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This paper investigates the environmental parameters favoring significant tornadoes in Bangladesh through a simulation of ten high-impact events. A climatological perspective is first presented on classifying significant tornadoes in Bangladesh, noting the challenges since reports of tornadoes are not documented in a formal manner. The statistical relationship between United States and Bangladesh tornado-related deaths suggests that significant tornadoes do occur in Bangladesh so this paper identifies the most significant tornadic events and analyzes the environmental conditions associated with these events. Given the scarcity of observational data to assess the near-storm environment in this region, high-resolution (3-km horizontal grid spacing) numerical weather prediction simulations are performed for events identified to be associated with a significant tornado. In comparison to similar events over the United States, significant tornado environments in Bangladesh are characterized by relatively high convective available potential energy, sufficient deep-layer vertical shear, and a propensity for deviant (i.e., well to the right of the mean flow) storm motion along a low-level convergence boundary. [ABSTRACT FROM AUTHOR]

Climate classifications based on temperature and precipitation measurements are increasingly being used for environmental and climate change studies. Using three classification methods (Köppen, Extended Köppen, and Holdridge) and one observational dataset for present climate (CRU, Climate Research Unit), we show that GCMs have bridged the gap that led to the emergence of RCMs thirty years ago, as GCMs can now provide global climate classifications whose accuracy and precision are comparable to those of regional outputs of the RCMs. Projections of high-resolution GCMs for future climates under the assumptions of three Representative Concentration Pathways (RCP26, RCP45 and RCP85) can therefore be used as a primary source for climate change and global warming studies at high resolution. This paper provides comprehensive, model-derived climate classifications for the entire planet, using RCMs and two GCMs for present and future climate-change scenarios, and discusses how well the models actually represent the climates of the world when compared with reference, ground validation data. It turns out that both GCMs and RCMs appear still limited to provide practical estimates of the world climates even for present climate conditions. The modeling of precipitation remains the Achilles' heel of models and thus of multidimensional indices, which are very sensitive to this variable. The conclusion is that model outputs at regional scale need to be taken with extreme caution without venturing into informing policies presenting potentially large societal impacts. Nonetheless, the role of models as privileged tools to advance our scientific knowledge of the Earth's system remains undisputed. • A systematic analysis of climate classifications with RCMs and two high resolution GCMs is presented. • Three classification methods are used to illustrate the differences between models and observations. • Current high-resolution GCMs can provide climate classifications with the same accuracy as those from RCMs. • Uncertainties in modeling precipitation persist for large parts of the world. [ABSTRACT FROM AUTHOR]

Abstract Elucidating the vertical structures of clouds is key to reducing the uncertainty in cloud-induced climate forcing; however, we do not yet know explicitly about the cloud structures that form over Beijing, China, and how they vary. In this paper, we document vertically resolved cloud structures and their climatological variation based on three-year (1 January 2015 to 31 December 2017) ground-based Ka-band (35 GHz) millimeter-wave cloud radar (MMCR) measurements. Cloud top height (CTH) and radar reflectivity are higher in summer and autumn than in spring and winter. We verified the MMCR measurements of the CTH and cloud base height (CBH) independently based on observations from the Himawari-8 and CL51 ceilometer at the same station, respectively. The CTH decreased as the thickness of the cloud layer increased. This may have been due to the mechanism used to determine the CTH, which was based on data from the satellite infrared channel. There were two peaks in the CBH distribution, located at 0–1 km and 5–6 km, whereas the frequency of the CTH peaked at an altitude of 9–10 km, between 1200 and 1800 LST. Precipitating clouds tended to be located at lower altitudes and exhibited stronger radar reflectivities than non-precipitating clouds. The cloud frequency tended to peak in autumn and reached a minimum in winter. In terms of the seasonality of clouds at various levels, middle- and high-level clouds were dominant over Beijing, with high and low (precipitating) clouds concentrated between May and October. Mid-altitude clouds occurred more frequently in winter and spring. CTH peaked in summer due to the strong solar radiation received by the surface. In contrast, minimum CBH occurred in winter. The cloud heights were higher and radar reflectivities larger than reported in previous studies. These results are correlated with urban warming effects. Understanding the properties of clouds will not only be of benefit to researchers carrying out cloud forcing studies, but will also provide key validation data for climate model simulations. Graphical abstract Unlabelled Image Highlights • The cloud vertical structures in Beijing are elucidated using 3-year observations • Cloud structures from MMCR, Himawari-8 and ceilometer agree with each other very well • Cloud top heights are shown to decrease as the cloud layer thickness increases [ABSTRACT FROM AUTHOR]

Abstract Climate classifications of climate models' outputs have been used to assess environmental changes but systematic analyses of the differences between models, scenarios and classification methods are scarce. Here, the results of applying the most commonly used climate classifications to the outputs of 47 Global Climate Models (GCM) of different physical parameterizations and varied grid size are presented. The extent and intensity of changes for present climate, three different Representative Pathways Scenarios (RCP26, RCP45 and RCP85) and three increasingly-fine classification methods show that there is a consensus between models, and that climate classifications are indeed useful tools to translate physical climatology variables into environmental changes. The main conclusions are that climate classifications can indeed be used to gauge model performance at several grid sizes and that the classification method does not decisively affects the potential global changes in future climates under increasing greenhouse gas emissions. The analyses also reveal that there are several uncertainties that are not attributable to model grid size or to limitations in the reference datasets but more likely to deficiencies in the physics of the models. Highlights • This paper provides a systematic analysis of climate classifications for present and future climates. • Several classifications methods are used to illustrate the consensus between models and observations. • Overall, global models show consistent changes but uncertainties persist with large differences in individual model skill. [ABSTRACT FROM AUTHOR]

Abstract: This is the first paper to report a detailed comparison between ground based measurements of cloud enhanced ultraviolet (UV) radiation at sites located in both hemispheres. The measurements were undertaken at a Northern Hemisphere site (at the campus of the University of Girona in Spain) and a Southern Hemisphere site (campus of the University of Southern Queensland, Toowoomba, Australia). These sites were chosen as they have both simultaneous sky camera and solar radiation (including UV) measurement campaigns operating for at least 5 years over the same time period. Although hemispheric comparisons of UV radiation are not new, this paper focuses on a study of enhancements, including a comparison of the frequency of occurrence and associated cloud types, at the locations with differing climatology. It is found that although there are commonalities between the sites, such as the correlation of the frequency of enhancements with larger solar zenith angle (SZA) both at Girona and Toowoomba, there are differences too, e.g. the frequency of occurrence of the enhancements, and the type of clouds that produce most enhancements (cirrus at Girona, cumulus at Toowoomba). It is suggested that this is due to the differences in latitude coupled with a large amount of Cirriform cloud, especially corresponding to large SZA, at Girona. Possible explanations for this major difference were examined, with the most likely hypothesis relating to multiple scattering due to Cirriform cloud and the subsequent effect on UV radiation on a horizontal surface at large SZA. [Copyright &y& Elsevier]

