Lagrangian Backward Trajectory Research Articles

Moisture Sources of the Tohoku Heavy Rainfalls in August 2022 and the Influences of Tropical Storms

AbstractIn August 2022, the Tohoku region of Japan suffered three major heavy rainfall events, while one was related to an anticyclone, and the others were induced by a “Baiu‐like” stationary front. By using the Lagrangian backward trajectories, we found the moisture sources of the three events varied much due to the rapidly changing atmospheric conditions. In general, moisture from the Sea of Japan contributed the most in all events, while the western Pacific also played a certain role. Surprisingly, moisture from tropical storms and the East China Sea did not contribute much due to the along‐transport precipitation. But, based on forward‐traced trajectories, we confirmed that tropical storms did enhance moisture transport from subtropical regions via their southerly wind. Overall, this study highlights the role of the moisture gains nearby and the outer flows of tropical storms during heavy rainfalls in a northern region.

European heatwaves in present and future climate simulations: a Lagrangian analysis

Abstract. Heatwaves are prolonged periods of anomalously high temperatures that can have devastating impacts on the environment, society and economy. In recent history, heatwaves have become more intense and more numerous over most continental areas, and it is expected that this trend will continue due to the ongoing global temperature rise. This general intensification may be modified by changes also in the underlying thermodynamical and dynamical processes. In order to study potential changes in heatwave characteristics and dynamics, we compare Lagrangian backward trajectories of airstreams associated with historic (1991–2000) and future (2091–2100) heatwaves in six different European regions. We use a percentile-based method (Heat Wave Magnitude Index daily) to identify heatwaves in a large ensemble of climate simulations (Community Earth System Model Large Ensemble (CESM-LE) with 35 members). The simulations have been forced by historical representative concentration pathways (RCPs) up to 2005 and by the RCP8.5 scenario afterwards. In general, we find that air parcels associated with heatwaves are located to the east or inside the respective regions 3 d prior to the events. For future heatwaves, the model projects a north-/northeastward shift of the origin of the air masses in most study regions. Compared to climatological values, airstreams associated with heatwaves show a larger temperature increase along their trajectory, which is connected to stronger descent and/or stronger diabatic heating when the air parcels enter the boundary layer. We find stronger descent associated with adiabatic warming in the northern, more continental regions and increased diabatic heating in all regions (except of the British Isles) in the simulated future climate. The enhanced diabatic heating is even more pronounced for heatwaves over continental regions. Diabatic temperature changes of near-surface air are driven by sensible heat fluxes, which are stronger over dry soils. The amplified diabatic heating associated with future heatwaves may thus be explained by an additional drying of the land surface.

Springtime nitrogen oxides and tropospheric ozone in Svalbard: results from the measurement station network

Abstract. Svalbard is a remote and scarcely populated Arctic archipelago and is considered to be mostly influenced by long-range-transported air pollution. However, there are also local emission sources such as coal and diesel power plants, snowmobiles and ships, but their influence on the background concentrations of trace gases has not been thoroughly assessed. This study is based on data of tropospheric ozone (O3) and nitrogen oxides (NOx) collected in three main Svalbard settlements in spring 2017. In addition to these ground-based observations and radiosonde and O3 sonde soundings, ERA5 reanalysis and BrO satellite data have been applied in order to distinguish the impact of local and synoptic-scale conditions on the NOx and O3 chemistry. The measurement campaign was divided into several sub-periods based on the prevailing large-scale weather regimes. The local wind direction at the stations depended on the large-scale conditions but was modified due to complex topography. The NOx concentration showed weak correlation for the different stations and depended strongly on the wind direction and atmospheric stability. Conversely, the O3 concentration was highly correlated among the different measurement sites and was controlled by the long-range atmospheric transport to Svalbard. Lagrangian backward trajectories have been used to examine the origin and path of the air masses during the campaign.

Diabatic processes modulating the vertical structure of the jet stream above the cold front of an extratropical cyclone: sensitivity to deep convection schemes

