ALBERT

Description: Thermokarst lakes are typical features of the northern permafrost ecosystems, and play an important role in the thermal exchange between atmosphere and subsurface. The objective of this study is to describe the main thermal processes of the lakes and to quantify the heat exchange with the underlying sediments. The thermal regimes of five lakes located within the continuous permafrost zone of northern Siberia (Lena River Delta) were investigated using hourly water temperature and water level records covering a 3-year period (2009–2012), together with bathymetric survey data. The lakes included thermokarst lakes located on Holocene river terraces that may be connected to Lena River water during spring flooding, and a thermokarst lake located on deposits of the Pleistocene Ice Complex. Lakes were covered by ice up to 2 m thick that persisted for more than 7 months of the year, from October until about mid-June. Lake-bottom temperatures increased at the start of the ice-covered period due to upward-directed heat flux from the underlying thawed sediment. Prior to ice break-up, solar radiation effectively warmed the water beneath the ice cover and induced convective mixing. Ice break-up started at the beginning of June and lasted until the middle or end of June. Mixing occurred within the entire water column from the start of ice break-up and continued during the ice-free periods, as confirmed by the Wedderburn numbers, a quantitative measure of the balance between wind mixing and stratification that is important for describing the biogeochemical cycles of lakes. The lake thermal regime was modeled numerically using the FLake model. The model demonstrated good agreement with observations with regard to the mean lake temperature, with a good reproduction of the summer stratification during the ice-free period, but poor agreement during the ice-covered period. Modeled sensitivity to lake depth demonstrated that lakes in this climatic zone with mean depths 〉 5 m develop continuous stratification in summer for at least 1 month. The modeled vertical heat flux across the bottom sediment tends towards an annual mean of zero, with maximum downward fluxes of about 5 W m−2 in summer and with heat released back into the water column at a rate of less than 1 W m−2 during the ice-covered period. The lakes are shown to be efficient heat absorbers and effectively distribute the heat through mixing. Monthly bottom water temperatures during the ice-free period range up to 15 °C and are therefore higher than the associated monthly air or ground temperatures in the surrounding frozen permafrost landscape. The investigated lakes remain unfrozen at depth, with mean annual lake-bottom temperatures of between 2.7 and 4 °C.

Description: Boltzmann electron PIC simulation of the E-sail effect Annales Geophysicae, 33, 1507-1512, 2015 Author(s): P. Janhunen The solar wind electric sail (E-sail) is a planned in-space propulsion device that uses the natural solar wind momentum flux for spacecraft propulsion with the help of long, charged, centrifugally stretched tethers. The problem of accurately predicting the E-sail thrust is still somewhat open, however, due to a possible electron population trapped by the tether. Here we develop a new type of particle-in-cell (PIC) simulation for predicting E-sail thrust. In the new simulation, electrons are modelled as a fluid, hence resembling hybrid simulation, but in contrast to normal hybrid simulation, the Poisson equation is used as in normal PIC to calculate the self-consistent electrostatic field. For electron-repulsive parts of the potential, the Boltzmann relation is used. For electron-attractive parts of the potential we employ a power law which contains a parameter that can be used to control the number of trapped electrons. We perform a set of runs varying the parameter and select the one with the smallest number of trapped electrons which still behaves in a physically meaningful way in the sense of producing not more than one solar wind ion deflection shock upstream of the tether. By this prescription we obtain thrust per tether length values that are in line with earlier estimates, although somewhat smaller. We conclude that the Boltzmann PIC simulation is a new tool for simulating the E-sail thrust. This tool enables us to calculate solutions rapidly and allows to easily study different scenarios for trapped electrons.

Description: Coupling global models for hydrology and nutrient loading to simulate nitrogen and phosphorus retention in surface water – description of IMAGE–GNM and analysis of performance Geoscientific Model Development, 8, 4045-4067, 2015 Author(s): A. H. W. Beusen, L. P. H. Van Beek, A. F. Bouwman, J. M. Mogollón, and J. J. Middelburg The Integrated Model to Assess the Global Environment–Global Nutrient Model (IMAGE–GNM) is a global distributed, spatially explicit model using hydrology as the basis for describing nitrogen (N) and phosphorus (P) delivery to surface water, transport and in-stream retention in rivers, lakes, wetlands and reservoirs. It is part of the integrated assessment model IMAGE, which studies the interaction between society and the environment over prolonged time periods. In the IMAGE–GNM model, grid cells receive water with dissolved and suspended N and P from upstream grid cells; inside grid cells, N and P are delivered to water bodies via diffuse sources (surface runoff, shallow and deep groundwater, riparian zones; litterfall in floodplains; atmospheric deposition) and point sources (wastewater); N and P retention in a water body is calculated on the basis of the residence time of the water and nutrient uptake velocity; subsequently, water and nutrients are transported to downstream grid cells. Differences between model results and observed concentrations for a range of global rivers are acceptable given the global scale of the uncalibrated model. Sensitivity analysis with data for the year 2000 showed that runoff is a major factor for N and P delivery, retention and river export. For both N and P, uptake velocity and all factors used to compute the subgrid in-stream retention are important for total in-stream retention and river export. Soil N budgets, wastewater and all factors determining litterfall in floodplains are important for N delivery to surface water. For P the factors that determine the P content of the soil (soil P content and bulk density) are important factors for delivery and river export.

