Showing papers in "International Journal of Climatology in 2006"

Guidelines for assessing the suitability of spatial climate data sets

Abstract: Spatial climate data are often key drivers of computer models and statistical analyses, which form the basis for scientific conclusions, management decisions, and other important outcomes. The recent availability of very high-resolution climate data sets raises important questions about the tendency to equate resolution with realism. This paper discusses the relationship between scale and spatial climate-forcing factors, and provides background and advice on assessing the suitability of data sets. Spatial climate patterns are most affected by terrain and water bodies, primarily through the direct effects of elevation, terrain-induced climate transitions, cold air drainage and inversions, and coastal effects. The importance of these factors is generally lowest at scales of 100 km and greater, and becomes greatest at less than 10 km. Except in densely populated regions of developed countries, typical station spacing is on the order of 100 km. Regions without major terrain features and which are at least 100 km from climatically important coastlines can be handled adequately by most interpolation techniques. Situations characterized by significant terrain features, but with no climatically important coastlines, no rain shadows, and a well-mixed atmosphere can be reasonably handled by methods that explicitly account for elevation effects. Regions having significant terrain features, and also significant coastal effects, rain shadows, or cold air drainage and inversions are best handled by sophisticated systems that are configured and evaluated by experienced climatologists. There is no one satisfactory method for quantitatively estimating errors in spatial climate data sets, because the field that is being estimated is unknown between data points. Perhaps the best overall way to assess errors is to use a combination of approaches, involve data that are as independent from those used in the analysis as possible, and use common sense in the interpretation of results. Data set developers are encouraged to conduct expert reviews of their draft data sets, which is probably the single most effective way to improve data set quality. Copyright  2006 Royal Meteorological Society.

Temperature and precipitation variability in Italy in the last two centuries from homogenised instrumental time series

Abstract: The Italian monthly temperature (mean, maximum and minimum) and precipitation secular data set was updated and completely revised. Station density and metadata availability were greatly improved and the series were subjected to a detailed quality control and homogenisation procedure. The data homogenisation is described in detail. The bias affecting original data is quantified by studying the temporal evolution of the mean adjustments applied to the series and examined in the light of the stations history. The results stress the importance of homogenisation in climate change studies. The final data set was clustered into climatically homogeneous regions by means of a Principal Component Analysis. Yearly and seasonal trend analyses were performed both on regional average series and on the mean Italian series. The results highlight a positive trend for mean temperature of about 1 K per century all over Italy; it is generally higher for minimum temperature than for the maximum temperature. The progressive application of trend analysis shows that, in the last 50 years, behaviour is the opposite; the maximum temperature trend being stronger than that of the minimum temperature. This has led to a negative trend in the daily temperature range that for the last 50 years has become positive. Precipitation shows a decreasing tendency, even if low and rarely significant, the negative trend being only 5% per century on a yearly basis. Copyright  2006 Royal Meteorological Society.

Comparison of seven meteorological indices for drought monitoring in Iran

Abstract: Drought monitoring is an essential component of drought risk management. It is normally performed using various drought indices that are effectively continuous functions of rainfall and other hydrometeorological variables. A number of drought indices have been introduced and applied in different countries to date. This paper compares the performance of seven indices for drought monitoring in the Tehran province of Iran. The indices used include deciles index (DI), percent of normal (PN), standard precipitation index (SPI), China-Z index (CZI), modified CZI (MCZI), Z-Score and effective drought index (EDI). The comparison of indices is based on drought cases and classes that were detected in the province over the 32 years of data, as well as over the latest 1998–2001 drought spell. The results show that SPI, CZI and Z-Score perform similarly with regard to drought identification and respond slowly to drought onset. DI appears to be very responsive to rainfall events of a particular year, but it has inconsistent spatial and temporal variation. The SPI and EDI were found to be able to detect the onset of drought, its spatial and temporal variation consistently, and it may be recommended for operational drought monitoring in the Province. However, the EDI was found to be more responsive to the emerging drought and performed better. Copyright © 2006 Royal Meteorological Society

Downscaling from GCM precipitation: a benchmark for dynamical and statistical downscaling methods

Abstract: A precipitation downscaling method is presented using precipitation from a general circulation model (GCM) as predictor. The method extends a previous method from monthly to daily temporal resolution. The simplest form of the method corrects for biases in wet-day frequency and intensity. A more sophisticated variant also takes account of flow-dependent biases in the GCM. The method is flexible and simple to implement. It is proposed here as a correction of GCM output for applications where sophisticated methods are not available, or as a benchmark for the evaluation of other downscaling methods. Applied to output from reanalyses (ECMWF, NCEP) in the region of the European Alps, the method is capable of reducing large biases in the precipitation frequency distribution, even for high quantiles. The two variants exhibit similar performances, but the ideal choice of method can depend on the GCM/reanalysis and it is recommended to test the methods in each case. Limitations of the method are found in small areas with unresolved topographic detail that influence higher-order statistics (e.g. high quantiles). When used as benchmark for three regional climate models (RCMs), the corrected reanalysis and the RCMs perform similarly in many regions, but the added value of the latter is evident for high quantiles in some small regions. Copyright © 2006 Royal Meteorological Society.