Abstract. The effect of deep convection parameterisation on the jet stream above the cold front of an explosive extratropical cyclone is investigated in the global numerical weather prediction model ARPEGE, operational at Météo-France. Two hindcast simulations differing only in the deep convection scheme used are systematically compared with each other, with (re)analysis datasets and with NAWDEX airborne observations. The deep convection representation has an important effect on the vertical structure of the jet stream above the cold front at 1-d lead time. The simulation with the less active scheme shows a deeper jet stream, associated with a stronger potential vorticity (PV) gradient in the middle troposphere. This is due to a larger deepening of the dynamical tropopause on the cold air side of the jet and a higher PV destruction on the warm air side, near 600 hPa. To better understand the origin of this stronger PV gradient, Lagrangian backward trajectories are computed. On the cold air side of the jet, numerous trajectories undergo a rapid ascent from the boundary layer to the mid-levels in the simulation with the less active deep convection scheme, whereas they stay at mid-levels in the other simulation. This ascent explains the higher PV noted on that side of the jet in the simulation with the less active deep convection scheme. These ascending air masses form mid-level ice clouds that are not observed in the microphysical retrievals from airborne radar-lidar measurements. On the warm air side of the jet, in the warm conveyor belt ascending region, the Lagrangian trajectories with the less active deep convection scheme undergo a higher PV destruction due to a stronger heating occurring in the lower and middle troposphere. In contrast, in the simulation with the most active deep convection scheme, both the heating and PV destruction extend further up into the upper troposphere.

Atmospheric moisture sources of drought and wet events during 1979–2019 in the Three-River Source Region, Qinghai-Tibetan Plateau

The soil moisture observation data from the Qinghai-Tibetan Plateau permafrost and alpine wetland (PAW) monitoring network and from the National Center for Environmental Prediction (NCEP) Final Analysis (FNL) from 1989 to 2019 were adopted to drive the Lagrangian backward trajectory model to simulate the water vapour transportation paths and source regions to the three-river source region during dry and wet episodes. The results showed that the water vapour transport paths mainly came from the land and ocean in the west during the extreme drought episodes based on the standardized precipitation index (SPI), including the northern African continent, the eastern European plains, and the mid-latitudes of the Atlantic Ocean. The water vapour source regions were mainly concentrated in the Kashmir region, northeast India and southern Motuo on the Qinghai-Tibet Plateau. When the SPI indicated an extremely wet state, the transport mainly occurred to the east and south, including the Pacific Ocean and the South China Sea, where water vapour entered the mainland from East/South China, passed through the Yangtze River Basin, and finally reached the three-river source region from east or south of the Qinghai-Tibet Plateau, while the water vapour from the Indian Ocean and Arabian Sea passed through the Indian Peninsula and the Bay of Bengal and entered the three-river source region from the western and southern sides of the Qinghai-Tibet Plateau.

How is the El Niño–Southern Oscillation signal recorded by tree‐ring oxygen isotopes in southeastern China?

AbstractPrevious studies universally found that tree‐ring oxygen isotope (δ18OTR) in southeastern China (SEC) significantly recorded the El Niño–Southern Oscillation (ENSO) signal. However, the correlation between the climate in SEC and ENSO is insignificant, so the local climatic response of SEC δ18OTR fails to explain the significant positive correlations between these δ18OTR records and ENSO. In this study, based on four δ18OTR series from SEC, meteorological data, simulated precipitation oxygen isotope (δ18OP) data, and a moisture uptake atlas deduced from Lagrangian backward trajectory experiments, a conceptual model was established to explain how the ENSO signal is recorded by SEC δ18OTR. During the El Niño decaying years, the Northwest Pacific (NWP) convective activity weakens; the convective weakening areas coincide with the main moisture sources for SEC, which enriches 18O in moisture that is transported to SEC, resulting in positive anomalies of δ18OP in SEC; finally, the ENSO‐modulated δ18OP signal is inherited by SEC δ18OTR. The opposite situations occur in the La Niña events. It was also indicated that the change in the moisture contribution percentage from different sources may not be a primary factor that connects SEC δ18OTR with ENSO. These findings contribute to understanding climatic signals represented by δ18OTR and δ18OP in the East Asian monsoon region.

A Lagrangian Backward Air Parcel Trajectories Clustering Framework

Many studies concerning atmosphere moisture paths use Lagrangian backward air parcel trajectories to determine the humidity sources for specific locations. Automatically grouping trajectories according to their geographical position simplifies and speeds up their analysis. In this paper, we propose a framework for clustering Lagrangian backward air parcel trajectories, from trajectory generation to cluster accuracy evaluation. We employ a novel clustering algorithm, called DenLAC, to cluster troposphere air currents trajectories. Our main contribution is representing trajectories as a one-dimensional array consisting of each trajectory’s points position vector directions. We empirically test our pipeline by employing it on several Lagrangian backward trajectories initiated from Břeclav District, Czech Republic.

