Showing papers in "International Journal of Climatology in 2012"

A refined index of model performance

Abstract: In this paper, we develop, present and evaluate a refined, statistical index of model performance. This ‘new’ measure (dr) is a reformulation of Willmott's index of agreement, which was developed in the 1980s. It (dr) is dimensionless, bounded by − 1.0 and 1.0 and, in general, more rationally related to model accuracy than are other existing indices. It also is quite flexible, making it applicable to a wide range of model-performance problems. The two main published versions of Willmott's index as well as four other comparable dimensionless indices—proposed by Nash and Sutcliffe in 1970, Watterson in 1996, Legates and McCabe in 1999 and Mielke and Berry in 2001—are compared with the new index. Of the six, Legates and McCabe's measure is most similar to dr. Repeated calculations of all six indices, from intensive random resamplings of predicted and observed spaces, are used to show the covariation and differences between the various indices, as well as their relative efficacies. Copyright © 2011 Royal Meteorological Society

A comparison of statistical downscaling methods suited for wildfire applications

Abstract: Place-based data is required in wildfire analyses, particularly in regions of diverse terrain that foster not only strong gradients in meteorological variables, but also complex fire behaviour. However, a majority of downscaling methods are inappropriate for wildfire application due to the lack of daily timescales and variables such as humidity and winds that are important for fuel flammability and fire spread. Two statistical downscaling methods, the daily Bias corrected Spatial Downscaling (BCSD) and the Multivariate Adapted Constructed Analogs (MACA) that directly incorporate daily data from global climate models, were validated over the western US using global reanalysis data. While both methods outperformed results obtained from direct interpolation from reanalysis, MACA exhibited additional skill in temperature, humidity, wind, and precipitation due to its ability to jointly downscale temperature and dew point temperature, and its use of analog patterns rather than interpolation. Both downscaling methods exhibited value added information in tracking fire danger indices and periods of extreme fire danger; however, MACA outperformed the daily BCSD due to its ability to more accurately capture relative humidity and winds. Copyright © 2011 Royal Meteorological Society

Recent developments on the South American monsoon system

Abstract: Fil: Marengo, J. A.. Centro de Previsao de Tempo e Estudos Climaticos. Instituto Nacional de Pesquisas Espaciais; Brasil

Sky view factor analysis of street canyons and its implications for daytime intra-urban air temperature differentials in high-rise, high-density urban areas of Hong Kong: a GIS-based simulation approach

Abstract: Hong Kong is a high-density sub-tropical city with 7 million people living in an urban area of just over 260 km2. Tall and closely packed buildings are the common urban morphology. How the urban geometry influences the microclimate in summer daytime is a primary planning concern. The sky view factor (SVF) has been commonly used to indicate the impact of urban geometry on air temperature differences in cities. However, only limited discussions in this aspect have been addressed for daytime course in high-rise and high-density urban environment such as Hong Kong. This paper firstly provides a comprehensive review of SVF analysis in urban climatology studies and then presents a simulation approach to investigate the role of SVF in determining summer daytime intra-urban air temperature differences in urban Hong Kong. An ArcGIS-embedded computer program is developed for calculating continuous SVF values for an entire urban environment and an SVF map is generated. The result is evaluated against meteorological data observed in field measurements. The regression analysis shows that the spatial average of SVF values has a close negative relationship with daytime intra-urban temperature differences. The study indicates that SVF is a significant factor for understanding the microthermal climate in Hong Kong's street canyons. The paper further raises discussions on the application of SVF analysis to urban planning. The study demonstrates that the SVF analysis is a useful and effective tool for planners and urban climatologists conducting studies on high-rise and high-density sub-tropical cities. The understanding can provide support for the development of planning standards and good practice. Copyright © 2010 Royal Meteorological Society

Development and analysis of a 50-year high-resolution daily gridded precipitation dataset over Spain (Spain02)

Abstract: In this paper, we present a new publicly available high-resolution daily precipitation gridded dataset developed for peninsular Spain and the Balearic islands using 2756 quality-controlled stations (this dataset is referred to as Spain02). The grid has a regular 0.2° (approx. 20 km) horizontal resolution and spans the period from 1950 to 2003. Different interpolation methods were tested using a cross-validation approach to compare the resulting interpolated values against station data: kriging, angular distance weighting, and thin plane splines. Finally, the grid was produced applying the kriging method in a two-step process. First, the occurrence was interpolated using a binary kriging and, in a second step, the amounts were interpolated by applying ordinary kriging to the occurrence outcomes. This procedure is similar to the interpolation method used to generate the E-OBS gridded data—the state-of-the-art publicly available high-resolution daily dataset for Europe—which was used in this study for comparison purposes. Climatological statistics and extreme value indicators from the resulting grid were compared to those from the 25 km E-OBS dataset using the observed station records as a reference. Spain02 faithfully reproduces climatological features such as annual precipitation occurrence, accumulated amounts and variability, whereas E-OBS has some deficiencies in the southern region. When focusing on upper percentiles and other indicators of extreme precipitation regimes, Spain02 accurately reproduces the amount and spatial distribution of the observed extreme indicators, whereas E-OBS data present serious limitations over Spain due to the sparse data used in this region. As extreme values are more sensitive to interpolation, the dense station coverage of this new data set was crucial to get an accurate reproduction of the extremes. Copyright © 2010 Royal Meteorological Society

