Frequency Of Weather Types Research Articles

Types of Artemisia pollen season depending on the weather conditions in Wrocław (Poland), 2002–2011

The aim of the study was to characterise Artemisia pollen season types according to weather conditions in Wrocław (south-western Poland) in the years 2002–2011. Over the period analysed, the start date of the pollen season (determined by the 95 % method) ranged from 10 July 2002 to 28 July 2010. The start date of the pollen season can be determined by using Crop Heat Units (CHUs). During the period 2002–2011, the Artemisia pollen season started after the cumulative value of CHUs had reached 2,000–2,100 °C. The three distinguished types of Artemisia pollen season are best described by the frequency of weather types defined by the type of circulation, mean daily air temperature, and the occurrence of rain. The variation in these factors affected the dynamics of the pollen season. The noteworthy frequency of days with rain and high seasonal sum of precipitation totals as well as the dominance of cyclonic weather from the westerly direction had an impact on the extension of the pollen season. The meteorological factors that directly affect pollen release and transport primarily include air humidity, expressed as vapour pressure (r > 0.3, p < 0.01), temperature(r from 0.2 to 0.4, p < 0.01). The relationships between averaged meteorological data and daily pollen concentration were stronger (r > 0.5, p < 0.01). Based on the correlation analysis, the meteorological variables were selected and regression equations were established using stepwise backward regression analysis.

Influence of atmospheric circulation patterns on the oxygen isotope ratio of tree rings in the Alpine region

The oxygen isotope ratio of precipitation and tree rings is a complex function of climate variables and atmospheric dynamics, which often makes the interpretation of δ18O for palaeoclimate research challenging. Here we analyzed monthly precipitation δ18O series for 1973–2004 and annually resolved tree ring δ18O chronologies for 1945–2004 for three sites in Switzerland: one north of the Alps, one at high‐elevation within the Alps, and one south of the Alps. The goal of the study was to improve the understanding of the tree ring archive by a systematic analysis of nonlocal parameters related to atmospheric circulation, in particular, geopotential height field anomalies and the frequency of synoptic weather situations, in addition to the usual local climate parameters like temperature, sunshine duration, and relative humidity. We observed that on average high‐pressure situations during summer were associated with relatively high δ18O and low‐pressure situations were associated with relatively low δ18O, for both the isotope ratio in precipitation and tree rings. However, correlations to the frequency of weather types were not higher than simple correlations to local temperature. Accordingly, we constructed a combined index from temperature and air pressure that proved to be a good predictor of δ18O in precipitation and used this as the source water term in a tree ring isotope fractionation model. This enabled us to use the model beyond the period where isotope values for precipitation are available, opening new perspectives in the interpretation of long tree ring δ18O chronologies.

Spatial variability in large-scale and regional atmospheric drivers of Pinus halepensis growth in eastern Spain

In this study we analyzed the influence of general atmospheric circulation patterns and the frequency of weather types on the spatio-temporal variability of tree-ring growth in Pinus halepensis forests in eastern Spain. Three atmospheric circulation patterns affecting the western Mediterranean region were included in the study: the North Atlantic Oscillation (NAO), the Western Mediterranean Oscillation (WeMO) and the Mediterranean Oscillation (MO). In addition, the particular circulation pattern affecting eastern Spain was quantified using the frequency of weather types. The variability of radial growth (width) of earlywood and latewood in P. halepensis was quantified at 19 sites using dendrochronological methods. Two distinct patterns, reflecting growth variability in the northern and southern areas involved in the study, were identified for both earlywood and latewood tree-ring series. The influence of atmospheric circulation modes on tree growth resembled the spatial patterns identified, as earlywood and latewood formation in northern sites was determined by the NAO variability, whereas the WeMO dominated growth at the southern sites. Winter, summer and autumn weather types also exerted a control over tree radial growth. We conclude that both atmospheric circulation indices and weather types exert significant control on the formation of earlywood and latewood, because of their influence on precipitation patterns. The findings also suggest that wet and mild conditions during winter and the following autumn enhance P. halepensis earlywood and latewood formation, respectively. Thus, winter atmospheric patterns may indirectly influence latewood growth through direct effects on previous earlywood development driven by precipitation variability.

