Isentropic Back Trajectories Research Articles

Projected changes in precipitation recycling over the Tibetan Plateau based on a global and regional climate model

AbstractPrecipitation recycling, as represented by the precipitation contributed by locally evaporated water vapor, is a key indicator of regional changes in the water cycle. The Quasi Isentropic Back-Trajectory method, combined with a global climate model [Community Climate System Model (CCSM)] and regional climate model [Weather Research and Forecasting (WRF) model simulation forced by CCSM (WRF-CCSM)], was used to analyze historical (1982–2005) and future (2090–2099) precipitation recycling over the Tibetan Plateau (TP). The study focuses on the differences in the projection of precipitation recycling ratio (PRR) changes and relevant mechanisms between the fine-resolution (30 km) WRF-CCSM and coarse-resolution (~110 km) CCSM simulations. Compared with CCSM, the biases and root-mean-square errors of the historical evapotranspiration and precipitation over the TP were greatly reduced in the WRF-CCSM simulation, particularly in precipitation. Using WRF-CCSM outputs, higher PRRs in all elevation bands, as well as the opposite seasonal pattern and linear trend of PRR for the river basins in the northern TP, were revealed. Unlike the CCSM projections, WRF-CCSM projects increasing trends of PRR changes with elevation under the RCP4.5 and RCP8.5 scenarios, with the largest increase at an elevation of about 5000 m. WRF-CCSM projects a diverse spatial and seasonal pattern of PRR changes, in contrast to the uniform decrease projected by CCSM. The larger fractional increases of future evapotranspiration contribution (precipitation contributed by local evapotranspiration) per unit warming than precipitation changes in WRF-CCSM suggests an enhanced contribution of locally evaporated moisture to total precipitation in the future under the RCP4.5 and RCP8.5 scenarios.

Black carbon profiles from tethered balloon flights over the southeastern Tibetan Plateau

Black carbon (BC) can change the energy budget of the earth system by strongly absorbing solar radiation: both suspended in the atmosphere, incorporated into cloud droplets, or deposited onto high-albedo surfaces. BC’s direct radiative forcing is highly dependent on its vertical distribution. However, due to large variabilities and the small number of vertical profile measurements, there is still large uncertainty in this forcing value. Moreover, the vertical profile of BC and its relative elevation to clouds determine BC’s lifetime in the atmosphere and its transport and removal processes. Experimental measurements of BC vertical profiles over the Tibetan Plateau are very important: not only for studies of BC effects on regional climate, but also for studies of BC transport from surrounding regions with strong anthropogenic emissions. In November-December 2017, a series of tethered balloon flights was launched at the Southeast Tibet Observation and Research Station for the Alpine Environment of the Chinese Academy of Sciences (“SETORS”, located at 29°46′N, 94°44′E, 3300 m a.s.l.). A cylindrical balloon with a diameter of 7.9 m and maximum volume of 1100 m3 was used. A 7-channel Aethalometer (Model AVIO-33, Magee Scientific®, USA) was installed in the gondola attached to the balloon, together with several other instruments including a GPS for altitude, and sensors for temperature and relative humidity. The airborne Aethalometer measured BC mass concentration (ng/m3) on a on a 1-second timebase at 7 wavelengths ranging from 370 nm to 950 nm. Meanwhile, another Aethalometer (Model AE-33, Magee Scientific®, USA) was used to monitor BC mass concentration near the surface, at a height of about 10 m above the ground. From the tethered balloon flights, we derived three profiles designated as “F1”, “F3-ASC”, and “F3-DES”. The maximum height for the F1 flight was 500 m a.g. l., namely 3800 m a.s. l.; while the maximum height for the F3 flight was 1950 m a.g. l., namely 5250 m a.s. l. Based on the potential temperature and relative humidity data, the profiles were divided into three layers: the stable boundary layer (SBL), the residual layer (RL), and the free troposphere (FT). The vertical distribution of BC shows a prominent peak within the SBL. The mean BC concentration in SBL (1000±750 ng/m3) was one order of magnitude higher than in RL and FT, which were 140±50 ng/m3 and 120±50 ng/m3, respectively. The BC concentration measured in the present study in FT over the southeastern Tibetan Plateau is comparable to measurements in Arctic regions, but lower than values in South Asia. Analysis of the wavelength dependence of the data yields an estimate of the biomass burning contribution. This showed a maximum value in SBL of 44%±37%, and was 16%±6% in RL and 14%±5% in FT. Analysis of 24-h isentropic back trajectories showed that BC in SBL and RL was dominated by local sources, while in the FT, BC is mainly influenced by mid- to long-distant transport by the westerlies. In addition, analysis of the variations of BC concentration and biomass burning contribution on a high-resolution time scale showed that BC concentrations and the nature of their sources are largely influenced by air mass origins and transport. To our knowledge, this is the first ever in situ measurement of BC concentration over the Tibetan Plateau in the atmospheric boundary layer and free troposphere up to 5000 m a.s. l.

