A drought hazard assessment index based on the VIC–PDSI model and its application on the Loess Plateau, China

Drought is a complex natural hazard that is poorly understood and difficult to assess. This paper describes a VIC–PDSI model approach to understanding drought in which the Variable Infiltration Capacity (VIC) Model was combined with the Palmer Drought Severity Index (PDSI). Simulated results obtained using the VIC model were used to replace the output of the more conventional two-layer bucket-type model for hydrological accounting, and a two-class-based procedure for calibrating the characteristic climate coefficient (K _j ) was introduced to allow for a more reliable computation of the PDSI. The VIC–PDSI model was used in conjunction with GIS technology to create a new drought assessment index (DAI) that provides a comprehensive overview of drought duration, intensity, frequency, and spatial extent. This new index was applied to drought hazard assessment across six subregions of the whole Loess Plateau. The results show that the DAI over the whole Loess Plateau ranged between 11 and 26 (the greater value of the DAI means the more severe of the drought hazard level). The drought hazards in the upper reaches of Yellow River were more severe than that in the middle reaches. The drought prone regions over the study area were mainly concentrated in Inner Mongolian small rivers, Zuli and Qingshui Rivers basin, while the drought hazards in the drainage area between Hekouzhen–Longmen and Weihe River basin were relatively mild during 1971–2010. The most serious drought vulnerabilities were associated with the area around Lanzhou, Zhongning, and Yinchuan, where the development of water-saving irrigation is the most direct and effective way to defend against and reduce losses from drought. For the relatively humid regions, it will be necessary to establish the rainwater harvesting systems, which could help to relieve the risk of water shortage and guarantee regional food security. Due to the DAI considers the multiple characteristic of drought duration, intensity, frequency, and spatial extent, and because it is based on the VIC–PDSI model and GIS technologies, the DAI could provide some new way on directly comparing the drought hazards over different regions during a long-term period. The result of this study may be useful to decision makers when formulating drought management policies to alleviate the risk of water shortages and guarantee regional food security.     

Spatial–temporal variations of spring drought based on spring-composite index values for the Songnen Plain,Northeast China

A spring-composite index (s-CI) is proposed in this study that involves slightly altering the use of the accumulated precipitation from the composite index (CI) comparing the value with other three commonly used indices (standardized precipitation index, SPI; self-calibrated Palmer drought severity index, sc-PDSI; and CI). In addition, the spatial–temporal variation of the s-CI in the Songnen Plain (SNP) was investigated using the Mann–Kendall test and empirical orthogonal function (EOF) methods. The results indicated that the proposed s-CI could identify most drought events in 1990s and 2000s and performed relatively better than SPI, sc-PDSI, and CI in this region. Compared with the other three indices, the s-CI had a higher correlation with relative soil moisture in April and May. The recent spring droughts (2000s) were the most severe in April or May. The weather was drier in May compared with April in the 1980s, whereas the weather was wetter in May than in April in the 1960s and 1970s. Moreover, the spatial patterns of the first EOFs for both April and May indicated an obviously east–west gradient in the SNP, whereas the second EOFs displayed north–south drought patterns. The proposed index is particularly suitable for detecting, monitoring, and exploring spring droughts in the Songnen Plain under global warming.     

Precipitation and temperature space–time variability and extremes in the Mediterranean region: evaluation of dynamical and statistical downscaling methods

