Hydro-meteorological Climate Research Articles

Spatiotemporal Variation Characteristics of Droughts and Their Connection to Climate Variability and Human Activity in the Pearl River Basin, South China

Droughts have damaging impacts on human society and ecological environments. Therefore, studying the impacts of climate variability and human activity on droughts has very important scientific value and social significance in order to understand drought warnings and weaken the adverse impacts of droughts. In this study, we used a combined drought index based on five Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On solutions to characterize droughts in the Pearl River basin (PRB) and its sub-basins during 2003 and 2020. Then, we accurately quantified the impact of climate variability and human activity on droughts in the PRB and seven sub-basins by combining the hydrometeorological climate index and in situ human activity data. The results show that 14 droughts were identified in the PRB, particularly the North River basin with the most drought months (52.78%). The El Niño-Southern Oscillation and the Indian Ocean Dipole were found to have important impacts on droughts in the PRB. They affect the operation of the atmospheric circulation, as well as the East Asia summer monsoon, resulting in a decrease in precipitation in the PRB. This impact shows a significant east–west difference on the spatial scale. The middle and upper reaches of the PRB were found to be dominated by SM, while the lower reaches were found to be dominated by GW. Human activity was found to mainly exacerbate droughts in the PRB, but also plays a significant role in reducing peak magnitude. The sub-basins with a higher proportion of total water consumption experienced more droughts (more than 11), and vice versa. The Pearl River Delta showed the highest drought intensification. Reservoir storage significantly reduces the drought peak and severity, but the impact effect depends on its application and balance with the total water consumption. Our study provides a reference for analyzing the drought characteristics, causes, and impacts of sub-basins on a global scale.

Changes in the Thermal and Hydrometeorological Forest Growth Climate During 1948–2017 in Northern Germany

Recent severe droughts and climate change projections have caused rising worries about the impacts of a warmer and drier climate on forests and the future of timber production. While recent trends in thermal and hydrometeorological climate factors have been studied in many regions on earth, less is known about long-term change in climate variables most relevant for tree health and productivity, i.e., temperature (T), precipitation (P), climatic water balance (CWB), and SPEI aridity index in early and mid-summer, when leaf unfolding and peak stem growth take place. Here, we analyze T, P, CWB, and SPEI trends separately for all growing season months (April-September) during the 1948–1982 (before the recent warming) and 1983–2017 periods (after the onset of warming) in their spatial variation across the North German Lowlands based on a dense climate station network. While trends in thermal and hydrometeorological variables were weak from 1948 to 1982, we find a significant decrease in April precipitation and increase in July precipitation from 1983 to 2017 throughout much of the study region, while June precipitation has decreased locally by 10 mm or more (or up to 20%). The cumulated growing-season CWB has deteriorated by up to 30 mm from 1948–1982 to 1983–2017 in most of the region except at the North Sea coast, where it became more favorable. Recent climate aridification is more pronounced in the drier South-east of the study region with a more continental climate, as indicated by stronger negative P, CWB, and SPEI trends for April, May, and June. We conclude that water availability especially in the physiologically important months April and June has deteriorated in the larger part of the North German Lowlands since the 1980s, increasingly impairing hydrometeorological forest growth conditions. The identified trends may serve as early-warning signals of anticipated future loss in tree vitality.

Advances in Urban Meteorological Research in China

Over the past decades, a large number of studies have been carried out in the field of urban meteorology in China. This paper summarizes the main progress in urban meteorology research from four aspects: urban meteorological observation network and field campaign, multi-scale model of urban meteorology, interaction between urban meteorology and atmospheric environment, and the impacts of urbanization on weather and climate. Major advances are as follows. China’s major cities have established or are improving comprehensive urban meteorological observation networks characterized by multi-platform, multi-variable, multi-scale, multi-link, and multi-function. Beijing, Nanjing, Shanghai, and other cities carried out urban meteorological field campaigns, which were included in the WMO research demonstration project. Wind tunnel experiments and scale-model outdoor experiments were successfully conducted. Multi-scale urban meteorological and air quality prediction numerical model systems have been developed and put into operational use. The urban heat island effect; urban impacts on precipitation, regional climate, and air quality; urban planning; and interaction between urban meteorology and atmospheric environment are extensively investigated. Finally, efforts to improve observational technology, data assimilation, and urban system modeling, to explore the impacts of urbanization on environment and human health, and to provide integrated urban hydro-meteorological climate and environmental services are planned ahead.

Does the Drakensberg dehydrate southwestern Africa?

We have investigated the impacts of the high-elevated orography of the Drakensberg on the hydro-meteorological climate in the southwestern Africa, which is characterized by an annual aridity. The focus has been in the summer season (November to March), which contributes 70% of the annual rainfall of the region. A sensitivity experiment performed using a regional climate modeling, in which the Drakensberg is eliminated from the lower boundary condition, shows that precipitation is significantly enhanced over the southwestern Africa and its amplification exceeds 200% in a benchmark experiment. Without the Drakensberg, the lower-tropospheric easterly, associated with the subtropical anti-cyclone over the Indian Ocean, is allowed to penetrate and transport the moisture more westward. Correspondingly, a regime of the intense upward motion also shifts westward to Namibian coastal region and instead, the weak subsidence is formed around the Drakensberg in southeastern Africa. The westward-shifted upward motion is attributed to the enhanced rainfall, which is generated by local daily-anomaly circulation with the help of the enriched moisture. Consequently, the aridity over the southwestern Africa changes from severe arid/arid to semi-arid based on a traditional aridity index. We conclude that the Drakensberg partially contributes to the dry climate over the southwestern Africa.