Recent climate change in the Arabian Peninsula: annual rainfall and temperature analysis of Saudi Arabia for 1978–2009

Abstract: The rainfall and temperature climatology over the Arabian Peninsula are analysed on an annual basis using various gridded datasets. For Saudi Arabia, the area of which represents almost 80% of the Peninsula, the climatic datasets from its 27 ground observations are analysed for the period 1978–2009, with additional gridded datasets used to describe the observed state and change of the present climate. The gridded datasets represent well the very dry (40–80 mm) area over the world's largest sand desert (Rub Al-Khali), the dry (80–150 mm) area over middle-to-north of Saudi Arabia, and the wettest (>150 mm) region in the southwest of the Peninsula. The annual temperature is relatively high (24–27 °C) in the middle-to-south of the Peninsula and low ( 27 °C) is obtained over the Rub Al-Khali. Over Saudi Arabia, the observed annual rainfall showed a significant decreasing trend (47.8 mm per decade) in the last half of the analysis period, with a relatively large interannual variability, while the maximum, mean and minimum temperatures have increased significantly at a rate of 0.71, 0.60, and 0.48 °C per decade, respectively. This information is invaluable to consider in any climate impact assessment studies in Saudi Arabia. Copyright © 2012 Royal Meteorological Society

Exceedance of heat index thresholds for 15 regions under a warming climate using the wet-bulb globe temperature

Abstract: Thermal comfort is quantified in 15 regions using the wet-bulb globe temperature (WBGT), examining past and future rates of thresholds exceedance corresponding to moderate, high, and extreme heat (28, 32, and 35 °C, respectively). As recent changes to thermal comfort appear to be dominated by temperature and humidity, a WBGT approximation based only on these is used. A new homogenised dataset from 1973 to 2003 is developed which provides WBGT daily means, daily maximums averaged over 5-day periods, and the highest extreme for each 5-day period; recent trends are positive for all regions except northeast USA and northeast Australia. A simple model for predicting summertime threshold exceedance rates, with a fixed distribution of anomalies about the seasonal mean, is found to adequately predict changes for the above quantities given seasonal mean values. This model is used to predict the impact of regional 1–5 °C temperature increases on WBGT exceedance rates with no change in relative humidity. Results show that heat events may worsen as much, or more, in humid tropical and mid-latitude regions even if they warm less than the global average, due to greater absolute humidity increases. A further 2 °C warming from the present is sufficient to push peak WBGT above 35 °C, an extreme heat event, in all regions except the UK. An ensemble of HadCM3 climate model simulations is used to investigate likely regional changes in mean summertime temperature, relative humidity and WBGT under an A1B scenario for the 2020s and 2050s. Unsurprisingly, simulated regional changes often depart significantly from the global average, and the impact of regional changes in relative humidity is not always negligible. Increases in WBGT are nonetheless expected in all regions, and are more predictable than increases in temperature at least in mid-latitude regions owing to the compensating effects of humidity. © 2010 Royal Meteorological Society and Crown Copyright

Trends in European precipitation extremes over 1951–2010

Abstract: Significant trends in precipitation extremes over Europe since the middle of the 20th century have been found in earlier studies. Most of these studies are based on descriptive indices of moderate extremes that occur on average a few times per year. Here we have analyzed rarer precipitation events which occur on average once in 5, 10 and 20 years in the 1950s and 1960s using extreme value theory. We have focused on the 1-d and 5-d precipitation amounts in Northern and Southern Europe in all four seasons. Changes over the time period 1951–2010 are studied by considering five consecutive 20-year time intervals with 10-year overlap. Despite considerable decadal variability, our results indicate that 5-, 10- and 20-year events of 1-d and 5-d precipitation for the first 20-year period generally became more common during this 60-year period. For all regions, seasons and return periods, the median reduction in return period between the first and last 20-year periods is ∼21% with variations between a decrease of ∼2% and ∼58%.

