ALBERT

Description: Waterbodies in the arctic permafrost zone are considered a major source of the greenhouse gas methane (CH4). Spatial extrapolation of these CH4 fluxes to a region or the circum-Arctic, however, are still associated with large uncertainties. Here, we address this issue by using a combination of airborne CH 4 flux measurements and waterbody mapping based on TerraSAR-X and Sentinel-1 data across two study areas (1000 km2) in the Mackenzie Delta, Canada. Our results indicate that permafrost waterbodies, even if they seem to be strong emitters on an individual basis, do not necessarily translate into significant CH 4 emission hot spots on a regional scale. Our results show inconsistent patterns in the correlations between waterbody types and the CH 4 flux in the two study areas and across different spatial resolutions. Technical advances enabling the determination of the CH4 flux of individual waterbodies across a region provide a prospective direction to improve our understanding.

Description: The goal of this study is to scale aircraft measured fluxes of sensible and latent heat to the North Slope of Alaska and develop high resolution flux maps. For this purpose we analyzed an eddy-covariance data set obtained by the research aircraft POLAR 5 as part of the AIRMETH-2012 campaign, and investigated the spatial patterning of energy fluxes. Environmental response functions between flux observations and corresponding biophysical and meteorological drivers were estimated using a combination of time-frequency decomposition, dispersion modeling and machine learning. The extracted relationships are then used to scale observational data across heterogeneous Arctic landscapes, thus improving the spatial coverage and representativeness of the energy fluxes. Maps of projected energy fluxes are used to asses energy partitioning in northern ecosystems and to determine dominant energy exchange processes of permafrost area.

Description: A non‐eddy‐resolving microscale model is applied to simulate convection over three different leads (elongated channels in sea ice), which were observed by aircraft over the Arctic Marginal Ice Zone in 2013. The study aims to evaluate the quality of a local and a non‐local turbulence parametrization. The latter represents a lead‐width‐dependent approach for the turbulent fluxes designed for idealised conditions of a lead‐perpendicular, near‐neutral inflow in an atmospheric boundary layer (ABL) capped by a strong inversion at around 250 to 350 m height. The observed cases considered here are also characterised by an almost lead‐perpendicular flow but, in comparison to the idealised conditions, our analysis covers effects in stable inflow conditions and a much shallower ABL. The model simulations are initialised with observed surface parameters and upwind profiles, and the results are compared with measurements obtained above and downwind of the leads. The basic observed features related to the lead‐generated convection can be reproduced with both closures, but the observed plume inclination and vertical entrainment near the inversion layer by the penetrating plume are underestimated. The advantage of the non‐local closure becomes obvious by the more realistic representation of regions with observed vertical entrainment or where the observations hint at counter‐gradient transport. It is shown by comparison with the observations that results obtained with the non‐local closure can be further improved by including the determination of a fetch‐dependent inversion height and by specifying a parameter determining the plume inclination as a function of the upwind ABL stratification. Both effects improve the representation of fluxes, boundary‐layer warming, and vertical entrainment. The model is also able to reproduce the observed vanishing of a weak low‐level jet over the lead, but its downwind regeneration and related momentum transport are not always well captured, irrespective of the closure used.

Description: There is as yet no standard methodology for measuring wind gusts from a moving platform. To address this, we have developed a method to derive gusts from research aircraft data. First we evaluated four different approaches, including Taylor’s hypothesis of frozen turbulence, to derive the gust length scales that correspond to the gust time scales, namely, the gust duration (seconds) and the sample period (typically 10 min). The novelty of our method is in using peak factors (deviation of the gust from the mean wind speed normalized by the local turbulence) to convert between the scales. After devising a way to derive the gust length scales, we calculated the gust factors from aircraft observations and tested them against those from four parameterizations originally developed for weather stations. Three of them performed well (R2=0.66 or higher), while the fourth overestimated the gust factors in unstable conditions(R2=0.52). The mean errors for all methods were low, from -0.02 to 0.05, indicating that wind gust factors can indeed be measured from research aircraft. Moreover, we showed that aircraft can provide gust measurements within the whole boundary layer, if horizontal legs are flown at multiple levels over the same track. This is a significant advance, as gust measurements are usually limited to heights reached by weather masts. In unstable conditions over the open ocean the gust factor was nearly constant with height throughout the boundary layer, the near-surface values only slightly exceeding those at upper levels. Furthermore, we found gust factors to be strongly dependent on surface roughness conditions, which differed between the open ocean and sea ice in the Arctic marine environment. The roughness effect on the gust factor was stronger than the effect of boundary-layer stability.

Description: Leads are elongated channels in sea ice which play animportant role for theheat andmoisture exchange between the polar ocean and atmosphere. The aircraft campaign STABLE aimed to improve our current understanding of the formation of convective plumes over leads and their impact on the polar atmospheric boundary layer. It was carried out over the pack ice in the northern Fram Strait in March 2013.Wepresent case studies of the boundary layermodification and turbulent fluxes over four wide leads, which differed strongly with respect to lead characteristics and environmental conditions. The observed near-surface sensible heat fluxes ranged from15 to 180Wm−2. The leads also induced an increase of the near-surface temperature of up to 3.2 ◦C and a humidity increase of up to 0.2 g kg−1. In one of the cases, large entrainment fluxes exceeding 30% of the surface fluxes were observed. Vertical profiles of turbulent sensible heat and momentum fluxes were nonlinear downstream of the leads with a distinct flux maximum in the core of the convective plumes. In two cases, the plumes also strongly affected the wind field within the atmospheric boundary layer. Low-level jets that existed in those cases in the region upstream of the leads disappeared in the plume region. Finally, it is shown that large errors can occur when flux measurements are derived from lead orthogonal flight legs only. Therefore, complex flight patterns, as presented in this study, are necessary to accurately determine the energy fluxes in the environment of leads.