Skill of 6HR and 12HR thunderstorm forecasts based on assessment of larger-scale information

Summary The relationship between information, contained in aerological data from the European area, and a thunderstorm occurrence in the area of the Czech Republic was investigated with input data from the period of May–September 1989–1991. SYNOP reports from Czech ground stations were utilized to assess event occurrence. TEMP 00UTC and TEMP 12UTC reports from European stations were used to determine potential diagnostic predictors, and the TEMP00 data served as the input data set for the 12hr mesoscale model forecast to gain prognostic predictors. Each of the two diagnostic data sets from 00UTC and 12UTC and of the prognostic data set comprised about 400 predictors/predictand elements. The categorical forecast of thunderstorm occurrence, based on the application of linear regression and a simple version of pattern recognition, is discussed. The critical success index was determined for every type of forecast and used to assess forecast skill.     

On very-low-frequency atmospherics at Roburent,Northern Italy

Summary The diurnal and seasonal variation of the VLF sferics pulse rate recorded at Roburent, Northern Italy, on 27 kHz since 1966 is derived. By analyzing the sunrise effect, the minimum distance of the thunderstorm centers providing at that time an unperturbed behaviour of the sferics pulse rate is obtained: this distance varies in azimuth and with the seasons and lies between 600 and 1300 km. In the time of the equinoxes, however, it is more stable between 700 and 900 km. The role of the groundwave propagation, generated by thunderstorm center within a radius of less than 500 km, is emphasized.

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Zusammenfassung Der Tagesgang und der Jahresgang der Sferics-Impulshäufigkeit werden abgeleitet. Diese Sferics-Häufigkeit ist seit 1966 auf 27 kHz in Roburent, in Nord-Italien, registriert worden. Der Mindestabstand der Gewitterherde, welche zur Zeit des Sonnenaufgangs ein ungestörte Bild der Impulshäufigkeit geben, wird mit Hilfe des Sonnenaufgangseffekts abgeleitet: der Abstand liegt — abhängig von der Jahreszeit und der Einfallsrichtung — zwischen 600 und 1300 km; jedoch bewegt er sich in dem engeren Rahmen zwischen 700 und 900 km zur Zeit der Tag-und Nachtgleichen. Betont wird die Rolle der Bodenwelle, welche von Gewitterherden in einem Abstand von weniger als 500 km erzeugt wird.

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Contribution No. 241 of the Istituto Geofisico e Geodetico, Università di Genova,Genoa, Italy. This work has been supported by a grant from the Consiglio Nazionale delle Ricerche (Comitato Nazionale per le Scienze Fisiche), Roma.     

Variations in thunderstorm VLF emissions propagating over epicenters of earthquakes

In this study, the variations of average amplitude of the very-low-frequency (VLF) signal thunderstorm origin propagating over areas of strong (magnitude >5) earthquakes are considered. Despite the non-stationary state of thunderstorms, a decrease in the amplitude of its VLF signals was usually detected 3–6 days before an earthquake, with subsequent recovery on the day of the event. The effect is similar to the attenuation of the signal amplitude of low-frequency radio stations that is observed for several days before the occurrence of an earthquake. These influences of earthquakes on thunderstorm VLF signals are recorded along different paths (different azimuths and different distances to the earthquake epicenter). The probability that an earthquake of strong magnitude will induce changes in the thunderstorm VLF signal is 60–70%.     

The June 2013 Alberta Catastrophic Flooding Event: Part 1—Climatological aspects and hydrometeorological features