Consensus between GCM climate change projections with empirical downscaling: precipitation downscaling over South Africa

Abstract: This paper discusses issues that surround the development of empirical downscaling techniques as context for presenting a new approach based on self-organizing maps (SOMs). The technique is applied to the downscaling of daily precipitation over South Africa. SOMs are used to characterize the state of the atmosphere on a localized domain surrounding each target location on the basis of NCEP 6-hourly reanalysis data from 1979 to 2002, and using surface and 700-hPa u and v wind vectors, specific and relative humidities, and surface temperature. Each unique atmospheric state is associated with an observed precipitation probability density function (PDF). Future climate states are derived from three global climate models (GCMs): HadAM3, ECHAM4.5, CSIRO Mk2. In each case, the GCM data are mapped to the NCEP SOMs for each target location and a precipitation value is drawn at random from the associated precipitation PDF. The downscaling approach combines the advantages of a direct transfer function and a stochastic weather generator, and provides an indication of the strength of the regional versus stochastic forcing, as well as a measure of stationarity in the atmosphere–precipitation relationship. The methodology is applied to South Africa. The downscaling reveals a similarity in the projected climate change between the models. Each GCM projects similar changes in atmospheric state and they converge on a downscaled solution that points to increased summer rainfall in the interior and the eastern part of the country, and a decrease in winter rainfall in the Western Cape. The actual GCM precipitation projections from the three models show large areas of intermodel disagreement, suggesting that the model differences may be due to their precipitation parameterization schemes, rather than to basic disagreements in their projections of the changing atmospheric state over South Africa. Copyright  2006 Royal Meteorological Society.

Temporal dynamics of the urban heat island of Singapore

Abstract: The temporal variability of the canopy-level urban heat island (UHI) of Singapore is examined for different temporal scales on the basis of observations during a 1-year period. Temperature data obtained from different urban areas (commercial, Central Business District (CBD), high-rise and low-rise housing) are compared with ‘rural’ reference data and analysed with respect to meteorological variables and differences in land use. The results indicate that the peak UHI magnitude occurs 3–4 h (>6 h) after sunset in the commercial area, (at other urban sites). Higher UHI intensities generally occur during the southwest monsoon period of May–August, with a maximum of ∼7 °C observed in the commercial area under ideal meteorological conditions. Variations in seasonal precipitation explain some of the differences in urban–rural cooling. No clear relationship between urban geometry and UHI intensity can be seen, and intra-urban variations of temperature are also shown to be influenced by other site factors, e.g. the extent of green space and anthropogenic heat. Lastly, results from the present study are compared with UHI data from other tropical and mid-latitude cities. Copyright © 2006 Royal Meteorological Society.

The western mediterranean oscillation and rainfall in the iberian peninsula

Abstract: Seasonal precipitation variability in the east of the Iberian Peninsula is weakly linked to the North Atlantic Oscillation (NAO) during autumn and winter. For the purpose of improving the study of its performance, low-frequency variability patterns specific to the Mediterranean basin have been searched for. In this way, the Western Mediterranean Oscillation (WeMO) has been defined by means of the dipole composed, in its positive phase, by the anticyclone over the Azores and the depression over Liguria, and its index (WeMOi), as a result of the difference of the standardised values in surface atmospheric pressure in San Fernando (Spain) and Padua (Italy). This new index allows the detection of the variability relevant to the cyclogenesis next to the western Mediterranean basin, which determines in a predominant way the types of rainfall in the Gulf of Valencia. In this area, the WeMO is significantly better than the NAO to explain the monthly pluviometric anomalies during these seasons. Also, a daily resolution of the WeMOi can provide a useful tool to forecast torrential rainfall events in the north-western zones of the Mediterranean (eastern part of the Iberian Peninsula and the south of France), and such significantly daily rainfall frequencies for different thresholds. Copyright © 2006 Royal Meteorological Society.

Thin plate smoothing spline interpolation of daily rainfall for New Zealand using a climatological rainfall surface

Abstract: This study presents a method for estimating daily rainfall on a 0.05° latitude/longitude grid covering all of New Zealand for the period 1960–2004 using a second order derivative trivariate thin plate smoothing spline spatial interpolation model. Use of a hand-drawn (and subsequently digitised) mean annual rainfall surface as an independent variable in the interpolation is shown to reduce the interpolation error compared with using an elevation surface. This result is confirmed when long-term average annual rainfall data, derived from the daily interpolations, are validated using long-term river flow data. Copyright © 2006 Royal Meteorological Society

Downscaling heavy precipitation over the United Kingdom: A comparison of dynamical and statistical methods and their future scenarios

Abstract: Six statistical and two dynamical downscaling models were compared with regard to their ability to downscale seven seasonal indices of heavy precipitation for two station networks in northwest and southeast England. The skill among the eight downscaling models was high for those indices and seasons that had greater spatial coherence. Generally, winter showed the highest downscaling skill and summer the lowest. The rainfall indices that were indicative of rainfall occurrence were better modelled than those indicative of intensity. Models based on non-linear artificial neural networks were found to be the best at modelling the inter-annual variability of the indices; however, their strong negative biases implied a tendency to underestimate extremes. A novel approach used in one of the neural network models to output the rainfall probability and the gamma distribution scale and shape parameters for each day meant that resampling methods could be used to circumvent the underestimation of extremes. Six of the models were applied to the Hadley Centre global circulation model HadAM3P forced by emissions according to two SRES scenarios. This revealed that the inter-model differences between the future changes in the downscaled precipitation indices were at least as large as the differences between the emission scenarios for a single model. This implies caution when interpreting the output from a single model or a single type of model (e.g. regional climate models) and the advantage of including as many different types of downscaling models, global models and emission scenarios as possible when developing climate-change projections at the local scale.

Development of scale‐free climate data for Western Canada for use in resource management

Abstract: Applying climate data in resource management requires matching the spatial scale of the climate and resource databases. Interpolating climate data in mountainous regions is difficult. In this study, we present methodology to generate scalefree climate data through the combination of interpolation techniques and elevation adjustments. We apply it to monthly temperature and precipitation normals for 1961–90 produced by the Parameter-elevation Regressions on Independent Slopes Model (PRISM) for British Columbia, Yukon Territories, the Alaska Panhandle, and parts of Alberta and the United States. Equations were developed to calculate biologically relevant climate variables including various degree-days, number of frost-free days, frost-free period, and snowfall from monthly temperature and precipitation data. Estimates of climate variables were validated using an independent dataset from weather stations that were not included in the development of the model. Weather station records generally agreed well with estimated climate variables and showed significant improvements over original PRISM climate data. A stand-alone MS Windows application was developed to perform all calculations and to integrate future climate predictions from various global circulation models. We demonstrate the use of this application by showing how climate change may affect lodgepole pine seed planning zones for reforestation in British Columbia. Copyright  2006 Royal Meteorological Society.

Flood frequency in China's Poyang Lake region: trends and teleconnections

Abstract: Poyang Lake in Jiangxi Province is the largest freshwater lake in China and is historically a region of significant floods. Annual events of peak lake stage and of severe floods have increased dramatically during the past few decades. This trend is related primarily to levee construction at the periphery of the lake and along the middle of the Changjiang (Yangtze River), which protects a large rural population. These levees reduce the area formerly available for floodwater storage resulting in higher lake stages during the summer flood season and catastrophic levee failures. The most severe floods in the Poyang Lake since 1950, and ranked in descending order of severity, occurred in 1998, 1995, 1954, 1983, 1992, 1973, and 1977. All of these floods occurred during or immediately following El Ni˜ no events, which are directly linked to rainfall in central China. The 2-year recurrence interval for maximum annual lake stage during El Ni˜ no years is 1.2 m higher than during non-El Ni˜ no years. The 10-year recurrence interval is 1.4 m higher during El Ni˜ no years than during non-El Ni˜ no years. Copyright  2006 Royal Meteorological Society.

Variability of precipitation in the Atacama Desert: its causes and hydrological impact

Abstract: An analysis of the variability of rainfall at 27 stations and run-off at 4 stations between 18° and 28°S in the Atacama Desert has been carried out. A diagonal boundary zone between summer- and winter-dominated areas is related to the provenance of the rainfall: Amazonia to the north and east, and Pacific moisture to the south. It is shown that winter rainfall tends to be higher during El Nino years, while heavy summer rainfall tends to be more common during La Nina. However, rather than the precipitation being directly controlled by El Nino-Southern Oscillation (ENSO), previous studies have shown that it is the regional synoptic conditions towards the source areas that largely control temporal precipitation variations, and these are in turn either facilitated or inhibited by ENSO. The spatio-temporal variability of precipitation leads to a complex hydrological regime. Perennial rivers in the north and central Atacama Desert tend to flood in summer, especially during La Nina conditions, from source to sea. Perennial rivers in the south tend to flood in summer, but as a result of melt from the previous years snowfall, especially during El Nino conditions, again from source to sea. However, while inland areas may also experience flooding of ephemeral rivers in summer associated with La Nina, coastal areas on the other hand experience winter flooding of ephemeral rivers associated with El Nino. Surface water flood events, and groundwater recharge events reported in the literature, are generally less frequent than ENSO events, confirming the requirement for specific synoptic conditions and making the use of averages unsound for present-day hydrological studies and water resource evaluations. Copyright © 2006 Royal Meteorological Society.

Two‐dimensional indices of atmospheric blocking and their statistical relationship with winter climate patterns in the Euro‐Atlantic region

Abstract: The statistical relationship between the leading climate patterns of mid-tropospheric flow and atmospheric blocking over the Euro–Atlantic region during winter is investigated using three new two-dimensional blocking indicators. The focus is on the leading climate pattern of the 500-hPa geopotential variability, i.e. the North Atlantic Oscillation (NAO). The results indicate that the blocking-NAO relation is not restricted to the North Atlantic region, where blocking and the NAO are known to be out of phase. All three indicators show that the positive NAO phase is characterised by an enhanced occurrence of blocking-type high-pressure systems over the European mainland. The sign change of the NAO-blocking relation from west to east is well detectable with the two-dimensional blocking indicators and it is found further south than at the traditionally studied blocking latitudes near 60°N. The analysis of blocking events by seasonal NAO indices leads to similar (albeit less significant) results as with a daily NAO index stratification. This indicates that the relation between the NAO and blocking is fairly insensitive to the chosen time resolution. The investigation is extended from the second to fourth pattern of the mid-tropospheric flow variability using empirical orthogonal function (EOF) patterns. It reveals that one phase of each of the major Euro–Atlantic climate patterns is collocated with the region of maximum blocking frequency. The clearest separation between positive (negative) EOF phases and blocking (no blocking) situations is found for EOF × 2 and 3 and is associated with changes from zonal to ridge-like flow, similar to the so-called northern European ‘blocking signature’. This is an indication that the purely statistically defined EOF patterns are related to the physical blocking phenomenon. Copyright © 2005 Royal Meteorological Society.

Atmospheric conditions associated with the exceptional fire season of 2003 in Portugal

Abstract: The summer of 2003 was characterised by exceptional warm weather in Europe, particularly during the first two weeks of August, when a devastating sequence of large fires was observed, reaching an amount of circa 450 000 ha, the largest figure ever recorded in Portugal in modern times. They were concentrated in two relatively confined regions of Portugal and a considerable proportion of burnt area was due to fires started on the 2nd and 3rd of August. It is shown that the 850 hPa temperature values observed over Portugal for the 1st and 2nd of August 2003 were the highest recorded since 1958. Using data from synoptic stations covering the entire Portuguese territory, the event was characterised in fine detail, including the evolution of both maximum and minimum temperatures, surface relative humidity, and wind anomaly fields. The different spatial extent of maximum and minimum temperatures is analysed leading to the new all-time Portuguese records of 47.3 °C for maximum and 30.6 °C for minimum temperatures that were recorded on the 1st of August near the main area of occurrence of the largest fire. Finally, it is shown that the summer of 2003 was preceded by a wet winter followed by a very dry month of May, a precipitation anomalous regime that contributed to a climatic background that favoured the role played by the early August heatwave and the associated meteorological surface conditions. Copyright © 2006 Royal Meteorological Society

Climatic behavior of various urban parks during hot and humid summer in the mediterranean city of Tel Aviv, Israel

Abstract: This study examines the climatic behavior of different designs of urban parks during hot and humid summer conditions, and their influence on human comfort in Tel Aviv, Israel. The research was conducted in three different types of urban parks: a park with grass and a few low trees, a park with medium sized trees and a park with high and wide-canopied trees. The results showed that an urban park that contains high trees with a wide canopy has the maximum cooling effect during daytime, reduces temperatures by up to 3.5 °C and lowers heat stress values despite increasing relative humidity values. An urban park that contains dense, medium sized trees can also reduce temperatures during daytime by up to 2.5 °C as well as slightly lower heat stress. However, during nighttime it can create uncomfortable climatic conditions owing to the reduction of wind velocity and increase in relative humidity. An urban park covered with grass can be warmer and sometimes even more humid than the built-up area during the day, which increases heat stress values. Copyright © 2006 Royal Meteorological Society.

Europe's 2003 heat wave: a satellite view of impacts and land–atmosphere feedbacks

Abstract: A combination of satellite imagery, meteorological station data, and the NCEP/NCAR reanalysis has been used to explore the spatial and temporal evolution of the 2003 heat wave in France, with focus on understanding the impacts and feedbacks at the land surface. Vegetation was severely affected across the study area, especially in a swath across central France that corresponds to the Western European Broadleaf (WEB) Forests ecological zone. The remotely sensed surface temperature anomaly was also greatest in this zone, peaking at +15.4 °C in August. On a finer spatial scale, both the vegetation and surface temperature anomalies were greater for crops and pastures than for forested lands. The heat wave was also associated with an anomalous surface forcing of air temperature. Relative to other years in record, satellite-derived estimates of surface-sensible heat flux indicate an enhancement of 48–61% (24.0–30.5 W m−2) in WEB during the August heat wave maximum. Longwave radiative heating of the planetary boundary layer (PBL) was enhanced by 10.5 W m−2 in WEB for the same period. The magnitude and spatial structure of this local heating is consistent with models of the late twenty-first century climate in France, which predict a transitional climate zone that will become increasingly affected by summertime drought. Models of future climate also suggest that a soil-moisture feedback on the surface energy balance might exacerbate summertime drought, and these proposed feedback mechanisms were tested using satellite-derived heat budgets. Copyright © 2006 Royal Meteorological Society.

The behavior of extreme cold air outbreaks under greenhouse warming

Abstract: Climate model output is used to analyze the behavior of extreme cold-air outbreaks (CAOs) under recent and future climatic conditions. The study uses daily output from seven GCMs run under late-twentieth century and projected twenty-first century radiative conditions (SRES A1B greenhouse gas emission scenario). We define a CAO as an occurrence of two or more consecutive days during which the local mean daily surface air temperature is at least two standard deviations below the local wintertime mean temperature. In agreement with observations, the models generally simulate modern CAOs most frequently over western North America and Europe and least commonly over the Arctic. These favored regions for CAOs are located downstream from preferred locations of atmospheric blocking. Future projections indicate that CAOs—defined with respect to late-twentieth century climatic conditions—will decline in frequency by 50 to 100% in most of the Northern Hemisphere during the twenty-first century. Certain regions, however, show relatively small changes and others actually experience more CAOs in the future, due to atmospheric circulation changes and internal variability that counter the thermodynamic tendency from greenhouse forcing. These areas generally experience greater near-surface wind flow from the north or the continent during the twenty-first century and/or are especially prone to atmospheric blocking events. Simulated reductions in CAOs are smallest in western North America, the North Atlantic, and in southern regions of Europe and Asia. The Eurasian pattern is driven by a strong tendency for the models to produce sea-level pressure (SLP) increases in the vicinity of the Mediterranean Sea (intermodel mean of 3 hPa), causing greater advection of continental air from northern and central Asia, while the muted change over western North America is due to enhanced ridging along the west coast and the increased frequency of blocking events. The North Atlantic response is consistent with a slowdown of the thermohaline circulation, which either damps the warming regionally or results in a cooler mean climate in the vicinity of Greenland. Copyright © 2006 Royal Meteorological Society.

On the Asian expression of the PDO

Abstract: The causes and mechanisms of Pacific Decadal Variability (PDV) are still being investigated. One reason that such variability is not better understood is the scarcity of high-resolution paleoclimatic records from the Asian side of the North Pacific. Here we present a reconstruction of the boreal spring (March–May) Pacific Decadal Oscillation (PDO) index, spanning A.D. 1565–1988, that is the first to represent the large-scale Asian expression of this phenomenon using tree-ring data. Intervention analysis reveals significant (95% level) regime shifts corresponding to those in the instrumental PDO during the mid-1920s, mid-1940s and mid-1970s, the latter period associated with the famed 1976 shift in Pacific climate. Shifts in the preinstrumental period show varying correspondence with those of a North American-based treering reconstruction of the North Pacific index (NPI), another indicator of Pacific decadal climate variability. Differences between these two time series hint at modulation of local climate from Asian monsoon, El Ni˜ no-Southern Oscillation (ENSO) and volcanic forcing, which could be partly masked by combining data from these sensitive regions in future PDO reconstructions. Overall, however, comparison of the reconstructions from both Asia and North America (NA) is useful for evaluating the distinct expressions of the PDO on both sides of the North Pacific and their interactions with the tropics. Copyright  2006 Royal Meteorological Society.

The development of a new dataset of Spanish Daily Adjusted Temperature Series (SDATS) (1850–2003)

Abstract: Here we present the development of a new adjusted dataset composed of the 22 longest and most reliable Spanish daily temperature records (maximum and minimum temperatures and derived daily mean temperature) covering the period 1850–2003. The paper describes the approaches followed for compiling, controlling the quality and homogenising these 22 daily Spanish records, leading to the creation of a dataset called ‘Spanish Daily Adjusted Temperature Series’ (SDATS). An assessment of the sources of data and metadata used is followed by a reliability assessment of the selected network. Data quality control (QC) procedures applied to raw daily maximum (Tmax) and minimum (Tmin) temperatures and their results are then considered. For the very first time, an empirical minimisation of the bias related to the impact of changing exposure of thermometers on the records has been undertaken. The application of the Standard Normal Homogeneity Test (SNHT) to check homogeneity of raw Tmax and Tmin data on a monthly basis is presented, together with a discussion of the causes, magnitudes and timings of the various inhomogeneities. All 22 records contained a number of inhomogeneities (2.6 on average), mainly associated with documented station relocations confirmed by the metadata available. Monthly adjustments calculated for both screen developments and from the SNHT were linearly interpolated to a daily basis following the Vincent et al. (2002) scheme. Finally, the procedures adopted for creating the regional average, the Spanish Temperature Series (STS), together with an exploratory analysis of long-term trends of each Tmax and Tmin records, are provided. The final analysis shows that over mainland Spain highly significant rates of temperature increases have occurred for Tmax and Tmin (0.12 °C/decade and 0.10 °C/decade, respectively) over 1850–2003. Copyright © 2006 Royal Meteorological Society.

Observed trends in daily precipitation indices in South Africa: 1910-2004

Abstract: The spatial variations of trends of relevant extreme precipitation indices were examined for 138 rainfall stations in South Africa for the period 1910 to 2004, a follow-up study of various others on precipitation trends in the region. This study aims to inform on trends in daily extreme precipitation indices, over as long a period with instrumental data as possible, while also retaining as sufficient a spatial coverage of results as possible. Data should also be forthcoming from climate stations with the time series spanning the complete study period, avoiding patching of data which can cause inhomogeneities. Some significant changes in indices, averaged over specific areas in South Africa, could be identified. These include areas with significant increases and decreases in annual precipitation, increases in the longest annual dry spell indicating more extreme dry seasons, increases in the longest annual wet spells indicating more extreme wet seasons, and increases in high daily precipitation amounts. The conclusion is that, while in the largest part of South Africa there has been no real evidence of changes in precipitation over the past century, there are however some identifiable areas where significant changes in certain characteristics of precipitation have occurred over the period 1910 to 2004. Copyright © 2006 Royal Meteorological Society

Recreation climate assessment

Abstract: The study examines methods for assessing the atmospheric resource component of recreation environments. Beach use is selected as it is a highly weather sensitive recreational activity. The study area is King's Beach in Queensland, Australia. The nature of the relationship between beach climate and the enjoyment of recreational pursuits is taken to be a function of thermal, physical, and aesthetic components of the atmospheric environment. A body-atmosphere heat-budget model is used to integrate and isolate the thermal component of beach weather and enable identification of important non-thermal recreational resource attributes of the atmosphere. Beach-user sensory perception of on-site atmospheric conditions expressed verbally is used to assess the physical and aesthetic components of the atmospheric environment. The immediate thermal environment of the beach user is the main contributing factor to assessments of the desirability of on-site meteorological conditions, followed by the non-thermal effects of cloud and wind. Rainfall events of half-hour duration or longer have an overriding effect.

Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic

Abstract: An analysis of the late twentieth century and the twenty-first century predictions of Antarctic circulation patterns in a ten model ensemble of global climate system models, using the method of self-organizing maps (SOMs), is presented. The model simulations, and this analysis, were conducted in support of the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report. An application of the methodology to twentieth century reanalyses resulted in conclusions congruent with earlier synoptic climatologies that used different techniques. It was found that the SOM method is a useful tool for diagnosing differences among a large cohort of often quite divergent simulations, allowing the rapid identification of outliers. Some models do rather poorly at simulating the present day Antarctic circulation, but taking these outliers into account and considering the models as an ensemble, the simulations of twentieth century circulation are reasonable. The most notable outlier was the Centre National de Recherches Meteorologiques model (also known as ARPEGE), which was unable to simulate Southern Ocean cyclones. Also of interest was the excessive cyclogenesis displayed by the National Center for Atmospheric Research CCSM3 model. The trend towards increasing cyclonicity and stronger zonal winds is quite consistent among models, and is also reflected in an increase in positive Antarctic oscillation (AAO) index. The coherence of temperature and precipitation anomaly patterns and their trends reflects the extent to which these are related to circulation. It is clear from this analysis that several of the models in this ensemble are capable of predicting the Antarctic Peninsula warming as the rest of the continent cools—this is an important advance in our simulation capacity. Copyright © 2006 Royal Meteorological Society.

An event‐based jet‐stream climatology and typology

Abstract: A novel climatology is developed for upper-tropospheric jet streams, which is complementary to and an alternative for the traditional depictions of the time-mean jets. It entails identifying the occurrence of a jet event at a given location and then compiling the spatial frequency distribution of such events. The resulting climatology, derived using the ERA-15 reanalysis data set of the ECMWF for the period 1979–1993 indicates that (1) in both hemispheres the annual cycle of jet events takes the form of comparatively smooth transition from a quasi-annular structure in summer to a more spiral-like structure in winter with a temporally asymmetric return to the summer pattern; (2) the hemispheres differ primarily in the amplitude of the frequencies and the longitudinal overlap of the spiral portion of the pattern. In addition, the jet events are subdivided using a two-class typology comprising shallow and deep jets whose vertical shear (sic. baroclinicity) are/are not confined principally to the upper troposphere. This provides a conceptually simple and dynamically meaningful classification since deep jets are more likely to spawn tropospheric-spanning cyclones. The accompanying climatology displays important longitudinal variations and significant inter-hemispheric differences. A comparison is drawn between these new and conventional climatologies and typologies. Also, comments are proffered on the relationship between, on the one hand, the patterns of jet frequency including the differing distributions of the shallow and deep types and, on the other hand, the location of the time-mean jets and the downstream storm tracks. Copyright © 2006 Royal Meteorological Society.