A spatio‐temporal structure‐based approach to drought characterisation

Abstract: Drought characterisation is an intrinsically spatio-temporal problem. A limitation of previous approaches to characterisation is that they discard much of the spatio-temporal information by reducing events to a lower-order subspace. To address this, an explicit 3-dimensional (longitude, latitude, time) structure-based method is described in which drought events are defined by a spatially and temporarily coherent set of points displaying standardised precipitation below a given threshold. Geometric methods can then be used to measure similarity between individual drought structures. Groupings of these similarities provide an alternative to traditional methods for extracting recurrent space-time signals from geophysical data. The explicit consideration of structure encourages the construction of summary statistics which relate to the event geometry. Example measures considered are the event volume, centroid, and aspect ratio. The utility of a 3-dimensional approach is demonstrated by application to the analysis of European droughts (15 °W to 35°E, and 35 °N to 70°N) for the period 1901–2006. Large-scale structure is found to be abundant with 75 events identified lasting for more than 3 months and spanning at least 0.5 × 106 km2. Near-complete dissimilarity is seen between the individual drought structures, and little or no regularity is found in the time evolution of even the most spatially similar drought events. The spatial distribution of the event centroids and the time evolution of the geographic cross-sectional areas strongly suggest that large area, sustained droughts result from the combination of multiple small area (∼106 km2) short duration (∼3 months) events. The small events are not found to occur independently in space. This leads to the hypothesis that local water feedbacks play an important role in the aggregation process.

Revisiting Swiss temperature trends 1959–2008

Abstract: Temperature is a key variable for monitoring global climate change. Here we perform a trend analysis of Swiss temperatures from 1959 to 2008, using a new 2 × 2 km gridded data-set based on carefully homogenised ground observations from MeteoSwiss. The aim of this study is twofold: first, to discuss the spatial and altitudinal temperature trend characteristics in detail, and second, to quantify the contribution of changes in atmospheric circulation and local effects to these trends. The seasonal trends are all positive and mostly significant with an annual average warming rate of 0.35 °C/decade (∼1.6 times the northern hemispheric warming rate), ranging from 0.17 in autumn to 0.48 °C/decade in summer. Altitude-dependent trends are found in autumn and early winter where the trends are stronger at low altitudes (<800 m asl), and in spring where slightly stronger trends are found at altitudes close to the snow line. Part of the trends can be explained by changes in atmospheric circulation, but with substantial differences from season to season. In winter, circulation effects account for more than half the trends, while this contribution is much smaller in other seasons. After removing the effect of circulation, the trends still show seasonal variations with higher values in spring and summer. The circulation-corrected trends are closer to the values simulated by a set of ENSEMBLES regional climate models, with the models still tending towards a trend underestimation in spring and summer. Our results suggest that both circulation changes and more local effects are important to explain part of recent warming in spring, summer, and autumn. Snow-albedo feedback effects could be responsible for the stronger spring trends at altitudes close to the snow line, but the overall effect is small. In autumn, the observed decrease in fog frequency might be a key process in explaining the stronger temperature trends at low altitudes.

Derivation of Birmingham's summer surface urban heat island from MODIS satellite images

Abstract: This study investigates the summer (June, July, August) night urban heat island (UHI) of Birmingham, the UK's second most populous city. Land surface temperature remote sensing data is used from the MODIS sensor on NASA's Aqua satellite, combined with UK Met Office station data to map the average variation in heat island intensity over the Birmingham conurbation. Results are presented of average UHI events over four Pasquill-Gifford stability classes D, E, F, and G between 2003 and 2009, as well as a specific heatwave event in July 2006. The results quantify the magnitude of the Birmingham surface UHI as well as the impact of atmospheric stability on UHI development. During periods of high atmospheric stability, a UHI of the order of 5 °C is evident with a clear peak in the central business district. Also identified, are significant cold spots in the conurbation. In one city park, recorded surface temperatures are up to 7 °C lower than the city centre. Copyright © 2010 Royal Meteorological Society

The significant climate warming in the northern Tibetan Plateau and its possible causes

Abstract: We have identified the northern Tibetan Plateau as having experienced the most significant warming of any region in the entire plateau domain since 1961. Warming in the northern plateau violates the previously suggested elevation dependency of warming trends. Further analysis shows that the increase in surface air temperature in summer has played a primary role in the rapid increase of the annual mean air temperature in the northern Tibetan Plateau since the mid-1980s. In addition, the summer air temperature is correlated with ozone in the region, a result which is statistically significant. This correlation seems to have a relationship with solar radiation and ozone depletion ratios. Further discussion shows that the most significant warming in the northern plateau may be related to radiative and dynamical heating that are results of pronounced stratospheric ozone depletion. Copyright © 2011 Royal Meteorological Society

A numerical study of the Urban Heat Island over Madrid during the DESIREX (2008) campaign with WRF and an evaluation of simple mitigation strategies