Source Apportionment of Regional Ozone Pollution Observed at Mount Tai, North China: Application of Lagrangian Photochemical Trajectory Model and Implications for Control Policy

AbstractTo better understand the characteristics and trace the sources of regional ozone (O3) in the North China Plain (NCP), we analyzed 1‐year continuous observations obtained at Mount Tai in 2018 and compared with previous data from 2006–2009. In the warm seasons (April–September), O3 pollution (defined as a maximum daily 8‐h average O3 mixing ratio that exceeds 75 ppbv) occurred frequently (59%–92% of days) and O3 concentrations significantly increased (especially in July–September) from 2006–2009 to 2018. We applied the Lagrangian photochemical trajectory model, built on the coupling of the Lagrangian backward trajectory model and Master Chemical Mechanism box model, to identify the source regions, key precursors, and emission sectors. The NCP was identified as the major source region with an average contribution of 74% ± 27% to the regional O3 concentrations during the O3 episodes in April–September. Regional O3 formation was highly NOx‐sensitive in air masses traveling from the southern part of the NCP but limited by anthropogenic hydrocarbons (especially alkenes) in air masses from the northern part of the NCP. The reduction of emissions from transportation and industry sectors would significantly reduce the regional O3 concentrations. Biomass burning also exerts a significant influence on regional O3 concentrations under certain circumstances. This study demonstrates that the regional background O3 at mountaintop levels is a good indicator of surface O3 pollution over a wide spatial coverage, and provides guidance for regional collaboration on emission control to mitigate photochemical air pollution over the NCP.

Identification of pedestrian-level ventilation corridors in downtown Beijing using large-eddy simulations

Ventilation corridor, which is defined as air path that is composed of urban open spaces with low aerodynamic roughness and airflow resistance, has been discussed in Beijing previously. This study links up with different scales of ventilation characteristics in Beijing using multi-scale numerical simulations. Validated Weather Research and Forecasting simulations demonstrate the spatial wind patterns in two seasons and the impact of urban roughness on wind speed. Combined with a Lagrangian backward trajectory model, boundary layer ventilation corridors of Beijing are identified. To provide more fundamental and precise supporting information to tackle the complexity of ventilation environment in the capital city, a set of building-resolved large-eddy simulations (LES) was conducted. Each simulation covers a neighbourhood of 3 km by 3 km and the combined model domain covers the major part of downtown Beijing. Urban canopy ventilation and the influences of urban parameters are investigated. A multiple linear regression model between site-averaged velocity ratios and morphological parameters including building coverage ratio, frontal area index, and building volume is established to identify the importance of different urban factors. Open spaces in urban areas play an important role in urban ventilation. Pedestrian-level ventilation corridors are identified by combining all LES-computed velocity ratios in downtown Beijing. For both annual prevailing (South-south-west) and winter prevailing (North-north-west) winds, three pedestrian-level ventilation corridors were identified. Land use on the upstream and along these corridors should be protected for climatic and environmental enhancement.

Assessing moisture sources of precipitation in the Western Pamir Mountains (Tajikistan, Central Asia) using deuterium excess

Air moisture of oceanic origin can be subject to long-range transport and could contribute to precipitation at distant places. With ongoing climate change the relationship between water vapour sources and sinks is under constant evolution and plays an important role for water budget assessments. This study analysed monthly integrated and event-based precipitation samples from two field sites in the Western Pamir Mountains (Tajikistan) for stable isotope compositions of oxygen (δ18O) and hydrogen (δ2H) of water. The aim was to investigate water vapour sources and, in particular, to evaluate contributions of water vapour from the Mediterranean region. The latter has been often postulated as a potential moisture source region for Central Asia. Deuterium excess values (d), that serve as a fingerprint of moisture origin, were below 13‰ in monthly integrated samples, for most of the year. This indicates a smaller contribution of high Mediterranean moisture (∼20‰) to the Western Pamir Mountains than originally expected. A maximum d value of 19‰ was observed in March, which is not in agreement with the common observation of increased Mediterranean contribution during winter (DJF). A Lagrangian backward trajectory model (HYSPLIT) was applied to infer the general air mass origin for sampled precipitation events. A maximum contribution of ‘western’ moisture of 40% was also detected in March, while 40%–60% of moisture that contributed to precipitation events in winter was transported by trajectories that originated from the Northern Indian Ocean.

