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Willis I.C.,University of Cambridge | Fitzsimmons C.D.,University of Cambridge | Melvold K.,Norwegian Water Resources and Energy Directorate NVE | Andreassen L.M.,Norwegian Water Resources and Energy Directorate NVE | And 2 more authors.
Hydrological Processes | Year: 2012

Digital elevation models of the surface and bed of Midtdalsbreen, Norway are used to calculate subglacial hydraulic potential and infer drainage system structure for a series of subglacial water pressure assumptions ranging from atmospheric to ice overburden. A distributed degree-day model is used to calculate the spatial distribution of melt on the glacier surface throughout a typical summer, which is accumulated along the various drainage system structures to calculate water fluxes beneath the glacier and exiting the portals for the different water pressure assumptions. In addition, 78 dye-tracing tests were performed from 33 injection sites and numerous measurements of water discharge were made on the main proglacial streams over several summer melt seasons. Comparison of the calculated drainage system structures and water fluxes with dye tracing results and measured proglacial stream discharges suggests that the temporally and spatially averaged steady-state water pressures beneath the glacier are ~70% of ice overburden. Analysis of the dye return curves, together with the calculated subglacial water fluxes shows that the main drainage network on the eastern half of the glacier consists of a hydraulically efficient system of broad, low channels (average width/height ratio≈75). The smaller drainage network on the west consists of a hydraulically inefficient distributed system, dominated by channels that are exceptionally broad and very low (average width/height ratio≈350). The even smaller central drainage network also consists of a hydraulically inefficient distributed system, dominated by channels that are very broad and exceptionally low (average width/height ratio≈450). The channels beneath the western and central glacier must be so broad and low that they can essentially be thought of as a linked cavity system. © 2011 John Wiley & Sons, Ltd.

Kruit R.J.W.,National Institute for Public Health and the Environment RIVM | Kruit R.J.W.,Wageningen University | van Pul W.A.J.,National Institute for Public Health and the Environment RIVM | Sauter F.J.,National Institute for Public Health and the Environment RIVM | And 6 more authors.
Atmospheric Environment | Year: 2010

New parameterizations for surface-atmosphere exchange of ammonia are presented for application in atmospheric transport models and compared with parameterizations of the literature. The new parameterizations are based on a combination of the results of three years of ammonia flux measurements over a grassland canopy (dominated by Lolium perenne and Poa trivialis) near Wageningen, the Netherlands and existing parameterizations from literature. First, a model for the surface-atmosphere exchange of ammonia that includes the concentration at the external leaf surface is derived and validated. Second, a parameterization for the stomatal compensation point (expressed as Γs, the ratio of [NH4 +]/[H+] in the leaf apoplast) that accounts for the observed seasonal variation is derived from the measurements. The new, temperature-dependent Γs describes the observed seasonal behavior very well. It is noted, however, that senescence of plants and field management practices will also influence the seasonal variation of Γs on a shorter timescale. Finally, a relation that links Γs to the atmospheric pollution level of the location through the 'long-term' NH3 concentration in the air is proposed. © 2009 Elsevier Ltd. All rights reserved.

Mensah A.A.,Julich Research Center | Mensah A.A.,ETH Zurich | Holzinger R.,Institute for Marine and Atmospheric Research Utrecht IMAU | Otjes R.,Energy Research Center of the Netherlands | And 5 more authors.
Atmospheric Chemistry and Physics | Year: 2012

Observations of aerosol chemical composition in Cabauw, the Netherlands, are presented for two intensive measurement periods in May 2008 and March 2009. Sub-micron aerosol chemical composition was measured by an Aerodyne Aerosol Mass Spectrometer (AMS) and is compared to observations from aerosol size distribution measurements as well as composition measurements with a Monitor for AeRosol and GAses (MARGA) based instrument and a Thermal-Desorption Proton-Transfer-Reaction Mass-Spectrometer (TD-PTR-MS). An overview of the data is presented and the data quality is discussed. In May 2008 enhanced pollution was observed with organics contributing 40% to the PM1 mass. In contrast the observed average mass loading was lower in March 2009 and a dominance of ammonium nitrate (42%) was observed. The semi-volatile nature of ammonium nitrate is evident in the diurnal cycles with maximum concentrations observed in the morning hours in May 2008 and little diurnal variation observed in March 2009. Size dependent composition data from AMS measurements are presented and show a dominance of organics in the size range below 200 nm. A higher O:C ratio of the organics is observed for May 2008 than for March 2009. Together with the time series of individual tracer ions this shows the dominance of OOA over HOA in May 2008. © 2012 Author(s).