Impacts of climate change on European hydrology at 1.5, 2 and 3 degrees mean global warming above preindustrial level

Impacts of climate change at 1.5, 2 and 3 °C mean global warming above preindustrial level are investigated and compared for runoff, discharge and snowpack in Europe. Ensembles of climate projections representing each of the warming levels were assembled to describe the hydro-meteorological climate at 1.5, 2 and 3 °C. These ensembles were then used to force an ensemble of five hydrological models and changes to hydrological indicators were calculated. It is seen that there are clear changes in local impacts on evapotranspiration, mean, low and high runoff and snow water equivalent between a 1.5, 2 and 3 °C degree warmer world. In a warmer world, the hydrological impacts of climate change are more intense and spatially more extensive. Robust increases in runoff affect the Scandinavian mountains at 1.5 °C, but at 3 °C extend over most of Norway, Sweden and northern Poland. At 3 °C, Norway is affected by robust changes in all indicators. Decreases in mean annual runoff are seen only in Portugal at 1.5 °C warming, but at 3 °C warming, decreases to runoff are seen around the entire Iberian coast, the Balkan Coast and parts of the French coast. In affected parts of Europe, there is a distinct increase in the changes to mean, low and high runoff at 2 °C compared to 1.5 °C, strengthening the case for mitigation to lower levels of global warming. Between 2 and 3 °C, the changes in low and high runoff levels continue to increase, but the changes to mean runoff are less clear. Changes to discharge in Europe’s larger rivers are less distinct due to the lack of homogenous and robust changes across larger river catchments, with the exception of Scandinavia where discharges increase with warming level.

Impacts of weighting climate models for hydro-meteorological climate change studies

Weighting climate models is controversial in climate change impact studies using an ensemble of climate simulations from different climate models. In climate science, there is a general consensus that all climate models should be considered as having equal performance or in other words that all projections are equiprobable. On the other hand, in the impacts and adaptation community, many believe that climate models should be weighted based on their ability to better represent various metrics over a reference period. The debate appears to be partly philosophical in nature as few studies have investigated the impact of using weights in projecting future climate changes. The present study focuses on the impact of assigning weights to climate models for hydrological climate change studies. Five methods are used to determine weights on an ensemble of 28 global climate models (GCMs) adapted from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database. Using a hydrological model, streamflows are computed over a reference (1961–1990) and future (2061–2090) periods, with and without post-processing climate model outputs. The impacts of using different weighting schemes for GCM simulations are then analyzed in terms of ensemble mean and uncertainty. The results show that weighting GCMs has a limited impact on both projected future climate in term of precipitation and temperature changes and hydrology in terms of nine different streamflow criteria. These results apply to both raw and post-processed GCM model outputs, thus supporting the view that climate models should be considered equiprobable.

Hydro-Meteorological Drought Projections into the 21-st Century for Selected Polish Catchments

The nature of drought conditions is estimated using a range of indices describing different aspects of drought events. Three drought indices are evaluated, namely the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Runoff Index (SRI), using observed hydroclimatic data and applying them to hydro-meteorological projections into the 21st century. The first two indices are evaluated using only meteorological variables and from this point of view, are better suited to meteorological drought projections than the third index, SRI, which is based on catchment discharge and represents hydrological drought. We assess information contained in those indices and their suitability to catchment scale climate projection drought assessment in ten selected Polish catchments, representing different hydro-climatic conditions, which are used as a case study. Projections of climatic variables (precipitation and temperature) are obtained from the EURO-CORDEX initiative derived from seven climate models at a grid resolution of 12.5 km for the time period 1971–2100. Future runoff projections for the catchments are obtained using a conceptual rainfall-runoff model (HBV). The results of analyses of indices based on observations in the reference period show consistent estimates for most of the catchments. Hydro-meteorological climate model projections for three periods, including the reference period 1971–2000, and two 30-year periods, near-future 2021–2050 and far-future 2071–2100, are used to estimate changes of future drought conditions in the catchments studied. The results show a substantial variation of temporal drought patterns over the catchments and their dependence on projected precipitation and temperature variables and the type of indices applied. Of the three indices studied, only SPEI projections indicate drier conditions in the catchments in the far-future period. The other two indices, SPI and SRI, indicate wetter climates in the future.

Assessment of land use/land cover dynamics vis-à-vis hydrometeorological variability in Wular Lake environs Kashmir Valley, India using multitemporal satellite data

Wular Lake, one of the largest freshwater lakes of Jhelum River Basin, is showing signs of deterioration due to the anthropogenic impact and changes in the land use/land cover (LULC) and hydrometeorological climate of the region. The present study investigated the impacts of temporal changes in LULC and meteorological and hydrological parameters to evaluate the current status of Wular Lake environs using multisensor, multitemporal satellite and observatory data. Satellite images acquired for the years 1992, 2001, 2005, and 2008 were used for determining changes in the LULC in a buffer area of 5 km2 around the Wular Lake. LULC mapping and change analysis using the visual interpretation technique indicated significant changes around the Wular Lake during the last two decades. Reduction in lake area from 24 km2 in 1992 to 9 km2 in 2008 (−62.5 %) affected marshy lands, the habitat of migratory birds, which also exhibited drastic reduction from 85 km2 in 1992 to 5 km2 in 2008 (−94.117 %). Marked development of settlements (642.85 %) in the peripheral area of the Wular Lake adversely affected its varied aquatic flora and fauna. Change in climatic conditions, to a certain extent, is also responsible for the decrease in water level and water spread of the lake as witnessed by decreased discharge in major tributaries (Erin and Madhumati) draining into the Wular Lake.