Balloon-borne measurements of temperature, water vapor, ozone and aerosol backscatter on the southern slopes of the Himalayas during StratoClim 2016–2017

Abstract. The Asian summer monsoon anticyclone (ASMA) is a major meteorological system of the upper troposphere–lower stratosphere (UTLS) during boreal summer. It is known to contain enhanced tropospheric trace gases and aerosols, due to rapid lifting from the boundary layer by deep convection and subsequent horizontal confinement. Given its dynamical structure, the ASMA represents an efficient pathway for the transport of pollutants to the global stratosphere. A detailed understanding of the thermal structure and processes in the ASMA requires accurate in situ measurements. Within the StratoClim project we performed state-of-the-art balloon-borne measurements of temperature, water vapor, ozone and aerosol backscatter from two stations on the southern slopes of the Himalayas. In total, 63 balloon soundings were conducted during two extensive monsoon-season campaigns, in August 2016 in Nainital, India (29.4∘ N, 79.5∘ E), and in July–August 2017 in Dhulikhel, Nepal (27.6∘ N, 85.5∘ E); one shorter post-monsoon campaign was also carried out in November 2016 in Nainital. These measurements provide unprecedented insights into the UTLS thermal structure, the vertical distributions of water vapor, ozone and aerosols, cirrus cloud properties and interannual variability in the ASMA. Here we provide an overview of all of the data collected during the three campaign periods, with focus on the UTLS region and the monsoon season. We analyze the vertical structure of the ASMA in terms of significant levels and layers, identified from the temperature and potential temperature lapse rates and Lagrangian backward trajectories, which provides a framework for relating the measurements to local thermodynamic properties and the large-scale anticyclonic flow. Both the monsoon-season campaigns show evidence of deep convection and confinement extending up to 1.5–2 km above the cold-point tropopause (CPT), yielding a body of air with high water vapor and low ozone which is prone to being lifted further and mixed into the free stratosphere. Enhanced aerosol backscatter also reveals the signature of the Asian tropopause aerosol layer (ATAL) over the same region of altitudes. The Dhulikhel 2017 campaign was characterized by a 5 K colder CPT on average than in Nainital 2016 and a local water vapor maximum in the confined lower stratosphere, about 1 km above the CPT. Data assessment and modeling studies are currently ongoing with the aim of fully exploring this dataset and its implications with respect to stratospheric moistening via the ASMA system and related processes.

Impact of Aerosol‐PBL Interaction on Haze Pollution: Multiyear Observational Evidences in North China

AbstractAtmospheric aerosols have been found to influence the development of planetary boundary layer (PBL) and hence to enhance haze pollution in megacities. Previous works on aerosol‐PBL interaction were mainly based on model simulation for short‐term cases; so far, there is a lack of long‐term observational evidences. In this study, based on multiyear measurements and reanalysis meteorological data, we give observational evidences on aerosol‐PBL interaction and its impact on pollution aggravation. We found a significant heating in upper PBL with maximum temperature change about 0.7 °C on average and a substantial dimming near surface with a mean temperature drop of −2.2 °C under polluted condition. By integrating Eulerian forward simulation and Lagrangian backward trajectory calculation, we demonstrated that the atmospheric heating was mainly induced by light‐absorbing aerosols like black carbon. Then an index representing such effect was proposed, which could well characterize aerosol‐PBL interaction and its impact on air pollution.

Global distribution of CO<sub>2</sub> in the upper troposphere and stratosphere

Abstract. In this study, we construct a new monthly zonal mean carbon dioxide (CO2) distribution from the upper troposphere to the stratosphere over the 2000–2010 time period. This reconstructed CO2 product is based on a Lagrangian backward trajectory model driven by ERA-Interim reanalysis meteorology and tropospheric CO2 measurements. Comparisons of our CO2 product to extratropical in situ measurements from aircraft transects and balloon profiles show remarkably good agreement. The main features of the CO2 distribution include (1) relatively large mixing ratios in the tropical stratosphere; (2) seasonal variability in the extratropics, with relatively high mixing ratios in the summer and autumn hemisphere in the 15–20 km altitude layer; and (3) decreasing mixing ratios with increasing altitude from the upper troposphere to the middle stratosphere ( ∼ 35 km). These features are consistent with expected variability due to the transport of long-lived trace gases by the stratospheric Brewer–Dobson circulation. The method used here to construct this CO2 product is unique from other modelling efforts and should be useful for model and satellite validation in the upper troposphere and stratosphere as a prior for inversion modelling and to analyse features of stratosphere–troposphere exchange as well as the stratospheric circulation and its variability.

