Seasonality and spatial variability of dynamic precipitation controls on the Tibetan Plateau

The Tibetan Plateau (TP) is the origin of many large Asian rivers, which provide water resources for large regions in south and east Asia. Therefore, the water cycle on the TP and adjacent high mountain ranges, in particular the precipitation distribution and variability play an important role for the water availability for billions of people in the downstream regions of the TP. The High Asia Refined analysis (HAR) is used to analyse the dynamical factors that influence precipitation variability in the TP region, including the factors resulting in the enhancement and suppression of precipitation. Four dynamical fields that can influence precipitation are considered: the 300 hPa wind speed and wind speed 2 km above ground, the 300 hPa vertical wind speed, and the atmospheric water transport. The study focusses on the seasonality and the spatial variability of the precipitation controls and their dominant patterns. Results show that different factors have different effects on precipitation in different regions and seasons. This depends mainly on the dominant type of precipitation, i.e. convective or frontal/cyclonic precipitation. Additionally, the study reveals that the midlatitude westerlies have a high impact on the precipitation distribution on the TP and its surroundings year-round and not only in winter

Climatology of Tibetan Plateau Vortices in reanalysis data and a high-resolution global climate model

The Tibetan Plateau (TP) and surrounding high mountains constitute an important forcing of the atmospheric circulation due to their height and extent, and thereby impact weather and climate in downstream regions of East Asia. Mesoscale Tibetan Plateau Vortices (TPVs) are one of the major precipitation-producing systems on the TP. A fraction of TPVs moves off the TP to the east and can trigger extreme precipitation in parts of China, e.g. the Sichuan province and the Yangtze River valley, that can result in severe flooding. In this study, the climatology of TPV occurrence is examined in two reanalyses and, for the first time, in a high-resolution global climate model using an objective feature tracking algorithm. Most TPVs are generated in the north-western part of the TP; the centre of this main genesis region is small and stable throughout the year. The strength and position of the subtropical westerly jet is correlated to the distance TPVs can travel eastwards and therefore could have an effect on whether or not a TPV is moving-off the TP. TPV-associated precipitation can account for up to 40% of the total precipitation in parts of China in selected months, often due to individual TPVs. The results show that the global climate model is able to simulate TPVs at N512 (~25 km) horizontal resolution and in general agrees with the reanalyses. The fact that the global climate model can represent the TPV climatology opens a wide range of options for future model-based research on TPVs

Climatology of near-surface wind speed from observational, reanalysis and high-resolution regional climate model data over the Tibetan Plateau

As near-surface wind speed plays a role in regulating surface evaporation and thus the hydrological cycle, it is crucial to explore its spatio-temporal characteristics. However, in-situ measurements are scarce over the Tibetan Plateau, limiting the understanding of wind speed climate across this high-elevation region. This study explores the climatology of near-surface wind speed over the Tibetan Plateau by using for the frst time homogenized observations together with reanalysis products and regional climate model simulations. Measuring stations across the center and the west of the plateau are at higher elevations and display higher mean and standard deviation, confrming that wind speed increases with increasing altitude. By exploring wind characteristics with a focus on seasonal cycle through cluster analysis, three regions of distinct wind regimes can be identifed: (1) the central Tibetan Plateau, characterized by high elevation; (2) the eastern and the peripheral areas of the plateau; and (3) the Qaidam basin, a topographic depression strongly infuenced by the blocking efect of the surrounding mountainous terrain. Notably, the ERA5 reanalysis, with its improvements in horizontal, vertical, and temporal spacing, model physics and data assimilation, demonstrates closer agreement to the measured wind conditions than its predecessor ERA-Interim. It successfully reproduces the three identifed wind regimes. However, the newest ERA5-Land product does not show improvements compared to ERA5, most likely because they share most of the parametrizations. Furthermore, the two dynamical downscalings of ERA5 analyzed here fail to capture the observed wind statistics and exhibit notable biases and discrepancies also when investigating the diurnal variations. Consequently, these high-resolution downscaling products do not show add value in reproducing the observed climatology of wind speed compared to ERA5 over the Tibetan Plateau

Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation

The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments

Comparison of a manual and an automated tracking method for Tibetan Plateau vortices

Tibetan Plateau vortices (TPVs) are mesoscale cyclones originating over the Tibetan Plateau (TP) during the extended summer season (April-September). Most TPVs stay on the TP while a small number can move off the TP to the east. TPVs are known to be one of the main precipitation-bearing systems on the TP and moving-off TPVs have been associated with heavy precipitation and flooding downstream of the TP (e.g. Sichuan province, Yangtze River Valley). Identifying and tracking TPVs is difficult both due to their comparatively small horizontal extent (400 – 800 km) and the limited availability of soundings over the TP, which, in turn, constitutes a challenge for short-term predictions of TPV-related impacts and for the climatological study of TPVs. In this study, (i) manual tracking (MT) results using radiosonde data from a network over and downstream of the TP are compared with (ii) results obtained by an automated tracking (AT) algorithm applied to ERA-Interim reanalysis. Ten MT-TPV cases are selected based on method (i) and matched to and compared with the corresponding AT-TPVs identified with method (ii). Conversely, ten AT-TPVs are selected and compared with the corresponding MT-TPVs. In general, the comparison shows good results in cases where the underlying data are in good agreement, but considerable differences are also seen in some cases and explained in terms of differences in the tracking methods, data availability/coverage and disagreement between sounding and ERA-Interim data. Recommendations are given for future efforts in TPV detection and tracking, including in an operational weather forecasting context

Precipitation seasonality and variability over the Tibetan plateau as resolved by the High Asia reanalysis

Because of the scarcity of meteorological observations, the precipitation climate on the Tibetan Plateau and surrounding regions (TP) has been insufficiently documented so far. In this study, the characteristics and basic features of precipitation on the TP during an 11-yr period (2001–11) are described on monthly-to-annual time scales. For this purpose, a new high-resolution atmospheric dataset is analyzed, the High Asia Reanalysis (HAR), generated by dynamical downscaling of global analysis data using the Weather Research and Forecasting (WRF) model. The HAR precipitation data at 30- and 10-km resolutions are compared with both rain gauge observations and satellite-based precipitation estimates from the Tropical Rainfall Measurement Mission (TRMM). It is found that the HAR reproduces previously reported spatial patterns and seasonality of precipitation and that the high-resolution data add value regarding snowfall retrieval, precipitation frequency, and orographic precipitation. It is demonstrated that this process-based approach, despite some unavoidable shortcomings, can improve the understanding of the processes that lead to precipitation on the TP. Analysis focuses on precipitation amounts, type, seasonality, and interannual variability. Special attention is given to the links between the observed patterns and regional atmospheric circulation. As an example of an application of the HAR, a new classification of glaciers on the TP according to their accumulation regimes is proposed, which illustrates the strong spatial variability of precipitation seasonality. Finally, directions for future research are identified based on the HAR, which has the potential to be a useful dataset for climate, glaciological, and hydrological impact studies

Modelling human choices: MADeM and decision‑making

Research supported by FAPESP 2015/50122-0 and DFG-GRTK 1740/2. RP and AR are also part of the Research, Innovation and Dissemination Center for Neuromathematics FAPESP grant (2013/07699-0). RP is supported by a FAPESP scholarship (2013/25667-8). ACR is partially supported by a CNPq fellowship (grant 306251/2014-0)

Atmosphärischer Wassertransport und dynamische Kontrollfaktoren des Niederschlags auf dem Tibetischen Plateau