Abstract: In 1986, the Tornado and Storm Research Organisation (TORRO) developed and published an eleven point Hailstorm Intensity Scale and this has been used to characterise more than 2500 hailstorms known to have occurred in Britain and Ireland since the first documented hailstorm event of 1141 AD; this scale has recently (2005) been revised with additional parameters. Most analyses relate to Great Britain, but reference is made to the more extreme recorded events in Ireland. The most intense hailstorm in these Islands reached intensity H8 on the TORRO international scale which extends from intensities H0 to H10. For the 75 years 1930 to 2004 this paper examines significant, damaging (H2 or more intensity) storms in respect of seasonal frequency and geographical distribution. This paper also discusses the climatology (including diurnal incidence) of the less common, but potentially devastating storms of H5 intensity or more, using the much longer period 1800 to 2004 and with particular attention to hail swath lengths and widths. The synoptic origins for these major events, where known, are discussed, with particular reference to Lamb''s classification of weather types across the British Isles from 1861 onwards. More than 2500 British and Irish hailstorms are recorded in TORRO''s database of which over 1300 are of hailstorm intensity H2 or more, including 156 British storms since 1800 that reached intensity H4–5 or more on the TORRO H scale. The geographical distribution of hailstorms in the British Isles shows that the highest frequency of significant, damaging storms (H2 or more intensity with hailstones usually over 15 mm diameter) is in central and eastern England, with the East Midlands, East Anglia and the lower Thames Valley most conspicuous when the incidences are mapped. Since 1870 the severest (H5 intensity or greater) events have been predominantly associated with cyclonic, southerly or south-easterly weather types; however a significant minority (19%) occurred with an anticyclonic element in the classification. For storms of this severity (e.g. with potential for severe structural damage, pitting of aircraft bodywork and even the risk of serious injuries), reporting will have been more consistent over a longer period. These destructive storms, (the most recent “cluster” of which was between 1992 and 1997) have typically followed a track from the S, SSW or SW to the N, NNE or NE with a swath length often 25 km or more (reaching 335 km in one case) and a swath width sometimes in excess of 10 km. Analysis of the database since 1800 indicates a very conspicuous July maximum for these more extreme (H5 intensity plus) events. [Copyright &y& Elsevier]

Abstract: Records of UV data started in Madrid at the beginning of the 90''s decade. After some quality control on the data a seven year period from 1996 to 2002 was selected to perform an analysis of daily and seasonal variability of solar ultraviolet B (UVB) values at the centre of the Iberian Peninsula. Instruments used are a Brewer MKIV spectrophotometer and a YES UVB-1 broadband pyranometer. Both instruments provided integrated daily values according with the Diffey spectrum although they have different measurement procedures. Data statistics from each instrument for the same period and statistical relationships between daily values from both instruments are shown. As broadband YES provided a record with fewer time gaps, analysis of UVI extreme values is performed using that instrument. The relationship between UVB values with ozone, cloudiness and visible radiation has been described in several papers for different places in the world. In this paper, those relationships are shown for Madrid. A discussion about specific problems found when trying to isolate every effective factor is also included. Some of those relationships could be helpful to retrieve UVB values from other ancillary data as visible radiation and cloudiness. Finally, a detailed study for 10 days in June 1997 when the maximum record of daily values from the present time series was obtained, highlight the close correlation between total ozone content and the daily variability of UVB for similar amounts of incoming radiation. [Copyright &y& Elsevier]

Abstract: A climatology of various parameters associated with severe convective storms has been constructed for Europe. This involves using the reanalysis data base from ERA-40 for the period 1971–2000 and calculating monthly means, variability range and extremes occurrence of fields such as convective available potential energy, convective inhibition energy, mid-tropospheric lapse rate, low-tropospheric moisture content and storm relative helicity for different layers. This process is a first step towards development of a synthetic climatology of European severe weather, and is publicly available at the web site http://ecss.uib.es. Preliminary results derived from these products were presented during the ECSS 2004 conference. This paper is devoted to a more detailed presentation and discussion of the main results. It is hypothesized that preferred areas for severe thunderstorms occurrence in Europe would extend along a zonal belt over the south-central regions, where high helicity associated with the extratropical storm tracks and thermodynamically-favourable profiles established over the southern Atlantic and Mediterranean Sea would most likely be concatenated. Further, this effort has been complemented with a collection of existing reports of significant (at least F2) tornadoes in Europe during the period 1971–2003. We present this data set in this paper and it also can be found at the website http://ecss.uib.es. Thus, the tornado collection can be used to test the appropriateness of the parameters selected for the synthetic climatology. In particular, it is found that the convective available potential energy, low-tropospheric moisture content and environmental shear, when related to the monthly climatology, are reasonably good descriptors of the tornadic environments. [Copyright &y& Elsevier]

The physical properties of hailstones registered by a hailpad network (size distribution, mass, kinetic energy) are essential data for the establishment of a regional hail climatology. Nevertheless, when comparing these data to the same properties of hailstones inside the cloud, the melting process must be taken into account. This paper presents a brief theoretical study of the changes effected on hailstone size distribution due to the melting process. The paper is based on previous studies dealing with the melting of hailstones before they reach the ground. The aim is to analyze the influence of this melting process on hailstone size distribution. An initial melting simulation was carried out in order to achieve this aim. Despite the common assumption that hailstone size distribution on the ground is exponential, it was found that when the in-cloud size distribution is exponential, on the ground, there are fewer small hailstones than what would be expected in an exponential distribution.The data registered by the hailpad network in Leo´n (Spain) for 1 year were used to estimate the size of every hailstone before the melting process. The results show that the hailstone size distribution simulated inside the cloud resembles more closely an exponential distribution than the hailstones on the ground.The type of hailstone size distribution inside the cloud will be the starting point for calculating the hailstone size distribution on the ground. Several equations describing the melting processes are used to calculate a new probability density function that initially corresponds to an exponential distribution that undergoes a partial melting process. The result is a function that is not monotonously decreasing like the exponential function, but rather a function that has a peak for a given size. This new function fits better the data found than the exponential function and actually resembles the gamma function. [Copyright &y& Elsevier]

Understanding the underlying processes causing large-scale dust storms and their transport in North Africa is essential for their accurate prediction. Different atmospheric mechanisms have been identified to govern the emission of dust and its transport, ranging in scale and season, from Rossby wave breaking to local, low-level cold and dry jets. Connecting these features in a Lagrangian sense is a coherent airflow from the midlatitude upper troposphere to the surface where dust concentrations peak. Such dry intrusions (DIs) are linked to reduced static stability and strong, dry winds and indeed have been recently shown to play a central role in four of the largest dust storm in the region. Yet, the climatological link between DIs and dust storms over a long time period has not been studied yet. The aim of this paper is to identify objectively dust events that co-occur with DIs, and understand their spatio-temporal characteristics and underlying dynamical drivers. Combining Lagrangian-based DI identification in ERA-Interim and dust optical depth data in CAMS reanalyses, 325 events during 2003–2018 are identified. The events occur mostly in late winter and spring, when they are also larger in size and last longer, compared to summer events. When occurring with DIs, dust optical depth is generally higher, compared to events that are not accompanied by DIs. We focus on March events, and find coherent large-scale precursors and atmospheric conditions of dust-DI events: a northward jet shift over the North Atlantic with anticyclonic Rossby wave breaking occurs on average 4–5 days prior to the events. The lower troposphere responds with dry and cold conditions in northwest Africa, coinciding with the outflow of the DI airstream that is initially guided by the upper trough. Finally, elevated near-surface dust concentrations prevail on the leading edge of the DI in tropical west Africa, and in northeast Africa, where dust is exported to the Mediterranean in association with a Mediterranean cyclone. • 325 dust storms related to DIs in North Africa were compiled using objective identification and a matching algorithm. • The largest, longest and most frequent events occur in February and March. They are more intense compared to non-DIs events. • Large-scale precursor jet anomalies related to anticyclonic Rossby wave breaking exist 4–5 days prior to the events. • DIs descend from upper-tropospheric anomalies towards the surface in Africa where the highest dust concentrations prevail. [ABSTRACT FROM AUTHOR]