Cluster Analysis of Atmospheric Dynamics and Pollution Transport in a Coastal Area

Summertime atmospheric dynamics in the coastal zone of the industrialized Dunkerque agglomeration in northern France was characterized by a cluster analysis of back trajectories in the context of pollution transport. The MESO-NH atmospheric model was used to simulate the local dynamics at multiple scales with horizontal resolution down to 500 m, and for the online calculation of the Lagrangian backward trajectories with 30-min temporal resolution. Airmass transport was performed along six principal pathways obtained by the weighted k-means clustering technique. Four of these centroids corresponded to a range of wind speeds over the English Channel: two for wind directions from the north-east and two from the south-west. Another pathway corresponded to a south-westerly continental transport. The backward trajectories of the largest and most dispersed sixth cluster contained low wind speeds, including sea-breeze circulations. Based on analyses of meteorological data and pollution measurements, the principal atmospheric pathways were related to local air-contamination events. Continuous air quality and meteorological data were collected during the Benzene–Toluene–Ethylbenzene–Xylene 2006 campaign. The sites of the pollution measurements served as the endpoints for the backward trajectories. Pollutant transport pathways corresponding to the highest air contamination were defined.

Adjoint of the global Eulerian–Lagrangian coupled atmospheric transport model (A-GELCA v1.0): development and validation

Abstract. We present the development of the Adjoint of the Global Eulerian–Lagrangian Coupled Atmospheric (A-GELCA) model that consists of the National Institute for Environmental Studies (NIES) model as an Eulerian three-dimensional transport model (TM), and FLEXPART (FLEXible PARTicle dispersion model) as the Lagrangian Particle Dispersion Model (LPDM). The forward tangent linear and adjoint components of the Eulerian model were constructed directly from the original NIES TM code using an automatic differentiation tool known as TAF (Transformation of Algorithms in Fortran; http://www.FastOpt.com), with additional manual pre- and post-processing aimed at improving transparency and clarity of the code and optimizing the performance of the computing, including MPI (Message Passing Interface). The Lagrangian component did not require any code modification, as LPDMs are self-adjoint and track a significant number of particles backward in time in order to calculate the sensitivity of the observations to the neighboring emission areas. The constructed Eulerian adjoint was coupled with the Lagrangian component at a time boundary in the global domain. The simulations presented in this work were performed using the A-GELCA model in forward and adjoint modes. The forward simulation shows that the coupled model improves reproduction of the seasonal cycle and short-term variability of CO2. Mean bias and standard deviation for five of the six Siberian sites considered decrease roughly by 1 ppm when using the coupled model. The adjoint of the Eulerian model was shown, through several numerical tests, to be very accurate (within machine epsilon with mismatch around to ±6 e−14) compared to direct forward sensitivity calculations. The developed adjoint of the coupled model combines the flux conservation and stability of an Eulerian discrete adjoint formulation with the flexibility, accuracy, and high resolution of a Lagrangian backward trajectory formulation. A-GELCA will be incorporated into a variational inversion system designed to optimize surface fluxes of greenhouse gases.

Turbulent vertical diffusivity in the sub-tropical stratosphere

Abstract. Vertical (cross-isentropic) mixing is produced by small-scale turbulent processes which are still poorly understood and paramaterized in numerical models. In this work we provide estimates of local equivalent diffusion in the lower stratosphere by comparing balloon borne high-resolution measurements of chemical tracers with reconstructed mixing ratio from large ensembles of random Lagrangian backward trajectories using European Centre for Medium-range Weather Forecasts analysed winds and a chemistry-transport model (REPROBUS). We focus on a case study in subtropical latitudes using data from HIBISCUS campaign. An upper bound on the vertical diffusivity is found in this case study to be of the order of 0.5 m2 s−1 in the subtropical region, which is larger than the estimates at higher latitudes. The relation between diffusion and dispersion is studied by estimating Lyapunov exponents and studying their variation according to the presence of active dynamical structures.

Sulphur deposition on the Italian area: A trajectory model study

A national-scale modelling study of the transport and deposition of sulphur oxides emitted from the Italian sources is presented. A regional Lagrangian backward trajectory model, using standard available meteorological (WMO and ECMWF) and emission (CORINAIR) data, has been set up, allowing to simulate the behaviour of pollutants of indigenous origin. The contribution from other European countries is provided by RAINS/EMEP modelling system, and used in this context as a «continental background». The predicted deposition values have been compared with the measurements provided by the Italian Acid-Deposition Network, thus allowing model calibration and verification. The relative importance of depositions of emissions located in different districts of the country has then been assessed, as well as an estimate of the amount exported outside the country boundaries.

A lagrangian backward trajectory model and its application to the study of acid rain in the Emei mountainous district

A Lagrangian backward trajectory model is developed to study the variation of acidic pollutant concentrations at a preselected site. The model is one level, with linear chemical transformations. However, simple cloud processes are included. The application is made to the study of acid rain in the Emei Mountainous District, southwest China. With limited research conditions, the results show that the distant sources have significant effects on the acid rain at Emei and these sources are relatively restricted in the Sichuan Basin, especially along the direction northeast of Emei.