In June 2013, excessive rainfall associated with an intense weather system triggered severe flooding in southern Alberta, which became the costliest natural disaster in Canadian history. This article provides an overview of the climatological aspects and large‐scale hydrometeorological features associated with the flooding event based upon information from a variety of sources, including satellite data, upper air soundings, surface observations and operational model analyses. The results show that multiple factors combined to create this unusually severe event. The event was characterized by a slow‐moving upper level low pressure system west of Alberta, blocked by an upper level ridge, while an associated well‐organized surface low pressure system kept southern Alberta, especially the eastern slopes of the Rocky Mountains, in continuous precipitation for up to two days. Results from air parcel trajectory analysis show that a significant amount of the moisture originated from the central Great Plains, transported into Alberta by a southeasterly low level jet. The event was first dominated by significant thunderstorm activity, and then evolved into continuous precipitation supported by the synoptic‐scale low pressure system. Both the thunderstorm activity and upslope winds associated with the low pressure system produced large rainfall amounts. A comparison with previous similar events occurring in the same region suggests that the synoptic‐scale features associated with the 2013 rainfall event were not particularly intense; however, its storm environment was the most convectively unstable. The system also exhibited a relatively high freezing level, which resulted in rain, rather than snow, mainly falling over the still snow‐covered mountainous areas. Melting associated with this rain‐on‐snow scenario likely contributed to downstream flooding. Furthermore, above‐normal snowfall in the preceding spring helped to maintain snow in the high‐elevation areas, which facilitated the rain‐on‐snow event. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatial and temporal characteristics of droughts in the western part of Bangladesh

Spatial and temporal characteristics of droughts in the western part of Bangladesh have been analysed. Standardized precipitation index method is used to compute the severity of droughts from the rainfall data recorded in 12 rainfall gauge stations for the period of 1961–1999. An artificial neural network is used to estimate missing rainfall data. Geographic Information System (GIS) is used to map the spatial extent of droughts of different severities in multiple time scales. Critical analysis of rainfall is also carried to find the minimum monsoon and dry months rainfall require in different parts of the study area to avoid rainfall deficit. The study shows that the north and north‐western parts of Bangladesh are most vulnerable to droughts. A significant negative relationship between multiple ENSO index and rainfall is observed in some stations. Analysis of seasonal rainfall distribution, rainfall reliability and long‐term rainfall trend is also conducted to aid prediction of future droughts in the area. Copyright © 2007 John Wiley & Sons, Ltd.     

Solar flares and their impact on potential gradient and air-earth current characteristics at high mountain stations

Summary Recordings of potential gradient and air-earth current in fair-weather conditions were made at high-elevation mountain stations; the latter were generally above the vertical-mixing layer and not disturbed by local sources. Both electrical quantities increased significantly from the day of appearance of a solar flare (or a maximum incidence of flares) till the fourth day after the event. Peak potential gradient and air-earth current exceed the normal levels measured during quiet-sun conditions by 50–60% in terms of averages. This result was compared with sferics incidence data and daily geomagnetic coefficients grouped around the selected (flare) dates. Both of the latter quantities show time histories (for the days preceding and following the selected dates) closely correlated with the atmospheric-electrical histories of the same intervals. We may conclude from these results that the total potential between the ground and the upper atmosphere increases for several days after solar flares. The results obtained by previous studies on Mauna Loa confirm this inference. The cause of the phenomenon is presumed to be enhanced thunderstorm activity attendant on solar disturbances.We wish to thank the Deutsche Forschungsgemeinschaft for a grant taht enabled us to carry out the studies described in this paper     

The response of thunderstorm VLF radio noise to solar activity according to observations in Yakutsk

The relation between the long-term variations in the thunderstorm VLF radio noise intensity and solar activity in 1979–2006 has been studied. The sunspot number (the Wolf number) was used as a parameter characterizing solar activity. The intensity of VLF noise registered in Yakutsk characterizes thunderstorm activity in Eastern Siberia (0100–0500 UT) and in the African thunderstorm center (1300–1700 UT). Using the results of a correlation analysis, it has been found that thunderstorm activity in Eastern Siberia and in the African world centre is in antiphase with a change in the sunspot number. The highest anticorrelation coefficients between solar activity and thunderstorm discharge intensity were obtained for thunderstorms in Eastern Siberia. In this case the maximal correlation coefficients (R = −0.59 and −0.75) were obtained for the average monthly values of the VLF radio noise intensity in August, measured at 0400 UT and 1600 UT, respectively.     

Analysis of recent changes in rainfall and drought indices in Nigeria, 1981–2015

ABSTRACT

This study assesses the climate change impact on rainfall and drought incidents across Nigeria. Linear regression, Mann-Kendall tests and lag-1 serial correlation were adopted to analyse the trends and variability of rainfall and drought at 18 synoptic stations. Analysis of annual precipitation series indicates an increase in rainfall amounts at all stations, except Minna, Gusau and Yola. Seventeen of the 18 stations recorded at least one main drought period, between 1983 and 1987. A decreasing trend for the standardized precipitation index SPI-12 series was seen at Yola station, while the other stations showed an increasing trend. Also, Nigeria witnessed more annual rainfall totals but with high variability within the rainy months of the year in the first 15 years of the 21st century compared to the 20th century. Such variability in rainfall may have a significant effect on groundwater resources and the hydrology of Nigeria.     