Abstract: Nowadays, mesoscale meteorological models coupled to Urban Canopy Parameterizations (UCP) can be used to complement and interpret the information gathered from intensive meteorological campaigns on the behaviour of the Urban Boundary Layer (UBL). Moreover, the impact of the air conditioning (AC) systems on the air temperature, the relationships existing between energy consumption (EC) and meteorological conditions, and the evaluation of strategies to mitigate the Urban Heat Island (UHI) phenomenon can be evaluated using detailed UCP. In this work, a new UCP implemented in the Weather Research and Forecasting (WRF) model (version V3.2) has been tested over the city of Madrid using two different turbulent parameterisations of the Planetary Boundary Layer (PBL) under atmospheric conditions that were favourable for a large UHI. Two selected days were analysed coinciding with the Dual-use European Security IR Experiment (DESIREX) campaign that took place in the summer of 2008, and focused on Urban Heat Island (UHI) and Urban Thermography (UT) monitoring and assessment. For the two simulated days (30 June and 1 July) a high UHI intensity (5-6 °C) was observed and modelled. Numerical results for the surface air temperature and wind speed were compared against measurements showing a global satisfactory performance of the model. Some differences in the air temperature predictions were observed within the two turbulent schemes. Subsequently, the impact of the AC systems and the EC were evaluated for the simulated period. The heat fluxes coming from AC systems were responsible of an increase in the air temperature up to 1.5–2 °C in some dense urban areas. Effects of modifications in the roof albedo and building material properties reduced the total EC by 4.8 and 3.6%, respectively, affecting the intensity of the UHI. When AC systems were not ejecting the heat fluxes out in the atmosphere, the EC was reduced to 2.5%. Copyright © 2011 Royal Meteorological Society

The future of dry and wet spells in Europe: a comprehensive study based on the ENSEMBLES regional climate models

Abstract: Under the aspect of future climate change, it is important for decision makers to know how drought conditions might change on the regional scale in order to map out adequate adaptation and mitigation strategies. Recent RCM simulations provided by the ENSEMBLES project are used to analyse changes in dry and wet conditions in Europe by the mid of the 21st century under the A1B emission scenario. Eight RCMs are selected to capture the uncertainties of the projected changes. An empirical statistical error correction approach is applied to modelled daily mean air temperature and precipitation amount to account for RCM errors, and commonly used drought indices—the Standardized Precipitation Index (SPI), the self calibrated Palmer Z-Index (scZI) and Palmer Drought Severity Index (scPDSI)—are calculated. Changes in the mean, in interannual variability, and in frequency, length, distance, magnitude, and area of dry and wet events are investigated. The statistical significance of the projected multi-model mean changes and the according uncertainties are analysed for nine European subregions. Furthermore, distributional changes of the dry and wet spell characteristics are assessed. Changes in the mean, and in dry and wet event characteristics show the most pronounced changes towards drier and wetter conditions in the southern- and northernmost European subregions, respectively. Here, the changes are highly significant and confident, while the projected changes are more dissonant for the other subregions. Severe changes in the extremes of event length, distance, magnitude, and area particularly arise in the southern- and northernmost European subregions. The projected changes in interannual variability are less significant and confident. However, significantly increasing interannual variability is projected in regions with pronounced changes in the mean towards wetter as well as towards drier conditions. Copyright © 2011 Royal Meteorological Society

Possible effects of the North Atlantic Oscillation on the strengthening relationship between the East Asian Summer monsoon and ENSO

Abstract: In contrast to the weakened relationship between the Indian summer monsoon and El Nino–Southern Oscillation (ENSO) since 1970s, the East Asian summer monsoon (EASM) has exhibited a strengthened relationship with ENSO. In this study, observational and numerical evidences manifest that spring NAO may exert notable impacts on the enhancement of the EASM–ENSO relationship. Anomalous spring NAO induces a tripole sea surface temperature anomaly (SSTA) pattern in North Atlantic which persists into ensuring summer. The tripole SSTA excites downstream tele-connections of a distinct Rossby wave train prevailing over the northern Eurasia and a simple Gill-Matsuno-type quadrupole response over western Pacific. The former modulates the blocking highs over the Ural Mountain and the Okhotsk Sea. The latter enhances the linkage between the western Pacific subtropical high and ENSO. The co-effects of the two tele-connection patterns help to strengthen (or weaken) the subtropical Meiyu-Baiu-Changma front, the primary rain-bearing system of the EASM. As such, spring NAO is tied to the strengthened connection between ENSO and the EASM. Copyright © 2011 Royal Meteorological Society

Precipitation variability and change in the Calabria region (Italy) from a high resolution daily dataset

Abstract: The present study aims at improving data availability and quality for the last 80–90 years for daily precipitation in the Calabria region (southern Italy). First, the original database was homogenised and the gaps filled in for 129 daily rain gauges for the 1916–2006 period. Then, precipitation variability and change were evaluated at an adequate spatial resolution. Monthly and annual total precipitation (P), number of wet days (WDs), and precipitation intensity (PI) were calculated for each series. With regard to the monthly total precipitation a general negative trend, albeit not everywhere significant, was detected, in particular for the autumn–winter period, while in summer the tendency was toward an increase in total precipitation. The monthly behaviour of WDs was not very different from that observed for P: a diffuse negative trend was detected in most months, particularly evident and significant in January, with the exception of April and the summer months, for which the tendency was toward an increase. Regarding the PI, a general negative and often significant trend was found for the entire region and for almost all the months, except summer. Attention was also focused on tendencies in the different PI categories, revealing negative trends in high-intensity categories, especially coming from the winter season. Finally, running trend analysis revealed that the previously discussed tendencies were not persistent throughout the series length, but depended on the period examined. This important aspect should be taken into account when different results based on different time windows are compared. Copyright © 2010 Royal Meteorological Society