In recent years, climate extreme events have caused increasing direct economic and social losses in the arid region of northwestern China. Based on daily temperature and precipitation data from 1960 to 2010, this paper discussed the linear trend and abrupt changes of climate indices. The general evolution was obtained by the empirical orthogonal function (EOF), the Mann-Kendall test, and the distribution-free cumulative sum chart (CUSUM) test. The results are as follows: (1) climate showed a warming trend at annual and seasonal scale, with all temperature indices exhibiting statistically significant changes. The warm indices have increased, with 1.37%days/decade of warm days (TX90p), 0.17 °C/decade of warmest days (TXx) and 1.97 days/decade of warm spell duration indicator (WSDI), respectively. The cold indices have decreased, with − 1.89%days/decade, 0.65 °C/decade and − 0.66 days/decade for cold nights (TN10p), coldest nights (TNn) and cold spell duration indicator (CSDI), respectively. The precipitation indices have also increased significantly, coupled with the changes of magnitude (max 1-day precipitation amount (RX1day)), frequency (rain day (R0.1)), and duration (consecutive dry days (CDD)). (2) Abrupt changes of the annual regional precipitation indices and the minimum temperature indices were observed around 1986, and that of the maximum temperature indices were observed in 1996. (3) The EOF1 indicated the overall coherent distribution for the whole study area, and its principal component (PC1) was also observed, showing a significant linear trend with an abrupt change, which were in accordance with the regional observation results. EOF2 and EOF3 show contrasts between the southern and northern study areas, and between the eastern and western study areas, respectively, whereas no significant tendency was observed for their PCs. Hence, the climate indices have changed significantly, with linear trends and abrupt changes noted for all climate indices, which will have serious impacts on water resources and ecological environment in the arid region of China. [ABSTRACT FROM AUTHOR]

Although many theoretical and observational studies have investigated the mechanism of conditional symmetric instability (CSI) release and associated it with mesoscale atmospheric phenomena such as frontal precipitation bands, cloud heads in rapidly developing extratropical cyclones and sting jets, its climatology and contribution to precipitation have not been extensively documented. The aim of this paper is to quantify the contribution of CSI release, yielding slantwise convection, to climatological precipitation accumulations for the North Atlantic and western Europe. Case studies reveal that CSI release could be common along cold fronts of mature extratropical cyclones and the North Atlantic storm track is found to be a region with large CSI according to two independent CSI metrics. Correlations of CSI with accumulated precipitation are also large in this region and CSI release is inferred to be occurring about 20% of the total time over depths of over 1 km. We conclude that the inability of current global weather forecast and climate prediction models to represent CSI release (due to insufficient resolution yet lack of subgrid parametrization schemes) may lead to errors in precipitation distributions, particularly in the region of the North Atlantic storm track. [ABSTRACT FROM AUTHOR]

• Long-term air temperature prediction is carried out by a novel two-phased approach. • eXplainable Artificial Intelligence models are applied to better analyse the results. • The approach is applied to the Southern part of the Iberian Peninsula from ERA5 data. • A comprehensive comparison against state-of-the-art techniques has been carried out. • XAI models outperform the compared techniques and benefit from being interpretable. In this paper we have tackled the problem of long-term air temperature prediction with eXplainable Artificial Intelligence (XAI) models. Specifically, we have evaluated the performance of an Artificial Neural Network (ANN) architecture with sigmoidal neurons in the hidden layer, trained by means of an evolutionary algorithm (Evolutionary ANNs, EANNs). This XAI model architecture (XAI-EANN) has been applied to the long-term air temperature prediction at different sub-regions of the South of the Iberian Peninsula. In this case, the average August air temperature has been predicted from ERA5 Reanalysis data variables, obtaining good predictions skills and explainable models in terms of the input climatological variables considered. A cluster analysis has been first carried out in terms of the average air temperature in the zone, in such a way that a number of sub-regions with different air temperature behaviour have been defined. The proposed XAI-EANN model architecture has been applied to each of the defined sub-regions, in order to find significant differences among them, which can be explained with the XAI-EANN models obtained. Finally, a comprehensive comparison against some state-of-the-art techniques has also been carried out, concluding that there are statistically significant differences in terms of accuracy in favour of the proposed XAI-EANN model, which also benefits from being an XAI model. [ABSTRACT FROM AUTHOR]

Based on the 18 realizations of three state-of-the-art climate models participated in the Coupled Model Intercomparison Project phase 6 (CMIP6), this paper examines the changes in climatology and trends of winter diurnal temperature range (DTR) over East Asia in response to natural and anthropogenic forcings. In the observation, the climatology of DTR during 1979–2014 displays a horizontal tile from southwest to northeast over East Asia. In addition, the observed DTR also presents an obviously declining trend in recent decades, with the magnitude of roughly −0.10 °C decade−1. The historical all-forcing simulations well reproduce the geographical distributions and magnitude of climatology and trends of winter DTR over East Asia in 1979–2014. Furthermore, the winter DTR exhibits a continuous declining trend from the historical scenario to the future SSP245 scenario over East Asia, with the most significant declining during the first half of twenty-first century (i.e., 2021–2060). Based on the individual forcing experiments, it suggests that the greenhouse gases forcing plays a primary role in driving the climatology and trends of DTR over East Asia, especially for the SSP245 scenario. Further analysis indicates that the increasing precipitation under greenhouse gases forcing largely explain the declining DTR trends over East Asia, especially over the northern East Asia both in historical and SSP245 scenarios. • The observed decreasing DTR trends are reasonably captured in the all-forcing historical simulations in CMIP6 models. • The greenhouse gas forcing plays a primary role in driving the declining DTR trends over East Asia • The declining of DTR is partly attributed to the increasing precipitation over East Asia in the greenhouse gas forcing. [ABSTRACT FROM AUTHOR]

Research presents an overview on thunderstorm occurrence in Poland and focuses mainly on the relationship between human observations of thunderstorms (SYNOP daily summaries) and instrumental lightning detection data (PERUN network) in the timeframe between 2002 and 2013. The total of 4,952,203 cloud-to-ground flashes (2082 days with thunderstorms) derived from the PERUN lightning database, and 12,419 daily thunderstorm SYNOP reports from 44 meteorological stations (1417 days with thunderstorms) are compared. Within the use of two different computational methods, we define the threshold value of the human average observational thunderstorm detection range within a meteorological station. Results indicate that the average of this value ranges from 16.9 km (Delta computational method) to 18.3 km (threat score computational method). Given the limitations of both methods, we believe that the average of these two (17.5 km) may be the most reliable estimate that expresses how lightning is perceived by humans. Large differences in observational range values between some of the stations (e.g. from 12 km in Bielsko-Biała to 24 km in Łeba) indicate that thunderstorm measurements performed by humans are not homogeneous and are prone to errors. We estimate that an average increase/decrease of observational range by approximately 1 km results in 1 additional/redundant day in the average annual number of thunderstorm days in the climatological sense. Results indicate that already existing thunderstorm climatology papers that are based on SYNOP thunderstorm reports may present presumably not entirely reliable results and overestimate or underestimate values from the real distribution. [ABSTRACT FROM AUTHOR]