Further evidence for impact of solar flares on potential gradient and air-earth current characteristics at high mountain stations

Summary For the period February 1967–May 1969, a period of increasing solar activity, unrestricted substantiation was obtained of earlier results which had indicated that subsequent to H-flares potential gradient and air-earth current will increase significantly. A presupposition of this result is that the atmospheric electrical elements are recorded at stations with only minimal local climatic and aerosol physical disturbance factors, and that only meteorologically undisturbed days are used. It was ascertained that the exclusive use of fine weather days is of no influence on the results. The confirmed solar-terrestrial relation is even more pronounced when measurements are based on flares in the vicinity of the central meridian of the sun. It was further substantiated that the frequency of sferics pulses is significantly increased from the flare day on. As the cause for this a solar-triggered increase in thunderstorm frequency is suggested which, in turn, leads to an increase of the potential gradient and earth current. Proton flares have an effect analogous to that of H-flares. If H-flare key days are used on which significant changes were observed in the distribution of pressure over the northern hemisphere, a significant increase of potential gradient and air-earth current is found even with these flare key days. Thus for the first time a connection is made between solar-caused atmospheric electrical effects and synchronous global meteorological changes.     

Number and volume raindrop size distributions in Taiwan

Knowledge of rainfall characteristics is very important for the accurate estimation of rainfall kinetic energy and prediction of soil erosion. In this study, a reliable and efficient data collection and analysis system was developed to analyse the natural raindrop data collected in subtropical Taiwan. Both raindrop size distributions by number and volume were carefully analysed. The seasonal variations of the rainfall erosivity factor R, which is an index of the erosive potential of rainfall and a function of rainfall kinetic energy, was also discussed. An isoerodent map of Taiwan was also developed based on the rainfall data recorded by 158 automated rainfall‐measuring stations within 26 years. Copyright © 2007 John Wiley & Sons, Ltd.     

Some aspects of the monsoon circulation and monsoon rainfall

Summary The south Asian summer monsoon from June to September accounts for the greater part of the annual rainfall over most of India and southeast Asia. The evolution of the summer and winter monsoon circulations over India is examined on the basis of the surface and upper air data of stations across India. The salient features of the seasonal reversals of temperature and pressure gradients and winds and the seasonal and synoptic fluctuations of atmospheric humidity are discussed. The space-time variations of rainfall are considered with the help of climatic pentad rainfall charts and diagrams. The rainfall of several north and central Indian stations shows a minimum around mid-August and a maximum around mid-February which seem to be connected with the extreme summer and winter positions of the ITCZ and the associated north-south shifts in the seasonal circulation patterns. Attention is drawn to the characteristic features of the monsoon rainfall that emerge from a study of daily and hourly rainfall of selected stations. Diurnal variations of temperature, pressure, wind and rainfall over the monsoon belt are briefly treated.     

Some features of the arid area variations over India: 1871–1984

Spatial variability of the annual rainfall over drier regions of India is studied by examining the variations in the arid areas. A long period (1871–1984) arid area series has been prepared for the entire country, including the two broad subregions of North India and Peninsular India, using annual rainfall data from 306 well distributed stations. Following an objectively determined criterion based on rainfall amount alone, the yearly area under arid conditions is obtained by totalling areas which received annual rainfall totals less than 560 mm. The interannual variability of the arid area series is large and its distribution is highly right-skewed, demonstrating large spatial variations in the annual rainfall over India. Statistical tests do not suggest any significant long-term trend in the arid area series, but persistently low values of the arid area after 1941 are noteworthy. Implications for the study of risk analysis and assessment of drought and desertification processes are discussed.     