Study of surface ozone at Port Blair, India, a remote marine station in the Bay of Bengal

This paper presents seasonal variation of surface ozone monitored continuously at site of the meteorological observatory at Port Blair, a maritime site of the Bay of Bengal for the period of August, 2005–March, 2007. Present observation depicts the characteristics of surface ozone at the remote marine site and the long range transport of pollutants from three different sides i.e., Indian Subcontinent, South East Asia and Indian Ocean. Very high ozone mixing ratio (~70–80ppbv) is occasionally observed during March and November at this site. A campaign mode of observation of trace gases (surface ozone, CO, NOx, CO2), aerosol concentration and its size, UV radiation at Port Blair was made to understand the role of transport on pollutants during March 16–26, 2002. During this period of observation, a near zero surface ozone of different time scales (~few hours) has been observed several times during the period of midnight to early morning. Simultaneously NOx (NO+NO2) (~40ppbv) and carbon monoxide was observed very high (300–600ppbv) during this period. Source of this high pollutant are not expected at this remote marine sites although wind patterns, 7-days isentropic back Trajectory analysis and MATCH Model output suggest that polluted air mass has come from eastern side of Indian subcontinent.

Deuterium and oxygen 18 in precipitation and atmospheric moisture in the upper Urumqi River Basin, eastern Tianshan Mountains

The contribution of stable isotopes in meteorological, climatological and hydrological research is well known. This study analyzed the deuterium and oxygen 18 contents (δD and δ18O) of precipitation in event-based samples at three stations (Glacier No. 1, Zongkong, Houxia) along the upper Urumqi River Basin from May 2006 to August 2007. The δ18O in precipitation revealed a wide range and a distinct seasonal variation at all three stations, with enriched values occurring in summer and depleted values in winter. A statistically significant positive correlation was observed between the δ18O and δD and local surface air temperature, and better linear relationship existed between δ18O and air temperature than that of δD. This suggests that paleoclimatic archives relating to precipitation δ18O and δD can be useful for qualitative temperature reconstruction. The d-excess in precipitation also exhibited a seasonal variability. Based on NCEP/NCAR reanalysis data, three-dimensional isentropic back-trajectories in HYSPLIT model were employed to determine the moisture source for each precipitation event. Results indicate a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter, and d-excess can be used as a sensitive tracer of the moisture transport history.

Large‐scale equatorward transport of ozone in the subtropical lower stratosphere

Anomalous vertical profiles of ozone were observed in the subtropical lower stratosphere (near south Florida) in July 2002 during the NASA‐sponsored Cirrus Regional Study of Tropical Anvils and Cirrus Layers‐Florida Area Cirrus Experiment measurement campaign. It is shown that there is an enhancement of ozone (initially >150%) above the tropopause extending up to ∼410 K potential temperature. This ozone increase is the result of recent transport of middle‐and high‐latitude lower stratospheric air into the subtropics. This meridional transport was a consequence of a geostrophic flow pattern established by a quasi‐stationary anticyclone centered over the south central United States that persisted for much of July 2002. We show the spatial and temporal extent of meridional isentropic transport into the subtropics by examining the ozone vertical profiles in combination with the NOy:O3 correlations as well as isentropic back trajectory calculations. The anomalous ozone profiles are also reproduced in a global chemical transport model.

