Aerosol transport

Research on Desert Dust Transport

Desert dust is an important component of the climate system. It can directly reflect incoming sunlight, influence the development of precipitation in clouds, or fertilise the ocean with iron and other nutrients. Modelling the long-range transport of mineral aerosol is however complicated by the different scale of the involved processes. This includes the mobilisaiton of dust in the desert, uplift into the free atmosphere, long-range transport, and finally wet or dry deposition back to the surface. The aim of this work is to understand the full transport history of the desert dust.

Sahara dust transport events to the Alps

On several occasions a year dust from the Sahara desert crosses the Mediterranean, and is deposited over central Europe. In a study we examined the occurence of desert dust in an ice core from Piz Zupo in the Swiss Alps. Curiously, the desert dust was associated with high concentrations of MSA, a substance that is known to originate from phytoplankton, i.e. organisms living in ocean surface waters.
A detailed modelling study of the dust transport based on backward trajectories revealed that dust can be transported over quite long distances and several days to the Alps. It appears most likely that the MSA was incorporated into the dust plume during the transport over the Mediterranean, possibly due to convective mixing with moist marine boundary layer air.

See also the animations of two dust transport events, which is an electronic supplement to the corresponding paper (Sodemann et al., 2006).

Figure 1: Chlorophyll-a and selected dust transport trajectories. (a) March 2000, (b) October 2000.

Seasonality of Sahara dust export

In a further study, it was explored which synoptic features typically are associated with dust export from the Sahara during one particular year. A one-year climatology of dust export based on the same back-trajectory method as for stuying the transport events revealed a compilcated pattern of dust export. Large dust eruptions of several days into the eastern Atlantic dominate along the African west coast. Dust transport into the Mediterranean is very variable. Short episodes are mostly associated with mid-latitude or Mediterranean cyclones, that are associated with dust transport to the north.

Figure 2: Seasonality of dust export during 2000 along boundaries of the Sahara desert.

Relevant publications

  • Sodemann, H., Palmer, A. S., Schwierz, C., Schwikowski, M. and Wernli, H., 2006: The transport history of two Saharan dust events archived in an Alpine ice core, Atmos. Chem. Phys.,6, 667-688. [Abstract, Paper, PDF, 2MB, Electronic supplement, Online discussion]
  • Sodemann, H., 2006: Tropospheric transport of water vapour: Lagrangian and Eulerian perspectives, Diss. No. 16623, ETH Zurich. Logos Verlag, Berlin, 250 pp. [order a print copy] [request a PDF]
  • Bürzle, S., October 2004: Untersuchung des Saharastaubtransportes an die nordafrikanische Küste im Jahre 2000, Diploma Thesis, IACETH, ETH Zürich, 71 pp.
  • Sodemann, H., Schwierz, C., Wernli, H., Palmer, A. S. and Schwikowski, M., 2005: Transport history of Saharan dust archived in an Alpine ice core, Annual Report 2004, Paul-Scherrer Institute, pg. 29.

Stable boundary layers

Research on Stable Boundary Layers

The boundary layer is the interface between the earth surface and the free atmosphere. As such, it is a key component of the climate system. Energy fluxes of momentum, heat and moisture couples the atmosphere to the surface in this layer. Depending on daytime, latitude, and other conditions, its depth ranges between a few metres (polar night in Antarctica) to a few kilometres (Sahara during mid-day). As many processes in this layer are governed by turbulence, it is very challenging to represent them appropriately in coarse resolution NWP and climate models.

Energy fluxes in the Stable Boundary Layer

Turbulence is generally to small to resolve it explicity in coarse-resolution models. Most current NWP and climate models currently represent the fluxes in the boundary layer with a relatively straightforward and simplified bulk parametrisation (e.g. Louis et al., 1982). It is however known that there are a range of problems with this simple parameterisation. Effectively, it can lead to a de-coupling between the earth surface and the free atmosphere under very stable conditions. Stable boundary layers often persist at high latitudes and during polar night.

In this work we examined how predicted energy fluxes according to the parameterisation of Zilitinkevich et al., 2002 compared to measurement data from the FINTUREX field campaign. During this campaign, which took place in summer 1994 on the Ekstrom ice shelf near the German Neumayer station, eddy correlation measurements and tower measurements were carried out for a period of about 1 month.

Figure 1: Time series of the stratification at the Neumayer Station, Antarctica, during several days of the FINTUREX experiment. Gray: stable stratification, blue: unstable stratification.

Figure 1 shows a temporal sequence of the stability profile in the lowest 10m above the surface during the FINTUREX measurement campaign. As can be seen from the gray colours, the stable stratification in the middle and upper surface layer becomes neutral to unstable (blue colours) with the diurnal cycle. In the lowest half metre above ground, however, stable stratification persists. This persistent near-surface inversion layer was confirmed by other studies in Greenland, and shows some similarities with stability profiles above the ocean. The inversion layer effectively decouples the heat fluxes from the surface temperature. It could therefore be important to include this phenomenon in energy flux parameterisations for high latitudes.

