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  • 35 Reads
The origin of moisture feeding up Atmospheric Rivers over the Arctic

The Arctic system is one of the most vulnerable region under climate change conditions and it has suffered important changes on last decades. Several recent studies have suggested the influence of moisture transport in the observed sea ice loss on this region. Changes in moisture transport could affect the arctic region by changing the cloud cover, by affecting river discharge or by direct effect of precipitation over the sea ice, for example. Atmospheric rives (ARs) represent one of the main mechanism of global moisture transport, being especially relevant on the connection between lower and higher latitudes. Despite this importance, the influence of ARs over the Arctic system has not been widely study.

The objective of this is work is to establish a first step on the study of the influence of the occurrence of ARs over the polar region. For this purpose, the lagrangian model FLEXPART was used to analyze moisture sources for those regions of maximum occurrence of ARs for the period 1994-2014 in order to analyze the origin of moisture transported by these meteorological structures. The location of ARs affecting the Arctic was realized using an automated algorithm and the region of maximum occurrence was defined taking into account the number of ARs detected for August and September (when sea ice is minimum over the Arctic ocean) over a band of 10° of latitude centered on 60°N. For these regions and considering those days of ARs occurrence, the anomalous moisture sources was defined in relation with mean situation for the complete period.

From the results, main moisture sources for ARs events extends over the North Atlantic and North Pacific oceans, moreover local input of moisture over the region of maximum ARs occurrence seems to be especially relevant. It is interesting to highlight the moisture uptake from Eastern Asia for the month of August. In general it could be conclude that, for ARs events the moisture uptake around and over the maximum occurrence area highly increase becoming relevant sources of moisture feeding up the event.

The location of the origin of the moisture that feed up Arctic ARs is an important step forward on the study of the influence of these structures over the region. Further analysis regarding the contribution of moisture from ARs over the region should be realized in order to complete the relation ARs-sea ice; being this study suitable for a future work.

  • Open access
  • 14 Reads
Prediction of Annual Inflow to Karkheh Dam Reservoir using Time Series Models

The optimal exploitation of water from a dam reservoir requires a comprehensive knowledge of future availability of water resources. In this case the amount of water that will be available in the future is important. Also, we need to examine the flows at the dam from a short-term perspective. This is necessary to avoid overflowing and to minimize damage. In order to facilitate forecasting of the water resources, many different techniques have been developed through the years. In this paper, using annual mean flow data (since 1958-2005) obtained from jelogir majin Hydrometric station at Karkheh River (upstream of Karkheh Dam), the Auto Regressive Integrated Moving average (ARIMA) model, for prediction of annual mean inflow to Karkheh Dam reservoir was accomplished. On the basic of comparison the results of the model with measured data, the performance of ARIMA (4, 1, 1) model by conditional least square (CLS) estimation parameter method is acceptable.The SAS and SPSS softwares were used to implement of the models.

  • Open access
  • 27 Reads
A lagrangian analysis of the moisture transport during the 2003 drought episode over the Mediterranean region

In the last decades many studies have pointed out an increasing number of natural hazards associated with extremes in precipitation and drought conditions. Generally, dry and hot conditions across the Europe impact on the Mediterranean region. The Mediterranean is located at the border between the tropical climate zone and the mid latitude climate belt. Due to its large extension and diverse topography, it shows large climatic differences that make its climate scientifically interesting.  

The aim of this study is to analyze the moisture transport during the 2003 drought episode observed over the surroundings of the Mediterranean. The region was defined according to the 5th Intergovernmental Panel on Climate Change (IPCC) Assessment Report. The episode was identified using Standardized Precipitation Evapotranspiration Index (SPEI), calculated using monthly CRU (TS3.24.01) precipitation and potential evapotranspiration (PET). One of the crucial advantages of the SPEI over the other widely used drought indexes is its multi-scalar characteristics, which enable identification of different drought types. Therefore, the monthly SPEI-1, SPEI-3, SPEI-6, SPEI-12 and SPEI-24 indexes were used to identify the episodes on different time scales. This episode was the most severe during the period 1980-2015 according to the SPEI-1 analysis. Analyses of precipitation, potential evapotranspiration, omega at 500hPa, and vertically integrated moisture flux have been conducted to characterize the anomalous patterns over the region during the event. A Lagrangian approach was then applied in order to investigate possible changes in the moisture transport from and toward the Mediterranean region during the episode. This approach is based on the FLEXPART model integrated with the ERA-Interim data set.

