State Mesoscale Model (MM5), the QUADRICS BOlogna Limited Area Model (QBOLAM) and the Regional Atmospheric Modelling System (RAMS), over the Calabria region (Italy) for the period October 2000–May 2002. Contiguous rain area (CRA) analysis is the diagnostic tool used to assess and quantify the position errors of the precipitation forecasts with respect to the observed precipitation patterns. Observation gridded analyses were obtained by means of the Barnes algorithm on the available rain gauge observations. Moreover, an approach to measure the quality of precipitation forecasts routinely by means of a global indicator called CRA Mean Shift (CMS) that summarizes the CRA verification outcomes is proposed. The CMS index would represent a statistical indicator of model quality in forecasting the correct positions of precipitation patterns. The model's tendency to misplace the forecast precipitation patterns towards a particular direction was tested by using a bootstrap procedure. All models seem to show statistically poor abilities in forecasting the correct precipitation pattern position over the verification domain considered. As far as the tendency towards a particular direction is concerned, only the RAMS model seems to show a systematic horizontal misplacement of precipitation patterns towards a particular direction. Copyright © 2008 Royal Meteorological Society

The impact of the temperature distribution on the Hadley circulation simulated by an axisymmetric model is studied. The temperature distributions that drive the model are modulated here by two parameters, n and k, the former controlling the horizontal broadness and the latter defining change in the vertical lapse rate. In the present study, the changes of the temperature distribution mimic changes of the energy input of the atmospheric system leaving as an invariant the equator-poles difference. Both equinoctial and time-dependent Hadley circulations are simulated and results compared. The results give evidence that concentrated temperature distributions enhance the meridional circulation and jet wind speed intensities even with a lower energy input. The meridional circulation and the subtropical jet stream widths are controlled by the broadness of horizontal temperature rather than the vertical lapse rate k, which is important only when the temperature distribution is concentrated at the equator. The jet stream position does not show any dependence with n and k, except when the temperature distribution is very wide and in such a case the jet is located at the mid-latitude. Using n = 2 and k = 1 we have the formulation of the potential temperature adopted in classical literature. A comparison with other works is performed and our results show that the model running in different configurations (equinoctial, solstitial and time-dependent) yields results similar to one another.

This article investigates the role of sea surface temperature (SST) as well as the effects of evaporation and moisture convergence on the evolution of cyclone Klaus, which occurred on January 23 and 24, 2009. To elucidate the role of sea surface temperature (SST) and air–sea fluxes in the dynamics of the cyclone, ten hydrostatic mesoscale simulations were performed by Bologna Limited Area Model (BOLAM). The first one was a control experiment with European Centre for Medium-Range Weather Forecasts (ECMWF) SST analysis. The nine following simulations are sensitivity experiments where the SST are obtained by adding a constant value by 1 to 9 K to the ECMWF field. Results show that a warmer sea increases the surface latent heat fluxes and the moisture convergence, favoring the development of convection in the storm. Convection is affected immediately by the increased SST. Later on, drop of mean sea level pressure (MSLP) occurs together with increasing of surface winds. The cyclone trajectory is not sensitive to change in SST differently from MSLP and convective precipitation.

This study has been carried out in order to investigate the predictability of a rapidly developing storm over the Mediterranean area, by comparing the behaviour of three different meteorological models. The models chosen are the global, variable mesh, model ARPEGE, which is running operationally at Meteo-France, and two limited area models, namely ALADIN (also developed at Meteo-France) and BOLAM (developed in Italy, mainly at FISBAT). This comparison study has been performed by a cooperation among the French Meteorological Service and the National Research Council of Italy (CNR) FISBAT and IFA institutes. In this paper we present the results obtained by integrating the models on a short time-range, corresponding to the developing stage of a strong cyclone that moved rather quickly to the North from Northern Africa, associated with perturbed weather and distinct mesoscale features. In particular, this cyclone was characterized by a low-level warm core, associated with very strong winds and precipitation. All the three models were successful in describing the main characteristics of this cyclogenesis event, in particular the rapid evolution of the low level vortex. However, important differences emerged in “forecasting” the value of the pressure minimum, its position and time evolution, and the associated distribution of precipitation. When the large scales are treated by using analyzed rather than forecast lateral boundary conditions, the two LAMs perform better than the global model in simulating the low evolution but the global high resolution model better describes the general features of the cyclogenesis. Differences in model performance are found especially in the distribution of precipitation forecasts, suggesting two main conclusions: insufficient descriptions of mountains can change the location of the precipitation release, the precipitation scheme of the ALADIN model is sensitive to the lateral boundary conditions. The overestimation of the cyclone in BOLAM seems due to an oversensitivity to the latent heat release that intensifies the circulation in the cyclone core.