A new objective procedure for classifying New Zealand synoptic weather types during 1958–2008

AbstractA set of 12 daily weather types has been obtained for the New Zealand region from the 51‐year NCAR/NCEP dataset by using a two‐stage classification procedure (rotated T‐mode principal component analysis followed by convergent K‐means clustering). Analysis of this classification confirms previous studies and provides further insight into the variation features of the regional synoptic weather on the seasonal, interannual and longer‐time scales.There are seasonal variations in both intensity and frequency of synoptic weather types. For example, southern highs are more frequent in summer and autumn, while troughs and lows occur more often in winter and secondarily spring. The high and low centres of weather types tend to be more intense in winter than summer.The frequency and lifetime of synoptic weather types fluctuate on the interannual and decadal scales; there exist significant correlations between frequencies of individual weather types and Southern Oscillation Index (SOI). In all seasons, the frequencies of weather types can change with phases of SOI. The most significant changes in weather type frequencies during the negative SOI phase occur in spring, and characterized by more frequent occurrence of weather types which provide prevailing southwesterly to westerly winds and reduced prominence of anticyclonic situations. The most significant changes in type frequencies during the positive SOI phase occur in summer, with more frequent anticyclonic types but less frequent cyclonic types. The frequency change in winter seems different from other seasons, although consistent with previous studies. In addition, the probability of extremely frequent occurrence or lack of occurrence of certain weather types during a month also varies significantly with SOI phases and with seasons, indicating that the chance for extreme weather conditions to occur is conditional on SOI.The results can be useful for climate forecasts, studies of climate extremes, air pollution, climatic impacts on human health and other environmental problems. Copyright © 2010 Royal Meteorological Society

Weather Types and Rainfall over Senegal. Part I: Observational Analysis

AbstractA k-means cluster analysis is used to summarize unfiltered daily atmospheric variability at regional scale over the western Sahel and eastern tropical North Atlantic during the boreal summer season [July–September (JAS)] 1961–98. The analysis employs zonal and meridional regional wind fields at 925, 700, and 200 hPa from the European Centre for Medium-Range Weather Forecasts reanalyses. An eight-cluster solution is shown to yield an integrated view of the complex regional circulation variability, without the need for explicit time filtering. Five of the weather types identified characterize mostly typical phases of westward-moving wave disturbances, such as African easterly waves (AEWs), and persistent monsoon surges, while the three others describe mostly different stages of the seasonal cycle. Their temporal sequencing describes a systematic monsoonal evolution, together with considerable variability at subseasonal and interannual time scales.Daily rainfall occurrence at 13 gauge stations in Senegal is found to be moderately well conditioned by the eight weather types, with positive rainfall anomalies usually associated with southerly wind anomalies at 925 hPa. Interannual variability of daily rainfall frequency is shown to depend substantially on the frequency of occurrence of weather types specific to the beginning and end of the JAS season, together with the number of persistent monsoon surges over the western Sahel. In contrast, year-to-year changes in the frequency of the weather types mostly associated with westward-moving waves such as AEWs are not found to influence seasonal frequency of occurrence of daily rainfall substantially.The fraction of seasonal rainfall variability related to weather-type frequency is found to have a strong relationship with tropical Pacific sea surface temperatures (SSTs): an El Niño (La Niña) event tends to be associated with a higher (lower) frequency of dry weather types during early and late JAS season with enhanced trade winds over the western Sahel, together with lower (higher) prevalence of persistent monsoon surges. The component of seasonal rainfall variability not related to weather-type frequency is characterized by changes in rainfall probability within each weather type, especially those occurring in the core of the JAS season; it exhibits a larger decadal component that is associated with an SST pattern previously identified with recent observed trends in Sahel rainfall.