  • Open access
  • 63 Reads
A proposed robust approach for calculating the Standardized Evapotranspiration Deficit Index (SEDI) at the global scale

Here we propose a new methodology for calculating the Standardized Evapotranspiration Deficit Index (SEDI) at the global scale using the difference between the actual evapotranspiration (ET) and the atmospheric evaporative demand (AED). ET was estimated by the Global Land Evaporation Amsterdam Model (GLEAM) v3a. The SEDI has been proposed recently to quantify drought severity based on the difference between actual evapotranspiration (ET) and the atmospheric evaporative demand (AED). Our findings demonstrate that, regardless of the AED dataset used for calculations, a log-logistic distribution is needed in order to fit the ED time series. As such, in many regions worldwide, the SEDI is insensitive to the AED method used for calculation. The SEDI showed significant correlations with the Standardized Precipitation Evapotranspiration Index (SPEI) across a wide range of regions, particularly for short SPEI time-scales. Overall, while this work provides a robust approach for calculating spatially and temporally comparable SEDI estimates, regardless of the climate region and land surface conditions, further studies remain needed to assess the performance and the applicability of the SEDI to quantify drought severity across varying crop and natural vegetation areas.

  • Open access
  • 24 Reads

Western Iberia is frequently struck by intense mid-latitude cyclones coming from the North Atlantic basin and often impinging extreme weather over large swaths of the Iberian Peninsula (IP). The spatial distribution and characterization of past floods and landslides with important social consequences in Portugal for the period 1865-2015 was performed within the context of the DISASTER project (Zezere et al., 2014). From this database, a major hydro-geomorphologic event was selected, the February 1979, in order to study its atmospheric forcings and to analyze its societal impacts.

The February 1979 event is a top ranked event in the DISASTER database regarding the total number of affected (18578), displaced (14322) and evacuated (4244) people in Portugal and in the Tagus basin (7677, 4816 and 2834, respectively).

Most of the days considered in this event produced daily precipitation values over or within the 90th-95th percentile of the corresponding long term daily precipitation series (available at high resolution between 1950 and 2008). Most of the event precipitation occurred in days characterized by wet Circulation Weather Types, i.e. cyclonic (C), west (W) or southwest (SW) types, which agrees with the assessment of wet days obtained by Trigo and DaCamara (2000) and Ramos et al. (2014) for the IP domain.

Also, throughout this period, the North Atlantic Ocean is crossed several times by narrow and prolonged bands of high moisture concentration, with cores above 9 g/kg, that originate near the Caribbean islands and move towards extratropical latitudes by the influence of southwestern low-level jets of medium or high intensity. These are mostly persistent Atmospheric Rivers (ARs) reaching the western IP coast and affecting most of the month of February until the 16th.

Overall, regarding the large-scale circulation, a deep low-pressure system located over the North Atlantic and reaching western IP, allowed for the frequent passage of frontal systems over the territory which was responsible for this precipitation event. In addition, local convective instabilities and strong moisture transport from the Tropical Atlantic produced an extremely intense 15-day precipitation event over western IP, that establishes as the meteorological trigger of the February 1979 Disaster event.


Acknowledgements: This work was financed by national funds through FCT - Portuguese Foundation for Science and Technology, I.P., under the framework of the project FORLAND Hydro-geomorphologic risk in Portugal: driving forces and application for land use planning (PTDC/ATPGEO/1660/2014). A. M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).

  • Open access
  • 14 Reads
Adaptation strategies of the hydrosocial cycles in the Mediterranean regions

The Spanish regions located in the Mediterranean have been affected by several factors over the years (climatic conditions of aridity, strong demands, quick and intense urban and population growth) that have generated a negative water balance in which water contributions are unable to meet the demands. This problem is aggravated by future scenarios estimated as a consequence of climate change, which predicts a decrease in precipitation and an increase in temperature and frequency of maximum events. The aim of this work is to evaluate the adaptation strategies that have been conducted in these cities in order to adjust their hydrosocial cycles to development, as well as new actions that are being implemented in order to mitigate the effects of climate change. Diversification of supply sources has been one of the keys to this evolution, using both own and external resources. These adaptive measures have made the search for new supply sources a necessity, stimulating the expansion and integration of non-conventional water sources (desalination and reuse of reclaimed water). Likewise new strategies, such as the implementation of sustainable urban drainage systems (SUDS), seem to gain positions in the water planning of these regions. The theoretical analysis developed in this work is corroborated by the study of the hydrosocial cycle evolution of three cities of the province of Alicante (southeast Spain), and the adaptive measures that the different actors involved in the cycle have developed in each one of them. The input and output are considered in this system thanks to the information provided by the management companies in each of the phases (urban consumption, treated and reused volumes). The results obtained evidence the complexity of this hybrid socio-natural process where water and society influence each other and where an adequate planning is essential in order to overcome new scenarios.