Located in a transition zone between the northern and southern climates in China, the Huai River Basin is prone to extreme events such as drought and flood. Based on the daily precipitation data at 134 stations between 1961 and 2013, this paper analyzed the spatial and temporal patterns of the dry and wet conditions in the Huai River Basin through the statistical analysis of the rainfall stations' annual and seasonal standard precipitation index (SPI) series. Annual SPI series exhibited a decreasing trend at 86 stations and an increasing trend at the remaining stations. None of the increasing trend was significant, while the decreasing trend was significant at two stations at 5% significance level (α = 0.05) and one station at 10% level. Seasonal-wise, there has been a prevailing trend of drying in spring and autumn, and wetting in summer and winter. The trends in the spring and summer SPI series have been mostly insignificant, while those in autumn and winter, significant (α = 0.10) at over 30 stations. The Pettitt test results indicated that the significant transitions (α = 0.10) in the autumn and winter SPI series mostly occurred in the middle to late 1980s. Comparison of the average number of dry and wet years between the two sub-periods of 1961–1984 and 1990–2013 suggested a significant increase (α = 0.05) in the average number of severely wet years across much of the basin. Overall, significant changes have already occurred in the dry and wet conditions of the Huai River Basin, which could have profound impacts on the food and water safety situation of the region. [ABSTRACT FROM AUTHOR]

Understanding the lightning climatological distribution is of great significance but remains challenging which is in part due to the lack of continuous and uniform lightning observations. In this paper, for the first time, an 11-year ground-based cloud-to-cloud lightning dataset of China National Lightning Detection Network from 2010 to 2020 is used to investigate the lightning climatology over China's land area in terms of spatial distribution, seasonal, monthly and diurnal variations. Lightning activity in China's land area generally decreases from south to north and from east to west. The mean cloud-to-ground lightning density of China's land area is 0.9 fl km−2 yr−1, with the average maximums (exceeding 10 fl km−2 yr−1) are found in the south and east coastal provinces, such as Guangdong, Hainan, Fujian and Zhejiang Province. The lightning activity in spring, summer and autumn seems consistent throughout the daytime cycle, with the lightning activity is rather weak in the morning and most active in the afternoon, while winter lightning appears to be more active in the nighttime and in the very early morning produced from nocturnal thunderstorms. The cloud-to-ground lightning activity in the tropical and subtropical areas within 15–25°N is mainly concentrated in the late spring and early summer, with two weak peaks in May and August, while lightning in mid latitudes within 25–55°N mainly occurs in the summer (June–July-August). The percentage of positive cloud-to-ground lightning flashes with peak current higher than 75 kA is more than 3 times than that of negative cloud-to-ground lightning flashes. Meanwhile, the mean peak current of cloud-to-ground lightning flashes shows an inverse diurnal pattern to the cloud-to-ground lightning activity. • Cloud-to-ground lightning climatology over Chinese continent is first investigated. • Lightning activity generally decreases from south to north and from east to west. • More lightning occurs in the summer in mid latitudes within 25–55°N. • Winter lightning is unique, usually produced from nocturnal thunderstorms. [ABSTRACT FROM AUTHOR]

Systematic high quality oceanic in-situ precipitation measurements are requested on an international science level and are essential for improved understanding and validation of hydrological processes in satellite, re-analysis and model data. OceanRAIN, the shipboard “Ocean Rainfall And Ice-phase precipitation measurement Network” for surface validation is, to date, the only systematic long-term disdrometer-based oceanic shipboard precipitation data collection effort to establish a comprehensive statistical basis of precipitation for all climate related hotspots over the global oceans. OceanRAIN utilizes automated disdrometer systems (ODM470) capable of measuring precipitation occurrence, intensity and accumulation and discriminates for rain, snow and mixed-phase precipitation through minute-based particle size distributions. The ODM470 was especially designed for shipboard operation under high and frequently varying wind speeds and rough sea states. This paper provides an overview on the OceanRAIN project, the instrumentation, algorithms, methodology, and data products. The procedure of the data processing chain is outlined, including calibration, shipboard operation, data ingest and quality control. The selected research ships do not circumvent high impact weather, allowing for a collection of the full precipitation spectrum including extremes. By October 2014 the fast growing OceanRAIN database comprised more than 3.7 million minutes of precipitation measurements (including true zeros) since its start in 2010. OceanRAIN aims at increasing knowledge about oceanic precipitation, improving error characterization of GPM (Global Precipitation Measurement) era precipitation retrievals, adding to the continual improvement of the satellite retrieval algorithms, as well as benchmarking existing satellite-based climatologies, re-analysis and model data. The accumulating data volume can be utilized for statistical and process study applications on different temporal and spatial scales, microphysical studies of rain and snow formation, and yields insight to the point-to-area problem of precipitation. Information on OceanRAIN including data products is available via http://www.oceanrain.org . [ABSTRACT FROM AUTHOR]

The forecasting of drought based on cumulative influence of rainfall, temperature and evaporation is greatly beneficial for mitigating adverse consequences on water-sensitive sectors such as agriculture, ecosystems, wildlife, tourism, recreation, crop health and hydrologic engineering. Predictive models of drought indices help in assessing water scarcity situations, drought identification and severity characterization. In this paper, we tested the feasibility of the Artificial Neural Network (ANN) as a data-driven model for predicting the monthly Standardized Precipitation and Evapotranspiration Index ( SPEI ) for eight candidate stations in eastern Australia using predictive variable data from 1915 to 2005 (training) and simulated data for the period 2006–2012. The predictive variables were: monthly rainfall totals, mean temperature, minimum temperature, maximum temperature and evapotranspiration, which were supplemented by large-scale climate indices (Southern Oscillation Index, Pacific Decadal Oscillation, Southern Annular Mode and Indian Ocean Dipole) and the Sea Surface Temperatures (Nino 3.0, 3.4 and 4.0). A total of 30 ANN models were developed with 3-layer ANN networks. To determine the best combination of learning algorithms, hidden transfer and output functions of the optimum model, the Levenberg–Marquardt and Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi-Newton backpropagation algorithms were utilized to train the network, tangent and logarithmic sigmoid equations used as the activation functions and the linear, logarithmic and tangent sigmoid equations used as the output function. The best ANN architecture had 18 input neurons, 43 hidden neurons and 1 output neuron, trained using the Levenberg–Marquardt learning algorithm using tangent sigmoid equation as the activation and output functions. An evaluation of the model performance based on statistical rules yielded time-averaged Coefficient of Determination, Root Mean Squared Error and the Mean Absolute Error ranging from 0.9945–0.9990, 0.0466–0.1117, and 0.0013–0.0130, respectively for individual stations. Also, the Willmott's Index of Agreement and the Nash–Sutcliffe Coefficient of Efficiency were between 0.932–0.959 and 0.977–0.998, respectively. When checked for the severity ( S ), duration ( D ) and peak intensity ( I ) of drought events determined from the simulated and observed SPEI , differences in drought parameters ranged from − 1.41–0.64%, − 2.17–1.92% and − 3.21–1.21%, respectively. Based on performance evaluation measures, we aver that the Artificial Neural Network model is a useful data-driven tool for forecasting monthly SPEI and its drought-related properties in the region of study. [ABSTRACT FROM AUTHOR]

This paper establishes and analyses the principal circulation patterns in and around Saudi Arabia (domain: 10°E–70°E; 5°N–40°N) based on the daily mean sea-level pressure (MSLP) 0.75° × 0.75° gridded data derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis ERA-Interim for the period 1979–2012. Association of the circulation patterns to the local climate is investigated using observed daily temperature and rainfall data at 27 locations over Saudi Arabia for the period 1979–2010. Using the widely-used mathematical technique principal component analysis (PCA) we determine with MSLP data the principal patterns. We only analyse data for the wet season (October–May). The automated typing procedure establishes 12 circulation types, which represent all the main synoptic features especially those that originate from the Mediterranean, the European and African continents, Siberia, and also the Red and Arabian seas. There is a strong link between the circulation types and surface climate that is synoptically and spatially interpretable. Particular circulation types are associated with specific climatic conditions across the country: rainfall is linked with Types 9, 10 and 11, whilst warm days (nights) with 1, 2, 3, 9, 10, 11 and 12 (1, 2, 3, 10, 11 and 12), and cold days (nights) with 2, 3, 4, 5, 6, 7, 11 and 12 (2, 4, 5, 6, 7 and 8). In terms of regional influences, the circulation types also exhibit specific links between certain types and climatic divisions across Saudi Arabia. [ABSTRACT FROM AUTHOR]