This study evaluates how statistical and dynamical downscaling models as well as combined approach perform in retrieving the space–time variability of near-surface temperature and rainfall, as well as their extremes, over the whole Mediterranean region. The dynamical downscaling model used in this study is the Weather Research and Forecasting (WRF) model with varying land-surface models and resolutions (20 and 50 km) and the statistical tool is the Cumulative Distribution Function-transform (CDF-t). To achieve a spatially resolved downscaling over the Mediterranean basin, the European Climate Assessment and Dataset (ECA&D) gridded dataset is used for calibration and evaluation of the downscaling models. In the frame of HyMeX and MED-CORDEX international programs, the downscaling is performed on ERA-I reanalysis over the 1989–2008 period. The results show that despite local calibration, CDF-t produces more accurate spatial variability of near-surface temperature and rainfall with respect to ECA&D than WRF which solves the three-dimensional equation of conservation. This first suggests that at 20–50 km resolutions, these three-dimensional processes only weakly contribute to the local value of temperature and precipitation with respect to local one-dimensional processes. Calibration of CDF-t at each individual grid point is thus sufficient to reproduce accurately the spatial pattern. A second explanation is the use of gridded data such as ECA&D which smoothes in part the horizontal variability after data interpolation and damps the added value of dynamical downscaling. This explains partly the absence of added-value of the 2-stage downscaling approach which combines statistical and dynamical downscaling models. The temporal variability of statistically downscaled temperature and rainfall is finally strongly driven by the temporal variability of its forcing (here ERA-Interim or WRF simulations). CDF-t is thus efficient as a bias correction tool but does not show any added-value regarding the time variability of the downscaled field. Finally, the quality of the reference observation dataset is a key issue. Comparison of CDF-t calibrated with ECA&D dataset and WRF simulations to local measurements from weather stations not assimilated in ECA&D, shows that the temporal variability of the downscaled data with respect to the local observations is closer to the local measurements than to ECA&D data. This highlights the strong added-value of dynamical downscaling which improves the temporal variability of the atmospheric dynamics with regard to the driving model. This article highlights the benefits and inconveniences emerging from the use of both downscaling techniques for climate research. Our goal is to contribute to the discussion on the use of downscaling tools to assess the impact of climate change on regional scales.     

Stationarity of atmospheric waves and blocking over Europe—based on a reanalysis dataset and two climate scenarios

In this investigation, a circulation index was used which is capable of tracing blocked states of the hemispheric circulation which has a relevance for a mid-latitude window between 40° and 60°N and between 30°W and 30°E. Results with respect to the seasonality of blocking situations are presented. Additionally, it is shown to which degree the circulation conditions, as they are simulated by the ECHAM5-MPI/OM1 climate model, have a tendency to exhibit blocking. Finally, results are presented which indicate the developments in blocking if the index is applied to scenario runs of the model.     

Summertime warming in the European part of Russia and extreme heat in 2010 as manifestation of large-scale atmospheric circulation trends in the late 20th–early 21st centuries

Considered is a relationship between long-term variations of the surface air temperature and of the field of geopotential at the level of 500 hPa in the middle of summer in the European part of Russia and the variations of large-scale atmospheric circulation described by the indices of North Atlantic and North Pacific centers of atmospheric action. The considerable inhomogeneity in the course of average temperature in the European part of Russia in July divides it into two periods, before and after the 1980s. Unlike stationary fluctuations of temperature before the 1980s, the 6–10-year fluctuations are typical of the period of 1981–2010 against a background of its considerable rise by 0.8°C per 10 years with the contribution of 20% to the total variance. During this period, about 80% of temperature variability are caused by the circulation variations, and 55% of them are associated with the North Pacific centers of atmospheric action, in particular, with the WP index in July, May, and April. Revealed regression dependences between circulation indices and the air temperature in the European part of Russia explain rather accurately the linear trend, the fluctuations with the period of 6–10 years, and the extremes in 1988, 2001–2003, and 2010. The analysis of the time series (1950–2012) of WP, PDO, and SOI indices demonstrates that changes in the atmospheric circulation took place in the extratropical zone of the Northern Hemisphere at the turn of the 1980s and this caused the formation of blocking situations and, hence, steady heat and drought in summer in the mid-latitude zone including the European part of Russia. These variations can be interpreted as a change in the regimes of strong (1950–1980) and weak (1981–2012) circulation. The heat in July 2010 was an extreme display of weak circulation, and this is indicated by the unprecedented low values of the WP index in July and May.     