Relevant publications

  • Sodemann, H. and Foken, T., 2005: Special characteristics of the temperature structure near the surface, Theor. Appl. Climatol.,80(2-4),81-89,doi:10.1007/s00704-004-0092-1. [Abstract, Paper, PDF, 500KB]
  • Sodemann, H. and Foken, T., 2004: Parameter estimation for an extended theory for stable atmospheric boundary layers, Quart. J. Royal Meteorol. Soc.,130(602A),2665-2672. [Abstract, Paper, PDF, 190KB]
  • Sodemann, H.,2002. Evaluation of a parameterisation for turbulent fluxes of momentum and heat in stably stratified surface layers. Diploma Thesis, Department of Micrometeorology, University of Bayreuth, Germany, 98 pp. [ PDF, 3MB]

Water vapour transport

Research on Atmospheric Water Vapour Sources and Transport

The atmospheric branch of Earth’s water cycle couples the major water reservoirs, namely oceans, ice shields, rivers, and the land surface via evaporation and precipitation. Water vapour is a crucial ingredient for many weather-related phenomena, such as rain, snow, and clouds. As a greenhouse gas, it is also important for the radiative budget of the Earth. However, many physical processes related to water vapour are associated with large uncertainties. This research aims at improving the understanding of weather and climate processes by studying the evaporation and transport processes of atmospheric water vapour.

Lagrangian diagnostic for water vapour origin

We have developed a method which allows to trace water vapour in the atmosphere backward in time to identify the location from which it has evaporated. The method is a diagnostic based on backward trajectories, and provides quantitative information on the various contributions from moisture sources to the water vapour in traced air parcels. This provides, for example, a physical link between anomalous sea surface temperatures and extreme precipitation events.

We applied the method to identify the inter-annual variability of moisture sources for water transport to Greenland. Our results based on the ERA-40 reanalyis data show that the North Atlantic and the Nordic Seas are Greenlands main moisture sources during winter. However, moisture source locations and strengths can change strongly from winter to winter, in correspondence with the North Atlantic Oscillation index. In a further study, we investigated the implications for stable isotopes in Greenland winter precipitation. The method was also applied to study the moisture source seasonality of the European Alps.

Figure 1: Sketch of the Lagrangian water vapour transport diagnostic.

Water vapour tracer simulations in a regional model

An alternative approach to tracing water transport in the atmosphere is explored by means of a regional numerical weather prediction model (CHRM). In this apporach, we follow water vapour tracers that only evaporate from restricted areas of the surface to follow the distribution of the water vapour during specific meteorological events. The advantage of this method is the detailed consideration of physical processes which are not resolved at the model grid scale, such as convective precipitation.

Our analyis currently focuses on the examination of the Elbe flood, which during which large parts of Central Europe suffered in August 2002. Figure 2 shows how sub-tropical moisture was advected into the domain and contributed to precipitation in the model domain. Details about the method and this particular case study are available in Sodemann et al., 2009.

Figure 2: Tracer from a Sub-tropical water vapour source region is advected into the domain during the Elbe flood (12 August 2002).

Relevant publications

  • Sodemann, H., Wernli, H. and Schwierz, C., 2009: Sources of water vapour contributing to the Elbe flood in August 2002 – A tagging study in a mesoscale model, Quart. J. Royal Meteorol. Soc., 135, 205-223, doi:10.1002/qj.374. [Abstract and PDF]
  • Sodemann, H., Masson-Delmotte, V., Schwierz, C., Vinther, B. M. and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Part II: Effects of North Atlantic Oscillation variability on stable isotopes in precipitation, J. Geophys. Res., 113, D12111, doi:10.1029/2007JD009416. [Abstract, PDF, 1.6MB]
  • Sodemann, H., Schwierz, C., and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Lagrangian moisture diagnostic and North Atlantic Oscillation influence, J. Geophys. Res., 113, D03107, doi:10.1029/2007JD008503. [Abstract, PDF, 6.4MB]
  • Stohl, A., Forster, C. and Sodemann, H., 2007: Remote sources of water vapor forming precipitation on the Norwegian west coast at 60°N – a tale of hurricanes and an atmospheric river, J. Geophys. Res., 113, D05102, doi:10.1029/2007JD009006. [Abstract, PDF, 7MB]
  • Sodemann, H. and Zubler, A., 2007: Herkunft des Niederschlagswassers im Alpenraum. DACH Meteorologentagung 2007, Hamburg. [Extended Abstract, PDF, 656KB]

Atmospheric Transport

Research on Atmospheric Transport

Transport of trace components of the Earth’s atmosphere, such as gases, smoke or dust, is governed by the winds and their turbulent properties. Simulating the transport and dispersion of such substances as realisticly as possible by means of atmospheric models is a persistent challenge due to the interaction of physical processes at very disparate scales. In-situ measurements and LIDAR observations from research aircraft are an increasingly powerful means to validate model simulations. Spaceborne remote-sensing observations place the aircraft measurements in a horizontally coherent perspective. This research aims at bringing together models and data from different sources to improve our understanding of atmospheric transport processes.