The paper examines the temporal and spatial distribution of intense convective cores as a function of CAPE and vertical wind shear. C band weather radar data are exploited over the complex orography of Austria. Further ERA-Interim data are used for the classification of synoptic flow and instability. A 5-year period of convective seasons shows the presence of severe thunderstorms over Austria. The spatial distribution of high radar reflectivity differs from the radar derived precipitation field due to the contribution of stratiform rain, weak convective events, and radar related measurement errors. Westerly and southerly flow classes are associated with more widespread thunderstorm development. One of the key results is that the strong deep-layer shear environment leads to organized, line oriented pattern over wide areas of Austria, except the observed minima over the Alpine crest. These preferred areas for severe storm occurrence can be well used for nowcasting. Especially during low CAPE conditions the magnitude of deep-layer shear is very important for the spatial arrangement, maximum size of the convective system, and time of occurrence. For the eastern part of Austria and the Alps, high deep-layer shear tends to produce larger cell cores in terms of high radar reflectivity. For the Alps during low CAPE conditions and for the eastern part of Austria for all CAPE classifications, the strong deep-layer shear increases the frequency of severe storms and shifts the peak of occurrence from afternoon toward the evening. [ABSTRACT FROM AUTHOR]

Santiago de Chile (33°27′ S–70°41′ W) is a mid-latitude city of 6 million inhabitants with a complicated surrounding topography. Aerosol extinction in Santiago is determined by the semi-arid local climate, the urban pollution, a regional subsidence thermal inversion layer, and the boundary-layer wind airflow. In this paper we report on spectral measurements of the surface irradiance (at 290–600 nm wavelength range) carried out during 2013 in the heart of the city by using a double monochromator-based spectroradiometer system. These measurements were used to assess the effect of local aerosols, paying particular attention to the ultraviolet (UV) range. We found that the aerosol optical depth (AOD) exhibited variations likely related to changes in the subsidence thermal inversion and in the boundary-layer winds. Although the AOD at 350 nm typically ranged from 0.2 to 0.3, peak values of about 0.7 were measured. The AOD diminished with the wavelength and typically ranged from 0.1 to 0.2 at 550 nm. Our AOD data were found to be consistent with measurements of the particulate matter (PM) mass concentration. [ABSTRACT FROM AUTHOR]

Satellite data provide the opportunity for systematic and continuous observation of cloud cover over large spatial scales. In this paper, we describe the generation of two new high spatial resolution (0.05°) daytime cloud fraction data sets over Switzerland. The data sets are based on the Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask products. The data sets cover the period from March 1, 2000 to February 29, 2012 (Terra/MODIS) and July 1, 2002 to February 29, 2012 (Aqua/MODIS) and represent mid-morning and early-afternoon cloud cover over Switzerland. Time series clearly reflected seasonal variations in cloud fraction over Switzerland. A comparison with cloud fraction observations at four Synop stations (Chur, Locarno/Monti, Payerne, Zurich/Kloten) revealed an agreement of monthly mean mid-morning cloud fraction (MMCF) within ±1 octa (i.e., 12.5%). Relative to Synop observations, MMCF was positively biased by 0.3–5.0%, except at Payerne (−2.5%). Linear correlation coefficients ranged from 0.878 to 0.972. Results were similar for monthly mean early-afternoon cloud fraction (MACF). Cloud fraction was found to be higher in the early-afternoon when compared to mid-morning, except at Payerne and Zurich/Kloten in fall, which is explained by typical daytime cloud cover patterns in Switzerland. Analysis of daily mid-morning cloud fraction showed that largest discrepancies were observed in partly cloudy conditions, which is mainly explained by differences in observation times and observation geometry. Our results demonstrate that the newly processed cloud fraction data sets from the MODIS sensor can play an important role in complementing traditional Synop observations in support of systematic cloud cover monitoring within the National Climate Observing System (GCOS Switzerland). [ABSTRACT FROM AUTHOR]

Abstract: A drop counter catching-type gauge and a tipping-bucket rain intensity gauge are compared in this paper in order to investigate the performance of suitable correction algorithms. Comparison in the field is performed at the resolution of 1min by assuming the drop counter as a working reference at low to medium rainfall rates. The observed deviations are mainly the result of sampling errors due to the low resolution (0.5mm) of the tipping-bucket. Two correction algorithms for this gauge are compared based on the knowledge of the time of tipping: since this is not actually available for the instrument in hand, numerical simulation is performed using information from the drop counter at the resolution of 10s. The first correction is obtained by distributing the tip volume over the inter-tip periods. The second algorithm further applies a “smoothing” procedure that redistributes the volume of initial, isolated and final types of tips according to climatology. Relative deviations of the one-minute rainfall intensity figures are used to assess improvements in the observed measurement performance. Sensitivity of the results upon varying the sampling resolution of a virtual tipping-bucket rain gauge is also investigated using numerical simulation. It is concluded that the knowledge of the time of tipping is essential to improve the performance of the tipping-bucket rain gauge. Distributing the tip volume over the inter-tip period is indeed sufficient to considerably improve the measurement accuracy, especially at low to medium intensities and a fine sampling resolution. The influence of the correction at high rainfall intensity increases with decreasing sampling resolution. Although the proposed smoothing algorithm provides further improvements at low intensities for gauges characterized by coarse sampling resolution, where the first algorithm is less effective, the results are still affected by a significant spread of deviation. [Copyright &y& Elsevier]

Abstract: Classification of weather maps at various isobaric levels as a methodological tool is used in several problems related to meteorology, climatology, atmospheric pollution and to other fields for many years. Initially the classification was performed manually. The criteria used by the person performing the classification are features of isobars or isopleths of geopotential height, depending on the type of maps to be classified. Although manual classifications integrate the perceptual experience and other unquantifiable qualities of the meteorology specialists involved, these are typically subjective and time consuming. Furthermore, during the last years different approaches of automated methods for atmospheric circulation classification have been proposed, which present automated and so-called objective classifications. In this paper a new method of atmospheric circulation classification of isobaric maps is presented. The method is based on graph theory. It starts with an intelligent prototype selection using an over-partitioning mode of fuzzy c-means (FCM) algorithm, proceeds to a graph formulation for the entire dataset and produces the clusters based on the contemporary dominant sets clustering method. Graph theory is a novel mathematical approach, allowing a more efficient representation of spatially correlated data, compared to the classical Euclidian space representation approaches, used in conventional classification methods. The method has been applied to the classification of 850hPa atmospheric circulation over the Eastern Mediterranean. The evaluation of the automated methods is performed by statistical indexes; results indicate that the classification is adequately comparable with other state-of-the-art automated map classification methods, for a variable number of clusters. [Copyright &y& Elsevier]