The influence of atmospheric circulation at different spatial scales on winter drought variability through a semi‐arid climatic gradient in Northeast Spain

AbstractThis paper analyses the spatial and temporal variability of winter droughts in a semi‐arid geographic gradient in Northeast Spain, from the Pyrenees in the north to the Mediterranean coastland in the south. Droughts that occurred between 1952 and 1999 were analysed by means of the Standardised Precipitation Index (SPI). The influence of the weather‐type frequency and of the general North Atlantic atmospheric circulation patterns was analysed. The results indicate that winter droughts show an important spatial variability in the study area, differentiating three well‐defined patterns. These correspond to the Pyrenees, the centre of the Ebro Valley, and the Mediterranean coastland. General negative trends in winter SPI have been found, which are indicative of the increase in winter drought conditions in the study area. Nevertheless, important spatial differences have also been recorded. Dominant north–south gradients in the influence of weather types are shown. Moreover, the negative trends in winter‐SPI values agree with the negative trend in the frequency of the weather types prone to cause precipitation, such as the C, SW and W weather types and the increase in the frequency of A weather types. Nevertheless, in the Mediterranean coastland, the positive trend in SPI values agrees with the increase in the frequency of weather types of the east (E, SE), which are prone to cause precipitation in this area. Interannual variations in the frequency of the different weather types have been highly determined by different general atmospheric circulation patterns, mainly the North Atlantic Oscillation (NAO). Nevertheless, the correlation between the time series of weather‐type frequency and the winter SPI is higher than that found between the SPI and the NAO. Thus, although the interannual NAO variability explains a high percentage of the interannual differences in the frequency of different weather types, it is not sufficient to explain the spatial and temporal variability of droughts, which respond better to atmospheric variability at more detailed (synoptic) spatial scales. Copyright © 2006 Royal Meteorological Society.

Modeling the impacts of climatic change and variability on the reliability, resilience, and vulnerability of a water resource system

During the last decade, there have been increasing concerns over water resource drought in northern England, brought about by the 1995 Yorkshire drought with an estimated 5‐month rainfall return period of 200 years. The impacts of climatic change and variability on water resource reliability, resilience, and vulnerability in this region are examined by modeling changes to weather type frequency, mean rainfall statistics, and potential evapotranspiration. Results indicate future improvements in water resource reliability due to increased winter rainfall but reductions in resource resilience and an increased vulnerability to drought. Severe droughts comparable to that of 1995 show only a slight increase in frequency by 2080. However, there are significant increases in both the magnitude and duration of severe water resource drought, as a consequence of summer rainfall reductions and increased climatic variability. This research provides a basis for the future planning and management of the Yorkshire water resource system.

Precipitation and the North Atlantic Oscillation: a study of climatic variability in northern England

AbstractVariations in precipitation in Yorkshire, UK, are linked to large‐scale atmospheric circulation using the North Atlantic Oscillation (NAO) index, objective Lamb weather types, and long‐term seasonal and annual precipitation indices at seven sites. Since the 1960s, a significant decrease in summer precipitation has been observed across the region, with increasing winter precipitation at western locations since 1970. At western sites, precipitation is highly correlated to the NAO, with positive and negative winter‐NAO indices producing anomalies from the long‐term winter precipitation average of 112% and 77% respectively. The recent winter half‐year precipitation increase is concentrated in March and related to a large increase in monthly NAO and the increased occurrence of westerly weather types. The decrease in summer precipitation, however, is caused by both declining receipt from westerly weather types and their decreased occurrence. This is significantly correlated to a decreasing summer index for the NAO. The NAO, the frequency of weather types and precipitation totals are all intimately linked on an annual and monthly time scale. Strong connections between winter precipitation and the NAO may prove very important, particularly in western Yorkshire where most surface water supplies are located. If the NAO can be predicted using sea‐surface temperature, it may allow the estimation of winter reservoir replenishment in western Yorkshire a few months in advance. This may provide an important predictor of future hydrological drought episodes and facilitate forward planning and management of water resource systems located in climatologically similar regions of the UK. Copyright © 2002 Royal Meteorological Society

Effects of ENSO on weather-type frequencies and properties at New Orleans, Louisiana, USA