Reconstructions of annual summer precipitation and temperature in north-central China since 1470 AD based on drought/flood index and tree-ring records

The understanding of the ongoing climate change needs high-resolution records of the past, which are difficult to obtain in north-central China. Historical documents are unique materials for high-resolution (up to season) climate change reconstruction. Here, we report an attempt of quantitative climate reconstruction covering the main part of north-central China, by combining historical drought/flood index and tree-ring data. The rigorous verification tests confirm the fidelity of transfer functions used in the reconstructions. The precipitation and temperature anomalies/intervals were then defined based on the reconstructions. Finally, the intensity of several big droughts recorded in historical documents was re-examined and the dominant and recessive patterns of heat/water changes within the study area were identified. We concluded that (1) the droughts, occurred during the years of 1484 AD, 1585–1587 AD, 1689–1691 AD, 1784–1786 AD and 1876–1878 AD, were the results of rainless and torrid combination; (2) the droughts, occurred during the years of 1560–1561 AD, 1599–1601 AD, 1609 AD, 1615–1617 AD, 1638–1641 AD and 1899–1901 AD, were first caused by rainless summer, and then controlled by low precipitation and/or high temperature; (3) the droughts, occurred during the years of 1527–1529 AD, 1720–1722 AD, 1813–1814 AD, 1856–1857 AD and 1926–1930 AD, were first caused by torrid summer, and then controlled by both low precipitation and high temperature; (4) the dominant climate pattern within the study area consisted of warm–dry and cold–wet alternations, and the recessive pattern consisted of cold–dry and warm–wet alternations. We also showed that the drought/flood index is a valuable climate proxy in quantitative reconstructions, especially in places where tree-ring data is not available.     

The influence of atmospheric circulation on the intensity of urban heat island and urban cold island in Poznań, Poland

The study has analyzed influence of an atmospheric circulation on urban heat island (UHI) and urban cold island (UCI) in Poznań. Analysis was conducted on the basis of temperature data from two measurement points situated in the city center and in the ?awica airport (reference station) and the data concerning the air circulation (Nied?wied?’s calendar of circulation types and reanalysis of National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR)). The cases with UHI constitute about 85 % of all data, and UCI phenomena appear with a frequency of 14 % a year. The intensity of UHI phenomenon is higher in the anticyclonic circulation types. During the year in anticyclonic circulation, intensity of UHI is 1.2 °C on average while in cyclonic is only 0.8 °C. The occurring of UHI phenomena is possible throughout all seasons of the year in all hours of the day usually in anticyclonic circulation types. The cases with highest UHI intensity are related mostly to nighttime. The cases of UCI phenomena occurred almost ever on the daytime and the most frequently in colder part of the year together with cyclonic circulation. Study based on reanalysis data indicates that days with large intensity of UHI (above 4, 5, and 6 °C) are related to anticyclonic circulation. Anticyclonic circulation is also promoting the formation of the strongest UCI. Results based on both reanalysis and the atmospheric circulation data (Nied?wied?’s circulation type) confirm that cases with the strongest UHI and UCI during the same day occur in strong high-pressure system with the center situated above Poland or central Europe.     

Spatial and temporal variability of soil biological activity in the Province of Québec, Canada (45–58 °N, 1960–2009)—calculations based on climate records

Climate records of air temperature (AT) and total precipitation (TP) are standard inputs for soil carbon dynamic models, i.e., for calculating temperature and moisture effects on soil biological activity. In this study our objective was to determine both spatial and temporal differences in soil biological activity in the Province of Québec, Canada. Soil biological activity was here calculated on a daily basis with the ICBM re_clim parameter using data from weather stations. When keeping soil and crop properties constant, re_clim (unitless) allows us to assess relative differences in soil biological activity. The magnitude of the temporal changes in re_clim, AT and TP were analyzed using Sen’s slope, which is a nonparametric method used to determine the presence of a trend component. The re_clim varied across Québec from 0.50 (58 °N) to a high of 1.66 (45 °N). Considering only the area with significant agricultural production, re_clim varied from 0.99 at Gaspé (48 °N) to 1.66 at Philipsburg (45 °N), i.e., soil organic carbon (SOC) decomposition rate is 68 % higher at the latter site (1.66/0.99) and correspondingly more C input is needed to maintain SOC. Soil biological activity increased from 1960 to 2009, with a mean slope variation in re_clim of about +10 %. The temporal variation in AT had more influence than that of TP. For 1980–2009 the mean annual slope of re_clim was significantly different from zero for 29 out of 49 climate records (mean?=?+14 %; N?=?29). We also emphasize that analysis of seasonal changes in AT is an issue that needs further attention, as well as modeling climate-induced changes in SOC dynamics based on future climate scenarios.