Abstract: The raindrop spectra observed in a precipitation system is a complex phenomenon that can help to explain the underlying physical processes of rainfall. This paper explores the characteristics of raindrop spectra in terms of drop size distributions (DSD) using seven years of Joss–Waldvogel disdrometer data within the mid-latitude UK region climatology. A total of 162,415 one-minute “filtered” raindrop spectra obtained from the disdrometer are fitted into a normalized gamma DSD model describing DSDs by the concentration parameter (N w ), the drop diameter (D m and D 0 ), and the shape parameter (μ). The results show that the rain rates retrieved from the normalized gamma DSD model are in good agreement with the disdrometer measured rain rates, implying the appropriateness of the raindrop spectra as normalized gamma distributions. The DSD characteristics are studied in different seasonal (“cold” and “warm”), atmospheric (“dry” and “wet”) as well as rain type (“stratiform” and “convective”) contexts in a long-term perspective. It has been revealed that the normalized gamma DSD parameters are very sensitive to the rain intensities. The mass weighted mean drop diameter D m clearly increases exponentially with respect to the rain intensities. Variation of the DSDs in different contexts is also exposed reflecting seasonal, atmospheric and rain type consequence on raindrop spectra. Particularly, the scatterplot between the concentration parameter log10 N w and the median drop diameter D 0 exhibits clear separation index between stratiform and convective DSDs. There is a large difference in averaged mass weighted mean drop diameters among stratiform and convective segments (stratiform < D m > = 0.988mm versus convective < D m > = 1.99mm). The DSD inconsistency in different contexts have been further examined in terms of the Z–R relationships (Z=aR b ) variability. Additionally, 10 selected events taken place during the study period are also investigated, in which each of the events has shown unique DSD characteristics. [Copyright &y& Elsevier]

Abstract: This paper discusses the potential of Regional Climate Models (RCMs) as reanalysis tools by presenting a reconstruction of the European climate using several RCMs with diverse physical parameterizations. The use of RCMs is intended to increase the spatial resolution of the analysis provided by Global Models through dynamic downscaling. At the same time, the use of several models allows us to characterize the uncertainties, as these can be estimated from the spread of the ensemble. When the RCMs are nested in reanalyses instead of in a Global Model it is possible to create climatologies of unprecedented robustness for variables such as temperature, precipitation, wind speed, and humidity, among others. While these climatologies are subject to further improvement as methods and computing power evolve, they point the way forward to the development of atmospheric information products suitable for a variety of studies including education, agriculture, renewable energies and climate change research, biogeography, insurance, risk assessment, hydrology, and regional planning. [Copyright &y& Elsevier]

Abstract: One of the most important problems in meteorology, physical oceanography, climatology, and related fields is the understanding and dynamical description of multi-scale interactions. Multi-scale interactions are closely related to extreme events in climate and, therefore, of great practical importance. Here an extreme event is defined in terms of the non-Gaussian tail (sometimes also called a weather or climate regime) of the data''s probability density function (PDF), as opposed to the definition in extreme value theory, where the statistics of time series maxima (and minima) in a given time interval are studied. The non-Gaussian approach used here allows for a dynamical view of extreme events in weather and climate, going beyond the solely mathematical arguments of extreme value theory. Extreme events are by definition scarce, but they can have a significant impact on people and countries in the affected regions. Understanding extremes has become an important objective in weather/climate variability research because weather and climate risk assessment depends on knowing the tails of PDFs. In recent years, new tools that make use of advanced stochastic theory have evolved to evaluate extreme events and the physics that govern these events. Stochastic methods are ideal to study multi-scale interactions and extreme events because they link vastly different time and spatial scales. One theory attributes extreme anomalies to stochastically forced dynamics, where, to model nonlinear interactions, the strength of the stochastic forcing depends on the flow itself (multiplicative noise). This closure assumption follows naturally from the general form of the equations of motion. Because stochastic theory makes clear and testable predictions about non-Gaussian variability, the multiplicative noise hypothesis can be verified by analyzing the detailed non-Gaussian statistics of atmospheric and oceanic variability. This review paper discusses the theoretical framework, observational evidence, and related developments in stochastic modeling of extreme events in weather and climate. [Copyright &y& Elsevier]

Abstract: The South American Low-Level Jet Experiment (SALLJEX) provided a unique dataset to investigate the existence of a mesoscale low-level circulation and of a diurnal cycle in its related convergence pattern over the Southeastern South American region east of the Andes, as well as its relationship with deep convection during the warm season. The present paper builds upon high-resolution analyses produced, assimilating the data collected during the SALLJEX field campaign using BRAMS, and explores their capability to reproduce mesoscale circulations not resolved by the low density observational network available in this region. Results of the analyses show a diurnal oscillation signal in the mean boundary layer convergence pattern over the plains with a nocturnal (daytime) convergence (divergence) maximum. These results are coherent with previous findings of a nocturnal phase in the mature stage of organized deep convection and related precipitation in subtropical latitudes east of the Andes during the warm season. The diurnal cycle of convergence/divergence in the boundary layer is described over a 15-day period, during which different synoptic conditions occurred. During weakly forced environments a regime characterized by nocturnal eastward anomaly flow and convergence and daytime westward anomaly flow and divergence related to a mesoscale northwestern mountain–central plain flow regime dominates over the plains between the Andes Mountains, the Parana River Valley, and the southern Brazil mountain range. In contrast, during synoptic conditions dominated by the presence of a deep thermal low over northwestern Argentina and a related low-level jet, convergence at night is mainly accomplished by the predominantly meridional low-level jet, which exhibits an anomalous weak wind speed diurnal cycle with respect to its summer climatological mean. On the other hand daytime divergence is completely produced by the zonal wind component as in the previous synoptic situation. Mesoscale circulations are altered (still effecting mean divergence in the domain, which exhibits a diurnal oscillation) upon the initiation of deep convective circulations in the evening in an increasingly convectively unstable atmosphere driven by a persistent horizontal advection of heat and moisture at low levels and forced by convergence generated by the low-level jet and the presence of a frontal zone. Convection intensifies at night when its related convergence over the plains comes in phase with the convergence related to the nocturnal maximum in the low-level jet. [Copyright &y& Elsevier]

Abstract: This paper presents the spatial and temporal distribution of cloud-to-ground lightning over Estonia and the adjacent sea for the period 2005–2009. Data collected by the NORDLIS (NORDic Lightning Information System) lightning detection network was used. The spatial distribution of lightning was calculated in a 10×10km grid. A total of 172,613 cloud-to-ground flashes were registered in the area under observation during 2005–2009. The annual mean cloud-to-ground flash density over the area was 0.34 flashes km−2 year−1. The lowest values found were less than 0.10 flashes km−2 year−1 and the highest flash densities were 0.80–1.01 flashes km−2 year−1. The monthly distribution of lightning showed the highest activity in July and August. Of all the registered flashes, 99.4% were reported from May to October. The daily distribution of flashes showed single days on which thunderstorm activity was very high, against a background of much lower everyday activity. The diurnal distribution of lightning showed an evident peak between 15:00 and 17:00 local time over land. Over the sea, a flatter maximum lay between 13:00 and 21:00. Minimum lightning activity occurred between 22:00 and 06:00 over land and from 02:00 to 09:00 over the sea. Our work revealed that the spatial and temporal characteristics of cloud-to-ground lightning over Estonia generally resemble the characteristics found at other mid-latitude study sites. [Copyright &y& Elsevier]

Abstract: A previous hypothesis advanced from observational studies such as METROMEX suggests that the intensity, frequency, and organization of cumulus convection may be impacted by the forcing of enhanced merger activity downstream of urban zones. A resulting corollary is that cities may exert an indirect anthropogenic “forcing” of parameters related to convection and associated phenomena such as lightning and precipitation. This paper investigates the urban-merger hypothesis by examining the role of convective cell mergers on the existence and persistence of the Houston lightning “anomaly”, a local maximum in cloud-to-ground (CG) lightning activity documented to exist over and east of Houston. Using eight summer seasons of peak columnar radar reflectivity, CG lightning data and a cell-tracking algorithm, a two-dimensional cell merger climatology is created for portions of Eastern Texas and Louisiana. Results from the tracking and analysis of over 3.8 million cells indicate that merger-driven enhancements in convection induce a positive response (O 46%) in ground flash densities throughout the domain, with areas of enhanced lightning typically being co-located with areas of enhanced merger activity. However, while mergers over the Houston area (relative to elsewhere in the domain) do result in more vigorous convective cells that produce larger CG flash densities, we find that CG lightning contributions due to mergers are distributed similarly throughout the domain. Hence while we demonstrate that cell mergers do greatly impact the production of lightning, the urban cell merger hypothesis does not uniquely explain the presence of a local lightning maximum near and downstream of Houston. [Copyright &y& Elsevier]

