| 1. |
- Kukulies, Julia, et al.
(författare)
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Kilometer-Scale Multimodel and Multiphysics Ensemble Simulations of a Mesoscale Convective System in the Lee of the Tibetan Plateau: Implications for Climate Simulations
- 2023
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 36:17, s. 5963-5987
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Tidskriftsartikel (refereegranskat)abstract
- Kilometer-scale climate model simulations are useful tools to investigate past and future changes in extreme precipitation, particularly in mountain regions, where convection is influenced by complex topography and land–atmosphere interactions. In this study, we evaluate simulations of a flood-producing mesoscale convective system (MCS) downstream of the Tibetan Plateau (TP) in the Sichuan basin from a kilometer-scale multimodel and multiphysics ensemble. The aim is to better understand the physical processes that need to be correctly simulated for successfully capturing downstream MCS formation. We assess how the ensemble members simulate these processes and how sensitive the simulations are to different model configurations. The preceding vortex evolution over the TP, its interaction with the jet stream, and water vapor advection into the basin are identified as key processes for the MCS formation. Most modeling systems struggle to capture the interaction between the vortex and jet stream, and perturbing the model physics has little impact, while constraining the large-scale flow by spectral nudging improves the simulation. This suggests that an accurate representation of the large-scale forcing is crucial to correctly simulate the MCS and associated precipitation. To verify whether the identified shortcomings systematically affect the MCS climatology in longer-term simulations, we evaluate a 1-yr WRF simulation and find that the seasonal cycle and spatial distribution of MCSs are reasonably well captured and not improved by spectral nudging. While the simulations of the MCS case highlight challenges in extreme precipitation forecasting, we conclude that these challenges do not systematically affect simulated climatological MCS characteristics.
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| 2. |
- Kukulies, Julia, et al.
(författare)
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Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation
- 2023
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Ingår i: Frontiers in Earth Science. - 2296-6463. ; 11
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Forskningsöversikt (refereegranskat)abstract
- 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.
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| 3. |
- Kukulies, Julia, et al.
(författare)
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The Role of Mesoscale Convective Systems in Precipitation in the Tibetan Plateau Region
- 2021
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X .- 2169-8996. ; 126:23
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Tidskriftsartikel (refereegranskat)abstract
- Mesoscale convective systems (MCSs) have been identified as an important source of precipitation in the Tibetan Plateau (TP) region. However, the characteristics and structure of MCS-induced precipitation are not well understood in this location. Infrared (IR) satellite imagery has been used for MCS tracking, but cirrus clouds or cold surfaces can lead to false MCS classification over mountain regions. Here, we combine brightness temperatures from IR imagery with satellite precipitation estimates from GPM IMERG and track MCSs over the TP, at the boundary of the TP (TPB), and in the surrounding lower-elevation plains (LE), between 2000 and 2019. In most parts of LE and TPB, MCSs produced 50%–80% of the total summer precipitation (60%–90% of summer heavy precipitation), whereas MCSs over the TP account for below 10% of the total summer precipitation (10%–30% of summer heavy precipitation). Our results also show that MCSs that produce the largest amounts of heavy precipitation are characterized by longevity and large extents rather than by high intensities. These are mainly located in the populous areas south and east of the TP. A tracking of meso-β convective systems over the TP shows that small-scale convection makes a large contribution to total and heavy precipitation. This suggests that more localized convective systems are important for the regional water cycle over the higher terrain and highlights the importance of convective-scale modeling to improve our understanding of precipitation dynamics in the TP region.
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| 4. |
- Minola, Lorenzo, et al.
(författare)
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Climatology of near-surface wind speed from observational, reanalysis and high-resolution regional climate model data over the Tibetan Plateau
- 2023
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Ingår i: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- 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 first 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, confirming 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 identified: (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 influenced by the blocking effect 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 identified 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.
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| 5. |
- Prein, Andreas F., et al.
