ALBERT

Description: The airborne laser scanning (ALS) datasets were acquired at the Arctic tundra site of Trail Valley Creek (TVC), Northwest Territories, Canada, which is underlain by continuous permafrost. Basic processing and filtering steps were applied to the ALS point cloud. Based on a classification into ground and vegetation points, a Digital Terrain Model (DTM) and rasters of mean and maximum vegetation heights are derived. Detailed metadata are included.

Description: Large uncertainties still exist in the global methane budget with clear disagreements between bottom-up and top-down estimates, limiting confidence in climate projections. This is particularly true in the Arctic, which is warming rapidly while storing vast amounts of organic carbon that could potentially be released as carbon dioxide and methane, adding a new greenhouse gas source of unknown magnitude. Regional scale methane emission estimates and functional relationships between potential drivers and methane fluxes are currently unavailable. The Airborne Measurements of Methane Fluxes (AIRMETH) campaigns are designed to quantitatively and spatially explicitly address this question. While ground-based eddy covariance (EC) measurements provide continuous in-situ observations of the surface-atmosphere exchange of energy and matter, they are rare in the Arctic permafrost zone and site selection is bound by logistical constraints among others. Consequently, these observations cover only small areas that are not necessarily representative of the region of interest. Airborne measurements can overcome this limitation by covering distances of hundreds of kilometers over time periods of a few hours. During the AIRMETH-2012 campaign aboard the research aircraft POLAR 5 we measured turbulent exchange fluxes of energy and methane along thousands of kilometers covering the North Slope of Alaska. Time-frequency (wavelet) analysis, footprint modeling, and machine learning techniques are used to extract spatially resolved turbulence statistics and fluxes, spatially resolved contributions of land cover and biophysical surface properties to each flux observation, as well as regionally valid functional relationships between environmental drivers and observed fluxes that can explain spatial flux patterns and – if available in temporal resolution – allow for spatio-temporal scaling of the observations. Here we present a 100 m resolution gridded methane flux map for the North Slope of Alaska, covering about 90.000 km2. We show that surface properties like elevation, temperature, and NDVI along with meteorological drivers such as shortwave radiation, water vapor mixing ratio, and horizontal wind speed are sufficient to explain and project the measured fluxes. The median methane flux for the campaign period (end of June/beginning of July) was 19.4 mg m−2 d−1 after excluding all values with 30 % standard error. The largest fluxes were observed along the coast and in the Arctic coastal plain, decreasing towards the Brooks Range.

Description: Arctic ecosystems are undergoing a very rapid change due to global warming and their response to climate change has important implications for the global energy budget. Therefore, it is crucial to understand how energy fluxes in the Arctic will respond to any changes in climate related parameters. Attribution of these responses, however, is challenging because measured fluxes are the sum of multiple processes that respond differently to environmental factors. Ground-based measurements of surface fluxes provide continuous in-situ observations of the surfaceatmosphere exchange. But these observations may be non-representative because of spatial and temporal heterogeneity, indicating that local observations cannot easily be extrapolated to represent global scales. Airborne eddy covariance measurements across large areas can reduce uncertainty and improve spatial coverage and spatial representativeness of flux estimates. Here, we present the potential of environmental response functions for quantitatively linking energy flux observations over high latitude permafrost wetlands to environmental drivers in the flux footprints. We used the research aircraft Polar 5 equipped with a turbulence probe as well as fast temperature and humidity sensors to measure turbulent energy fluxes across the Alaskan North Slope. We used wavelet transforms of the original highfrequency data, which enable much improved spatial discretization of the flux observations, and determine biophysically relevant land cover properties in the flux footprint. A boosted regression trees technique is then employed to extract and quantify the functional relationships between energy fluxes and environmental drivers. Using the extracted environmental response functions and meteorological fields simulated by the Weather Research and Forecasting (WRF) model, the surface energy fluxes were then projected beyond the measurement footprints across the entire North Slope of Alaska.