Abstract: This paper presents the climatology of severe local convective storms in Bangladesh. We collected sufficient severe local convective storm events over a long period (2,324 events from 1990 to 2005) to compile reliable climatological statistics by surveying the literature. Severe local convective storms frequently occur during the pre-monsoon season from March to May, peaking in April. The highest frequency was found to be at 16–17 Local Standard Time (LST) and 20–21 LST, with lower activity around midnight and early morning. It was also found that the occurrence of severe local convective storms is more concentrated in the central region of Bangladesh, and tends to spread northward during the pre-monsoon season. [Copyright &y& Elsevier]

Abstract: This review paper deals with four aspects of precipitation: measurement, remote sensing, climatology and modeling. The measurement of precipitation is summarized in terms of the instruments that count and measure drop sizes (defined as disdrometers) and the instruments that measure an average quantity proportional to the integrated volume of an ensemble of raindrops (these instruments are normally called rain gauges). Remote sensing of precipitation is accomplished with ground based radar and from satellite retrievals and these two approaches are separately discussed. The climatology of precipitation has evolved through the years from the traditional rain gauge data analyses to the more sophisticated data bases that result from a coalescence of data and information on precipitation that is available from several sources into amalgamated products. Recently, rain observations from both ground and space have been assimilated into regional and global numerical weather prediction models aiming at improved moisture analysis and better forecasts of extreme weather events. The current status and the main outstanding issues related to precipitation forecasting are discussed, providing a basic structure for research coordination aimed at the improvement of modeling, observation and data assimilation applicable to global and regional scales. [Copyright &y& Elsevier]

Abstract: Since climate trends are getting considerable attention in recent years, we aimed in this study to compare trends and rhythms of complexity (fractal dimension, FD) of rainfall data series between two continents: Latin America and Europe. Two parallel nonlinear methods for calculating FD of a temporal data series, Higuchi''s and consecutive differences, were combined with Fast Fourier Transform (FFT) to obtain FD oscillations of monthly accumulated rainfall. The data were collected for the last thirty years in Pastaza province, Ecuador and Veneto province, Italy. In order to calculate their FD time dependence, FD(t), moving windows of different lengths (short, 10–20 and long 21–350 samples), were applied. Both methods, each combined with FFT, detected identical (or very similar) rhythms of detrended FD(t) in the two data series, but frequencies with dominant amplitudes differed (4.4 years in Ecuador, 10.3 years in Italy). Long-term FD(t) trends were also studied using optimized long window lengths (~200 samples). A linear positive trend was obtained for the Ecuadorian rainfall data over the whole recorded period. Italian fractal trend profile was, however, characterized by two periods: a constant high value for years 1974–1993, followed by a linear decrease for 1993–2005. Trend results, obtained with two different methods, were also similar. Accordance of the results, reported in the present paper by applying these two methods, validates their use as a tool in future fractal meteorological measurements. As well, these results indicate that positive FD trend obtained for Pastaza (Ecuador) and negative trend computed for Veneto (Italy) account for a local or regional phenomenon, most probably caused by extensive deforestation and land use change (Ecuador) and continental or global atmospheric pattern variability (Italy). [Copyright &y& Elsevier]

Abstract: Severe thunderstorms constitute a major weather hazard in Europe, with an estimated total damage of 5–8 billion euros each year nowadays. Even though there is an upward trend in damage due to increases in vulnerability and possibly also due to climate change impacts, a pan-European database of severe thunderstorm reports in a homogeneous data format did not exist until a few years ago. The development of this European Severe Weather Database (ESWD) provided the final impetus for the establishment of the European Severe Storms Laboratory (ESSL) as a non-profit research organisation in 2006, after having started as an informal network in 2002. Our paper provides an overview of the first research results that have been achieved by ESSL. We start by outlining the reporting practice and quality-control procedure for the database, which has been enhanced by a major software upgrade in the fall of 2008. It becomes apparent that the state of reporting converges to a realistic description of the severe storms climatology, corroborating, for instance, earlier estimates of tornado occurrence in Europe. Nevertheless, a further rise in the number of reported events must be expected, even without the presence of any physical trends. The European tornado and damaging wind intensity distributions as a function of the Fujita scale are quantitatively similar to long-term distributions from the USA, except for a strong underreporting of weak events (F0) that still persists in Europe. In addition, the ESSL has recently proposed a new wind speed scale, the Energy- or “E-scale” which is linked to physical quantities and can be calibrated. Finally, we demonstrate the large potential of ESWD data use for forecast or nowcasting/warning verification purposes. [Copyright &y& Elsevier]

Abstract: The main goal of the present paper is to discuss the asymmetry of the rainfall distribution over the mountain range of the Fouta Djallon — Guinea — in relation with orography and local atmospheric circulation. The Fouta Djallon is the main West African hydrographic source area. The two principal rivers (Niger and Senegal) take birth in this region where, in addition, one of the highest cumulative rain heights in the world (>10 m) is observed. The data used are images of the Meteosat satellite, reanalyses of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) and ECMWF (European Centre for Medium-Range Weather Forecasts), and observations from ground-based raingauge networks. Over the Fouta Djallon, the convective activity is found to increase in frequency and intensity due to the orographic forcing of water vapor in an area of convergence between monsoon and Harmattan fluxes; heavy rain is released over the southwest side of the range. A conceptual model is proposed suggesting that convective systems developing over or east of the range are advected westward of the generating area, resulting in a strong pluviometric contrast and a maximum offshore. [Copyright &y& Elsevier]

Abstract: Due to the significant amount of severe storm damage from the mid 1990s, a practical need has arisen for updating risk assessment. For reliable and systematic sampling of events, data acquisition has been arranged through the disaster management official body using a pyramidal national coverage. Post-analysis, including its meteorological part, proceeds in a GIS environment. This paper focuses specifically on damaging tornadoes, since those are the most violent and best-documented phenomena. Different statistics are calculated and explained, such as seasonal, diurnal and magnitude distributions. Spatial occurrence and features are mapped. A complete synoptic climatology is given by typifying the generating conditions and categorizing events into certain classes, while discussing the role of the Carpathian Basin. In the end a conceptual issue in connection with self-similarity is raised for further discussion. [Copyright &y& Elsevier]

Abstract: The objective of this study was to investigate the correlation of visibility with chemical composition of Kaohsiung aerosols. Daytime visibility was observed around noon at two observation sites in metropolitan Kaohsiung, Taiwan in the years of 1999 and 2000. Both seasonal and diurnal variation patterns of visibility were observed in the region. Ambient aerosols were sampled and analyzed for 11 constituents, including water-soluble ionic species (Cl−, NO3 −, SO4 −2, NH4 +, K+, Na+, Ca+2, and Mg+2) and carbonaceous contents (OC, EC, and TC), to characterize the chemical composition of Kaohsiung aerosols. Furthermore, a stepwise multiple linear regression model was developed to elucidate the influence of aerosol species on visibility impairments. The results showed that sulfate was the dominant species that affected both light scattering coefficient and visibility. On average, the percentage contributions of visibility degrading species to light scattering coefficient were 29% for sulfate, 28% for nitrate, 22% for total carbon, and 21% for PM2.5-remainder. An empirical regression model of visibility based on sulfate, nitrate, and relative humidity was also developed. The model showed that sulfate in PM2.5 was the most sensitive species to visibility variation, suggesting that the reduction of sulfate in PM2.5 could effectively improve the visibility of metropolitan Kaohsiung. During the investigation period, an event of Asian dusts intruded metropolitan Kaohsiung and dramatically increased the aerosol loadings, especially in the coarse particles. However, local visual air quality did not degrade accordingly during the Asian dust event because both visibility and light scattering coefficient are affected mainly by the fine particles. The results are discussed in detail in the paper. [Copyright &y& Elsevier]