  • Open access
  • 24 Reads
Complex spatial and temporal influences of climatic drought time-scales on hydrological droughts in natural basins of U.S.

We have analysed the response of hydrological droughts to climatic droughts in different natural basins of United States of America. For this purpose we selected 289 complete series from an initial data set of 702 gauging stations, covering the period between 1940 and 2013. Drainage basins were obtained for each gauging station using a digital terrain model, and climate series (precipitation and the atmospheric evaporative demand) were obtained for each drainage series. A number of topographic, edaphic and remote sensing variables were quantified for each basin. A hydrological drought index (The Standardized Stream Flow Index-SSI) was obtained for each basin and the Standardized Precipitation Evapotranspiration Index (SPEI) was used as a metric of climatic drought severity. Relationship between different SPEI time scales and the SSI was quantified using Pearson’s correlations and the general patterns of response of hydrological droughts to climatic droughts were identified using a principal component analysis. In general there is a response of SSI to short SPEI time-scales but strong seasonality in this response is also found. Coherent geographic patterns were obtained from the analysis and the factors that control the different responses were analysed by means of a predictive discriminant analysis. Independent analysis for three periods (1940-1964, 1965-1989 and 1989-2013) were also performed and showed not important changes in the response of the SSI to the SPEI in the past 65 years.

  • Open access
  • 21 Reads
Seasonal and annual daily precipitation risk maps for the Andean region of Peru

We develop for the first time maps of annual and seasonal extreme precipitation risk in the Andean region of Peru. For this purpose, we used the complete daily precipitation records existing in Peru and after a careful quality control and homogeneity checking we selected 178 stations distributed across the mountainous chain. In each meteorological station, we obtained series of events of de-clustered daily intensity, total precipitation duration, total magnitude and dry-spell length. Using a peak-over-threshold approach we fitted the annual and seasonal series of these variables to a Generalized-Pareto distribution, obtained the distribution parameters and validated the performance of different thresholds to obtain reliable estimations of the precipitation probability. We found that a 90th percentile is in general the most suitable to develop the estimations for the different variables. The parameters obtained in the different meteorological stations were mapped using a universal krigging approach using the elevation and the distance to the ocean as co-variables. Maps of parameters were validated using a jack-knife approach and maximum expected precipitation intensity, magnitude, duration and dry-spell length estimated for a period of 25 and 50 years. The reliability of the spatial methodology was validated comparing observed precipitation and estimated by the spatial modelling in the different stations.

  • Open access
  • 14 Reads

Different ways to estimate future return levels for extreme rainfall are described and applied to the Iberian Peninsula (IP), based on Extreme Value Theory (EVT). This study is made for an ensemble of high quality rainfall time series observed in the Iberian Peninsula over the period 1961-2010. Both, peaks-over-threshold (POT) approach and block maxima with the Generalized Extreme Value (GEV) distribution will be used and their results compared when linear trends are assumed in the parameters: threshold and scale parameter for POT and location and scale parameter for GEV. Both all-days and rainy-days-only data sets were considered, because rainfall over the IP is a special variable in that a large number of the values are 0. Another methodology is then tested, for rainy days only, considering the role of how the mean, variance, and number of rainy days evolve. The 20-year return levels (RLs) expected in 2020 were estimated using these methodologies for three seasons: fall, spring and winter. GEV is less reliable than POT because fixed blocks lead to the selection of non-extreme values. Future RLs obtained with POT are higher than those estimated with GEV, mainly for some observatories showing significant positive trend for the number of rainy days. Fall becomes the season with heaviest rainfall, rather than winter nowadays, for some regions.

  • Open access
  • 11 Reads
Land use change over Amazon Forest and its impact on the climate

The purpose of this study is to analyze the Amazon (AMZ) deforestation impact on South America (SA) climate. Hence, two simulations with RegCM4, from 1979 to 2009, were carried out: one using a default land cover map (CTRL ) and the other one using deforestation scenario (LUC), i.e., all broadleaf evergreen trees tropical was changed to C3 grass. RegCM4 was integrated at a horizontal grid spacing of about 50 km and considering 18 sigma-pressure vertical levels for SA CORDEX domain. The lateral and boundary conditions for driving the model was obtained from the ERA-INTERIM reanalysis . The climate change signal due to AMZ deforestation is evaluated by comparing the climatology of the CTRL with the LUC. AMZ deforestation changes the precipitation pattern over northern South America, i.e., appears a dipolar response consisting of reduced precipitation over eastern AMZ and an increased precipitation over western AMZ. Concerning the temperature, there is a warmer anomaly over the deforested areas, and it contributes to decrease the surface pressure. The increase in air temperature is associated to an increase of the sensible heat flux and a decrease of the latent heat flux.

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