Evaluating Lagrangian and Eulerian model simulations in the Arctic
High latitude regions are notoriously difficult to simulate for atmospheric models. In part, this is because the polar regions are a harsh and inaccessible region and have remained so until present. In addition, however, the peculiar situation in these regions for example leads to very shallow and stably stratified boundary layers that are not well represented in models with limited vertical grid resolution. Remote sensing observations are hampered by the reflective ice and snow surfaces, and passive sensors are blind during a large part of the year because of the polar night. It is however known that the Arctic are a very sensitive region of the climate system, and at least the Arctic is a large sink for polluted air masses emitted from human activity in the mid-latitudes.

In our study, we investigated how a heavily polluted airmass originating from forest fires in Siberia was transported with a low-pressure system across the North Pole. Aircraft measurements and satellite observations indicated the horizontal and vertical extend of the pollution plume as it had risen to higher altitudes when arriving north of Greenland several days after emission. Comparison between two model simulations and measurements and observations showed very good agreement with the overall shape of the pollution feature. The Lagrangian transport model FLEXPART was however to retain more fine-scale structure and finer gradients than the Eulerian Chemical Transport Model (CTM) TOMCAT, in better agreement in particular with space-borne LIDAR observations and aircraft measurements.

Figure 1: Comparison of a FLEXPART model simulation (m) of total column carbon monoxide (CO) with satellite observations from the IASI satellite (n) and the TOMCAT Eulerian Chemistry Transport Model (CTM), panel (o).

Relevant publications

  • Sodemann, H., Pommier, M., Arnold, S. R., Monks, S. A., Stebel, K., Burkhart, J. F., Hair, J. W., Diskin, G. S., Clerbaux, C., Coheur, P.-F., Hurtmans, D., Schlager, H., Blechschmidt, A.-M., Kristjánsson, J. E., and Stohl, A., 2011: Episodes of cross-polar transport in the Arctic troposphere during July 2008 as seen from models, satellite, and aircraft observations, Atmos. Chem. Phys., 11, 3631-3651. [PDF, 7.0MB, Interactive Discussion]
  • Roiger, A., Schlager, H., Schäfler, A., Huntrieser, H., Scheibe, M., Aufmhoff, H., Cooper, O. R., Sodemann, H., Stohl, A., Burkhart, J., Lazzara, M., Schiller, C., Law, K. S., and Arnold, F., 2011: In-situ observation of Asian pollution transported into the Arctic lowermost stratosphere, Atmos. Chem. Phys., 11, 10975-10994 [PDF, 5.5MB, Interactive Discussion]
  • Schmale, J., Schneider, J., Ancellet, G., Quennehen, B., Stohl, A., Sodemann, H., Burkhart, J., Hamburger, T., Arnold, S. R., Schwarzenboeck, A., Borrmann, S., and Law, K. S., 2011: Source identification and airborne chemical characterisation of aerosol pollution from long-range transport over Greenland during POLARCAT summer campaign 2008, Atmos. Chem. Phys., 11, 10097-10123 [PDF, 5.4MB, Interactive Discussion]
  • Quennehen, B., Schwarzenboeck, A., Schmale, J., Schneider, J., Sodemann, H., Stohl, A., Ancellet, G., Crumeyrolle, S., and Law, K. S., 2011: Physical and chemical properties of pollution aerosol particles transported from North America to Greenland as measured during the POLARCAT summer campaign, Atmos. Chem. Phys., 11, 10947-10963 [PDF, 7.1MB, Interactive Discussion]
  • Brock, C. A., Cozic, J., Bahreini, R., Froyd, K. D., Middlebrook, A. M., McComiskey, A., Brioude, J., Cooper, O. R., Stohl, A., Aikin, K. C., de Gouw, J. A., Fahey, D. W., Ferrare, R. A., Gao, R.-S., Gore, W., Holloway, J. S., Hübler, G., Jefferson, A., Lack, D. A., Lance, S., Moore, R. H., Murphy, D. M., Nenes, A., Novelli, P. C., Nowak, J. B., Ogren, J. A., Peischl, J., Pierce, R. B., Pilewskie, P., Quinn, P. K., Ryerson, T. B., Schmidt, K. S., Schwarz, J. P., Sodemann, H., Spackman, J. R., Stark, H., Thomson, D. S., Thornberry, T., Veres, P., Watts, L. A., Warneke, C., and Wollny, A. G., 2011: Characteristics, sources, and transport of aerosols measured in spring 2008 during the aerosol, radiation, and cloud processes affecting Arctic climate (ARCPAC) project, Atmos. Chem. Phys., 11, 2423-2453. [PDF, 12.7MB, Interactive Discussion]
  • Hirdman, D., Burkhart, J. F., Sodemann, H., Eckhardt, S., Jefferson, A., Quinn, P. K., Sharma, S., Ström, S. and Stohl, A., 2010: Long-term trends of black carbon and sulphate aerosol in the Arctic: Changes in atmospheric transport and source region emissions, Atmos. Chem. Phys., 10, 9351-9368. [PDF, 4.3MB, Interactive Discussion]
  • Hirdman, D., Sodemann, H., Eckhardt, S., Burkhart, J. F., Jefferson, A., Mefford, T., Quinn, P. K., Sharma, S., Ström, S. and Stohl, A., 2010: Source identification of short-lived air pollutants in the Arctic using statistical analysis of measurement data and particle dispersion model output, Atmos. Chem. Phys., 10, 669-693. [PDF, 12MB, Interactive Discussion]
  • Warneke, C., Froyd, K. D., Brioude, J., Bahreini, R., Brock, C. A., Cozic, J., de Gouw, J. A., Fahey, D. W., Ferrare, R., Holloway, J. S., Middlebrook, A. M., Miller, L., Montzka, S., Schwarz, J. P., Sodemann, H., Spackman, J. R. and Stohl, A., 2010: An important contribution to springtime Arctic aerosol from biomass burning in Russia, Geophys. Res. Lett., 37, L01801, doi:10.1029/2009GL041816. [PDF]
  • Stohl, A., and Sodemann, H., 2010: Characteristics of atmospheric transport into the Antarctic troposphere, J. Geophys. Res., 115, D02305, doi:10.1029/2009JD012536. [PDF (open access), 5.3MB]
  • Hirdman, D., Aspmo, K., Burkhart, J. F., Eckhardt, S., Sodemann, H., and Stohl, A., 2009: Transport of mercury in the Arctic atmosphere: Evidence for a spring-time net sink and summer-time source, Geophys. Res. Lett., 36, L12814, doi:10.1029/2009GL038345. [PDF]

