Statistical Characteristics of Extreme Temperature Anomaly Groups in Hungary

Summary Investigation of diurnal temperature data series in Hungary indicates that extreme anomalies often occur in groups. Periods which include at least 3–6 extreme anomalous days with 3–5 times higher frequency relative to the climatological average are referred to as periods of extremity (PE). Although PEs cover only 5–12% of the whole time-series (1901–1993), they include 40–90% of all days with extreme temperatures. This refers both to the positive and negative anomalies. This paper shows a method for delimitation technique of PEs which is more suitable for the revelation of time-sequence structure of extreme temperature value occurrences, than traditional investigations of daily or monthly values. As it was checked by Monte-Carlo simulations, a one step autoregressive model, exhibiting nearly normal distribution gives a fairly good approach of extreme temperature occurrences. However, significant differences between the statistical characteristics of real and simulated PEs are also experienced. Statistical connections between PEs and macrocirculation are also investigated. Received November 12, 1996 Revised July 6, 1997     

Projections of Extreme Rainfall in Hong Kong in the 21st Century

The possible changes in the frequency of extreme rainfall events in Hong Kong in the 21st century wereinvestigated by statistically downscaling 30 sets of the daily global climate model projections (involvinga combination of 12 models and 3 greenhouse gas emission scenarios,namely,A2,A1B,and B1) of theFourth Assessment Report of the Intergovernmental Panel on Climate Change.To cater for the intermittentand skewed character of the daily rainfall,multiple stepwise logistic regression and multiple stepwise linearregression were employed to develop the downscaling models for predicting rainfall occurrence and rainfallamount,respectively.Verification of the simulation of the 1971-2000 climate reveals that the models ingeneral have an acceptable skill in reproducing past statistics of extreme rainfall events in Hong Kong.Theprojection results suggest that,in the 21st century,the annual number of rain days in Hong Kong is expectedto decrease while the daily rainfall intensity will increase,concurrent with the expected increase in annualrainfall.Based on the multi-model scenario ensemble mean,the annual number of rain day is expected todrop from 104 days in 1980-1999 to about 77 days in 2090-2099.For extreme rainfall events,about 90% ofthe model-scenario combinations indicate an increase in the annual number of days with daily rainfall 100mm (R100) towards the end of the 21st century.The mean number of R100 is expected to increase from 3.5days in 1980-1999 to about 5.3 days in 2090-2099.The projected changes in other extreme rainfall indicesalso suggest that the rainfall in Hong Kong in the 21st century may also become more extreme with moreuneven distributions of wet and dry periods.While most of the model-emission scenarios in general projectconsistent trends in the change of rainfall extremes in the 21st century,there is a large divergence in theprojections among different model/emission scenarios.This reflects that there are still large uncertainties inmodel simulations of future extreme rainfall events.     

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed. Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ¹³C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude on the CO₂ fertilisation feedback mechanism. The CO₂ changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows multi-millennial CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North.     

Dry/Wet Changes and Their Standing Characteristics in China During the Past 531 Years

Time series of the dryness-wetness(DW) index of 531 yr(AD 1470-2000) at 42 stations in regions A(most of North China and the east of Northwest China) and B(the Yangtze-Huaihe River valley) in China are applied to investigating the historical DW characteristics over various periods of the series with a relatively stationary average value using Bernaola-Galvan(BG) algorithm.The results indicate that region A/B underwent three drought-intensive periods(DIP;1471-1560,1571-1640,and 1920-2000/1501-1540,1631-1690,and 1911-1960) in the last 531 years.In the DIP of the last 130 years,the frequency of DW transition has increased in region A,but not obviously changed in region B in comparison with the other two historical DIPs.The dry period started in about 1920 in region A with severe drought events occurring from the late 1970s to the early 1980s.It lasted for about 50-70 yr in this century,and then a DW shift took place.The wet period in region B might maintain for the coming several decades.The variations of DW in region A are positively correlated with changes in temperature,but in region B,the correlation with temperature is weaker.It is found that the number of DW indices of various categories within a running window is an exponential function of the running window length.The dryness scale factor(DSF) is defined as the reciprocal of the characteristic value of the exponential distribution,and it has a band-like fluctuation distribution that is good for the detection of extreme drought(flood) clustering events.The results show that frequencies of the severe large-scale drought events that concurrently occurred in regions A and B were high in the late 12th century,the early 13th century,the early 17th century,and the late 20th century.This provides evidence for the existence of the time-clustering phenomena of droughts(floods).     