Description: Among other regions of the world, the Arctic is strongly affected by climate change. Globally, it is the region with the most pronounced warming, leading to permafrost warming and thawing. Part of the 1,300 Pg soil organic carbon currently stored in the frozen ground is already and might be further released as carbon dioxide (CO2) and methane (CH4). CO2 is released through aerobic soil respiration and from plant roots, but also sequestered through photosynthesis. CH4 emission can be attributed to either recent microbial activity or to past microbial or thermal decomposition and is spatially heterogeneous. To our knowledge, regional assessments of the total carbon flux (CO2 and CH4) based on high frequency airborne measurements do not exist. Here we determine the regional pattern of CO2 and total carbon emissions (CO2 + CH4) of the Mackenzie Delta region, Canada, based on the Airborne Measurements of Methane Fluxes Campaign (AIRMETH) in July 2013 [Kohnert et al., 2014]. The Mackenzie Delta is the second largest arctic delta (13,000 km2). Our measurements covered an area extending 320 km from west to east (140°58’ W to 133°22’W) and of 240 km from north to south (69°33’N to 67°26’N). The study area is heterogeneous and comprises the delta itself, the adjacent Yukon coastal plain, and Richards Island north east of the delta. Part of the delta is located north of the treeline. The area surrounding the delta is described as continuous permafrost zone where the permafrost reaches a thickness of 300 m along the coastal plain and 500 m on Richards Island. In the delta itself the discontinuous permafrost reaches a maximum thickness of 100 m. For the AIRMETH campaign we used the research aircraft Polar 5. Equipped with a 5-hole probe, the usual meteorological sensors, and a fast greenhouse gas analyser (GGA 24EP, Los Gatos Research Inc.) we flew at 30 - 60 m above ground at a true airspeed of 60 m s−1. CO2 and CH4 fluxes were calculated with a timefrequency resolved version of the eddy-covariance technique [Metzger et al., 2013]. We calculated flux topographies [Mauder et al., 2008] to resolve the fluxes along a linear flight track to the area within the footprint of the measurements. The result is a 100 m resolved gridded carbon flux map within the footprints of the flight tracks. Based on the flux topographies we produce a map of the regional pattern of peak growing season carbon fluxes.

Description: Arctic wetlands associated with permafrost as well as thawing permafrost emit the greenhouse gas methane (CH4). Two important contributors are recent microbial activity in the active layer or taliks (biogenic CH4), and deeper fossil sources where pathways through the permafrost exist (geologic CH4). Current emission estimates vary strongly between different models. Moreover, there is still disagreement between bottom-up estimates from local field studies, and topdown estimates from atmospheric measurements. Here, we quantify permafrost CH4 emissions directly on the regional scale, based on the Airborne Measurements of Methane Fluxes Campaigns (AIRMETH) in the Mackenzie River Delta region, Canada, in July 2012 and 2013 [Kohnert et al., 2014]. The Mackenzie Delta is the second largest Arctic delta (13,000 km2). Our measurements covered an area extending 320 km from west to east (140°58’W to 133°22’W) and of 240 km from north to south (69°33’N to 67°26’N). The study area comprises the delta itself, the adjacent Yukon coastal plain, and Richards Island north east of the delta. The area surrounding the delta is described as continuous permafrost zone where the permafrost reaches a thickness of 300 m along the coastal plain and 500 m on Richards Island. In the delta itself the discontinuous permafrost reaches a maximum thickness of 100 m. The northern part of the study area is crossed by geological faults and underlain by oil and natural gas deposits. We analyse the regional pattern of CH4 fluxes and estimate the contribution of geologic emissions to the total CH4 budget of the delta. CH4 fluxes were calculated with a time-frequency resolved version of the eddy-covariance technique [Metzger et al., 2013], followed by the calculation of flux topographies [Mauder et al., 2008]. The result is a 100 m resolved gridded flux map within the footprints of the flight tracks. The results provide the first regional estimate of CH4 release from the Mackenzie Delta and the adjacent coastal plain. We distinguish geological gas seeps from biogenic sources by their strength, and show that geologic sources contribute strongly to the annual CH4 budget of the study area: One percent of the covered area contains the strongest geological seeps which contribute disproportionately to an annual emission estimate. The contribution of geological sources to CH4 emission warrants further attention, in particular in areas where permafrost is vulnerable to increased geologic gas migration due to thawing and opening of new pathways. The presented map can be used as a baseline for future CH4 flux studies in the Mackenzie Delta. References Kohnert, K.; Serafimovich, A.; Hartmann, J. and Sachs, T. [2014]: Airborne measurements of methane fluxes in alaskan and canadian tundra with the research aircraft “polar 5”. In Reports on Polar and Marine Research, volume 673. Alfred Wegener Institue Bremerhaven, pp. 81. Mauder, M.; Desjardins, R.L. and MacPherson, I. [2008]: Creating surface flux maps from airborne measurements: Application to the Mackenzie area GEWEX study MAGS 1999. Boundary-Layer Meteorology, 129:431–450, 2008. Metzger, S.; Junkermann, W.; Mauder, M.; Butterbach-Bahl, K.; Trancón y Widemann, B.; Neidl, F.; Schäfer, K.; Wieneke, S.; Zheng, X. H.; Schmid, H. P. and Foken, T. [2013]: Spatially explicit regionalization of airborne flux measurements using environmental response functions. Biogeosciences, 10(4):2193–2217, 2013. doi:10.5194/bg-10-2193-2013.