The concept of stable isotopes

Stable water isotopes in atmospheric research
Stable water isotopes (oxygen 18 and deuterium) are an important observational tool for climate reconstructions from palaeoarchives, such as ice cores. Once callibrated with present-day temperatures, they can be used to reconstruct the temperature changes during the ice ages. These callibrations are possible because the concentration of stable isotopes in water vapour and precipitation depends on the meteorological conditions during the water’s transport history, in particular the temperatures during condensation.

While the climatic isotope information is widely used, it is not fully understood how for example seasonal or inter-annual variability of atmospheric circulation influence the isotope signal. This research focuses on the atmospheric conditions during isotope fractionation along its transport path, based on individual precipitation events. The aim is to gain a better physical understanding of the short-term isotope variability in palaeo archives, and use stable isotopes as an observational tool to validate the hydrological cycle in atmospheric models.

Figure 1: Sketch of stable isotopes fractionation processes during atmospheric transport. Point on the numbers to see a description of each process. From Sodemann (2006).

Inter-annual variability of stable water isotopes in Greenland
We investigated to what extent the inter-annual changes of stable isotopes in Greenland precipitation can be explained when the full moisture transport history is considered. To this end, we studied the transport of water to Greenland during winter months with different North Atlantic Oscillation (NAO) phases using a Lagrangian moisture diagnostic. The results show that modelled inter-annual variability is to 2/3 caused by the strong changes of atmospheric moisture transport to Greenland for different winters. One third is due to the fact that moisture from different evaporation sources is brought to Greenland. There remain however considerable uncertainties with respect to how stable water isotopes are brought into Lagrangian transport models.

Figure 2: Modelled isotopic composition (oxygen 18) of Greenland winter precipitation for the positive NAO phase (left), and the negative NAO phase (right).

Relevant publications

  • Sodemann, H., Schwierz, C., and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Lagrangian moisture diagnostic and North Atlantic Oscillation influence, J. Geophys. Res., 113, D03107, doi:10.1029/2007JD008503. [Abstract, PDF, 6.4MB]
  • Sodemann, H., Masson-Delmotte, V., Schwierz, C., Vinther, B. M. and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Part II: Effects of North Atlantic Oscillation variability on stable isotopes in precipitation, J. Geophys. Res., accepted February 2008. [Abstract, Manuscript, PDF, 1.6MB]