Association between extreme monsoons and the dipole mode over the Indian subcontinent

Summary The relationship of summer monsoon over India with the Indian Ocean Dipole Mode has been investigated applying simple statistical techniques. While a long time series of 132 years based on 1871–2002 for both summer monsoon rainfall as well as dipole mode index has been used in this study, the NCEP–NCAR reanalysis data (1948–2002) are used to examine the circulation features associated with the extreme dipole and monsoon phases. These flow patterns bring out the dynamics of the dipole – monsoon relationship. Lead/lag correlations between the dipole mode index and the Indian monsoon rainfall are computed. Results reveal that numerically the relationship is stronger following the monsoon. The lower troposphere flow patterns at 850 hPa associated with the extreme phases of the dipole and monsoon are consistent with the correlation analysis. Further a strong (weak) summer monsoon favours the development of the negative (positive) dipole event in autumn. The sliding correlations between Indian monsoon rainfall and the dipole mode index suggest that the impact of monsoon over dipole is weakening after 1960s. This weakening relationship has been evidenced by the composites of sea-surface temperature anomalies and circulation patterns. All the above analysis suggests that the summer monsoon has more influence on the dipole mode than vice-a-versa.     

Anticyclogenesis of surface pressure field in winter season,blocking, and variability of the Earth angular velocity

Analyzed are the anomalies of monthly mean values of surface pressure over Eurasia in winter seasons of 1901–2010. It is noted that the centers of large monthly positive pressure anomalies (16–24 hPa) are usually located within the 60°–70°N latitude zone. The Siberian high is well developed but the pressure anomalies in its center amount to 3–8 hPa only. The large monthly pressure anomalies at the isobaric surface of 500 hPa in the first natural synoptic area are mainly accompanied by the E-type of circulation (according to G.Ya. Vangengeim). The number of days with the western (W) type of circulation is extremely small and is practically absent when the anomalies in the center exceed 20 hPa (blocking process). The time periods of increase (decrease) in the annual number of days with W-circulation are well agreed with the periods of the Earth rotation acceleration (deceleration). The positive pressure anomalies were four or five times more frequent in the periods of the Earth angular velocity decrease: in 1933–1972 and after 2004.     

The Characteristics of Cold Air Outbreaks over the Eastern Highlands of Kenya

Summary Climatological statistics of extreme temperature events over Kenya are established from the analysis of daily and monthly maximum temperatures for a representative station (Nairobi Dagoretti Corner) over the period 1956–1997. The months of June to August were shown to be the coldest with a mean monthly maximum temperature of less than 22 °C. Seasonal (June to August) mean maximum temperature was 21.5 °C. Using this seasonal mean temperature for the period 1967–1997 delineated 1968 as the coldest year in this series and 1983 as the warmest year. Spectral analysis of the seasonal data, for both the coldest and the warmest years, revealed that the major periods were the quasi-biweekly (10 days) and the Intraseasonal Oscillations (23 days). Secondary peaks occurred at periods of 4–6 and 2.5–3.5 days. A temperature threshold of 16.7 °C during July was used to define cold air outbreaks over Nairobi. This threshold temperature of 16.7 °C was obtained from the mean July maximum temperature (20.9 °C) minus two standard deviations. Notable trends include a decrease in the frequency of station-days, between 1956 and 1997, with temperatures less than 16.7 °C during July. Surface pressure patterns indicate that the origin of the cold air is near latitude 25° S and to the east of mainland South Africa. The cold air near 25° S is advected northwards ahead of the surface pressure ridge. Received July 19, 1999 Revised January 11, 2000     

Forecast skill of the tropical intraseasonal oscillation in the NCEP GFS dynamical extended range forecasts