The Tibetan Plateau (TP) is the origin of many large Asian rivers, which provide water resources for large regions in south and east Asia. Therefore, the water cycle on the TP and adjacent high mountain ranges, in particular the precipitation distribution and variability play an important role for the water availability for billions of people in the downstream regions of the TP. The respective influence of the Indian and East Asian summer monsoon on TP precipitation and regional water resources, together with the detection of moisture transport pathways and source regions are the subject of recent research. The aim of this thesis is to gain a better understanding of the spatial and temporal precipitation variability on the TP. In order to do so we examine the moisture transport to and on the TP, analyse the underlying processes leading to enhancement or suppression of precipitation, and examine how those processes are affected by the mid-latitude westerlies and the monsoon system. A newly developed high-resolution dataset, the High Asia Refined analysis (HAR), is used to examine atmospheric water transport (AWT) and dynamical factors that influence precipitation variability in the TP region. The HAR is the result of dynamically downscaling an operational analysis. Due to the higher spatial and temporal resolution of the HAR, it better represents the complex topography of the TP and surrounding high mountain ranges than coarse-resolution data sets like reanalyses, thereby reducing precipitation biases. Analysing the AWT, we focus on spatiotemporal patterns, vertical distribution and transport through the TP boundaries. The results show that the mid-latitude westerlies have a higher share in summertime AWT over the TP than assumed so far. High mountain valleys in the Himalayas facilitate AWT from the south, whereas the high mountain regions inhibit AWT to a large extent and limit the influence of the Indian summer monsoon. Our results show that 36.86.3% of the atmospheric moisture needed for precipitation comes from outside the TP, while the remaining 63.2% is provided by local moisture recycling. We use monthly correlations of selected dynamic variables with the precipitation to analyse what controls precipitation variability on the TP and in the surrounding high mountain regions. The selected variables, called dynamic precipitation controls, are the wind speed at 300 hPa wind and the wind speed 2 km above ground, the vertical wind speed at 300 hPa, the vertically integrated atmospheric water transport, and the height of the planetary boundary layer. We focus on the seasonality and the spatial variability of the relationship between dynamic controls and precipitation. Results show that different controls have different effects on precipitation in different regions and seasons. For example, the 300 hPa wind speed has a positive effect in the western parts of the study region in winter and spring, while it has a negative effect on precipitation on the TP in summer by cutting off deep convection. The positive correlation of AWT with precipitation is higher in winter at the high mountain ranges in the western part of the study region, than in summer on the central TP. This result shows that on the central TP the strong convection in summer is able to produce precipitation with the moisture available from local sources, emphasising the importance of moisture recycling. Those results illustrate that the effect of dynamic controls on precipitation variability depends mainly on the dominant type of precipitation, i.e. convective or frontal/cyclonic precipitation. This thesis shows that the impact of the midlatitude westerlies on precipitation variability on the TP is strong, not only in winter, by enhancing moisture advection for orographic and frontal precipitation in the western parts of the study region but also in summer by cutting off deep convection on the TP and in other regions and seasons where and when precipitation is mainly convective. Additionally, the westerlies deliver more moisture to the TP in summer than assumed so far.Das Tibetische Plateau (TP) ist der Ursprung vieler großer asiatischer Flüsse, die große Gebiete in Süd- und Ostasien mit Wasser versorgen. Daher spielen der Wasserkreislauf auf dem TP und in den angrenzenden Hochgebirgen, insbesondere die Niederschlagsverteilung und -variabilität, eine wichtige Rolle für die Wasserverfügbarkeit für Milliarden von Menschen in den dem TP nachgelagerten Regionen. Der jeweilige Einfluss des