Occurrence Of Extreme Rainfall Research Articles

Changes in extreme rainfall in the Philippines (1911–2010) linked to global mean temperature and ENSO

ABSTRACTStatistical modelling of extreme values was used to detect potential changes in extreme rainfall in the Philippines and to investigate whether such changes are associated with the rising near‐surface global mean temperature and the El Niño–Southern Oscillation (ENSO). The generalized extreme value distribution was formulated to its stationary and non‐stationary forms, and then was fitted by the maximum likelihood method to the series of daily rainfall annual maxima (RX1day) spanning 100 years (1911–2010) at 23 meteorological stations across the country. Subsequently, statistically significant changes in extreme rainfall in the country were detected; such changes were further linked to the near‐surface global mean temperature and ENSO. Specifically, the study has revealed a country‐averaged increase in the median intensity of extreme rainfall by about 4.3% °C−1 rise in the near‐surface global mean temperature. Furthermore, a seasonal influence of ENSO on extreme rainfall in the Philippines has been shown. In particular, the stations located in the northwest section of the country, where 75–100% of the RX1day occurred in the summer monsoon season (July–September) during the entire period of 1911–2010, showed an average increase in the median intensity of extreme rainfall by about 6.5% °C−1 increase in the ENSO index. These findings imply a potential intensification and increase in the occurrence of extreme rainfall into the future as the global mean temperature continues to rise, and such trends should be considered in adaptation strategies to minimize the disasters caused by extreme rainfall events in the Philippines.

Relating changes in synoptic circulation to the surface rainfall response using self-organising maps

The climate of a particular region is governed by factors that may be remote, such as the El Nino Southern Oscillation or local, such as topography. However, the daily weather characteristics of a region are controlled by the synoptic-scale atmospheric state. Therefore changes in the type, frequency, duration or intensity of particular synoptic states over a region would result in changes to the local weather and long-term climatology of the region. The relationship between synoptic-scale circulation and the rainfall response is examined for a 31-year period at two stations in different rainfall regimes in South Africa. Dominant rain-bearing synoptic circulations are identified for austral winter and summer as mid-latitude cyclones and convective systems respectively whereas no circulations are dominantly associated with spring and autumn rainfall. Over the 31-year period a statistically significant increase in the frequency of characteristic summer circulation modes is observed during summer, winter and spring. During autumn a statistically significant shift towards characteristically winter circulation modes is evident. Seasonal rainfall trends computed at each station corroborate those of the circulation data. Extreme rainfall is associated with particular circulation modes and trends in both circulation and station data show an earlier occurrence of extreme rainfall during the rainy season.

Recent trends in the regime of extreme rainfall in the Central Sahel

Ongoing global warming raises the hypothesis of an intensification of the hydrological cycle, extreme rainfall events becoming more frequent. However, the strong time–space variability of extreme rainfall makes it difficult to detect meaningful trends in the regime of their occurrence for recent years. Using an integrated regional approach, it is shown that over the last 10 years, the Sahelian rainfall regime is characterized by a lasting deficit of the number of rainy days, while at the same time the extreme rainfall occurrence is on the rise. As a consequence, the proportion of annual rainfall associated with extreme rainfall has increased from 17% in 1970–1990 to 19% in 1991–2000 and to 21% in 2001–2010. This tends to support the idea that a more extreme climate has been observed over 2001–2010: this climate is drier in the sense of a persisting deficit of rainfall occurrence compared to 1950–1969, while at the same time there is an increased probability of extreme daily rainfall.

Seasonal and land use impacts on the nitrate budget and export of a mesoscale catchment in Southern Portugal