This study examines the forecast performance of tropical intraseasonal oscillation (ISO) in recent dynamical extended range forecast (DERF) experiments conducted with the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model. The present study extends earlier work by comparing prediction skill of the northern winter ISO (Madden-Julian Oscillation) between the current and earlier experiments. Prediction skill for the northern summer ISO is also investigated. Since the boreal summer ISO exhibits northward propagation as well as eastward propagation along the equator, forecast skill for both components is computed. For the 5-year period from 1 January, 1998 through 31 December, 2002, 30-day forecasts were made once a day. Compared to the previous DERF experiment, the current model has shown some improvements in forecasting the ISO during winter season so that the skillful forecasts (anomaly correlation>0.6) for upper-level zonal wind anomaly extend from the previous shorter-than 5 days out to 7 days lead-time. A similar level of skill is seen for both northward and eastward propagation components during the summer season as in the winter case. Results also show that forecasts from extreme initial states are more skillful than those from null phases for both seasons, extending the skillful range by 3–6 days. For strong ISO convection phases, the GFS model performs better during the summer season than during the winter season. In summer forecasts, large-scale circulation and convection anomalies exhibit northward propagation during the peak phase. In contrast, the GFS model still has difficulties in sustaining ISO variability during the northern winter as in the previous DERF run. That is, the forecast does not maintain the observed eastward propagating signals associated with large-scale circulation; rather the forecast anomalies appear to be stationary at their initial location and decay with time. The NCEP Coupled Forecast System produces daily operational forecasts and its predication skill of the MJO will be reported in the future.     

Characteristics of large-scale atmospheric circulation related to extreme monthly rainfall anomalies in the Pampa Region, Argentina, under non-ENSO conditions

Summary There is a widely held view that the Pampa region (PR) dry and wet periods are predominantly a consecuence of the El Ni?o-Southern oscillation (ENSO) phenomenom. The current paper focuses on non-ENSO rainfall anomalies for the period 1948–2000, the more recent of which have had catastrophic consequences throughout the region. We analyze horizontal water vapor transport, pressure and circulation anomalies occurring in Southern South America (SSA) during this type of event. Positive and negative (wet and dry) extreme events during the rainy and dry seasons in the region were registered. Based on NCEP reanalysis data it was established that under rainfall deficit, anomalies of similar intensity occurred simultaneously in the PR and in central Chile, whereas under excess rainfall the anomalies were mostly confined to the PR. The existence of a cyclone-anticyclone pair in the anomalous circulation pattern over mid latitudes of the Atlantic and Pacific oceans and straddling the southern portion of the continent maintains an intense and extense meridional circulation over the continental plains, which leads to the abnormal values in moisture transport and rainfall rate. The atmospheric water balance equation calculated for the PR indicates that anomalous water vapor is carried in from the continental equatorial region and from the subtropical Atlantic, its magnitude varying in accordance with the season and the sign of the anomaly. Furthermore, evidence of the important role of transient terms corroborates their contribution to the anomalous total moisture flux divergence under rainfall deficit during the dry season. The mean sea-level pressure anomaly fields of the extreme cases were further examined by principal component analysis to discern those circulation features directly linked to rainfall deviations.     

Empirical prediction of Indian summer monsoon rainfall with different lead periods based on global SST anomalies