Abstract: In the framework of the Cloud Liquid Water Network (CLIWA-NET) project, an attempt is made to reconstruct the spatial distribution of liquid water path (LWP) by combining data from a network of ground-based microwave radiometers with imagery of the Advanced Very High Resolution Radiometer (AVHRR). In order to provide atmospheric modelers sufficient high-quality reference data for model evaluation and improvement, measurements from three intensive measurement campaigns are analyzed. This paper describes the AVHRR quantitative cloud analysis environment that was developed and tested at the Royal Netherlands Meteorological Institute (KNMI) during the project. The environment is named KLAROS (KNMI Local Implementation of APOLLO in an Operational System). KLAROS includes retrieval of a suite of physical cloud parameters. In order to evaluate the liquid water path retrieval, ground-based radiometer measurements are used. A new technique, called the Russian doll method, is introduced to assess the relation between ground and satellite measurements. This relation depends on the variability in time and space of the observed cloud fields and shows up clearly in the Russian doll diagrams. First, results from the comparisons give confidence that KLAROS gives plausible liquid water path values. We find that AVHHR data from the NOAA-16 platform is much better suited for quantitative cloud analysis than its predecessors. In particular, the availability of a 1.6-μm channel and the good calibration contribute to the improved accuracies. The retrieval method for liquid water path will be implemented in the framework of the Satellite Application Facility of Climate Monitoring (CM-SAF). [Copyright &y& Elsevier]

Tornadoes and waterspouts occur in various parts of Greece and may often reach the media as remarkable events, depending of their intensity and the level of damage. This paper is based on a research started from the beginning of 2000 for identifying and recording tornadoes and waterspouts in Greece and evaluating their intensity and other fundamental characteristics. Search of historical events for obtaining more comprehensive tornado climatology is in progress. In an almost 3-year period, January 2000–September 2002, tornadoes and waterspouts have been reported from various parts of Greece, including western Greece, the Ionian islands and western Peloponnese mainly during autumn and winter. Northern Greece is affected by tornadic activity mostly during summer and early autumn. The geographical distribution showed a preference for tornado occurrence mainly in coastal areas. Based on this almost 3-year data sample, a mean number of six tornadoes per year has resulted, corresponding to a density of 0.45 10−4 km−2 year−1. Maximum intensity of tornadoes recorded was rated as T6 of the Tornado and Storm Research Organization (TORRO) scale (F3 of the Fujita scale). A basic synoptic typing scheme is presented, including four synoptic circulation patterns associated to tornado occurrence. [Copyright &y& Elsevier]

A brief overview of severe wind/hail storms over Bulgaria in the last 5 years (1997–2001) is presented. Synoptic and thermodynamic analyses of 27 cases during 1999–2001 (April–September) were performed. Among others, two events during this period produced extremely damaging winds. The first of them was associated with a tornado close to the Mediterranean coast on 15 May 1999 and has been already described in the literature.This paper illustrates the role of synoptic scale factors for generation of a second severe windstorm (that occurred near the Bulgarian capital Sofia on 22 May 2001) by using numerical model analysis fields. On the large scale, the circulation pattern was associated with a deformation middle-tropospheric field over the North Atlantic, Western Europe and Mediterranean. A zone of convergence in temperature and mass fields was formed over the Balkans, where there was an interleaving of warm, moist Mediterranean air advected in association with a warm front and cold dry air coming from the northwest in the eastern flank of an upper-level ridge. This synoptic circulation pattern produced deep convection along the boundary between the polar and subtropical high-level frontal systems. It is shown that synoptic- and meso-scale elements of the development may be revealed or confirmed by radiosounding data as well as by satellite and radar imagery. Data from the upper-air sounding of Sofia at 1200 UTC (available up to 650 hPa level) and other proximity soundings were processed and analysed. The obtained results show very high values of atmospheric instability indices (the energy of instability Ei=3785 J/kg, TT=59 °C, Z(ULC)=12.6 km). The updraft was up to 21.9 m/s and the difference of the wind velocity between 300- and 700-hPa levels was up to 12 m/s. Cloud water content reached up to 12.9 g/kg. [Copyright &y& Elsevier]

The main aim of this study was to present thunderstorms' occurrence in Poland in the 20th century. The paper also attempts to assess the impact of atmospheric circulation on the variability of number of days with thunderstorms and estimate synoptic conditions conducive to the formation of thunderstorms. The analysis was based on thunderstorm observations from 56 synoptic stations in Poland in the period 1951–2000 and 7 stations for the years 1885–2000. In the second part of the study, the types of synoptic situations elaborated by Osuchowska-Klein, the types of synoptic situations by T. Niedz´wiedz´, indices of atmospheric circulation by Z. Ustrnul and index of North-Atlantic Oscillation (NAO).As a result, it was determined that during the analysed period, no tendency was found in the multi-annual course of mean yearly number of days with thunderstorms in Poland, and the values usually do not differ much from the value typical for the climatic–geographical region in which Poland is situated. Synoptic situations have less influence on thunderstorm occurrence than that usually suspected. The eastern advection has the largest influence on the occurrence of the days with thunderstorms, especially in southern Poland. The analysis of relations between the variability of number of days with thunderstorms and the changes of the circulation indices proved these regularities. [Copyright &y& Elsevier]

Thunderstorms in the territories of the Czech Republic and neighbouring countries are almost exclusively the only phenomena occurring in the warm season. In the cold half of the year, from October to March, an average incidence of thunderstorms is only 2%, with the least occurrence being recorded in January. Yet, winter thunderstorms are dangerous particularly for air traffic because during them, the cloud base is rapidly falling down and visibility is suddenly worsening due to heavy snowfall. Notwithstanding these facts, the issue of their occurrence in the central European space has been paid little attention so far.Long years of study into historical weather extremes in the territory of the Czech Republic revealed over 10 chronicle entries on the occurrence of winter thunderstorms in the period between November and February from the 16th to the beginning of the 20th centuries. The irregular phenomenon was even devoted three occasional prints in central Europe in the second half of the 16th century, two of which were issued in Germany. Fires caused by winter thunderstorms were no sporadic cases.The occurrence of thunderstorms in winter was apparently associated with the passage of pronounced cold fronts. This can be documented on cases from the end of December 1555 when heavy thunderstorms and consequent fires were recorded within a short period of time in Holland, Germany and in Czech lands. It is assumed that the situation in 1627 was similar when a winter thunderstorm was recorded in Prague and in Holesˇov, southeastern Moravia on 28 December. In February 1581, a thunderstorm in Prague became one of three unusual events publicized by the local occasional newspaper.The beginning of modern studies into winter thunderstorms dates back to the 1960s with the use of lightning flash counters and later also with the use of systems for large-scale lightning flash detection and localization. However, more comprehensive meteorological and climatological assessments of their occurrence are still missing. The authors of the paper aim at outlining possibilities of the incidence of winter thunderstorms in the present and at contributing with some answers to the question of the long-term fluctuation of their frequency. [Copyright &y& Elsevier]