Comment on “Mercury concentrations in coastal California precipitation: Evidence of local and trans‐Pacific fluxes of mercury to North America” by D. J. Steding and A. R. Flegal

[1] In the paper by Steding and Flegal [2002] the authors claim to identify Hg in rain from trans-Pacific sources. Specifically, Steding and Flegal (paragraph [1]) state that ‘‘the Asian fluxes appear to enhance Hg concentrations both directly, through the emission of particle-bound Hg and reactive Hg, and indirectly, by increasing the rate of oxidation of Hg in the atmosphere.’’ They arrive at this conclusion by analysis of 46 event-based precipitation measurements of total Hg from two sites in California, combined with isentropic back-trajectories to identify the source region for the sampled air masses. [2] While this paper provides some potentially useful new data on Hg in rain, we found the arguments less then convincing. It is important to note that while the authors’ conclusion is plausible, the evidence presented by Steding and Flegal [2002] is insufficient to support this conclusion. Our concern with their analysis is based on three key points.

Role of temperature history in polar stratospheric cloud sightings

[1] In this paper, aerosol data from the Polar Ozone and Aerosol Measurement (POAM) satellites between 1994 and 1998 are used to explore the existence of polar stratospheric clouds (PSCs) in the Antarctic winter stratosphere. Multiwavelength aerosol extinction data are combined to yield aerosol volume density which is compared with that generated from theoretical equilibrium models for nitric acid trihydrate (NAT) particles and supercooled ternary solution (STS) droplets at 30 mb. As winter progresses, PSC volume falls, a strong indication of stratospheric denitrification or dehydration. Six-day isentropic back trajectories are generated from each of the POAM observation locations and are used to examine the recent temperature history of the aerosols. Statistical studies are carried out using multiple regression to determine whether temperature history plays a role in NAT cloud formation. We are unable to identify any factors which unquestionably affect the formation of PSCs at and just below the NAT condensation temperature, although the probability of PSC occurrence at this temperature is higher in air that had previously been colder. There is no significant evidence found for preferential cloud occurrence based upon the length of time that the aerosols were below the NAT condensation point. We conclude from this study that factors, other than temperature and temperature history, are partially responsible for the formation of NAT PSCs.

Low ozone event at Madrid in November 1996

A sudden transient increase of the solar ultraviolet (UV) radiation was detected with a ground-based photometer in Madrid (Spain, 41°N, 3°W) on 25 November 1996 (Cordoba et al., 2000. UV-B irradiance at Madrid during 1996, 1997 and 1998. J. Geophys. Res. 105 (D4) (2000) 4903–4906). The data obtained by the TOMS satellite instrument revealed that this event was related to a reduction of the total ozone column values. This low ozone episode has been analysed. The ozone decrease was accompanied by a reduction of air temperature in the lower stratosphere. The anti-correlation between total ozone and air parcel height along the three-dimensional isentropic back-trajectories suggests that the adiabatic uplift of air contributed significantly to both the ozone and the temperature decreases observed. The uplift could be caused by the propagation into the lower stratosphere of the high-pressure cell located over the mid-Atlantic region.