Future changes in intense precipitation over Canada assessed from multi-model NARCCAP ensemble simulations

Abstract: Annual maxima (AM) series of precipitation from 15 simulations of the North American Regional Climate Change Assessment Program (NARCCAP) have been analysed for gridpoints covering Canada and the northern part of United States. The NARCCAP Regional Climate Models' simulations have been classified into the following three groups based on the driving data used at the RCMs boundaries: (1) NCEP (6 simulations); (2) GCM-historical (5 simulations); and (3) GCM-future (4 simulations). Historical simulations are representative of the 1968-2000 period while future simulations cover the 2041-2070 period. A reference common grid has been defined to ease the comparison. Multi-model average intensities of AM precipitation of 6-, 12-, 24-, 72-, and 120-h for 2-, 5-, 10-, and 20-year return periods have been estimated for each simulation group. Comparison of results from NCEP and GCM-historical groups shows good overall agreement in terms of spatial distribution of AM intensities. Comparison of GCM-future and GCM-historical groups clearly shows widespread increases with median relative changes across all gridpoints ranging from 12 to 18% depending on durations and return periods. Fourteen Canadian climatic regions have been used to define regional projections and average regional changes in intense precipitation have been estimated for each duration and return period. Uncertainties on these regional values, resulting from inter-model variability, were also estimated. Results suggest that inland regions (e.g. Ontario and more specifically Southern Ontario, the Prairies, Southern Quebec) will experience the largest relative increases in AM intensities while coastal regions (e.g. Atlantic Provinces and the West Coast) will experience the smallest ones. These projections are most valuable inputs for the assessment of future impact of climate change on water infrastructures and the development of more efficient adaptation strategies. Copyright © 2011 Royal Meteorological Society

The influence of ENSO on winter rainfall in South Africa

Abstract: Whereas the impact of ENSO on the African summer rainfall regions is largely documented and still regularly investigated, little is known about its impact on the winter rainfall regions located at the southwestern and northwestern tips of Africa. Yet, these regions are densely inhabited and are net exporters of high-quality agricultural products. Here we analyze the relationship between El Nino Southern Oscillation (ENSO) and South Africa austral winter rainfall using a 682 raingauges daily rainfall database documenting the period 1950–1999. The May, June and July (MJJ) seasonal rainfall amount shows a positive correlation with the Nino3.4 index that becomes significant since the so-called 1976/1977 climate regime shift. Wet spells properties (length, frequency and intensity) at the raingauge scale indicate that high (low) MJJ seasonal rainfall amounts recorded during El Nino (La Nina) events are the result of longer (shorter) wet spells in the Cape Town area and more (less) frequent wet spells north of 33°S. Wet spells with daily rainfall amounts ranging between 10 and 50 mm are also more (less) frequent. Atmospheric dynamics fields during wet spells feature lower (higher) pressure and northwesterly (southerly) wind anomalies in the troposphere over the region. This suggests that rain-bearing systems are deeper (thinner) and larger (smaller) in extent, and located farther north (south) during El Nino (La Nina) events. Copyright © 2011 Royal Meteorological Society

A statistical model for the urban heat island and its application to a climate change scenario

Abstract: A linear statistical model relating the nocturnal urban heat island (UHI) intensity of Hamburg with meteorological conditions is constructed from observations taken by the German Meteorological Service (DWD). To find the appropriate predictors the relationship between different meteorological variables and the UHI of Hamburg is analyzed. Results and physical plausibility suggest that cloud cover, wind speed and relative humidity are the relevant variables and can be used to construct a statistical model. The parameters for the statistical model are determined with the generalized least square method. With the help of the statistical model up to 42% of the UHI variance can be explained. The statistical model is then used to statistically downscale results from climate runs of the regional climate models (RCM) REMO and CLM. Both RCMs were driven with the A1B SRES emission scenario runs of the global climate model ECHAM5/MPI-OM. The resulting values for the future UHI are analyzed with respect to monthly averages and the frequency distribution. Results show that changes in the UHI are different for the different months. Significant change (decrease of UHI) in the results of both RCMs and for both realizations of the A1B scenario can be found for April in at the middle and the end of the century and in December at the end of the century. For the summer months which are most relevant to the development of adaption strategies the results differ between the RCMs. REMO results show no significant changes for the summer, while analyses of CLM suggest significant increase in July and August. The frequency distribution of the summer UHI shows no significant changes for REMO and only in one realization of CLM a significant increase in moderate and strong UHI days can be found for the end of the century. Copyright © 2011 Royal Meteorological Society