Design rainfall estimation in Australia: a case study using L moments and Generalized Least Squares Regression

Design rainfall is an important input to rainfall runoff models and is used for many other water resources planning and design applications. The estimation of design rainfall is generally done by applying a regional frequency analysis technique that uses data from a large number of rainfall stations in the region. This paper presents a regional rainfall frequency analysis technique that uses an L moments based index method coupled with Generalized Least Squares Regression (GLSR). The particular advantages of the GLSR method are that it accounts for the differences in record lengths across various sites in the region and inter-station correlation in deriving regional prediction equations. The proposed method has been applied to a data set consisting of 203 rainfall stations across Australia. It has been found that the proposed method can be applied successfully in deriving reasonably accurate design rainfall estimates from 1 to 72 h durations. It has also been found that the proposed method provides quite consistent estimates where a third order polynomial is adequate in smoothing the intensity–frequency–duration (IFD) curves. The method can readily be extended to a larger data set of Australia and other countries to derive generalized IFD data.     

Entropy,consistency in rainfall distribution and potential water resource availability in Australia

Understanding the variability in monthly rainfall amounts is important for the management of water resources. We use entropy, a measure of variability, to quantify the rainfall variability in Australia. We define the entropy of stable rainfall (ESR) to measure the long‐term average rainfall variability across the months of the year. The stations in northern Australia observe substantially more variability in rainfall distributions and stations in southern Australia observe less variability in rainfall distribution across the months of the year. We also define the consistency index (CI) to compare the distribution of the monthly rainfall for a given year with the long‐term average monthly rainfall distribution. Higher value of the CI indicates the rainfall in the year is consistent with the overall long‐term average rainfall distribution. Areas close to the coastline in northern, southern and eastern Australia observe more consistent rainfall distribution in individual years with the long‐term average rainfall distribution. For the studied stations, we categorize the years into different potential water resource availability on the basis of annual rainfall amount and CI. For almost all Australian rainfall stations, El Niño years have a greater risk of having below median and relatively inconsistent rainfall distribution than La Niña years. The results may be helpful for developing area‐specific water usage strategies. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical investigation of the impact of uncertainties in satellite rainfall estimation and land surface model parameters on simulation of soil moisture

This study aims at evaluating the uncertainty in the prediction of soil moisture (1D, vertical column) from an offline land surface model (LSM) forced by hydro-meteorological and radiation data. We focus on two types of uncertainty: an input error due to satellite rainfall retrieval uncertainty, and, LSM soil-parametric error. The study is facilitated by in situ and remotely sensed data-driven (precipitation, radiation, soil moisture) simulation experiments comprising a LSM and stochastic models for error characterization. The parametric uncertainty is represented by the generalized likelihood uncertainty estimation (GLUE) technique, which models the parameter non-uniqueness against direct observations. Half-hourly infra-red (IR) sensor retrievals were used as satellite rainfall estimates. The IR rain retrieval uncertainty is characterized on the basis of a satellite rainfall error model (SREM). The combined uncertainty (i.e., SREM + GLUE) is compared with the partial assessment of uncertainty. It is found that precipitation (IR) error alone may explain moderate to low proportion of the soil moisture simulation uncertainty, depending on the level of model accuracy—50–60% for high model accuracy, and 20–30% for low model accuracy. Comparisons on the basis of two different sites also yielded an increase (50–100%) in soil moisture prediction uncertainty for the more vegetated site. This study exemplified the need for detailed investigations of the rainfall retrieval-modeling parameter error interaction within a comprehensive space-time stochastic framework for achieving optimal integration of satellite rain retrievals in land data assimilation systems.     

基于人工源的甚低频电磁波空间传播特征统计分析

利用2005年1月至2010年11月DEMETER卫星记录的NWC发射站的VLF电场功率谱数据,采用指数拟合的方法,分析了VLF电磁波在卫星高度激发的电场空间分布和衰减特征.研究结果表明:(1)VLF电场在发射站上空及其磁共轭区有着很强的对应关系,存在南、北2个强电场中心涡;(2)相对于发射站的位置,VLF电场中心点具有经度和纬度偏移,日侧地磁经度偏移均值大于夜侧,而地磁纬度偏移均值则小于夜侧;(3)日侧VLF电场强度呈现出周期性的年变化;(4)在VLF电场中心10°范围内,电场强度随距离快速衰减,衰减常数b在长达6年的时间内保持稳定.在以上研究结果基础上初步构建的卫星高度人工源电磁波空间分布特征,将为研究地表-电离层电磁波传播机理提供基础技术支撑.     