(författare)
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Towards Ensemble-Based Kilometer-Scale Climate Simulations over the Third Pole Region
- 2022
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau and its surrounding mountains have an average elevation of 4,400m and a glaciated area of ∼ 100,000km 2 giving it the name “Third Pole (TP) region”. The TP is the headwater of many major rivers in Asia that provide fresh water to hundreds of millions of people. Climate change is altering the energy and water cycle of the TP at a record pace but the future of this region is highly uncertain due to major challenges in simulating weather and climate processes in this complex area. The Convection-Permitting Third Pole (CPTP) project is a Coordinated Regional Downscaling Experiment (CORDEX) Flagship Pilot Study (FPS) that aims to revolutionize our understanding of climate change impacts on the TP through ensemble-based, kilometer-scale climate modeling. Here we present the experimental design and first results from multi-model, multi-physics ensemble simulations of three case studies. The five participating modeling systems show high performance across a range of meteorological situations and are close to having ”observational quality” in simulating precipitation and near-surface temperature. This is partly due to the large differences between observational datasets in this region, which are the leading source of uncertainty in model evaluations. However, a systematic cold bias above 2000m exists in most modeling systems. Model physics sensitivity tests performed with the Weather Research and Forecasting (WRF) model show that planetary boundary layer (PBL) physics and microphysics contribute equally to model uncertainties. Additionally, larger domains result in better model performance. We conclude by describing high-priority research needs and the next steps in the CPTP project.
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| 6. |
- Kukulies, Julia
(författare)
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Observing and Modeling Precipitation in the Tibetan Plateau region - from large-scale processes to convective storms
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Climate change in mountain regions has far-reaching societal impacts such as increased risks for natural hazards and water scarcity that may affect billions of people in the downstream regions. Precipitation changes play a critical role in these impacts due to their effects on river runoff and flooding. However, these changes remain hard to predict due to uncertainties in climate models and a lack of reliable observations. This dissertation aims to enhance the understanding of precipitation and its underlying large-scale and mesoscale processes in the Tibetan Plateau (TP) region, one of the most extensive and vulnerable mountain regions in the world. More specifically, the dissertation combines gauge measurements, satellite observations, reanalysis data, and high-resolution model simulations to investigate present-climate characteristics of clouds and precipitation over TP and its downstream regions. A key outcome is a data set of large storms, so-called mesoscale convective systems (MCSs), based on two decades of high-resolution satellite observations of clouds and precipitation. This data set is used to study MCS characteristics and their relation to large-scale atmospheric circulation systems and water vapor transport. Satellite observations reveal that MCSs are important precipitation producers in the river basins surrounding the TP, while convection over the TP occurs in a more scattered manner with significantly less precipitation. In addition, satellite observations are used to evaluate kilometer-scale simulations of MCSs. The model simulations capture the general spatial pattern and magnitude of MCS-associated precipitation but show also systematic biases in MCS frequencies in some regions south and east of the TP. It was found that interactions between large- and mesoscale processes affect the formation and evolution of MCSs over the TP and its downstream regions. The results identify several processes, e.g. interactions between the TP and the mid-latitude westerly circulation, that may drive future precipitation changes and need to be realistically represented in future climate model projections. As such, this dissertation constitutes a step towards reliable projections of climate change in the TP region.
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| 7. |
- Kukulies, Julia, et al.
(författare)
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Temporal and spatial variations of convection and precipitation over the Tibetan Plateau based on recent satellite observations. Part I: Cloud climatology derived from CloudSat and CALIPSO
- 2019
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 39:14, s. 5396-5412
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Tidskriftsartikel (refereegranskat)abstract
- This sequence of papers, consisting of two parts, examines temporal and spatial variations of convection and precipitation over the Tibetan Plateau (TP) based on recent satellite observations. Here in Part 1, seasonal and diurnal variations of cloud vertical structure and cloud properties have been derived from four combined CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite data sets and compared between three subregions in the TP which are marked by different dominating large-scale atmospheric circulations and moisture sources. The results show that the plateau is generally dominated by low-level single-layer clouds and stratiform cloud types. Cloud occurrence frequencies peak during the summer monsoon season between May and September and are generally higher during daytime compared with nighttime in all the three subregions. The fraction of detected ice cloud layers in the TP domain exceeds 50% during all months and 80% between January and April. While ice cloud layers occur as altostratus clouds in the westerly dominated north and transition zone, high-level cirrus cloud occurs frequently accompanied by lower level cumulus clouds in the monsoon-dominated south, especially during nighttime. This study complements previous satellite observations of clouds over the TP and reveals firstly the high contribution of stratiform ice cloud layers in the westerly dominated north, secondly the importance of the monsoon season which outweighs day-night differences and affects the examined cloud parameters in all regions and finally the significant regional differences of cloud characteristics within the plateau. It is therefore suggested to focus on the relative importance of stratification, mesoscale convective systems and advection in future studies on hydro-climatic changes in the TP region. © 2019 The Authors. International Journal of Climatology published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.
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| 8. |
- Kukulies, Julia, et al.