Description: Coupling aerosol optics to the chemical transport model MATCH (v5.5.0) and aerosol dynamics module SALSA (v1) Geoscientific Model Development Discussions, 8, 10735-10781, 2015 Author(s): E. Andersson and M. Kahnert Modelling aerosol optical properties is a notoriously difficult task due to the particles' complex morphologies and compositions. Yet aerosols and their optical properties are important for Earth system modelling and remote sensing applications. Operational optics models often make drastic and non realistic approximations regarding morphological properties, which can introduce errors. In this study a new aerosol optics model is implemented, in which more realistic morphologies and mixing states are assumed, especially for black carbon aerosols. The model includes both external and internal mixing of all chemical species, it treats externally mixed black carbon as fractal aggregates, and it accounts for inhomogeneous internal mixing of black carbon by use of a novel "core-grey shell" model. Simulated results of radiative fluxes, backscattering coefficients and the Ångström exponent from the new optics model are compared with results from another model simulating particles as externally mixed homogeneous spheres. To gauge the impact on the optical properties from the new optics model, the known and important effects from using aerosol dynamics serves as a reference. The results show that using a more detailed description of particle morphology and mixing states influences the optical properties to the same degree as aerosol dynamics. This is an important finding suggesting that over-simplified optics models coupled to a chemical transport model can introduce considerable errors; this can strongly effect simulations of radiative fluxes in Earth-system models, and it can compromise the use of remote sensing observations of aerosols in model evaluations and chemical data assimilation.

Description: Impact of environmental moisture on tropical cyclone intensification Atmospheric Chemistry and Physics, 15, 14041-14053, 2015 Author(s): L. Wu, H. Su, R. G. Fovell, T. J. Dunkerton, Z. Wang, and B. H. Kahn The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts relative to the storm path. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity that deforms the quasi-Lagrangian boundary of the storm and changes the moisture transport into the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. In contrast, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

Description: Exploring atmospheric blocking with GPS radio occultation observations Atmospheric Chemistry and Physics Discussions, 15, 35799-35822, 2015 Author(s): L. Brunner, A. K. Steiner, B. Scherllin-Pirscher, and M. W. Jury Atmospheric blocking has been closely investigated in recent years due to its impact on weather and climate, such as heat waves, droughts, and flooding. We use, for the first time, satellite-based observations from Global Positioning System (GPS) radio occultation (RO) and explore their ability to resolve blocking in order to potentially open up new avenues complementing models and re-analyses. RO delivers globally available and vertically high resolved profiles of atmospheric variables such as temperature and geopotential height (GPH). Applying a standard blocking detection algorithm we find that RO data robustly capture blocking as demonstrated for two well-known blocking events over Russia in summer 2010 and over Greenland in late winter 2013. During blocking episodes, vertically resolved GPH gradients show a distinct anomalous behavior compared to climatological conditions up to 300 hPa and sometimes even further up to the tropopause. The accompanied increase in GPH of up to 300 m in the upper troposphere yields a pronounced tropopause height increase. Corresponding temperatures rise up to 10 K in the middle and lower troposphere. These results demonstrate the feasibility and potential of RO to detect and resolve blocking and in particular to explore the vertical structure of the atmosphere during blocking episodes. This new observation-based view is available globally at the same quality so that also blocking in the Southern Hemisphere can be studied with the same reliability as in the Northern Hemisphere.