Source Regions for Atmospheric Aerosol Measured at Barrow, Alaska

Aerosol data consisting of condensation nuclei (CN) counts, black carbon (BC) mass concentration, and aerosol light scattering coefficient at the wavelength of 450 nm (SC) measured at Barrow, AK, from 1986 to 1997 have been analyzed. BC and SC show an annual cycle with the Arctic haze maxima in the winter and spring and the minima in the summer. The CN time series shows two maxima in March and August. Potential source contribution function (PSCF) that combines the aerosol data with air parcel backward trajectories was applied to identify potential source areas and the preferred pathways that give rise to the observed high aerosol concentrations at Barrow. Ten-day isentropic back trajectories arriving twice daily at 500 and 1500 m above sea level were calculated for the period from 1986 to 1997. The PSCF analyses were performed based on the 80th percentile criterion values for the 2- and 24-h averages of the measured aerosol parameters. There was a good correspondence between PSCF maps for the 2- and 24-h averages, indicating that 1-day aerosol sampling in the Arctic adequately represents the aerosol source areas. In winter, the high PSCF values for BC and SC are related to industrial source areas in Eurasia. The trajectory domain in winter and spring is larger than in summer, reflecting weaker transport in summer. No high PSCF areas for BC and SC can be observed in summer. The result is related to the poor transport into the Arctic plus the strong removal of aerosol by precipitation in summer. In contrast to the BC and SC maps, the CN plot for summer shows high PSCF areas in the North Pacific Ocean. High CN values appearto be mostly connected with the long-range transport from Eurasia in winter and spring and with the reduced sulfur compound emission from biogenic activities in the ocean in the summer. PSCF analysis was found to be effective in identifying potential aerosol source areas.

Ground‐based measurements of tropospheric CO, C2H6, and HCN from Australia at 34°S latitude during 1997–1998

High spectral resolution (0.004 cm−1) infrared solar absorption measurements of CO, C2H6, and HCN have been recorded with the Fourier transform spectrometer located at the Network for the Detection of Stratospheric Change complementary station at the University of Wollongong, Australia (34.45°S, 150.88°E, 30 m above sea level). The time series covers March 1997 to February 1998. Profile retrievals with maximum sensitivity in the upper troposphere show distinct seasonal cycles for all three molecules with maxima during October‐December 1997. Best fits to the time series of daily averages yield peak 0.03–14 km columns (molecules cm−2) of 1.54×1018 for CO, 8.56×1015 for C2H6, and 6.56×1015 for HCN during austral spring. Mixing ratio profiles of all three molecules during this time show maxima in the upper troposphere. Isentropic back trajectories suggest the elevated CO, C2H6, and HCN columns above Wollongong originated from southern Africa or South America with no significant contribution from the intense tropical Asian emissions during the strong El Niño event of 1997–1998.

The characteristics of ozone and related compounds in the boundary layer of the South China coast: temporal and vertical variations during autumn season

We present measurements of several trace gases made at a subtropical coastal site in Hong Kong in October and November 1997. The gases include O 3, CO, SO 2, and NO x. The surface measurement data are compared with those from an aircraft study [Kok et al. J. Geophys. Res. 102 (D15) (1997) 19043–19057], and a subset of the latter is used to show the vertical distribution of the trace gases in the boundary layer. During the study period, averaged concentrations at the surface site for O 3, CO, NO x , and SO 2 were 50, 298, 2.75, and 1.65 ppbv, respectively. Their atmospheric abundance and diurnal pattern are similar to those found in the “polluted” rural areas in North America. The measured trace gases are fairly well mixed in the coastal boundary layer in the warm South China region. Large variability is indicated from the data. Examination of 10-day, isentropic back trajectories shows that the measured trace gases are influenced by maritime air masses, outflow of pollution-laden continental air, and the mixing of the two. The trajectories capture the contrasting chemical features of the large-scale air masses impacting on the study area. CO, NO x and SO 2 all show higher concentrations in the strong outflow of continental air, as expected, than those in the marine category. Compared with previously reported values for the western Pacific, the much higher levels found in the marine trajectories in our study suggest the impacts of regional and/or sub-regional emissions on the measured trace gases at the study site. The presence of abundant O 3 and other chemically active trace gases in the autumn season, coupled with high solar radiation and warm weather, suggests that the South China Sea is a photochemically active region important for studying the chemical transformation of pollutants emitted from the Asian continent.

The Source, Size and Chemical Composition of the Winter Arctic Tropospheric Aerosol Layer Observed by Lidar at Eureka, Canada.