Summary The main objective of this study was to develop empirical models with different seasonal lead time periods for the long range prediction of seasonal (June to September) Indian summer monsoon rainfall (ISMR). For this purpose, 13 predictors having significant and stable relationships with ISMR were derived by the correlation analysis of global grid point seasonal Sea-Surface Temperature (SST) anomalies and the tendency in the SST anomalies. The time lags of the seasonal SST anomalies were varied from 1 season to 4 years behind the reference monsoon season. The basic SST data set used was the monthly NOAA Extended Reconstructed Global SST (ERSST) data at 2° × 2° spatial grid for the period 1951–2003. The time lags of the 13 predictors derived from various areas of all three tropical ocean basins (Indian, Pacific and Atlantic Oceans) varied from 1 season to 3 years. Based on these inter-correlated predictors, 3 predictor sub sets A, B and C were formed with prediction lead time periods of 0, 1 and 2 seasons, respectively, from the beginning of the monsoon season. The selected principal components (PCs) of these predictor sets were used as the input parameters for the models A, B and C, respectively. The model development period was 1955–1984. The correct model size was derived using all-possible regressions procedure and Mallow’s “Cp” statistics. Various model statistics computed for the independent period (1985–2003) showed that model B had the best prediction skill among the three models. The root mean square error (RMSE) of model B during the independent test period (6.03% of Long Period Average (LPA)) was much less than that during the development period (7.49% of LPA). The performance of model B was reasonably good during both ENSO and non-ENSO years particularly when the magnitudes of actual ISMR were large. In general, the predicted ISMR during years following the El Ni?o (La Ni?a) years were above (below) LPA as were the actual ISMR. By including an NAO related predictor (WEPR) derived from the surface pressure anomalies over West Europe as an additional input parameter into model B, the skill of the predictions were found to be substantially improved (RMSE of 4.86% of LPA).     

Évolution des indices des extrêmes climatiques en République du Tchad de 1960 à 2008

[Translated by the editorial staff] An analysis of climate trends and return levels for the period 1960–2008, using the ETCCDI-CLIVAR/JCOMM project approach, has been conducted for Chad, where droughts and flooding are recurrent. Using the RClimDex software, we show that almost all rainfall trends are decreasing, as in Central and Northern Africa. Rare extreme rain events (R99p) decrease significantly: 0.85?mm per decade. However, we note a slightly upward trend of 0.5 day per decade, in the number of consecutive dry days (CDD). Temperature indices are all positive except for the frequencies of very hot days (TX90p) and very cold nights (TN10p), which decrease significantly: ?0.39% day per year per decade, as in Central Africa and globally. Sequences of hot or cold days decrease as well, but by about 1% per year per decade. Return periods identified with the generalized extreme value (GEV) distributions show that they are well defined from 1 to 10 years. Extremely rare events from 10 to 50 years are associated with a mean return level of 660?mm of annual precipitation.     

2022年夏季中国高温干旱气候特征及成因探讨

2022年夏季,中国中东部发生了极端高温干旱气候异常,给经济、农业、人民生活造成了严重影响。本文回顾了此次高温干旱气候异常的时空特征,分析了其主要成因。2022年夏季,中国中东部区域平均的极端高温频次、日最高温度平均值、高温日数等指标均达到了1979年以来的最大值,区域平均降水则达到了1979年以来的最低值。此次气候异常主要是由于夏季中国中东部受强大的高压系统控制,与偏强的西太副高、中纬度的西风带扰动以及热带海温的影响有关。此外,本文探讨了全球增暖趋势对极端高温事件增多的影响,以及未来中国地区高温和干旱事件的可能变化。     

General Characteristics of the Atmospheric Boundary Layer Over a Flaw Lead Polynya Region in Winter and Spring

A time series of microwave radiometric profiles over Arctic Canada’s Cape Bathurst (70°N, 124.5°W) flaw lead polynya region from 1 January to 30 June, 2008 was examined to determine the general characteristics of the atmospheric boundary layer in winter and spring. A surface based or elevated inversion was present on 97% of winter (January–March) days, and on 77% of spring (April–June) days. The inversion was the deepest in the first week of March (≈1100 m), and the shallowest in June (≈250 m). The mean temperature and absolute humidity from the surface to the top of the inversion averaged 250.1 K (−23.1°C), and 0.56 × 10⁻³ kg m⁻³ in winter, and in spring averaged 267.5 K (−5.6°C), and 2.77 × 10⁻³ kg m⁻³. The median winter atmospheric boundary-layer (ABL) potential temperature profile provided evidence of a shallow, weakly stable internal boundary layer (surface to 350 m) topped by an inversion (350–1,000 m). The median spring profile showed a shallow, near-neutral internal boundary layer (surface to 350 m) under an elevated inversion (600–800 m). The median ABL absolute humidity profiles were weakly positive in winter and negative in spring. Estimates of the convergence of sensible heat and water vapour from the surface that could have produced the turbulent internal boundary layers of the median profiles were 0.67 MJ m⁻² and 13.1 × 10⁻³ kg m⁻² for the winter season, and 0.66 MJ m⁻² and 33.4 × 10⁻³ kg m⁻² for the spring season. With fetches of 10–100 km, these accumulations may have resulted from a surface sensible heat flux of 15–185 W m⁻², plus a surface moisture flux of 0.001–0.013 mm h⁻¹ (or a latent heat flux of 0.7–8.8 W m⁻²) in winter, and 0.003–0.033 mm h⁻¹ (or a latent heat flux of 2–22 W m⁻²) in spring.     