(författare)
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Temporal and spatial variations of convection, clouds and precipitation over the Tibetan Plateau from recent satellite observations. Part II: Precipitation climatology derived from global precipitation measurement mission
- 2020
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 40:11, s. 4858-4875
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Tidskriftsartikel (refereegranskat)abstract
- This sequence of papers examines spatio-temporal variations of precipitation over the Tibetan Plateau (TP) based on satellite observations. Here in Part 2, spatial patterns of seasonal and diurnal variations of precipitation have been examined based on the Global Precipitation Measurement Mission (GPM) and three additional satellite products. The results show a spatial dipole pattern of two distinct seasonalities: The central TP is marked by strong July peaks and exhibits rainfall contributions of the monsoon season (May-September) of more than 70%, whereas northwestern and southern regions of the plateau exhibit significantly smaller amplitudes in the annual cycle. In some southern regions which are characterized by very high summer mean precipitation and more extreme rain rates, winter months (October-April) contribute significantly to the total annual mean precipitation. In addition, there are larger differences in seasonal curves along a west-to-east axis, than along a north-to-south axis. The spatial patterns of diurnal precipitation over the TP are more complex compared to seasonality and point to multiple components, which construct the regional differences. These show also a seasonal dependence and are characterized by a stronger afternoon to early evening peak (17:00 LST time, 11:00 UTC) and weaker nighttime peak (23:00 LST, 17:00 UTC) during the monsoon season and over the plateau compared to its surroundings. Furthermore, it was shown that convective precipitation during the monsoon season contribute only up to 30% to the total precipitation, whereas more than 70% is produced by the 90th percentile of daily rain rates. An important characteristic of summer precipitation is hence that a significant part of the extreme precipitation is non-convective. This paper reveals new features of spatial patterns in seasonal and diurnal precipitation and highlights the importance of non-monsoonal components for seasonal precipitation variations.
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| 9. |
- Lai, Hui-Wen, et al.
(författare)
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Regionalization of seasonal precipitation over the Tibetan plateau and associated large-scale atmospheric systems
- 2021
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 34:7, s. 2635-2651
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation over the Tibetan Plateau (TP) has major societal impacts in South and East Asia, but its spatiotemporal variations are not well understood, mainly because of the sparsely distributed in situ observation sites. With the help of the Global Precipitation Measurement satellite product IMERG and the ERA5 dataset, distinct precipitation seasonality features over the TP were objectively classified using a self-organizing map algorithm fed with 10-day averaged precipitation from 2000 to 2019. The classification reveals three main precipitation regimes with distinct seasonality of precipitation: the winter peak, centered at the western plateau; the early summer peak, found on the eastern plateau; and the late summer peak, mainly located on the southwestern plateau. On a year-to-year basis, the winter peak regime is relatively robust, whereas the early summer and late summer peak regimes tend to shift mainly between the central and northern TP but are robust in the eastern and southwestern TP. A composite analysis shows that the winter peak regime experiences larger amounts of precipitation in winter and early spring when the westerly jet is anomalously strong to the north of the TP. Precipitation variations in the late summer peak regime are associated with intensity changes in the South Asian high and Indian summer monsoon. The precipitation in the early summer peak regime is correlated with the Indian summer monsoon together with anticyclonic circulation over the western North Pacific. The results provide a basic understanding of precipitation seasonality variations over the TP and associated large-scale conditions.
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| 10. |
- Ou, Tinghai, et al.
(författare)
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Wet bias of summer precipitation in the northwestern Tibetan Plateau in ERA5 is linked to overestimated lower-level southerly wind over the plateau
- 2023
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 61:5-6, s. 2139-53
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP), also called the Third Pole, is considered to be “the world water tower”. The northwestern TP (NWTP), which has an average elevation higher than 4800m, is an arid region where the summer precipitation is largely overestimated by the ERA5 global reanalysis product. We hypothesize that this wet bias is mainly caused by unrealistic lower-level winds that trigger strong convection over the region; it can be reduced by using a high-resolution regional climate model with a large domain that allows realistically representing interactions between the Westerlies and Asian summer monsoons. Here, downscaling using the Weather Research and Forecasting (WRF) model driven by ERA5 was conducted with a large domain (8°‒50° N, 65°‒125° E) at 9km for the period 1979‒2019 (WRF9km). Precipitation values from WRF9km and ERA5 were evaluated against satellite observations; compared with ERA5, WRF9km captured the climatological summer precipitation over the NWTP with a much-reduced wet bias. The ERA5 overestimation is mainly caused by excessive convective precipitation, likely linked to strong vertical motions over the NWTP induced by an overestimated lower-level southerly wind.
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