Bergamaschi P.,European Commission - Joint Research Center Ispra | Houweling S.,SRON Netherlands Institute for Space Research | Houweling S.,Institute for Marine and Atmospheric Research Utrecht IMAU | Segers A.,European Commission - Joint Research Center Ispra | And 18 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

The causes of renewed growth in the atmospheric CH4 burden since 2007 are still poorly understood and subject of intensive scientific discussion. We present a reanalysis of global CH4 emissions during the 2000s, based on the TM5-4DVAR inverse modeling system. The model is optimized using high-accuracy surface observations from NOAA ESRL's global air sampling network for 2000-2010 combined with retrievals of column-averaged CH4 mole fractions from SCIAMACHY onboard ENVISAT (starting 2003).Using climatological OH fields, derived global total emissions for 2007-2010 are 16-20 Tg CH 4/yr higher compared to 2003-2005. Most of the inferred emission increase was located in the tropics (9-14 Tg CH4/yr) and mid-latitudes of the northern hemisphere (6-8 Tg CH4/yr), while no significant trend was derived for Arctic latitudes. The atmospheric increase can be attributed mainly to increased anthropogenic emissions, but the derived trend is significantly smaller than estimated in the EDGARv4.2 emission inventory. Superimposed on the increasing trend in anthropogenic CH4 emissions are significant inter-annual variations (IAV) of emissions from wetlands (up to ±10 Tg CH4/yr), and biomass burning (up to ±7 Tg CH4/yr). Sensitivity experiments, which investigated the impact of the SCIAMACHY observations (versus inversions using only surface observations), of the OH fields used, and of a priori emission inventories, resulted in differences in the detailed latitudinal attribution of CH4 emissions, but the IAV and trends aggregated over larger latitude bands were reasonably robust. All sensitivity experiments show similar performance against independent shipboard and airborne observations used for validation, except over Amazonia where satellite retrievals improved agreement with observations in the free troposphere. Key Points A reanalysis of global CH4 emissions during the 2000s is presented derived global total emissions 2007-2010 16-20 Tg CH4/yr higher than 2003-2005 increase mainly in the tropics and NH mid-latitudes ©2013. American Geophysical Union. All Rights Reserved.

Ligtenberg S.R.M.,Institute for Marine and Atmospheric Research Utrecht IMAU | Kuipers Munneke P.,Institute for Marine and Atmospheric Research Utrecht IMAU | Van Den Broeke M.R.,Institute for Marine and Atmospheric Research Utrecht IMAU
Cryosphere | Year: 2014

A firn densification model (FDM) is used to assess spatial and temporal (1979-2200) variations in the depth, density and temperature of the firn layer covering the Antarctic ice sheet (AIS). A time-dependent version of the FDM is compared to more commonly used steady-state FDM results. Although the average AIS firn air content (FAC) of both models is similar (22.5 m), large spatial differences are found: in the ice-sheet interior, the steady-state model underestimates the FAC by up to 2 m, while the FAC is overestimated by 5-15m along the ice-sheet margins, due to significant surface melt. Applying the steady-state FAC values to convert surface elevation to ice thickness (i.e., assuming flotation at the grounding line) potentially results in an underestimation of ice discharge at the grounding line, and hence an underestimation of current AIS mass loss by 23.5% (or 16.7 Gt yr-1) with regard to the reconciled estimate over the period 1992-2011. The timing of the measurement is also important, as temporal FAC variations of 1-2m are simulated within the 33 yr period (1979-2012). Until 2200, the Antarctic FAC is projected to change due to a combination of increasing accumulation, temperature, and surface melt. The latter two result in a decrease of FAC, due to (i) more refrozen meltwater, (ii) a higher densification rate, and (iii) a faster firn-to-ice transition at the bottom of the firn layer. These effects are, however, more than compensated for by increasing snowfall, leading to a 4-14% increase in FAC. Only in melt-affected regions, future FAC is simulated to decrease, with the largest changes (-50 to -80 %) on the ice shelves in the Antarctic Peninsula and Dronning Maud Land. Integrated over the AIS, the increase in precipitation results in a similar volume increase due to ice and air (both ∼150 km3 yr-1 until 2100). Combined, this volume increase is equivalent to a surface elevation change of +2.1 cm yr-1, which shows that variations in firn depth remain important to consider in future mass balance studies using satellite altimetry. © Author(s) 2014.

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