Stream nitrate nitrogen exports are an important indicator of agricultural impacts on aquatic health in catchments. Quantitative assessment of factors and processes affecting stream nitrate loadings is complex because of the large number of causal factors and processes, such as weather and rainfall, catchment hydrological behavior, soils, land use practices and biogeochemical processes. An eco-hydrological catchment modeling approach, using the SWAT model driven by detailed field data, was used to analyze the nitrate export and the components of the nitrogen budget of the 352 km 2 upper Roxo river catchment in Southern Portugal. A detailed eight-year record (2001–2008) of the monitoring of weather, reservoir inflow, stream biogeochemistry , soils, in-stream and groundwater quality, and fertilizer application was used to calibrate and validate the streamflow and nitrate loadings obtained by the model. Results indicated a strong seasonal variation in nitrate exports, closely related to temperature and rainfall. Monthly nitrate loadings varied from 0.02 to 2.48 kg N ha −1 during summer and between 0.03 and 14 kg N ha −1 during late autumn and winter. Stream nitrate values, ranging from 1.5 to 16.5 mg N L −1 , were strongly related to extreme rainfall occurrences and wet periods. Detailed analysis of nitrate budget components at the sub-catchment level enabled evaluation of the impacts of the various processes affecting the nitrate nitrogen pool of the catchment. Besides high fertilizer inputs for annual crops, it was shown that biological nitrogen fixation and wet deposition by rainfall should be accounted for in input balances. Where denitrification naturally reduces nitrate levels in soils, streams and the reservoir, the largest contribution to stream nitrate originates from leached soil nitrate reappearing in groundwater baseflow, compared with less than 2% from direct surface runoff during high rainfall events. A fertilizer reduction scenario was effectively implemented to evaluate remedial nitrate control policies in accordance with the European Nitrate and Water Framework Directives. Agricultural practices and seasonal weather fluctuations were the main reasons for temporal variations in nitrate export via small streams to the main reservoir.

Annual maximum and peaks-over-threshold analyses of daily rainfall accumulations for Austria

[1] Daily rainfall data from 31 rain gauges in Austria with a record of at least 50 years are used to perform annual maximum and peaks-over-threshold analyses to address question concerning: (1) temporal nonstationarities in extreme rainfall; (2) dependence of extreme rainfall on elevation; (3) changes in the frequency, duration, and accumulation of extreme rainfall; and (4) relation between the occurrence of extreme rainfall and the North Atlantic Oscillation (NAO). Nonparametric tests are used to check for the presence of abrupt and slowly varying changes in annual maximum daily rainfall. The Pettitt test is used to detect change-points in the mean and variance of the quantity of interest, while Mann-Kendall and Spearman tests are used to detect monotonically increasing or decreasing patterns. Annual maximum daily rainfall from these 31 Austrian rain gauges provides limited evidence of the violation of the stationarity assumption, and we cannot make conclusive statements about the presence of a possible climate change signal. The parameters of the Generalized Extreme Value (GEV) distribution are used to examine the dependence of extreme rainfall on elevation. The results of this study do not suggest a strong relation between elevation and the GEV parameters. Changes over time in the frequency, duration, and intensity of extreme rainfall days, defined as days exceeding the 95th, 99th, and 99.5th percentiles, are tested using monotonic trend tests and change-point tests. There is little evidence of changes in these quantities. By modeling the frequency of extreme rainfall days in terms of the NAO with a Poisson regression model, we found that NAO is a more significant predictor of the occurrence of extreme rainfall days in western Austria.

Projection of changes in the frequency of heavy rain events over Hawaii based on leading Pacific climate modes

[1] This study investigates the frequency of heavy rainfall events in Hawaii during the wet season (October-April) 1958–2005 and their conditional dependence on the Pacific-North American (PNA) pattern and El Nino-Southern Oscillation (ENSO). Heavy rain events are defined by the 95% quantile in the rainfall distribution of the wet seasons. Twelve stations with daily reports of rainfall amounts were used to count the number of heavy rain days during wet seasons. Multiple linear regression (MLR) indicated that the PNA index (PNAI) and the Southern Oscillation Index (SOI) can explain a significant amount of the interannual to interdecadal variability for 9 out of 12 stations. Cross validation showed that PNAI and SOI together explain about 18–44% of the variability in the number of heavy rain events. Furthermore, the MLR model reproduces the trend toward fewer heavy rain events in the years after the Pacific climate shift in the mid-1970s. The MLR model was applied to the projected PNAI and SOI indices that were obtained from six IPCC AR4 climate models. The current suite of AR4 simulations based on the A1B and A2 emissions scenarios projects small and equivocal changes in the mean state of the SOI and PNAI during the 21st century. The covariance between PNAI and SOI in these simulations appears to be stable. To the extent that variations in the frequency and magnitude of ENSO and the PNA mode are responsible for modulating extreme rainfall occurrence in Hawaii, our results indicate small changes in the projected number of heavy rainfall days with large uncertainties resulting from disparities among the climate models.