How strong is the resemblance between two states of the atmosphere leading to the same outcome? In the following pages this question was addressed by studying the resemblance of the surface pressure fields, obtained by NCEP reanalyses, corresponding to “intense” precipitation events over 5 Italian climatic areas. Such “intense” precipitation events were selected by means of 45 daily precipitation records covering the period from 1951 to 2000.

On 16–17 November 2000, a relatively intense precipitation event on the north-western Italy was heavily underestimated, mainly due to shifting error, by three operational 10-km limited area models (LAMs) which differ about basic equations, domain size, and parameterisation schemes. The scope of the work is to investigate possible common error-sources independent from the single model, in particular the effect of initialisation. Thus, the complex evolution over the western Mediterranean Sea of the cyclone responsible for the event was investigated. Several objective and subjective verification techniques have been employed to check one of the LAMs' forecast against the available observations (precipitation from rain gauge and retrieved from ground-based radar, and satellite-retrieved atmospheric humidity patterns). Despite a clear statement is not achieved, results indicate that high sensitivity to the initial conditions, and the inadequacy of the observational network on the southern Mediterranean area, can play a major role in producing the forecast shifting error on the target area.

International audience; In the framework of the European VOLTAIRE project (Fifth Framework Programme), simulations of relatively heavy precipitation events, which occurred over the island of Cyprus, by means of numerical atmospheric models were performed. One of the aims of the project was indeed the comparison of modelled rainfall fields with multi-sensor observations. Thus, for the 5 March 2003 event, the 24-h accumulated precipitation BOlogna Limited Area Model (BOLAM) forecast was compared with the available observations reconstructed from ground-based radar data and estimated by rain gauge data. Since radar data may be affected by errors depending on the distance from the radar, these data could be range-adjusted by using other sensors. In this case, the Precipitation Radar aboard the Tropical Rainfall Measuring Mission (TRMM) satellite was used to adjust the ground-based radar data with a two-parameter scheme. Thus, in this work, two observational fields were employed: the rain gauge gridded analysis and the observational analysis obtained by merging the range-adjusted radar and rain gauge fields. In order to verify the modelled precipitation, both non-parametric skill scores and the contiguous rain area (CRA) analysis were applied. Skill score results show some differences when using the two observational fields. CRA results are instead quite in agreement, showing that in general a 0.27° eastward shift optimizes the forecast with respect to the two observational analyses. This result is also supported by a subjective inspection of the shifted forecast field, whose gross features agree with the analysis pattern more than the non-shifted forecast one. However, some open questions, especially regarding the effect of other range adjustment techniques, remain open and need to be addressed in future works.

The speed of Atlantic surface depressions, occurred during the autumn and winter seasons and that lead to intense precipitation over Italy from 1951 to 2000, was investigated. Italy was divided into 5 regions as documented in previous climatological studies (based on Principal Component Analysis). Intense precipitation events were selected on the basis of in situ rain gauge data and clustered according to the region that they hit. For each intense precipitation event we tried to identify an associated surface depression and we tracked it, within a large domain covering the Mediterranean and Atlantic regions, from its formation to cyclolysis in order to estimate its speed. "Depression speeds" were estimated with 6-h resolution and clustered into slow and non-slow classes by means of a threshold, coinciding with the first quartile of speed distribution and depression centre speeds were associated with their positions. Slow speeds occurring over an area including Italy and the western Mediterranean basin showed frequencies higher than 25%, for all the Italian regions but one. The probability of obtaining by chance the observed more than 25% success rate was estimated by means of a binomial distribution. The statistical reliability of the result is confirmed for only one region. For Italy as a whole, results were confirmed at 95% confidence level. Stability of the statistical inference, with respect to errors in estimating depression speed and changes in the threshold of slow depressions, was analysed and essentially confirmed the previous results.

The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM). The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband...