Showing papers in "International Journal of Climatology in 2010"

A 50-year high-resolution atmospheric reanalysis over France with the Safran system

Abstract: The assessment of regional climate change requires the development of reference long-term retrospective meteorological datasets. This article presents an 8-km-resolution atmospheric reanalysis over France performed with the the Safran-gauge-based analysis system for the period 1958–2008. Climatological features of the Safran 50-year analysis – long-term mean values, inter-annual and seasonal variability – are first presented for all computed variables: rainfall, snowfall, mean air temperature, specific humidity, wind speed and solar and infrared radiation. The spatial patterns of precipitation, minimum and maximum temperature are compared with another spatialization method, and the temporal consistency of the reanalysis is assessed through various validation experiments with both dependent and independent data. These experiments demonstrate the overall robustness of the Safran reanalysis and the improvement of its quality with time, in connection with the sharp increase in the observation network density that occurred in the 1990s. They also show the differentiated sensitivity of variables to the number of available ground observations, with precipitation and air temperature being the more robust ones. The comparison of trends from the reanalysis with those from homogenized series finally shows that if spatial patterns are globally consistent with both approaches, care must be taken when using literal values from the reanalysis and corresponding statistical significance in climate change detection studies. The Safran 50-year atmospheric reanalysis constitutes a long-term forcing datasets for land surface schemes and thus enables the simulation of the past 50 years of water resources over France. Copyright © 2009 Royal Meteorological Society

Climate change and the precipitation variations in the northwestern Himalaya: 1866–2006

Abstract: Using available instrumental records, this paper examines the variation of precipitation from 1866 to 2006 in the northwestern Himalaya (NWH). The study indicates no trend in the winter precipitation but significant decreasing trend in the monsoon precipitation during the study period. Periodicities on a multi-decadal scale (29–34 years and 58–64 years) obtained in power spectrum analyses point towards epochal behaviour in the precipitation series. Analyses of the temperature data show significant increasing trends in annual temperature in all three stations in the NWH during the data period. Warming effect is particularly noteworthy during the winter season. Negative relationships between mean winter air temperature and snowfall amounts recorded at different meteorological stations in this period reveal strong effect of rising temperatures on the decreasing snowfall component in total winter precipitation, reducing effective duration of winter on the windward side of the Pir Panjal Himalayan Range. The study also shows influence of global teleconnections [North-Atlantic Oscillation (NAO) during winter months and Southern Oscillation Index (SOI) during the monsoon months] on precipitation fluctuations in the NWH. The teleconnections that appear to exist between the precipitation and the temperature until the late 1960s seem to have weakened considerably in the last three decades. This may be ascribed to the diminishing effect of the natural factors such as quasi-biennial oscillations (QBO), El Nino Southern Oscillations (ENSO), double sunspot cycles (Hale), etc., in this period. Role of increasing concentration of greenhouse gases in the atmosphere cannot be ruled out. Copyright © 2009 Royal Meteorological Society

Urbanization signature in the observed heavy rainfall climatology over India.

Abstract: We assess the urbanization impacts on the heavy rainfall climatology during the Indian summer monsoon. While a number of studies have identified the impact of urbanization on local precipitation, a large-scale assessment has been lacking. This relation between urbanization and Indian monsoon rainfall changes is investigated by analyzing in situ and satellite-based precipitation and population datasets. Using a long-term daily rainfall dataset and high-resolution gridded analysis of human population, this study showed a significantly increasing trend in the frequency of heavy rainfall climatology over urban regions of India during the monsoon season. Urban regions experience less occurrences of light rainfall and significantly higher occurrences of intense precipitation compared to nonurban regions. Very heavy and extreme rainfall events showed increased trends over both urban and rural areas, but the trends over urban areas were larger and statistically more significant. Our analysis suggests that there is adequate statistical basis to conclude that the observed increasing trend in the frequency of heavy rainfall events over Indian monsoon region is more likely to be over regions where the pace of urbanization is faster. Moreover, rainfall measurements from satellites also indicate that urban areas are more (less) likely to experience heavier (lighter) precipitation rates compared to those in nonurban areas. While the mechanisms causing this enhancement in rainfall remain to be studied, the results provide the evidence that the increase in the heavy rainfall climatology over the Indian monsoon region is a signature of urban-induced rainfall anomaly. Copyright © 2009 Royal Meteorological Society

Local atmospheric decoupling in complex topography alters climate change impacts

Abstract: Supplementary material included in separate file. This article was originally published in International Journal of Climatology and is copyrighted by the Royal Meteorological Society

Effects of Amazon and Central Brazil deforestation scenarios on the duration of the dry season in the arc of deforestation

Abstract: Climate change predictions tied to Amazon deforestation scenarios are increasingly being used by government and non-government organisations for near-future planning applications. Despite incorporating a wide range of biophysical variables, these models neglect future scenarios of land use for adjoining regions, such as the Central Brazil Cerrado, which has been deforested by more than 50%. In this study, we investigate the impact of different Amazon and Central Brazil deforestation scenarios on the rainfall regime of the ‘arc-of-deforestation’ in Amazonia. We demonstrate that both Amazon and Cerrado deforestation contribute to an increase of the duration of the dry season in this region. Combining the effects of both scenarios, the dry season may increase from 5 months to 6 months, which may change the biosphere–atmosphere equilibrium in this region. This study demonstrates that the assessment of future Cerrado land use scenarios is also necessary to understand the future climate and ecosystem health of Amazonia. Copyright © 2009 Royal Meteorological Society

Changes in hot days and heat waves in China during 1961–2007

Abstract: Based on the daily maximum temperature (DMT) records at 512 stations during 1961–2007, the geographical patterns and temporal variations of hot days (HDs) and heat waves (HWs, including those persisting for 3–5 days and longer) over mainland China were studied. The HD (and hence HW) was defined in two ways, one by an absolute criterion, DMT > 35 °C, as applied in the nationwide meteorological agencies and another in a relative sense, DMT > the 90th percentile threshold of a local daily temperature distribution around the day. Two centers of high frequencies (over 5 days per year) of the absolute HDs during June–September were found in the regions of Xinjiang and the mid-lower reaches of the Yangtze River. The highest frequencies of the absolute HWs were about 1.5 times per year in the Xinjiang region and to the south of the mid-lower reaches of the Yangtze River. The frequencies of the relative HWs were about 1–1.5 times per year in most of China. The HDs and HWs increased significantly during the studied period in most of China, especially over the southeastern coast and northern China (by over 4 days per decade for relative HDs and 0.4 times per decade for relative HWs), but decreased significantly at some stations in the lower reaches of the Yellow River. Over most of China except northwestern China, the frequency of HDs was high during the 1960s–1970s, low in the 1980s, and high afterwards, with strong interannual variations. A remarkable increasing trend of HDs occurred after the 1990s in all regions. The changes in HDs and HWs were closely related to those in rain days and atmospheric circulation patterns at the interannual and interdecadal scales. Copyright © 2009 Royal Meteorological Society

Snow in the McMurdo Dry Valleys, Antarctica

Abstract: Snowfall was measured at 11 sites in the McMurdo Dry Valleys to determine its magnitude, its temporal changes, and spatial patterns. Annual values ranged from 3 to 50 mm water equivalent with the highest values nearest the coast and decreasing inland. A particularly strong spatial gradient exists in Taylor Valley, probably resulting from local uplift conditions at the coastal margin and valley topography that limits migration inland. More snow occurs in winter near the coast, whereas inland no seasonal pattern is discernable. This may be due, again, to local uplift conditions, which are common in winter. We find no influence of the distance to the sea ice edge. Katabatic winds play an important role in transporting snow to the valley bottoms and essentially double the precipitation. That much of the snow accumulation sublimates prior to making a hydrologic contribution underscores the notion that the McMurdo Dry Valleys are indeed an extreme polar desert.

Rainfall variability and the trends of wet and dry periods in Bangladesh

Abstract: Spatial patterns of annual and seasonal rainfall trends of Bangladesh over the time period 1958–2007 has been assessed using rainfall data recorded at 17 stations distributed over the country. Mann–Kendall trend test and the Sen's slope method are used to detect the significance and the magnitude of rainfall change, respectively. Historical dry and wet months are identified by using standardised precipitation index method and their trends are analysed to assess the possible change in wet and dry events in Bangladesh. The result shows a significant increase in the average annual and pre-monsoon rainfall of Bangladesh. The number of wet months is found to increase and the dry months to decrease in most parts of the country. Seasonal analysis of wet and dry months shows a significant decrease of dry months in monsoon and pre-monsoon. Copyright © 2009 Royal Meteorological Society

Variability and trends in the directional wave climate of the Southern Hemisphere

Abstract: The effect of interannual climate variability and change on the historic, directional wave climate of the Southern Hemisphere is presented. Owing to a lack of in situ wave observations, wave climate in the Southern Hemisphere is determined from satellite altimetry and global ocean wave models. Altimeter data span the period 1985 to present, with the exception of a 2-year gap in 1989–1991. Interannual variability and trends in the significant wave height are determined from the satellite altimeter record (1991 to present), and the dominant modes of variability are identified using an empirical orthogonal function (EOF) analysis. Significant wave heights in the Southern Ocean are observed to show a strong positive correlation with the Southern Annular Mode (SAM), particularly during Austral autumn and winter months. Correlation between altimeter derived significant wave heights and the Southern Oscillation Index is observed in the Pacific basin, which is consistent with several previous studies. Variability and trends of the directional wave climate are determined using the ERA-40 Waves Re-analysis for the period 1980–2001. Significant wave height, mean wave period and mean wave direction data are used to describe the climate of the wave energy flux vector. An EOF analysis of the wave energy flux vector is carried out to determine the dominant modes of variability of the directional seasonal wave energy flux climate. The dominant mode of variability during autumn and winter months is strongly correlated to the SAM. There is an anti-clockwise rotation of wave direction with the southward intensification of the Southern Ocean storm belt associated with the SAM. Clockwise rotation of flux vectors is observed in the Western Pacific Ocean during El-Nino events. Directional variability of the wave energy flux in the Western Pacific Ocean has previously been shown to be of importance to sand transport along the south-eastern Australian margin, and the New Zealand region. The directional variability of the wave energy flux of the Southern Ocean associated with the SAM is expected to be of importance to the wave-driven currents responsible for the transport of sand along coastal margins in the Southern Hemisphere, in particular those on the Southern and Western coastal margins of the Australian continent. Copyright © 2009 Royal Meteorological Society

Impacts of land use land cover on temperature trends over the continental United States: assessment using the North American Regional Reanalysis

Abstract: We investigate the sensitivity of surface temperature trends to land use land cover change (LULC) over the conterminous United States (CONUS) using the observation minus reanalysis (OMR) approach. We estimated the OMR trends for the 1979–2003 period from the US Historical Climate Network (USHCN), and the NCEP-NCAR North American Regional Reanalysis (NARR). We used a new mean square differences (MSDs)-based assessment for the comparisons between temperature anomalies from observations and interpolated reanalysis data. Trends of monthly mean temperature anomalies show a strong agreement, especially between adjusted USHCN and NARR (r = 0.9 on average) and demonstrate that NARR captures the climate variability at different time scales. OMR trend results suggest that, unlike findings from studies based on the global reanalysis (NCEP/NCAR reanalysis), NARR often has a larger warming trend than adjusted observations (on average, 0.28 and 0.27 °C/decade respectively). OMR trends were found to be sensitive to land cover types. We analysed decadal OMR trends as a function of land types using the Advanced Very High Resolution Radiometer (AVHRR) and new National Land Cover Database (NLCD) 1992–2001 Retrofit Land Cover Change. The magnitude of OMR trends obtained from the NLDC is larger than the one derived from the ‘static’ AVHRR. Moreover, land use conversion often results in more warming than cooling. Overall, our results confirm the robustness of the OMR method for detecting non-climatic changes at the station level, evaluating the impacts of adjustments performed on raw observations, and most importantly, providing a quantitative estimate of additional warming trends associated with LULC changes at local and regional scales. As most of the warming trends that we identify can be explained on the basis of LULC changes, we suggest that in addition to considering the greenhouse gases–driven radiative forcings, multi-decadal and longer climate models simulations must further include LULC changes. Copyright © 2009 Royal Meteorological Society

Spatial and temporal variability of precipitation concentration index, concentration degree and concentration period in Xinjiang, China

Abstract: This paper studied the spatial and temporal variability of the statistical structures of precipitation across Xinjiang, China, by analysing the time series of daily precipitation from 50 weather stations during the period from 1961 to 2008. Three indices precipitation concentration index (CI), precipitation concentration degree (PCD) and precipitation concentration period (PCP) were used to detect precipitation concentrations and the associated spatial patterns. The results show that higher precipitation CI values were mainly observed in Southern Xinjiang, whereas lower precipitation CI values were mostly detected in Northern Xinjiang. The precipitation CI values are noticeably larger in places where both annual total precipitation and number of rainy days are lower. The Mann–Kendall trend test demonstrates that the most parts of Xinjiang are characterized by no significant trends of precipitation CI at the 0.05 significance level. The periodicity characteristic of precipitation CI time series in Xinjiang could be detected by wavelet power spectrum analysis, and significant periods of that in most of Xinjiang were concentrated on 2–5 years band. The results of PCP reveal that rainfall in Xinjiang mostly occurs in summer, and the rainy season arrives earlier in Eastern Xinjiang than Western Xinjiang, whereas the results of PCD indicate that the rainfall in Northern Xinjiang was more dispersed within a year than that in Southern Xinjiang. Copyright © 2010 Royal Meteorological Society

Investigating the climate impacts of global land cover change in the community climate system model

Abstract: Recently, (Pitman et al., 2009) found a wide range of bio-geophysical climate impacts from historical land cover change when modelled in a suite of current global climate models (GCMs). The bio-geophysical climate impacts of human land cover change, however, have been investigated by a wide range of general circulation modelling, regional climate modelling, and observational studies. In this regard the IPCC 4th assessment report specifies radiative cooling of 0.2 W/m2 as the dominant global impact of human land cover change since 1750, but states this has a low to medium level of scientific understanding. To further contribute to the understanding of the possible climatic impacts of anthropogenic land cover change, we have performed a series of land cover change experiments with the community land model (CLM) within the community climate system model (CCSM). To do this we have developed a new set of potential vegetation land surface parameters to represent land cover change in CLM. The new parameters are consistent with the potential vegetation biome mapping of (Ramankutty and Foley, 1999), with the plant functional types (PFTs) and plant phenology consistent with the current day Moderate Resolution Imaging Spectroradiometer (MODIS) land surface parameters of (Lawrence and Chase, 2007). We found that land cover change in CCSM resulted in widespread regional warming of the near surface atmosphere, but with limited global impact on near surface temperatures. The experiments also found changes in precipitation, with drier conditions regionally, but with limited impact on average global precipitation. Analysis of the surface fluxes in the CCSM experiments found the current day warming was predominantly driven by changes in surface hydrology through reduced evapo-transpiration and latent heat flux, with the radiative forcing playing a secondary role. We show that these finding are supported by a wide range of observational field studies, satellite studies and regional and global climate modelling studies. Copyright © 2010 Royal Meteorological Society

Quantitative assessment of climate change and human impacts on long‐term hydrologic response: a case study in a sub‐basin of the Yellow River, China

Abstract: In this study we developed an impact factor formula (IFF) to quantitatively attribute separately the impacts of climate change and local human activities on hydrological response (i.e. run-off) in a sub-basin of Yellow River for the period 1950–2000. Using the daily climatic data, we first calibrated and verified the variable infiltration capacity (VIC) hydrological model to the baseline period 1955–1970. Then we developed the basin's natural run-off for the following three decades (1971–2000) using the VIC model without considering local human impacts, as the VIC model is benchmarked by the 1960's hydrological regime. On the basis of observed precipitation, run-off and reconstructed natural run-off data from 1971 to 2000, we quantified their long-term trend, decadal and annual variations. Using daily climatic observations, we showed that the precipitation and run-off have decreased from the baseline decade, the 1960s, indicating a drier hydrological regime for recent decades. We further applied the IFF to quantitatively attribute separately the impacts of reduced precipitation and increased temperatures from climate change and then of local human activities on hydrological run-off response. It was found that climate change has a greater impact than human activities on the basin's run-off for the three consecutive decades. The pCC (percentage change of run-off due to climate change impact) is found to be 89% followed by 66% and 56% in 1970s, 1980s and 1990s, respectively. Over the decades, pHA (percentage change of run-off due to human activities) has continuously increased from 11% to 44%. If the trend continues, in future, the pHA is going to outweigh pCC in this basin. This study provides a quantitative assessment methodology for water resources managers to understand the changing process of the hydrological cycle and attribute its causative factors in a sub-basin of the Yellow River. Copyright © 2009 Royal Meteorological Society

Microclimate modelling of street tree species effects within the varied urban morphology in the Mediterranean city of Tel Aviv, Israel

Abstract: Microclimate formation and its significance in urban planning was examined through two components that predominate in affecting the city's climate: built-up morphology and urban shade trees. The methodological approach focus is on a generalization procedure for quantifying the thermal effect of any studied situation through parameterization of the vegetated variables and the built-up forms. The analysis is integrative, using empirical climatic data followed by an analytical study for generalization and sensitivity analysis using an integrative model, the Green CTTC model. Three urban tree species predominant in the Tel Aviv gardens and streets, with different canopy characteristics, and three levels of building densities were analysed to determine their thermal effect on an urban street microclimate. The variables were parameterized according to six basic cooling attributes for the studied tree species in urban gardens in Tel Aviv, and according to three geometric built-up parameters for the studied urban street. The integrative modelling approach of considering all changes simultaneously was illustrated on an urban boulevard in Tel Aviv. The analysis demonstrates the shortcomings of piecemeal modelling and the merits of the integrative approach. The study indicates the importance of urban trees in alleviating the heat island effect in a hot and humid summer. The tree cooling effect was found to be strongly related to the built form geometry. In all the studied cases, the thermal effect of the tree was found to depend mainly on its canopy coverage level and planting density in the urban street and little on other species characteristics. The methodology of analysis presented in this paper can be applied to develop an operational tool in assessing for an urban open space the integrative thermal effects of different tree species, and of the varied urban morphology and the interaction between them. Copyright © 2009 Royal Meteorological Society

Recent severe heat waves in central Europe: how to view them in a long‐term prospect?

Abstract: The study examines whether recent occurrences of severe heat waves in central Europe were exceptional in the context of past fluctuations, and estimates their recurrence probabilities under several climate change scenarios. Using data from a network of meteorological stations in the Czech Republic since 1961, it is found that 1994 was the year with the most severe heat waves over majority of the area. The other seasons with enhanced heat wave characteristics were 1992, 2003 and 2006. Analysis of the long-term temperature series at Prague-Klementinum reveals that the July 2006 heat wave, covering 33 consecutive days, was the longest and most severe individual heat wave since 1775. Probabilities of long and severe heat waves are estimated from daily temperature series generated by a first-order autoregressive model with a deterministic component (incorporating a seasonal cycle and a long-term trend). The model is validated with respect to the simulation of heat waves in present climate (1961–2006) and subsequently run under several assumptions reflecting various rates of summer warming over 2007–2100. The return period of a heat wave reaching or exceeding the length of the 2006 heat wave in Prague is estimated to be around 120 years in 2006. Owing to an increase in mean summer temperatures, probabilities of very long heat waves have already risen by an order of magnitude over the recent 25 years, and are likely to increase by another order of magnitude by around 2040 under the summer warming rate assumed by the mid-scenario. Even the lower bound scenario yields a considerable decline of return periods associated with intense heat waves. Nevertheless, the most severe recent heat waves appear to be typical rather of a late 21st century than a mid-21st century climate. Copyright © 2009 Royal Meteorological Society

Comparison of different geostatistical approaches to map climate variables: application to precipitation

Abstract: The benefits of an integrated geographical information system (GIS) and a geostatistics approach to accurately model the spatial distribution pattern of precipitation are known. However, the determination of the most appropriate geostatistical algorithm for each case is usually neglected, i.e. it is important to select the best interpolation technique for each study area to obtain accurate results. In this work, the ordinary kriging (OK), simple kriging (SK) and universal kriging (universal kriging) methods are compared with three multivariate algorithms which take into account the altitude: collocated ordinary cokriging (OCK), simple kriging with varying local means (SKV) and regression-kriging (RK). The different techniques are applied to monthly and annual precipitation data measured at 136 meteorological stations in a region of southwestern Spain (Extremadura). After carrying out cross-validation, the smallest prediction errors are obtained for the three multivariate algorithms but, particularly, SKV and RK outperform collocated OCK, which needs a more demanding variogram analysis. These algorithms are easily implemented in a GIS, requiring the residual estimates and map algebra capability to generate the final maps. Results evidence the necessity of accounting for spatially dependent precipitation data and the collocated altitude, to accurately define monthly and annual precipitation maps. Copyright © 2009 Royal Meteorological Society

Trends in daily precipitation on the northeastern Iberian Peninsula, 1955–2006

Abstract: The temporal evolution of nine daily precipitation indices over the northeastern Iberian Peninsula was analysed for the period 1955–2006, using data from 217 observatories. Cross-tabulation analysis enabled detection of statistically significant overlap among spatial distributions of trends in the study area. There was a general decrease in annual precipitation at most observatories, a decrease in the number of rainy days and precipitation intensity and an increase in the duration of dry spells. The frequency and contribution to annual precipitation of moderate and heavy rainfall events did not change at most observatories, or showed a decreasing trend in these events. There was very large spatial and seasonal variability, with implications for water management and risk assessment in the region. The decrease in precipitation was very marked in headwaters during winter and spring, potentially affecting reservoir management in the region. Heavy rainfall events generally decreased in the west of the region in winter, and in the east of the region in autumn, when these areas are more exposed to hazards related to extreme rainfall. Large differences in the sign and magnitude of trends occurred over very short distances, suggesting the need for dense networks of observatories to ensure reliable and robust detection of regional trends. Copyright © 2009 Royal Meteorological Society

Impacts of land cover data quality on regional climate simulations

Abstract: The land surface influences local, regional and global climate across many time scales. Accurate representation of land surfaces is an important factor for climate modelling studies because land surfaces control the partitioning of available energy and water. Here we introduce new, up-to-date and accurate land cover data for the Marmara Region, Turkey derived from Landsat Enhanced Thematic Mapper (ETM+) images into the Weather Research and Forecasting (WRF) model. We used several image processing techniques to create accurate land cover data from Landsat sensor images obtained between 2001 and 2005. By comparing the new land cover data with the default WRF land cover data, we found that there are two types of error in WRF land cover data that caused misrepresentation of the study region. WRF uses Global Land Cover Characteristics (GLCC) data created from images acquired during 1992 and 1993 and it does not reflect current land cover. And the GLCC includes misclassifications. As a result of these errors, GLCC data do not represent urban areas in the cities of Istanbul, Izmit and Bursa and there are spectral mixing problems between classes, e.g. croplands, urban areas and forests. We used WRF land cover and our new land cover data to conduct numerical simulations. Using meteorological station data within the study area, we found that simulation with the new land cover dataset produces more accurate temperature simulations for the region, thus demonstrating the importance of accurate land cover data. Copyright © 2009 Royal Meteorological Society

Annual and seasonal mean temperatures in Finland during the last 160 years based on gridded temperature data

Abstract: The annual and seasonal mean temperature of Finland was calculated for 162 years based on spatially interpolated monthly mean temperature records The spatial interpolation method, known as kriging, was used with the following forcing parameters: the geographical coordinates, elevation of the terrain, and percentage share of lakes and sea Homogenised data was used, and thus the most important factor affecting the accuracy of the interpolated data was the uneven distribution of the available observation stations both in time and space The uncertainty due to the homogenisation adjustments made earlier was notably smaller In the mid-1800s, the uncertainty in the annual and seasonal mean temperatures was large, with a maximum in winter of over ± 20 °C, but the accuracy improved quickly with time as the number of the observation stations increased At the beginning of the 20th century, the uncertainty related to the limited station network was less than ± 02 °C, in winter less than ± 04 °C According to the data, the rise in Finland's annual mean temperature has been statistically significant during the last 100, 50 and 30 years During the last 100 years the increase in the mean temperature was largest during spring, but during the last 50 years winters have warmed up the most The temperature time series obtained are compatible with grid point values picked from the global temperature data grids starting from the 1880s, though the global data sets tend to smooth the extremes Copyright © 2009 Royal Meteorological Society

Influence of land use change on urban heat island derived from multi-sensor data

Abstract: Regional climate change was demonstrated to be likely influenced by anthropogenic dominated land surface processes. Urban heat island (UHI) is one of the important outcomes of such land surface processes induced by urbanization, and it is an urban climate phenomenon influenced by land use pattern and it represents the difference in albedo, roughness, and heat flux exchange of land surface. This study tries to examine the influence of land use change on UHI in greater Guangzhou from 1980–2007 by analysing Landsat MSS/TM/ETM+ and MODIS satellite data, meteorological records, and census data. An integrated and modified single-channel method was used to retrieve land surface temperature (LST). Decadal changes in land use fraction and UHI pattern show that cropland decreased in parallel to the increase in built-up area and the correlation coefficient reached 0.97. The UHI effect expanded from urban areas to surrounding suburban areas and countryside with an increase in land surface temperature (mean LST increased by 2.48 from 1990 to 2007) and a decrease in the green vegetation fraction (GVF) (mean GVF decreased by 0.16 from 1990 to 2007). The spatial heterogeneity of UHI expansion can be explained by spatial patterns of economic development, population increase, and abundance of vegetation cover. In addition, remarkable changes in air temperature due to relocation of meteorological stations are significant signals for detecting the influence of urbanization on urban heat island. Copyright © 2009 Royal Meteorological Society

Statistical assessment of changes in climate extremes over Greece (1955–2002)

Abstract: This article focuses on extreme events of high and low temperatures and precipitation and their amplitude and frequency changes over the last 50 years in Greece. Sixteen climate indices have been calculated and their trends were analysed to identify possible changes over a network of measurement stations covering the region quite homogeneously. Furthermore, the changes in the probability distribution of the indices were examined. In addition, we analyzed the temporal evolution of the amplitude and frequency of extreme events through the parameters of extreme value distribution. The temporal stability of the fitted distributions is examined and the spatial distribution of their trend as well as the changes in the 5-year return levels is investigated. Half of the examined climatic indices exhibit significant regional trend; most of them are sole functions of minimum temperature or precipitation. A shift was found in the tail distribution of the peak minimum temperatures, with the 5-year return value in the last quartile of the twientieth century to be equal to the 7-year return value in the previous quartile. Warming was also apparent for the maximum temperature, mainly in the summer months, but of smaller magnitude. Total precipitation shifted towards drier climate over the domain while extreme rainfall events exhibit increased variability without following any coherent regional pattern. The changes in the peak temperature extremes are most sensitive to the changes in the location of the distribution of annual extremes. Changes in the precipitation extremes are associated with changes in both the scale and location of the fitted distribution. The highest range of change was found for the scale parameter for both temperature and precipitation extremes, pointing out that the most influenced factor is the interannual variability of the extremes. Copyright © 2009 Royal Meteorological Society

A regional scale assessment of land use/land cover and climatic changes on water and energy cycle in the upper Midwest United States.

Abstract: This study examines the relative impact of regional land-cover/land-use patterns and projected future climate change on hydrologic processes. Historic, present and projected future land cover data were used to drive the variable infiltration capacity (VIC) model using observed meteorological forcing data for 1983–2007 over Wisconsin (USA). The current and projected future (year 2030) land cover data were developed using the land transformation model (LTM). The VIC model simulations were driven using downscaled and bias-corrected projected future climate forcing from three different Intergovernmental Panel for Climate Change (IPCC) AR4 general circulation models (GCMs): HadCM3, PCM and GFDL. Sensitivity results conducted on a single grid cell show that annual average surface runoff and baseflow were increased by 8 and 6 mm, respectively, while evapotranspiration was reduced by 15 mm when a fully forested grid was converted to cropland. Results also indicate that annual average net radiation and sensible heat flux were reduced considerably due to forest-to-cropland conversion, and the reduction was more prominent in winter and spring seasons due to effect of snow albedo. Forest-to-cropland conversion also resulted in increased latent heat flux in summer (JJA) while this land transformation increased the snow water equivalent in winter (DJF) and spring (MAM). Complete conversion of forest to cropland resulted in a decrease of the radiative surface temperature on an annual basis with more cooling occurring in winter and summer. Impacts of historic deforestation were similar to what was expected based on a single grid sensitivity analysis. Copyright © 2010 Royal Meteorological Society

Interannual and interdecadal variations of tropical cyclone activity in the South China Sea

Abstract: This study attempts to identify the factors affecting annual tropical cyclone (TC) activity in the South China Sea (SCS) using data during the period 1965–2005. The results indicate that the total number of TCs and number of TCs entering the SCS from the Western North Pacific are below normal in El Nino events but above normal during La Nina events. However, for TCs formed inside the SCS, the difference in numbers between the two phases of the El Nino-Southern Oscillation (ENSO) is not as obvious. In addition, the positive phase of the Pacific Decadal Oscillation (PDO) generally favours less TCs in all categories, while the negative PDO phase favours more. These results may be explained by the fact that the ENSO and the PDO affect TC behaviour through altering the conditions in the WNP to be favourable or unfavourable for TC genesis and movement into the SCS. Copyright © 2009 Royal Meteorological Society

Effects of site change and urbanisation in the Beijing temperature series 1977-2006.

Abstract: During 1977–1981 the Beijing (BJ) meteorological station was at a suburban location. In 1981 it was moved to a more urban location, but in 1997 it was subsequently moved back to the same suburban location. The daily BJ temperature series, together with those from 18 nearby stations, form a unique database for studying how site-change and possible urbanisation influences affect climate changes at a local scale. The site-change-induced biases were quantified, between 0.43 and 0.95 °C, based on comparisons between multi-year-mean seasonal temperature anomalies at BJ and the mean of those from a cluster of nearby stations. The annual mean urban-suburban difference was 0.81 °C around 1981 and 0.69 °C around 1997, indicating a growing urbanisation effect in the suburban compared to the downtown area. The linear warming trend in the adjusted (for site moves only) BJ temperature series during 1977–2006 was 0.78 °C/decade. Comparing with several rural and less-urban sites, we suggest that the BJ records include an urbanisation-related warming bias between 0.20 and 0.54 °C/decade, likely about 0.30 °C/decade, for the recent few decades. The climatic warming at BJ between 1977 and 2006 is likely, therefore, to be about 0.48 °C/decade. Caveats for using these estimates were discussed. Copyright © 2009 Royal Meteorological Society

Long-term changes and regional differences in temperature and precipitation in the metropolitan area of Hamburg

Abstract: Climate changes and the urban climate of the ‘green city’ Hamburg and its metropolitan region are analyzed using observational data for temperature and precipitation. Values for Hamburg's synoptic site HH-Fuhlsbuttel start in 1891 and are used to determine climate changes. Additional data from up to 45 climate stations are used to analyze the different aspects of the regional climate and urban effects on the temperature [urban heat island (UHI)] and precipitation [urban precipitation impact (UPI)]. The analysis of the long-term data shows that the climate has already changed. Annual precipitation significantly increases ∼0.8 mm/year when focusing on years 1891–2007 and ∼1.3 mm/year for 1948–2007. Precipitation increases are largest in November through March and March as well as June for 1978–2007. For April and July of this period, a precipitation decrease is found. The precipitation distribution shows that moderate daily precipitation amounts (≤10 mm/day) have increased by about 10% between 1948–1977 and 1978–2007. Precipitation amounts > 10 mm/day have increased by 20% in the same period. Average temperatures significantly increase by 0.07 K/decade (1891–2007), 0.19 K/decade (1948–2007), 0.6 K/decade (1978–2007) with largest significant increases in fall. For the UHI, it is found that the average temperature is higher up to 1.1 K in the densely build-up city area than outside. Values are about halved for more green urban areas but also depend on more local impacts. The minimum temperatures are up to 3 K higher and maximum temperatures slightly lower in the inner city than in the rural during summer. The winter temperatures are higher throughout the urban area. The UHI differences depend on wind speed; this dependence is best described by using the inverse square root of the wind speed. Classification using different wind directions shows that the precipitation is significantly higher (5–20%) for downwind of urban areas compared with the upwind side. Copyright © 2009 Royal Meteorological Society

Changes in Catchment-Scale Recession Flow Properties in Response to Permafrost Thawing in the Yukon River Basin

Abstract: Permafrost influences the hydrologic response of a catchment. In this study, we test the ability of recession flow analysis to reflect thawing of permafrost at the catchment scale for the well-studied Yukon river basin (YRB), covering large portions of Alaska, USA and parts of Canada. The changes in the recession flow properties detected in the YRB agree well with observations of permafrost thawing across central Alaska. In addition, there is good agreement between the relative increases in recession flow intercept (a proxy for effective depth to permafrost) and the relative annual increases in groundwater flow (independently assessed as a permafrost thawing effect) in the YRB catchments that have exhibited such groundwater flow increases. This study demonstrates the utility of recession flow analysis to reflect catchment-scale changes in permafrost across a variety of permafrost conditions. The strength of this method is that it requires only daily observations of streamflow to reflect permafrost thawing on much larger measurement support scales than the local scales of direct permafrost observations.