During the winter seasons from 1993-94 to 1998-99 at Eureka (80.0°N, 86.0°W) in the Canadian high arctic, we have observed tropospheric aerosol layers by using a Mie-scattering-polarization lidar system and we have investigated lidar parameters (Mie backscattering coefficients, aerosol depolarization ratios, and Angstrom exponents) of the layers. We calculated isentropic back trajectories in order to investigate the source regions of these layers. We also estimated the size of the particles in the layers by calculating modeled lidar parameters and comparing them with the observed lidar parameters. Daily sampling of aerosol particles was carried out in the late winter of 1998-99. Isentropic back trajectories of the aerosol layer observed over Eureka at different altitudes suggested that the source regions of the aerosol were Eurasia, the northern Pacific and Atlantic Oceans, the Arctic Ocean, and regions near the coast of the Arctic Ocean. The lidar parameters calculated by using the Mie theory and the modeled particle size distribution suggested that, although the Mie backscattering coefficient was determined largely by the number concentration of the accumulation mode particles of diameter 0.2 μm to 0.3 μm, the aerosol depolarization ratio and Angstrom exponent of the aerosol layers were influenced strongly by the number concentration and the geometric mean diameter of the coarse particles. The trajectories, characteristics, and chemical compositions of aerosols suggest that the arctic layers observed by lidar were composed of sulfate aerosol or sea salt.

<i>Letter to the Editor:</i> A strange cloud in the Arctic summer stratosphere 1998 above Esrange (68°N), Sweden

Abstract. When the University of Bonn lidar on the Esrange (68°N, 21°E), Sweden, was switched on in the evening of July 18, 1998, a geometrically and optically thin cloud layer was present near 14 km altitude or 400 K potential temperature, where it persisted for two hours. The tropopause altitude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmospheric temperatures near 230 K were approximately 40 K too high to support ice particles at stratospheric water vapour pressures of a few ppmv. The isentropic back trajectory on 400 K showed the air parcels to have stayed clear of active major rocket launch sites. The air parcels at 400 K had traveled from the Aleutians across Canada and the Atlantic Ocean arriving above central Europe and then turned northward to pass over above the lidar station. Parcels at levels at ±25 K from 400 K had come from the pole and joined the 400 K trajectory path above eastern Canada. Apparently the cloud existed in a filament of air with an origin different from those filaments both above and below. Possibly the 400 K level air parcels had carried soot particles from forest wild fires in northern Canada or volcanic ash from the eruption of the Korovin Volcano in the Aleutian Islands.Key words: Atmospheric composition and structure (aerosols and particles; biosphere-atmosphere interactions) · Meteorology and atmospheric dynamics (middle atmospheric dynamics)

A strange cloud in the Arctic summer stratosphere 1998 above Esrange (68°N), Sweden

When the University of Bonn lidar on the Esrange (68∞N, 21∞E), Sweden, was switched on in the evening of July 18, 1998, a geometrically and optically thin cloud layer was present near 14 km altitude or 400 K potential temperature, where it persisted for two hours. The tropopause altitude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmospheric temperatures near 230 K were approximately 40 K too high to support ice particles at stratospheric water vapour pressures of a few ppmv. The isentropic back trajectory on 400 K showed the air parcels to have stayed clear of active major rocket launch sites. The air parcels at 400 K had travelled from the Aleutians across Canada and the Atlantic Ocean arriving above central Europe and then turned north- ward to pass over above the lidar station. Parcels at levels at25 K from 400 K had come from the pole and joined the 400 K trajectory path above eastern Canada. Apparently the cloud existed in a filament of air with an origin diAerent from those filaments both above and below. Possibly the 400 K level air parcels had carried soot particles from forest wild fires in northern Canada or volcanic ash from the eruption of the Korovin Volcano in the Aleutian Islands.

Low‐ozone events and three‐dimensional transport at midlatitudes of South America during springs of 1996 and 1997

Sudden and transient decreases of ozone column values were detected over the midlatitudes of South America during the springs of 1996 and 1997 by a network of ground instruments. These low‐ozone events were accompanied by air temperature decreases in the lower stratosphere. The analysis of the events suggests a connection with the springtime Antarctic ozone hole. Three‐dimensional isentropic back trajectories showed that vertical motion also contributed to the decreases of total ozone observed at the midlatitudes of South America.