indischen und ostasiatischen Sommermonsuns auf TP-Niederschläge und regionale Wasserressourcen sowie die Identifizierung von Feuchtigkeitstransportwegen und Quellregionen sind Gegenstand aktueller Forschungsarbeiten. Ziel dieser Arbeit ist es, die räumliche und zeitliche Niederschlagsvariabilität auf dem TP besser zu verstehen. Zu diesem Zweck untersuchen wir den Feuchtigkeitstransport zum und auf dem TP, analysieren die zugrunde liegenden Prozesse, die zu einer Verstärkung oder Abschwächung des Niederschlags führen, und untersuchen, wie diese Prozesse durch die Westwinde der mittleren Breiten und das Monsunsystem beeinflusst werden. Ein neu entwickelter hochauflösender Datensatz, die High Asia Refined Analysis (HAR), wird verwendet, um den atmosphärischen Wassertransport (AWT) und dynamische Faktoren zu untersuchen, die die Niederschlagsvariabilität in der TP-Region beeinflussen. Die HAR ist das Ergebnis des dynamischen Downscaling einer operationellen Analyse. Aufgrund der höheren räumlichen und zeitlichen Auflösung der HAR wird die komplexe Topographie des TP und der umgebenden Hochgebirgsräume besser dargestellt als bei grob aufgelösten Datensätzen wie Reanalysen, wodurch systematische Niederschlagsfehler reduziert werden. Bei der Analyse des AWT konzentrieren wir uns auf die raumzeitlichen Muster, die vertikale Verteilung und den Transport durch die TP-Grenzen. Die Ergebnisse zeigen, dass die Westwinde der mittleren Breiten einen höheren Anteil am Sommer-AWT haben als bisher angenommen. Hochgebirgstäler im Himalaya begünstigen den AWT aus dem Süden, während die Hochgebirgsregionen den AWT weitgehend hemmen und den Einfluss des indischen Sommermonsuns begrenzen. Unsere Ergebnisse zeigen, dass 36,8±6,3% der für den Niederschlag benötigten Luftfeuchtigkeit von außerhalb des TP stammen, während die restlichen 63,2% durch lokales Feuchtigkeitsrecycling bereitgestellt werden. Wir verwenden monatliche Korrelationen ausgewählter dynamischer Variablen mit dem Niederschlag, um zu analysieren, wie die Niederschlagsvariabilität auf dem TP und in den umgebenden Hochgebirgsregionen gesteuert wird. Die ausgewählten Variablen, die als dynamische Kontrollfaktoren des Niederschlags bezeichnet werden, sind die Windgeschwindigkeit in 300hPa und die Windgeschwindigkeit 2km über dem Boden, die vertikale Windgeschwindigkeit in 300hPa, der vertikal integrierte atmosphärische Wassertransport und die Höhe der planetaren Grenzschicht. Wir konzentrieren uns auf die Saisonalität und die räumliche Variabilität des Zusammenhangs zwischen den dynamischen Kontrollfaktoren und dem Niederschlag. Die Ergebnisse zeigen, dass verschiedene Kontrollfaktoren in verschiedenen Regionen und Jahreszeiten unterschiedliche Auswirkungen auf den Niederschlag haben. Beispielsweise wirkt sich die Windgeschwindigkeit in 300hPa im Winter und Frühling im Westen des Untersuchungsgebiets positiv auf den Niederschlag aus, während sie sich im Sommer negativ auf die Niederschläge auf dem TP auswirkt, indem sie die Tiefenkonvektion unterbricht. Die positive Korrelation von AWT mit dem Niederschlag ist im Winter in den Hochgebirgsräumen im westlichen Teil der Untersuchungsregion höher als im Sommer auf dem zentralen TP. Dieses Ergebnis zeigt, dass die starke Konvektion auf dem zentralen TP im Sommer in der Lage ist Niederschlag mit der aus lokalen Quellen verfügbaren Feuchtigkeit zu erzeugen, was die Bedeutung des Feuchtigkeitsrecyclings unterstreicht. Diese Ergebnisse veranschaulichen, dass die Wirkung dynamischer Kontrollfaktoren auf die Niederschlagsvariabilität hauptsächlich von der vorherrschenden Art des Niederschlags abhängig ist, d. h. konvektiver oder frontaler/zyklonaler Niederschlag. Die vorliegende Arbeit zeigt, dass die Westwinde der mittleren Breiten einen großen Einfluss auf die Niederschlagsvariabilität auf dem TP haben. Dieser Einfluss ist nicht nur im Winter sichtbar durch die Verstärkung der Feuchteadvektion für den orografischen und frontalen Niederschlag im Westen des Untersuchungsgebiets, sondern auch im Sommer durch die Kappung der Tiefenkonvektion auf dem TP und in anderen Regionen und Jahreszeiten in denen der Niederschlag hauptsächlich konvektiv ist. Zusätzlich transportieren die Westwinde im Sommer mehr Feuchtigkeit auf das TP als bisher angenommen.BMBF, 03G084A, Variability and Trends in Benchmark Drainage Basins on the Tibetan Plateau (WET

Towards Ensemble-Based Kilometer-Scale Climate Simulations over the Third Pole Region

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