Examination of historical climate records indicates a significant relation between the El Nino/Southern Oscillation (ENSO) and seasonal temperature and precipitation in Louisiana. In this study, a 40 yr record of twice daily (06:00 and 15:00 h local time) weather types are used to study the effects of ENSO variability on the local climate at New Orleans, Louisiana. Tropical Pacific sea- surface temperatures (SSTs) for the NINO3.4 region are used to define ENSO events (i.e. El Nino and La Nina events), and daily precipitation and temperature data for New Orleans are used to define weather-type precipitation and temperature properties. Data for winters (December through Febru- ary) 1962-2000 are analyzed. The 39 winters are divided into 3 categories; winters with NINO3.4 SST anomalies 1°C (El Nino events), and neu- tral conditions (all other years). For each category, weather-type frequencies and properties (i.e. pre- cipitation and temperature) are determined and analyzed. Results indicate that El Nino events pri- marily affect precipitation characteristics of weather types at New Orleans, whereas the effects of La Nina events are most apparent in weather-type frequencies. During El Nino events, precipitation for some of the weather types is greater than during neutral and La Nina conditions and is related to increased water vapor transport from the Tropics to the Gulf of Mexico. The changes in weather-type frequencies during La Nina events are indicative of a northward shift in storm tracks and/or a decrease in storm frequency in southern Louisiana.

Frequency Changes of Weather Types in the Alpine Region since 1945

The annual occurrence of different weather types of Schuepp’s synoptic classification in the Alpine region has significantly changed since the beginning of its recording in 1945. The annual frequency (number of days per year) has shifted towards more convective and less advective weather types. Since 1945 the number of long-lasting convective episodes rose and the number of long-lasting advective episodes lessened. Most of these changes took place in winter. The annual frequencies of weather types and the annual mean of certain local meteorological parameters are significantly correlated. On the large scale there is a strong interdependence between the high pressure weather type and the North Atlantic Oscillation (NAO) index which is based on the sea-level pressure difference between Portugal and Iceland.

Winter anticyclone changes on the central Gulf Coast of the USA

A modified cyclone‒model‒based weather classification system is used to identify long‒term trends in anticyclonic weather type frequencies over the central Gulf Coast of the USA. Results suggest that days dominated by mid‒latitude cold‒season anticyclones of continental origin (CH days) have decreased in frequency from 1961 to 1989. At the same time, anticyclones of Pacific origin (PH days) have become more common over the same region. In both cases, most of the trends are characterized by abrupt ‘step changes’ rather than gradual changes. Principal components analysis in combination with a clustering procedure is used to analyse long‒term trends in the surface air mass properties at New Orleans during times of influence of these weather types. The intensities of these weather types as a whole do not display any significant trends. However, several individual weather variables do show significant change, especially on CH days. Although some trends may represent changes in trajectory and intensity of the air masses, many may be indicative of more general atmospheric tendencies not limited to CH and PH days, such as the urban heat island and reduced visibility associated with anthropogenic activity. Future research should be conducted to improve our understanding of how changes in the presence and properties of these anticyclones relate to changes in the larger scale steering flow patterns governing their trajectories and intensities. © 1997 The Royal Meteorological Society.

Validation of GCM control simulations using indices of daily airflow types over the British Isles

In this study, the control simulations of two general circulation model (GCM) experiments are assessed in terms of their ability to reproduce realistic ‘real world’ weather. The models examined are the UK Meteorological Office high-resolution atmospheric model (UKHI) and a coupled ocean/atmosphere model of the Max Planck Institut fur Meteorologic, Hamburg (MPI). An objective classification of daily airflow patterns over the British Isles is used as a basis for comparing the frequencies of model-generated weather types with the frequencies derived from 110 years of observed mean-sea-level pressure (MSLP) fields. The weather-type frequencies generated by the GCMs, and their relationships with simulated monthly mean temperatures and total precipitation over the UK, are compared, season by season, with similar results derived using the observational data. An index of gale frequencies over the British Isles, derived from a similar objective analysis of daily MSLP fields, is used to evaluate the ability of the GCMs to simulate the observed frequency of storm events. One advantage of using 110 years of observational data is that the observed decadal-scale variability of climate can be introduced into this type of validation exercise. Both the GCMs assessed here are too cyclonic in winter. The seasonality of both anticyclonic and cyclonic types is much too strong in MPI and summer precipitation in this model is greatly underestimated. MPI simulates the annual cycle of temperature well, while UKHI successfully reproduces the annual cycle of precipitation. The analysis also indicates that the summer temperature variability of the two models is not driven by circulation changes.