Stratospheric clouds over England

Aerosol measurements from the Halogen Occultation Experiment show stratospheric clouds over England on March 3 and 4, 1996. The measured cloud volume density profiles were compared to equilibrium volumes computed for nitric acid trihydrate (NAT) and liquid ternary H2SO4‐H2O‐HNO3 aerosols. The measurements on March 4 are confidently identified as NAT, and isentropic back‐trajectories show that conditions were favorable for ice formation roughly 11 hours upwind of the observed cloud. These results support the suggestion that ice formation could be a necessary precursor to NAT nucleation.

Transport of Asian air pollution to North America

Using observations from the Cheeka Peak Observatory in northwestern Washington State during March‐April, 1997, we show that Asian anthropogenic emissions significantly impact the concentrations of a large number of atmospheric species in the air arriving to North America during spring. Isentropic back‐trajectories can be used to identify possible times when this impact will be felt, however trajectories alone are not sufficient to indicate the presence of Asian pollutants. Detailed chemical and meteorological data from one of these periods (March 29th, 1997) indicates that the surface emissions were lifted into the free troposphere over Asia and then transported to North America in ∼6 days.

Trajectory Studies of Polar Stratospheric Cloud Lidar Observations at Sodankylä (Finland) During SESAME: Comparison with Box Model Results of Particle Evolution

Polar stratospheric clouds (PSC) were observed with the multi-wavelength lidar of the MOANA project (Modelling and Observations of Aerosols in the Northern Atmosphere) during SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment). The physical state, liquid or solid, of the cloud particles can be inferred from the lidar data. Using isentropic back-trajectories to obtain the thermal history of the sampled air masses, it is possible to reconcile most of the observations with current ideas on PSC formation and evolution. When the cloud particles were identified as liquid, changes in the size distribution of the droplets along the trajectory were calculated using a micro-physical box model. Backscatter ratios calculated from the size distributions are in broad agreement with the lidar data, giving confidence in current understanding of the evolution of ternary solution (H2SO4, HNO3 and H2O) droplets. Results from two soundings are shown which bear on the problem of the formation of solid particles. In the first, solid particles were detected. The air mass had cooled to the frost point 12 hours earlier. In the second no solid particles were detected although the air temperature was below the nitric acid trihydrate existence point, and had decreased by 12K in the previous 14 hours.

On the Antarctic origin of low ozone events at the South American continent during the springs of 1993 and 1994

This paper reports the detection of several low total ozone events at mid-latitudes in the South American continent during the springs of 1993 and 1994. During these ozone depletions the total ozone column reached values as low as 150 Dobson Units at high latitudes (56S). It was found that the temperature of the lower stratosphere for the 450 K isentropic surface decreased with each low ozone event observed, suggesting that masses of ozone poor air were transported from the cold polar vortex to the South American continent. Isentropic back trajectory analyses corroborate the Antarctic origin of the observed low ozone events.

Physical properties of stratospheric clouds during the Antarctic winter of 1995

Lidar observations collected during winter 1995 at McMurdo Station, Antarctica (78°S‐167°E), are analyzed to determine polar stratospheric cloud (PSC) physical properties. A scheme to infer PSC phase from lidar depolarization and backscatter profiles is presented. Interpretation is supported by collocated temperature soundings and by isentropic back trajectories. The analysis shows that first appearance of PSC is consistent with frozen sulfates, mixing with liquid ternary solutions (H2SO4‐HNO3‐H2O) when temperature lowers. Finally, solids consistent with HNO3 mixing ratios form as mixed phases first, then followed by full solid phases. Mixed phases (i.e., coexisting solid and liquid aerosols) are detected during the whole winter. While mixed phase PSCs form particularly in the altitude range 15–20 km and are the last to disappear, full solid phases are mainly observed above 20 km and last until the end of August. Mixed phases possess the largest PSC surface areas and, as a result of selective growth, can reach large, fast settling sizes. The considerable denitrification and halogen activation observed in the Antarctic lower stratosphere, where the ozone hole takes place, appears to be well correlated with the action of this kind of PSC.