Multi‐model forecast skill for mid‐summer rainfall over southern Africa

Abstract: Southern African December-January-February (DJF) probabilistic rainfall forecast skill is assessed over a 22-year retroactive test period (1980/1981 to 2001/2002) by considering multi-model ensembles consisting of downscaled forecasts from three of the DEMETER models, the ECMWF, Meteo-France and UKMO coupled ocean-atmosphere general circulation models. These models are initialized in such a way that DJF forecasts are produced at an approximate 1-month lead time, i.e. forecasts made in early November. Multi-model forecasts are obtained by: i) downscaling each model's 850 hPa geopotential height field forecast using canonical correlation analysis (CCA) and then simply averaging the rainfall forecasts; and ii) by combining the three models' 850 hPa forecasts, and then downscaling them using CCA. Downscaling is performed onto the 0.5° × 0.5° resolution of the CRU rainfall data set south of 10° south over Africa. Forecast verification is performed using the relative operating characteristic (ROC) and the reliability diagram. The performance of the two multi-model combinations approaches are compared with the single-model downscaled forecasts and also with each other. It is shown that the multi-model forecasts outperform the single model forecasts, that the two multi-model schemes produce about equally skilful forecasts, and that the forecasts perform better during El Nino and La Nina seasons than during neutral years. Copyright © 2010 Royal Meteorological Society

High-resolution (space, time) anthropogenic heat emissions: London 1970–2025

Abstract: The anthropogenic heat emissions generated by human activities in London are analysed in detail for 2005–2008 and considered in context of long-term past and future trends (1970–2025). Emissions from buildings, road traffic and human metabolism are finely resolved in space (30 min) and time (200 × 200 m2). Software to compute and visualize the results is provided. The annual mean anthropogenic heat flux for Greater London is 10.9 W m−2 for 2005–2008, with the highest peaks in the central activities zone (CAZ) associated with extensive service industry activities. Towards the outskirts of the city, emissions from the domestic sector and road traffic dominate. Anthropogenic heat is mostly emitted as sensible heat, with a latent heat fraction of 7.3% and a heat-to-wastewater fraction of 12%; the implications related to the use of evaporative cooling towers are briefly addressed. Projections indicate a further increase of heat emissions within the CAZ in the next two decades related to further intensification of activities within this area.

Analyses of extreme flooding in Austria over the period 1951–2006

Abstract: Analyses of extreme flooding in Austria is performed using daily discharge time series from 27 stations over the period 1951-2006. The main research questions revolve around: (1) temporal non-stationarities in the flood record, (2) upper tail and scaling properties of the flood peak records, and (3) relation between magnitude and frequency of flooding and the North Atlantic Oscillation (NAO). Two datasets are derived from the daily discharge time series: annual maximum daily discharge and peaks-over-threshold (POT) data. The validity of the stationarity assumption in the annual maximum discharge record is assessed by investigating the presence of abrupt and slowly varying changes using nonparametric tests. The time series are tested for abrupt changes both in the mean and variance of the flood peak distributions by means of the Pettitt test. The presence of monotonic trends is investigated by means of the Mann-Kendall and Spearman tests. Violations of the stationarity assumption are associated with abrupt rather than gradual changes. These step changes generally involve river regulation through construction of dams or other major engineering works. It is not possible to make conclusive statements about the presence of an anthropogenic climate change signal in the flood peak record. Similar conclusions are obtained when focussing on the frequency of POT floods. The Generalised Extreme Value distribution is used to study the upper tail and scaling properties of annual maximum daily discharge records. The location and scale parameters exhibit power-law behaviour as a function of drainage area. The shape parameters indicate that the flood peak distributions for Austria have a heavy tail. Non-stationary modelling of the annual maximum daily discharge and POT time series is used to explore the relation between flood magnitude and frequency and NAO. The results indicate that NAO is a significant covariate in explaining the magnitude and frequency of occurrence of flooding over a large part of Austria. Copyright © 2011 Royal Meteorological Society

CGCM projections of heavy rainfall events in China

Abstract: This paper discusses projections of heavy rainfall events in China during the 21st century based on daily precipitation data from the Fourth Assessment Report's (AR4) Coupled General Circulation Models (CGCM). Results show that all three experimental scenarios (scenarios A2, A1B, and B1) project consistent changes in frequency and intensity of heavy rainfall at the end of 21st century. In the regions of Northeast China and North China, there are no significant changes in frequency but there are remarkable increases in intensity of heavy rainfall, indicating that enhanced intensity is the main contributor to increased ratios of heavy rainfall to total annual precipitation in these regions. In regions of the lower reaches of Yangtze River and South China, increases in the amount of heavy rainfall are closely associated with increased frequency and increased intensity. Projected frequencies of heavy rainfall at the end of 21st century increase by 30.9 ∼ 56.6% in the Yangtze River and 35.9 ∼ 50.2% in South China compared to the period of 1980–1999, and projected intensities increase by 1.0 ∼ 5.7% and 2.8 ∼ 6.3%, respectively. Additionally, the ratios of heavy rainfall to total annual precipitation increase by 2.3 ∼ 5.4% in the Yangtze River and 1.8 ∼ 3.8% in South China. The significant increases of heavy rainfall ratios indicate that as the climate warms, heavy rainfall events are expected to increase at rates that are much faster than increases in total precipitation amounts, indicating that China will experience increased amounts of flooding. These results are substantially consistent among the three IPCC (Intergovernmental Panel on Climate Change) scenarios. The increased probability of heavy rainfall events in China is closely connected with increased transportation of water vapour from the Arabian Sea and the South China Sea. Additionally, atmosphere stratification has become increasingly unstable, which has provided a favorable background for the initiation of heavy rainfall at the end of the 21st century. Copyright © 2010 Royal Meteorological Society