The statistical relationship between weather-type frequencies and corn (maize) yields in southwestern Pennsylvania, USA

Several synoptic climatological classification techniques are used to relate atmospheric circulation and corn (maize) yields in two regions in southwestern Pennsylvania. The July frequency of occurrence of just one climate type is found to correlate highly with corn yield over a 10-year period. The correlation coefficients obtained through this approach are substantially higher than those obtained through more traditional methods relating yield to temperature and precipitation. While the synoptic climatological approach to yield prediction does not make the biological relationship between climatic factors and corn growth explicit, other features in addition to high correlations recommend it: the model does not require specification of particular climate variables and its emphasis on atmospheric circulation makes the approach more useful for climate change research than those based on simple weather observations without reference to atmospheric dynamics.

A procedure for removing the synoptic climate signal from environmental data

AbstractUsing weather‐type frequencies from a synoptic climatology, a technique is presented that discriminates between within‐type and between‐type variations in a time series of climate‐related environmental data. The removal of the synoptic climate signal, or declimatizing, is based on normalizing the data by the mean annual weather‐type frequencies for the study period. Declimatizing is illustrated symbolically and with a worked hypothetical example. An application of the procedure to visibility data from Pittsburgh, Pennsylvania demonstrates its utility in decomposing complex climate‐related environmental data into its component synoptic and non‐synoptic influences. The methodology can also distinguish the relative importance of between‐type and within‐type changes in a synoptic climatology.

Delimitation of the summer-dry period in Greece according to the frequency of weather-types

The mean absolute and relative frequencies of daily synoptic weather-types over the Greek area, have been calculated, for a period of 36 years (1950–1985), and for each decade of the seven-month period April to October. The real summer-dry period is delimited first empirically according to the relative frequencies of the weather-types for each period of 10 days; then objectively by use of the Principal Component Analysis. Results obtained by the two methods appear to converge. The summer-dry period lasts for six months: it begins in the third ten-day period of April and reaches to the second tenday period of October. From the second ten-day period of June to the third of August constitutes the high-summer-dry period; the periods before and after that stretch of timelasting for five ten day periods each-constitute the first and the second low-summer-dry periods.

Principal component analysis of the lamb catalogue of daily weather types: Part 1, annual frequencies

AbstractPrincipal component analysis is applied to annual weather type frequencies for the British Isles. The results objectively define characteristic relationships between the different types, and are used to determine changes in the atmospheric circulation in the region of the British Isles over the period 1861 to 1980. It is shown that, as far as annual frequencies are concerned, only six (of the 27) weather types are needed to define and monitor changes in the atmospheric circulation.The component amplitudes are correlated with temperature and rainfall data, and physically reasonable relationships are identified. The analysis is extended to the period 1781 to 1785, and similarities are found between the extremes experienced during that period and during the 1970s.

Fluctuations climatiques : une étude synoptique du climat, fin XVIIIe-début XIXe siècle

L'étude des fluctuations climatiques pendant les temps historiques est fonction de la découverte et de la confrontation de sources adéquates et fiables. E. Le Roy Ladurie, utilisant des données quantitatives soit directement, soit après traitement, a dessiné un synopsis magistral de l'histoire du climat au cours du dernier millénaire. On a enregistré, quoique de façon irrégulière au début, des observations instrumentales à travers une bonne partie de l'Europe pendant le dernier tiers de ce millénaire. Le regroupement et le recoupement minutieux d'un tel matériel ont permis de reconstituer des séries de longue durée pour plusieurs régions d'Europe ; ainsi pour l'Angleterre centrale à partir de 1659 et pour l'Europe centrale à partir de 1761.

A Synoptic Climatology for Environmental Baseline Analysis: New Orleans

Abstract Standard climatological observations made between 1971 and 1974 at Moisant Airport, New Orleans, have been organized into a calendar of eight synoptic weather types identified from the daily surface weather maps of the United States. The hourly data have been reorganized into tables representing monthly frequencies of weather types, mean properties at 0600 and 1500 CST, and relationships to precipitation. The climatic calendar and associated climatic properties by weather types represent climatic baseline inventories useful for evaluating favorable conditions for activities dependent upon weather such as construction and recreation, local and regional climatic variation by weather type and season, climatic variability through time, and the responses of environmental and biological processes and activities at the earth-atmosphere interface.