Monthly Synoptic Patterns Associated with Wet/Dry Conditions in the Eastern Mediterranean

Summary An investigation of the main features of large-scale synoptic patterns over Europe and the adjacent areas for extreme winter periods during 1980–1995 over the Eastern Mediterranean (EM) is performed. The NASA reanalysis data set is used to investigate composite sea level pressure (SLP), geopotential height of the 500 hPa surface (H-500) and precipitation–wet, normal and dry patterns for each month during the period October–March. It is found that the wet and dry cool seasons in the EM are associated with distinct SLP and H-500 anomaly patterns over Europe and the adjacent regions. During the dry spells large-scale positive SLP/H-500 anomaly areas prevail over Eastern Europe. A negative SLP anomaly is normally found during these periods over southwestern and Western Europe. During the wet cool seasons in the EM there are mainly negative SLP/H-500 anomaly areas over Eastern Europe to the north east of the EM. Positive SLP/H-500 anomalies are found over Western Europe. During wet months a trough zone between the Siberian and the Azorean Highs is positioned over the eastern part of the Mediterranean. During dry months the Siberian anticyclone is more intensive and the zone with low surface pressure is displaced to the central part of the northern Mediterranean. Received May 26, 1999 Revised August 26, 1999     

The climate of Namaqualand in the nineteenth century

Southern African climatic change research is hampered by a lack of long-term historical data sets. This paper aims to extend the historical climate record for southern Africa to the semi-arid area of Namaqualand in the Northern Cape province of South Africa. This is achieved through extensive archival research, making use of historical documentary sources such as missionary journals and letters, traveller’s writings and government reports and letters. References to precipitation and other climatic conditions have been extracted and categorised, providing a proxy precipitation data set for Namaqualand for the nineteenth century. Notwithstanding problems of data accuracy and interpretation the reconstruction enables the detection of severe and extreme periods. Measured meteorological data, available from the late 1870s, was compared to the data set derived from documentary sources in order to ascertain the accuracy of the data set and monthly rainfall data has been used to identify seasonal anomalies. Confidence ratings on derived dry and wet periods, where appropriate, have been assigned to each year. The study extends the geographical area of existing research and extracts the major periods of drought and climatic stress, from the growing body of historical climate research. The most widespread drought periods affecting the southern and eastern Cape, Namaqualand and the Kalahari were 1820–1821; 1825–1827; 1834; 1861–1862; 1874–1875; 1880–1883 and 1894–1896. Finally, a possible correspondence is suggested between some of the widespread droughts and the El Nino Southern Oscillation (ENSO).     

Signature of quasi–biennial oscillation and long-period equatorial disturbances in the lower atmosphere over Thiruvananthapuram

Summary An attempt has been made in this paper to examine different modes of oscillation in the wind field during different seasons over Thiruvananthapuram (lat. 8.29° N, long. 76.59° E, located at the extreme southwest coast of India) based on daily upper air observations for the period from January 1997 to December 1999. A power spectral analysis is carried out with the upper air data of the station. The study shows that one and half cycle of Quasi–Biennial Oscillation (QBO) and the power spectra of the meridional wind component exhibit peaks between the period of four days and seven days (corresponding frequency range between 0.25 day⁻¹ and 0.15 day⁻¹) during all seasons. The seasonal variation of these large-scale oscillations over the station depends upon the background mean zonal flow, which in turn closely related to the QBO structure. The time sequence of power spectra shows that the disturbances with periods between four days and seven days dominantly prevail in the upper troposphere and lower stratosphere throughout the year. The regimes of high power spectral intensity in this period range are maintained in the levels where the mean zonal flow (westerly or easterly) weakens and changes with height. The study establishes the fact that disturbances (mixed Rossby-gravity waves) acquire maximum power in the winter season whereas the south-west monsoon exhibits minimum spectral intensity when spreading of energy over a frequency range takes place.     