Regional and global atmospheric patterns governing rainfall in the southern Levant

Abstract: This study attempts to find a linkage between the interannual variations of the rainfall measured in 12 stations spread over the northern half of Israel (the southern Levant) and atmospheric circulations ranging from regional to global scale The analysis was done for the midwinter months, December–February, in which two-thirds of the annual rainfall occurs, during the years 1950–2002 The study is based on composite maps for extremely dry/wet seasons and on maps of correlation between atmospheric variables and the rainfall time series Our results show that an upper-level trough extending from Eastern Europe toward the Eastern Mediterranean (EM) is closely linked with the seasonal rainfall over the study area This is expressed by a correlation of −074 between the 500-hPa geopotential height at 325°N, 35°E and the rainfall This trough has two effects on the southern Levant rainfall: one is the dynamics implied by the upper trough and the other is the cool advection over the EM imparted by the northwesterly flows induced by the trough The latter presumably enhances the atmospheric instability when it sweeps over the warmer waters of the EM The upper trough was found to be associated with three global factors: the polar stratospheric jets, in both the Northern and Southern Hemispheres, and the SST variations over the western tropical Pacific Ocean, represented by the ‘Pacific Warm Pool’ index The EM trough is accompanied by a ridge covering western Europe, so that cold and wet winters in the southern Levant coincide with warm and dry winters over western Europe and vice versa Copyright © 2005 Royal Meteorological Society

An assessment of trends in UK runoff and low flows using a network of undisturbed catchments

Abstract: In many parts of the world attempts to discern climatic-driven changes in river flow patterns are hindered by the combined impact of other, more direct anthropogenic disturbances such as abstraction and impoundments. This study capitalises on a newly defined ‘benchmark’ network of natural catchments in the UK to discern natural variability in flow regimes. Trend tests were applied to time series of runoff and indicators of low-flow magnitude and duration for two study periods to assess the sensitivity of trends to the frame of reference over which tests were conducted. Notwithstanding the volatility of the recent past, the results demonstrate a general stability in runoff and low flows since the early 1960s. The strongest signal to emerge from the study is a positive runoff trend for catchments in Scotland, which was resilient to the effect of the study periods. There was also some indication of increasing runoff in maritime western areas of England and Wales. These increases in maritime areas are likely to reflect the dominance of westerly airflows in the recent past, associated with an increase in the North Atlantic Oscillation (NAO) Index. For low flows, there were no compelling trends—significant positive trends over the 1973–2002 period are influenced by a sequence of notably dry years at the start of the period and were not observed over a 40-year time-frame. There are some indications of a tendency towards decreasing low flows in some eastern catchments, but this is supported by few significant results. The results of this study provide a baseline against which to assess longer-term change from historical flow time series and to monitor future change in the benchmark network. Copyright © 2006 Royal Meteorological Society

The summer monsoon onset process over South Asia and an objective method for the date of monsoon onset over Kerala

Abstract: Eight pentads before the monsoon onset over Kerala (MOK), a spatially large area of deep convection formed near the equator south of the Bay of Bengal, which moved to Southeast Asia marking the onset of the South China Sea monsoon (SCSM) for many years. Three pentads before MOK, a similar area of convection formed near the equator south of the Arabian Sea. This heat source and the associated cross-equatorial low-level jet stream (LLJ) grew steadily in strength while moving north and at MOK the convective heat source passed through Kerala latitudes and the core of a well developed LLJ was located just south of Kerala. Eight pentads before MOK a warm pool was located over central Bay of Bengal and the area of active convection formed to its south near the equator in the region of large sea surface temperature (SST) gradient. Three pentads before MOK when the Bay of Bengal SST had cooled, a warm pool formed over central Arabian Sea and an active convection area was located south of it, also in the region of large SST gradient. A three-step method for objectively defining MOK has been developed in this paper. In step 1 of this operationally usable method, the date on which the zonal wind of 850 hPa, averaged over a box bounded by latitudes 5°N and 10°N and longitudes 70°E and 85°E, reached 6 m/s at 600 hPa is taken as the tentative date of MOK. Steps 2 and 3 checked whether the date thus chosen was a bogus monsoon onset or not and whether on that date there was widespread convection (low OLR) around Kerala, which moved north from the equatorial region. Copyright © 2006 Royal Meteorological Society.