Links between the rainfall regime in Israel and location and intensity of Cyprus lows

Abstract: The interannual variations and the spatial distribution of rainfall in the Mediterranean and semi-arid regions of Israel are analysed with respect to variations in the occurrence of the typical synoptic systems of the Eastern Mediterranean. The synoptic analysis is based on a daily, semi-objective synoptic classification (Alpert et al., 2004a). The study covers the months November–March, in which 90% of the annual rainfall is obtained, mostly resulting from Cyprus lows. The interannual variations of the rainfall are well explained by the synoptic types, and the occurrences of Cyprus lows are highly correlated with the rainfall. It was found that the daily and seasonal rainfall are highly dependent on the depth of the cyclone. Moreover, deep lows are more effective for the mountainous regions, due both to the enhanced orographic effect and to the fact that stronger winds, associated with deep lows, are more efficient in transporting rain-producing clouds from the Mediterranean Sea inland. The location of the cyclone determines the spatial distribution of the rain it produces over Israel. The cyclones located east of Cyprus were found productive mainly for the southern parts of the study region, while those located to the west and north of Israel were found productive for the north of the country. The high sensitivity of the rainfall to the location of the surface cyclones emphasizes the major role that lower level moisture transport plays in rain formation. Copyright © 2009 Royal Meteorological Society

A gridded hourly precipitation dataset for Switzerland using rain-gauge analysis and radar-based disaggregation.

Abstract: Rain gauges and weather radars both constitute important devices for operational precipitation monitoring. Gauges provide accurate yet spotty precipitation estimates, while radars offer high temporal and spatial resolution yet at a limited absolute accuracy. We propose a simple methodology to combine radar and daily rain-gauge data to build up a precipitation dataset with hourly resolution covering a climatological time period. The methodology starts from a daily precipitation analysis, derived from a dense rain-gauge network. A sequence of hourly radar analyses is then used to disaggregate the daily analyses. The disaggregation is applied such as to retain the daily precipitation totals of the rain-gauge analysis, in order to reduce the impact of quantitative radar biases. Hence, only the radar's advantage in terms of temporal resolution is exploited. In this article the disaggregation method is applied to derive a 15-year gridded precipitation dataset at hourly resolution for Switzerland at a spatial resolution of 2 km. Validation of this dataset indicates that errors in hourly intensity and frequency are lower than 25% on average over the Swiss Plateau. In Alpine valleys, however, errors are typically larger due to shielding effects of the radar and the corresponding underestimation of precipitation periods by the disaggregation. For the flatland areas of the Swiss Plateau, the new dataset offers an interesting quantitative description of high-frequency precipitation variations suitable for climatological analyses of heavy events, the evaluation of numerical weather forecasting models and the calibration/operation of hydrological runoff models.

Long-term hydrological changes of the Seine River flow (France) and their relation to the North Atlantic Oscillation over the period 1950–2008

Abstract: The variability of Seine River flow (France) was investigated using daily time series over the period of 1950–2008. The study aimed at characterizing the dominant modes explaining the variability of flow and at investigating their possible link with the North Atlantic Oscillation (NAO) as the dominant climate regime in the region. The Seine River flow was affected by a statistically significant increasing trend across the period. The hydrologic regime of the Seine River was found to be highly variable and seem to occur later in the year, approximately, since the end of the 1980s. Annual minimum, maximum and mean flow time series exhibited an obvious change around 1970. Continuous wavelet transform (CWT) of flow revealed energetic peaks highly localized in time, defining three time periods: before 1970, between 1970 and 1990 and after 1990. Two interannual modes (17 and 5–9 years) occur, respectively, around 1970 and in the early 1990s. The same observations on precipitation CWT ensure a climatic origin to these interannual modes. The annual oscillation was always strongly represented in flow and was affected by an increase in power from 1990 until the end of the time series. Comparison between NAO and annual flow anomalies revealed similar trend behaviour characterized by a change around 1970 towards more positive NAO indices and positive flow anomalies. In addition, CWT of NAO revealed similar modes of variability around 17 and 5–9 years as flow interannual modes. This highlighted a possible link between NAO and hydrometeorological processes in the region. The flow fluctuations (interannual modes and trend) expressing this potential link would explain approximately 23% of total variance of Seine River flow and would even reach 35% of the variance after removal of the annual cyclicity. These results would then emphasize the potential strong control of NAO on the variability of Seine River discharge. Copyright © 2009 Royal Meteorological Society

Improving estimation of missing values in daily precipitation series by a probability density function-preserving approach

Abstract: This work presents a novel method for estimating missing values in daily precipitation series. It is aimed at identifying the event time location with good accuracy and reconstructing the correct amount of daily rainfall. In addition, the statistical properties of the time series, i.e. both probability distribution and long-term statistics, are preserved. The completion method is based on a two-step algorithm that uses information from a cluster of neighboring stations. First, wet and dry days are tagged, and subsequently, the full precipitation amount for wet-classified days is estimated by a modified multi-linear regression approach. This method avoids overestimation of the number of wet days and underestimation of intense precipitation events, which are typical side effects of common regression-based approaches. Copyright © 2009 Royal Meteorological Society