Intercomparison of Two Hydroxyl Radical Measurement Techniques at the Atmosphere Simulation Chamber SAPHIR

At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10⁶ cm^–3) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10⁶ cm^–3, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 – 2) ×10⁶ cm^–3 higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO_x concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.     

Hydrometeorological conditions of the Volga region and current climate changes

The maps are presented of seasonal air temperature and precipitation amount anomalies averaged for the whole Volga region and adjacent territory for two time periods, 1946–1977 and 1978–2008. It is demonstrated that the considerable differences in the thermal and moistening regimes of the Volga region exist for these two periods. The relation is described between the variations of temperature and precipitation amount and the circulation types according to Vangengeim-Girs classification as well as the possibility to use these data for specifying the climatic scenarios obtained on the basis of physically complete hydrodynamic models.     

An evaluation of UKMO one-month ensemble forecasts of MSLP in the Southern Hemisphere

Summary ?We evaluate United Kingdom Meteorological Office (UKMO) one-month ensemble forecasts of mean sea-level pressure (MSLP) in the southern hemisphere (SH) to 60° S, with a special focus on their utility near New Zealand (NZ). There are 105 9-member ensembles, at approximately two-week intervals, between 1995 and 1999. Each forecast is averaged over two successive 15-day periods and verified against the NCEP/NCAR reanalysis data set. Compared to climatology, the skill of the ensemble mean is slightly positive in days 1–15, and slightly negative in days 16–30. Skill near NZ is slightly lower than the SH averages. For SH-scale circulation patterns (as seen in the first few principal components), skill is greater than for most individual grid points, but is still negligible or negative in days 16–30. Moderate skill-spread correlations (ρ ≈−0.5) were found for some skill scores. The way that skill varies with season and the Southern Oscillation Index is consistent with other research but not statistically significant for this small data set. Probabilistic forecasts of low and high pressures have skill similar to that of the ensemble mean. The ensemble spread is generally too small, in that the analysis lies within the ensemble less often than the theoretically optimum value of 80% of the time. Measured as a fraction of the natural variability, the spread increases substantially with time and latitude: it is less than 0.5 near the equator in days 1–15, and takes values near 1 only at higher latitudes during days 16–30. The initial sequential structure of the ensembles (a consequence of the use of time lags in their genesis) is still apparent in days 1–15 but has disappeared by days 16–30. Three potential alternatives to the ensemble mean were all found to have less skill than it. Received June 17, 2001; revised July 4, 2002; accepted November 22, 2002 Published online March 17, 2003     

Synoptic reasons for heavy snowfalls in the Polish – German lowlands

Summary Daily circulation patterns responsible for heavy snowfalls in the Polish – German lowlands were analysed. Composite maps of sea level pressure (SLP) and 500 hPa geopotential height means and anomalies were constructed for the days with an increase in snow cover depth by ≥5 cm. Contour maps show negative anomalies of SLP and 500 hPa level over central Europe, indicating a low pressure system. Strong positive anomalies of SLP appear over Scandinavia and the northern Atlantic with the centre of positive anomalies located over Iceland. Weaker negative anomalies are observed in the Azores region. This confirms the strong negative correlation between snow cover appearance and the North Atlantic Oscillation index in Europe. The days with heavy snowfalls were clustered using the Ward’s method. Three types of circulation patterns were distinguished, each of them characterised by a low pressure system over central Europe. Type 3 represents the northern position of the low with its centre over the Baltic Sea, Type 2 shows the southern position of the low with its centre over the Adriatic and the Ionic Sea and Type 1 represents the low location between the two previous patterns with a wide meridional trough over the Atlantic. Author’s address: Ewa Bednorz, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, ul. Dzięgielowa 27, 61-680 Poznań, Poland.