The role of synoptic‐scale circulation in the linkage between large‐scale ocean–atmosphere indices and winter surface climate in British Columbia, Canada

Abstract: In much of North America, variables such as temperature, precipitation, snowpack and streamflow are modulated by modes of large-scale ocean-atmosphere variability such as the Pacific Decadal Oscillation (PDO), El Nino-Southern Oscillation (ENSO) and the Pacific North American Pattern (PNA). In this study, we test the hypothesis that the influence of these modes on air temperature and precipitation in British Columbia (BC), Canada, can be explained in relation to changes in frequencies of synoptic-scale circulation types. A catalogue of 13 circulation types was derived by classifying daily mean sea-level pressure (MSLP) grids from 1948 to 2003. The grids cover BC and the North Pacific and were subjected to a standard pattern recognition algorithm employing principal component analysis followed by cluster analysis on the component scores. The circulation types are generally associated with distinctive patterns of precipitation and air temperature anomalies across BC. Multiple linear regressions for selected stations in BC using the type frequencies as predictors explain up to 75% of the variance of mean winter temperature and 65% of winter precipitation. The frequencies of most circulation types vary significantly between the different phases of ENSO, PDO and PNA in a manner consistent with the temperature anomalies associated with those modes and, to a lesser extent, with the more complex precipitation anomalies. In addition, however, average temperatures and precipitation amounts for some circulation types differ systematically between phases of ENSO and PDO. Subsequent analysis revealed distinct differences among ENSO and PDO phases in the upper-level circulation patterns associated with some surface types. A major part of the teleconnections can be explained through variations in the frequencies of synoptic-scale circulation types, but systematic within-type variability, particularly with PDO and PNA, can additionally influence the surface climate. Copyright © 2005 Royal Meteorological Society.

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

Abstract: This 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.

Episodes of alpine heavy precipitation with an overlying elongated stratospheric intrusion: a climatology

Abstract: This study discusses the role of stratospheric intrusions (determined as potential vorticity (PV) streamers) as upper-level instigators of heavy precipitation along the Swiss Alpine south side (AS) on a climatological timescale. A climatology of streamers is used compiled on the basis of the ECMWF 40-year re-analysis data set (ERA-40). Days of extreme and heavy precipitation along the Swiss AS are determined from an existing observational Alpine precipitation climatology. For these days, the presence of streamers over western Europe as well as their location and orientation is recorded. On 73% of the extreme precipitation days, a streamer is situated over western Europe. The mean spatial frequency distribution of the streamers on the extreme precipitation days exhibits a structure that resembles in its form and location the ‘archetypal heavy precipitation streamer’ known from case studies. The frequency maximum is situated over southern England and the west coast of France. The same analysis is applied to three sub-domains (Valais, Grisons, Ticino) along the Swiss AS. Significant differences in the location and the orientation of the streamers for the sub-domains are found. The majority of streamers associated with heavy rain in the western-most sub-domain (Valais) are oriented in a cyclonically-sheared fashion, while for the Ticino the streamers are more anti-cyclonically orientated. Differences for events of increasing severity are analysed by comparing the form, location, amplitude (PV), and persistence of the streamers on extreme and heavy precipitation days. The precipitation distribution is shifted to higher intensities for more persistent streamers. There is no detectable difference found in the form parameters, but the southerly moisture flux into the domain is significantly larger during extreme precipitation days than during heavy precipitation days. Likewise, the seasonal variation in the percentage of streamer-related heavy precipitation, which is highest in autumn (85%), can be related to the seasonal variation of southerly moisture fluxes. Copyright  2006 Royal Meteorological Society.

Simulating air temperature in an urban street canyon in all weather conditions using measured data at a reference meteorological station

Abstract: A model is proposed that adapts data from a standard meteorological station to provide realistic site-specific air temperature in a city street exposed to the same meso-scale environment. In addition to a rudimentary description of the two sites, the canyon air temperature (CAT) model requires only inputs measured at standard weather stations; yet it is capable of accurately predicting the evolution of air temperature in all weather conditions for extended periods. It simulates the effect of urban geometry on radiant exchange; the effect of moisture availability on latent heat flux; energy stored in the ground and in building surfaces; air flow in the street based on wind above roof height; and the sensible heat flux from individual surfaces and from the street canyon as a whole. The CAT model has been tested on field data measured in a monitoring program carried out in Adelaide, Australia, in 2000–2001. After calibrating the model, predicted air temperature correlated well with measured data in all weather conditions over extended periods. The experimental validation provides additional evidence in support of a number of parameterisation schemes incorporated in the model to account for sensible heat and storage flux. Copyright © 2006 Royal Meteorological Society