Statistical analysis of the relationship between summer monsoon precipitation extremes and foodgrain yield over India

Abstract: The Indian economy largely depends on agriculture which is highly influenced by the spatio-temporal variability of precipitation. Kharif and rabi are the two main crop-growing seasons which require major proportion of rainfall. Increase in heavy precipitation events, however, can have adverse effects on the crops. This study, therefore, is mainly focused on understanding the variation of extremes in precipitation during summer monsoon season and its impact on kharif foodgrain yield over India. For this, several objectively defined indices of observed precipitation extremes, in terms of frequencies and intensities, have been computed for the period 1951-2003 using daily gridded data of 1° latitude A� 1° longitude resolution. Efforts have also been made to prepare the climatology of extremes in precipitation (1961-1990) to determine their basic characteristics over the Indian region. Correlation analysis reveals that a large part of the country exhibits positive relationship between kharif foodgrain yield and the indices of extreme precipitation, significant at 5 level. Correlations with the indices of frequencies defined as seasonal count of days when rainfall exceeds 85th, 75th, and 65th percentiles (and 30, 20, and 10 mm) show that spatial extent and strength of the positive relationship decreases with increase in threshold values. This indicates that very heavy daily rainfall is less useful than the moderate daily rainfall. Strong negative relationship between the yield and indices of extreme precipitation observed over heavy precipitation areas of northeastern parts of the country indicate that very heavy daily rainfall have an adverse effect on crop. The study also points out that increase in heavy rainfall activities during July are favourable for the yield, but correlations become weak and statistically insignificant during the month of August. Individual crops show similar results as that obtained from aggregate kharif foodgrain, however, magnitude of the correlations varies from crop to crop.

Characteristic 20th and 21st century precipitation and temperature patterns and changes over the Greater Horn of Africa

Abstract: Characteristic patterns and changes in precipitation and temperature over the Greater Horn of Africa during the 20th and 21st century are analysed based on a sample of Coupled Model Intercomparison Project version 3 (CMIP3) models output. Analysis of the 11 CMIP3 models indicates that the equatorial eastern Africa region (including the entire Greater Horn of Africa (GHA)) have been experiencing a significant increase in temperature beginning in the early 1980s, in both A1B and A2 scenarios. All the Atmosphere Ocean Global Circulation Models (AOGCMs) analysed represent the correct mean annual cycle of precipitation, but there is a fairly large spread among the models in capturing the dominant bimodal peaks. In particular, all the models tend to overestimate the peak of the October–November–December (OND) season, while at the same time the peak of the March–April–May (MAM) season tends to be centered on May in the models instead of April as observed. The projected changes and probability distribution of minimum (Tmin) and maximum (Tmax) temperatures over the GHA sub-region based on PDFs constructed from daily values showed very diverse distributions for the present (1981–2000) and future (2046–2065; 2081–2100) periods. Whereas in the reference (1981–2000) the probability distribution functions (PDFs) constructed for both Tmin and Tmax, and during all the seasons had a near normal (but narrow) distribution, those of the future periods were quite diverse but generally very elongated, with significant shifts toward the positive tail. This generally implies that there is consensus among models and the ensemble mean about high likelihood of increase in extreme warmer Tmin and Tmax (more so Tmin) in the future over the GHA region. Our results also show significant increase in the number of days with Tmin and Tmax greater the 2 °C (above 1981–2000 average) by the middle as well as the end of 21st century in both the A1B and A1 scenarios. This is especially so during the June, July, and August (JJA) season where all the 92 days of the season indicate projected minimum temperature to increase by more than 2 °C above the 1981–2000 average by the end of 21st century in both scenarios. Copyright © 2011 Royal Meteorological Society

Spatial patterns and regimes of daily precipitation in Iran in relation to large-scale atmospheric circulation