Description: Response of OH airglow emissions to the mesospheric gravity waves and its comparisons with full wave model simulation at a low latitude Indian station Atmospheric Chemistry and Physics Discussions, 15, 35881-35906, 2015 Author(s): R. N. Ghodpage, M. P. Hickey, A. Taori, D. Siingh, and P. T. Patil The quasi-monochromatic gravity wave induced oscillations, monitored using the mesospheric OH airglow emission over Kolhapur (16.8° N and 74.2° E), India during January to April 2010 and January to December 2011, have been characterized using the Krassovsky method. The nocturnal variability reveals prominent wave signatures with periods ranging from 5.2–10.8 h as the dominant nocturnal wave with embedded short period waves having wave periods 1.5–4.4 h. The results show that the magnitude of the Krassovsky parameter, viz., |η| ranged from 2.1 to 10.2 for principal or long nocturnal waves (5.2 to10.8 h observed periods), and, from 1.5 to 5.4 for the short waves (1.5 to 4.4 h observed periods) during the years of 2010 and 2011, respectively. The phase, i.e., Φ values of the Krassovsky parameters exhibited larger variability and varied from −8.1 to −167°. The deduced mean vertical wavelengths are found to be approximately −60.2 ± 20 and −42.8 ± 35 km for long and short wave periods for the year 2010. Similarly, for 2011 the mean vertical wavelengths are found to be approximately −77.6 ± 30 km and −59.2 ± 30 km for long and short wave periods, respectively, indicating that the observations over Kolhapur were dominated by upward propagating waves. We use a full wave model to simulate the response of OH emission to the wave motion and compare the results with observed values. In the present report, we discuss the observed wave characteristics and cause of the noted differences.

Description: Effectiveness of replacing catalytic converters in LPG-fueled vehicles in Hong Kong Atmospheric Chemistry and Physics Discussions, 15, 35939-35990, 2015 Author(s): X. P. Lyu, H. Guo, I. J. Simpson, S. Meinardi, P. K. K. Louie, Z. H. Ling, Y. Wang, M. Liu, C. W. Y. Luk, N. Wang, and D. R. Blake Many taxis and public buses are powered by liquefied petroleum gas (LPG) in Hong Kong. With more vehicles using LPG, they have become the major contributor to ambient volatile organic compounds (VOCs) in Hong Kong. An intervention program aimed to reduce the emissions of VOCs and nitrogen oxides (NO x ) from LPG-fueled vehicles was implemented by the Hong Kong Government in September 2013. Long-term real-time measurements indicated that the program was remarkably effective in reducing LPG-related VOCs, NO x and nitric oxide (NO) in the atmosphere. Receptor modeling results further revealed that propane, propene, i -butane, n -butane and NO in LPG-fueled vehicle exhaust emissions decreased by 37.3 ± 0.4, 50.2 ± 0.3, 32.9 ± 0.4, 41.1 ± 0.4 and 75.9 ± 0.3 %, respectively, during the implementation of the program. In contrast, despite the reduction of VOCs and NO x , the O 3 production following the program increased by 0.25 ± 0.04 ppbv h −1 (4.8 %). Moreover, the production rate of HO x decreased due to the reduction of VOCs, whereas NO reduction resulted in a more significant decrease of the HO x in destruction compared to the decrease in production, and an increase of hydroxyl (OH) and hydroperoxyl (HO 2 ). Analysis of O 3 -VOCs-NO x sensitivity in ambient air indicated VOC-limited regimes in the O 3 formation before and during the program. Moreover, a maximum reduction percentage of NO x (i.e., 29.4 %) and the lowest reduction ratio of VOCs / NO x (i.e., ~ 3 : 1) in LPG-fueled vehicle emissions were determined to give a zero O 3 increment. The findings are of great help to future formulation and implementation of control strategies on vehicle emissions in Hong Kong.

Description: Mercury dynamics and mass balance in a subtropical forest, southwestern China Atmospheric Chemistry and Physics Discussions, 15, 35857-35880, 2015 Author(s): M. Ma, D. Wang, H. Du, T. Sun, Z. Zhao, Y. Wang, and S. Wei The mid-subtropical forest area in southwest China was affected by anthropogenic mercury (Hg) emissions over the past three decades. We quantified mercury dynamics on the forest field and measured fluxes and pools of Hg in litterfall, throughfall, stream water and forest soil in an evergreen broad-leaf forest field in southwestern China. Total Hg (THg) input by the throughfall and litterfall were assessed at 32.2 and 42.9 μg m −2 yr −1 , respectively, which were obviously higher than those formerly observed from other forest fields in the background of North America and Europe. Hg fluxes across the soil/air interface (18.6 μg m −2 yr −1 ) and runoff/stream flow (7.2 μg m −2 yr −1 ) were regarded as the dominant ways for THg export from the forest field. The forest field hosts an enormous amount of atmospheric Hg, and its reserves were estimated to 25 341 μg m 2 . The ratio of output to input Hg fluxes (0.34) is higher comparing with other study sites. The higher output/input ratio may represent an important ecological risk for the downstream aquatic ecosystems, even if~the forest field could be an effective sink of Hg.