Global Lightning Activity on the Basis of Inversions of Natural ELF Electromagnetic Data Observed at Multiple Stations around the World

The monitoring of global lightning activity and its spatial and temporal variations is known to be very essential for the study of global warming, the subject of greatest concern to human beings on planet Earth today. As a method of remote sensing for the global lightning distribution, we have proposed an inverse problem by using the data of natural electromagnetic noise in the ELF (extremely low frequency) Schumann resonance (SR) band observed simultaneously at a few stations around the world. The fundamentals of this inversion problem (or ELF tomography) to the SR data have been presented and the first attempt to deduce the global lightning distribution by means of the real SR data has been performed, which has indicated a possibility of snapshots of well-known thunderstorm centers on the globe. This ELF tomography consists of two stages. The first stage is the inversion of the ELF field power spectra to the distribution of lightning intensity by distance relative to an observation point. The obtained distance profiles of intensity of sources at a few stations are used as tomographic projections for reconstructing a spatial distribution of sources in the second stage. Maps of the global lightning distributions constructed by the result of inversions of ELF background field spectra obtained from three stations around the world show that the most active regions vary meridionally on the diurnal time scale being connected mainly with continental areas in the tropics. We do hope that this kind of inversion method to multi-stationed ELF data will be of great importance in the future.     

Cokriging for enhanced spatial interpolation of rainfall in two Australian catchments

Rainfall data in continuous space provide an essential input for most hydrological and water resources planning studies. Spatial distribution of rainfall is usually estimated using ground‐based point rainfall data from sparsely positioned rain‐gauge stations in a rain‐gauge network. Kriging has become a widely used interpolation method to estimate the spatial distribution of climate variables including rainfall. The objective of this study is to evaluate three geostatistical (ordinary kriging [OK], ordinary cokriging [OCK], kriging with an external drift [KED]), and two deterministic (inverse distance weighting, radial basis function) interpolation methods for enhanced spatial interpolation of monthly rainfall in the Middle Yarra River catchment and the Ovens River catchment in Victoria, Australia. Historical rainfall records from existing rain‐gauge stations of the catchments during 1980–2012 period are used for the analysis. A digital elevation model of each catchment is used as the supplementary information in addition to rainfall for the OCK and kriging with an external drift methods. The prediction performance of the adopted interpolation methods is assessed through cross‐validation. Results indicate that the geostatistical methods outperform the deterministic methods for spatial interpolation of rainfall. Results also indicate that among the geostatistical methods, the OCK method is found to be the best interpolator for estimating spatial rainfall distribution in both the catchments with the lowest prediction error between the observed and estimated monthly rainfall. Thus, this study demonstrates that the use of elevation as an auxiliary variable in addition to rainfall data in the geostatistical framework can significantly enhance the estimation of rainfall over a catchment.     

Regional frequency analysis of rainfall extremes in Southern Malawi using the index rainfall and L-moments approaches

Rainfall extremes often result in the occurrence of flood events with associated loss of life and infrastructure in Malawi. However, an understanding of the frequency of occurrence of such extreme events either for design or disaster planning purposes is often limited by data availability at the desired temporal and spatial scales. Regionalisation, which involves “trading time for space” by pooling together observations for stations with similar behavior, is an alternative approach for more accurate determination of extreme events even at ungauged areas or sites with short records. In this study, regional frequency analysis of rainfall extremes in Southern Malawi, large parts of which are flood prone, was undertaken. Observed 1-, 3-, 5- and 7-day annual maximum rainfall series for the period 1978–2007 at 23 selected rainfall stations in Southern Malawi were analysed. Cluster analysis using scaled at-site characteristics was used to determine homogeneous rainfall regions. L-moments were applied to derive regional index rainfall quantiles. The procedure also validated the three rainfall regions identified through homogeneity and heterogeneity tests based on Monte Carlo simulations with regional average L-moment ratios fitted to the Kappa distribution. Based on assessments of the accuracy of the derived index rainfall quantiles, it was concluded that the performance of this regional approach was satisfactory when validated for sites not included in the sample data. The study provides an estimate of the regional characteristics of rainfall extremes that can be useful in among others flood mitigation and engineering design.