The effect of high flow events on mussels (Mytilus edulis) in the Conwy estuary, North Wales, UK

One of the predicted consequences of climate change is an increase in the occurrence of extreme rainfall and a subsequent increase in frequency of high flow events in rivers. High flow events have the potential to impact estuarine communities like mussel assemblages due to decreased salinity and/or the transport of sediments, organic matter and nutrients from the terrestrial environment to the estuary. The impact of two high flow events was investigated using mussels located within the Conwy estuary, North Wales, using the ‘Beyond BACI’ approach. Three study sites were chosen, the potentially impacted site (Conwy) and two control sites located outside the estuary. Sampling took place over 18 months with samples being collected before and after each event. On each sampling occasion, the following data were collected: the total haemocyte count (THC) and condition index (CI) of the mussels and the diversity (Hloge) of their associated macrofauna. A significant effect of the first event (22nd October 2004) was found on the CI of the Conwy mussels, whereas a significant effect of the second event (10th October 2005) was found on mussel THC. No effect of either event was found on the diversity of the associated fauna. The results of this study suggest that any increase in the number or intensity of heavy precipitation or high flow events have potential implications for the health and resilience of estuarine mussel populations.

DYNAMICS OF GENERATION OF EXTREME PRECIPITATION IN THE NORTHERN OF PERU

This article studies the influential factors in the occurrence of extreme rainfall in the northern Peru. The favorable conditions are analyzed under the following hypothesis: warming up of the sea surface temperature (SST), the southward movement of the Inter Tropical Convergence Zone (ITCZ), and the advection of humidity transferred from the east part of the Andes. Extreme rainfall has been periodically observed, it occurs during El Nino and non El Nino events. The main goal is to quantify the critical threshold for this occurrence. These limits will be associated to the teleconnections of the SST, ITCZ and winds. The generation of a warm nucleus of SST facilitates the creation of convergence nucleus in low levels due to positive temperature advection. It may weaken the thermal inversion and raise the atmospheric instability. To explain the dynamics of generation of extreme precipitation, the relationship of these three important factors: SST, ITCZ and winds, is analyzed using MODIS, GOES, and simulation of winds with the Regional Atmospheric Modeling System (RAMS). The results of this study will help to alert extreme rainfall events

The South Atlantic Convergence Zone: Intensity, Form, Persistence, and Relationships with Intraseasonal to Interannual Activity and Extreme Rainfall

The characteristics of intensity, geographical location, and persistence of the South Atlantic convergence zone (SACZ) during the austral summer are investigated. Intensity and spatial features of the SACZ are identified by performing a factor analysis of structural properties of outgoing longwave radiation (OLR) data. The first two leading factors explain 65% of the total variance of structural properties and characterize the SACZ according to intensity and location (oceanic versus continental). An index is constructed based on the magnitude of the factor scores to identify intense (weak) and oceanic (continental) SACZ. The intense SACZ category is associated with negative OLR anomalies over a large area of tropical South America, extending from the western Amazon to the Atlantic Ocean. The weak SACZ category is observed with positive OLR anomalies over tropical South America and negative OLR anomalies over southeastern South America. Oceanic and continental aspects of the SACZ are related to a midlatitude wave train pattern. The Madden–Julian oscillation (MJO) modulates intense SACZ events with persistence longer than 3 days. Interannual variability of persistent events indicates that the ratio of oceanic to continental SACZ as well as their frequency depends on the phase of El Niño–Southern Oscillation (ENSO). Occurrence of extreme rainfall in Brazil is discussed in the context of variations in the SACZ and MJO. Intense (weak) SACZ increases (decreases) the 95th daily rainfall percentile over central-eastern Brazil compared to the climatology. Oceanic (continental) SACZ increases (decreases) the 95th daily rainfall percentile over southeastern Brazil. The MJO phase characterized by suppression of convective activity over Indonesia and enhancement over the central Pacific increases the 95th daily rainfall percentile over north-northeastern Brazil, whereas opposite features are observed for the phase of the MJO characterized by the enhancement of convection over Indonesia and suppression over the central Pacific.