Abstract: The relationships between large-scale atmospheric circulation types and seasonal regimes of daily precipitation over Iran are assessed using daily precipitation from a high-resolution gridded dataset provided by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) Project. Regional spatial modes of daily precipitation variability were identified by S-mode Principal Component Analysis (PCA) with Varimax rotation, applied to the subset of days when at least 10% of all grid-points over Iran received precipitation ≥ 5 mm. The study refers to the period 1961–2004 and is carried out for each season (excluding summer) separately. To characterize the dynamical features associated with each regional precipitation regime (PR), composites of daily atmospheric fields are computed by only averaging days with rotated PCA scores ≥ 1.5 (strong positive phase). In autumn and winter, Iran is divided into five PRs, while four PRs are identified in spring. Results suggest that the spatial distribution of precipitation over Iran is largely governed by the geographical position of both the mid-tropospheric trough over the Middle East and the Arabian anticyclone. In fact, in almost all PRs, the trough, as a pre-conditioning factor, leads to regional-scale ascending motions, whereas the Arabian anticyclone induces low-tropospheric moisture transports from southern water bodies into the cyclonic systems near Iran, triggering rain-generating conditions. Copyright © 2011 Royal Meteorological Society

How potentially predictable is northern European winter climate a season ahead

Abstract: We estimate the potential predictability of European winter temperature using factors based on physical studies of their influences on European winter climate. These influences include sea surface temperature patterns in different oceans, major tropical volcanoes, the quasi-biennial oscillation in the tropical stratosphere, and anthropogenic climate change. We first assess the predictive skill for winter mean temperature in northern Europe by evaluating statistical hindcasts made using multiple regression models of temperature for Europe for winter and the January–February season. We follow this up by extending the methodology to all of Europe on a 5° × 5° grid and include rainfall for completeness. These results can form the basis of practical prediction methods. However, our main aim is to develop ideas to act as a benchmark for improving the performance of dynamical climate models. Because we consider only potential predictability, many of the predictors have estimated values coincident with the winter season being forecast. However, in each case, these values are predictable on average with considerable skill in advance of the winter season. A key conclusion is that to reproduce the results of this paper, dynamical forecasting models will require a fully resolved stratosphere. Copyright © 2011 Royal Meteorological Society and British Crown copyright, the Met Office

Interannual variations of tropical cyclone activity over the north Indian Ocean

Abstract: An examination of the interannual variations of tropical cyclone (TC) activity over the North Indian Ocean during 1983–2008 has been carried out. The results suggest that instead of local sea surface temperatures, such variations, at least over the Bay of Bengal (BB) during October-November-December (OND), can be attributed to similar variations in the atmospheric flow patterns and moist static energy that are apparently forced largely by the El Nino/Southern Oscillation (ENSO). In an El Nino year, conditions for TC genesis and development, including 850-hPa relative vorticity, 200–850-hPa vertical shear of zonal wind, moist static energy, 500-hPa zonal wind, 500-hPa and 850-hPa geopotential height and 200-hPa divergence, are generally less favourable in BB and fewer intense cyclones are observed during OND. The reverse occurs during a La Nina event. However, causes of the variability of TC activity over BB during April-May-June and that over Arabian Sea have yet to be found, which may be due to the small sample size. Copyright © 2011 Royal Meteorological Society

Influence of the extent and genera of cloud cover on solar radiation intensity

Abstract: This paper aims to describe the influence of cloud cover, its extent and cloud genus, on solar radiation intensity measured at the Earth's surface. Solar radiation intensity values at varying degrees of cloudiness, based on observations and measurements performed between 2003 and 2007 in Krakow (Poland), are analysed in this paper. Analyses show that cloud cover impacts solar radiation intensity in two ways: usually weakening it, but intensifying in certain weather conditions. The greatest solar radiation intensity can be detected not when the sky is cloudless, but when it is partly cloudy (3/8-6/8), with convective clouds present. Copyright © 2011 Royal Meteorological Society

Climate change along the arid coast of northern Chile

Abstract: Long-term precipitation records from the extremely arid northern coast of Chile (18°S–30°S) were analysed to assess changes occurring at different time scales. Results are presented here along with a discussion on changes in the temperature and cloudiness regimes in order to offer a more comprehensive overview of the climate evolution in this extremely arid region. Apart from a significant influence of ENSO on the rainfall regime, characterized by a tendency for more frequent rainfall events during El Nino episodes, changes at the decadal time scale were identified in association with the Interdecadal Pacific Oscillation (IPO). Thus, the warm IPO-phase is associated with increased precipitation, while the opposite occurs during the cold IPO-phase. Changes occurring at the interannual and decadal time scales are superimposed on a long-term precipitation decline during the 20th century. Apart from the intensified dryness, the temperature records show a positive long-term trend resulting mainly from an abrupt warming in the mid-1970s, principally associated with a marked upwards shift of the minimum daily temperature, coinciding with the change from the cold to the warm phase of the IPO. However, the period following this step-like warming has been characterized by a persistent cooling trend, most evident in the maximum daily temperature, which is coherent with a negative trend in the sea surface temperature over a large oceanic region off the coast of northern Chile. In the northernmost region, this behaviour in the temperature regime was accompanied by a strong decrease in cloudiness since the 1970s. The negative trend in rainfall and the decrease in the total cloud cover are certainly important factors that could explain the coastal vegetation decline over the past decades in the coastal region north of 24°S. Copyright © 2011 Royal Meteorological Society