Characterization of drought development through remote sensing : a case study in Central Yunnan, China

2013-2014 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

A modelling perspective on precipitationin the Indus River Basin: from synopticto Holocene variability

The Indus River civilisation was the first urban society in South Asia. Its demise, ca. 4000 years ago, might have been caused by environmental factors, such as recurrent drought conditions. Yet, palaeoclimate archives present a fragmented picture of the climate at that time. This thesis explores the potential value of global climate models to further interpret the archaeological context. Precipitation variability is investigated at various timescales due to their different impact on human societies and their importance in the climate system: the synoptic scale, the seasonal cycle, the inter-annual to multi-decadal variability, and the multi-millennial Holocene trends. The precipitation from several climate model simulations is evaluated at each of these timescales. Indus River Basin precipitation is first explored in observational datasets. This study highlights the quality of ERA5 reanalysis, which is used as a reference in the subsequent chapters. Statistical tools show that more than 80\% of the precipitation in the Upper Indus Basin is related to cross-barrier moisture transport along the Himalayan foothills. The climate models analysed (IPSL-CM6-A, MRI-ESM2-0 and GISS-E2-1-G) generally reproduce this process well, but the seasonality of cross-barrier moisture transport is biased, resulting in precipitation biases: the main wet season, the summer monsoon, is shorter and significantly dryer, while the second wet season, in winter, is longer and more active. The link between winter precipitation, cross-barrier moisture transport and Western Disturbances is further explored, first in ERA5 and then compared to IPSL-CM6-A model output. Sub-daily resolution is needed to determine the origin of the positive precipitation bias in winter. This bias is related to differences in the atmospheric circulation associated with Western Disturbances. The stronger Subtropical Westerly Jet is also key to understand the precipitation overestimation. Centennial to millennial-scale precipitation variability is more difficult to evaluate due to the short length of observations and the paucity of climate records, but the results suggest that inter-annual variability in the IPSL climate model family is overly dominated by atmosphere-only processes, with a potentially large impact of the precipitation response to external forcings. In addition to biases in the representation of mean precipitation and synoptic to inter-annual variability, climate models are also limited by the representation of other internal processes such as ocean circulation, and vegetation-dust aerosols, as well as by the uncertainty in some external forcings such as volcanic eruptions and the solar activity. Hence, at the present state, past-climate simulations do not provide the nuanced information of precipitation changes and variability that is needed to understand impact of precipitation variability on archaeological contexts, and will not do so until significant breakthroughs are achieved. Nevertheless, climate models remain a powerful tool for climatologists to investigate large-scale processes that can eventually better characterise climate variability.This research was carried out as part of the TwoRains project, which is supported by funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant 648609)

Understanding Hydrological Repartitioning and Shifts in Drought Regimes in Central and South-West Asia Using MODIS Derived Perpendicular Dought Index and TRMM Data

In this paper, we use perpendicular drought indices (PDIs) and precipitation patterns to identify hydrological repartitioning in agricultural regions located in central and south-west Asia over the last decade. The spatio-temporal patterns of surface drought derived from perpendicular drought index (PDI) and modified perpendicular drought index (MPDI) from 250-mMODerate Resolution Imaging Spectroradiometer (MODIS) data in 8-day time steps are compared against two other drought indices: the Standardized Precipitation Index (SPI) as a meteorological drought index and the potential evapotranspiration (ET ) as an agro-meteorological drought index, along with 8-day MODIS Normalized Difference Vegetation Index (NDVI). The coefficient of variation as the ratio of standard deviation over inter-annual mean drought indices, precipitation from Tropical Rainfall Measuring Mission (TRMM) data are employed to analyse trends and shifts of hydrological regimes and its connection to both climate change and anthropogenic activities in the study area. Significant correlations are found between the PDI, the MPDI and precipitation and other applied meteorological and agro-meteorological drought indices. The results show that Central Asia is one of the areas across the globe experiencing dramatic shifts in hydrological regimes, and is being severely impacted by population growth and climate change. While acknowledging previous studies that used the PDI and the MPDI over some study regions in Iran, this paper, however, confirms implementation of higher resolution data (MOD09Q1) in both spatial (250 m) and temporal (8-days) dimensions shows a greater agreement between the drought information extracted by the PDI, MPDI, and field meteorological measurements. Supported by satellite based precipitation observations, we conclude that the perpendicular drought indices can be used as a drought early warning system over arid and semi-arid climatological conditions