Wind slab formation in snow: experimental setup and first results.

Geophys . Res. Abstr. 18: EGU2016‐11274. The formation of wind-hardened surface layers, also known as wind slabs or wind crusts, is studied. Better knowledge about which processes and parameters are important will lead to an improved understanding of the mass balances in polar and alpine areas. It will also improve snow-cover models (i.e. SNOWPACK) as well as the forecast of avalanche danger. A ring-shaped wind tunnel has been built and instrumented. The facility is ring-shaped to simulate an infinitely long snow surface (infinite fetch). A SnowMicroPen (SMP) is used to measure the snow hardness. Other sensors measure environmental conditions such as wind velocity, air temperature, air humidity, the temperature of the snow and of the snow surface. A camera is used to detect drifting particles and to measure the Specific Surface Area (SSA) at the snow surface via near-infrared photography. First experiments indicate that mechanical fragmentation followed by sintering is the most efficient process to harden the surface. The hardness increased rapidly during drifting snow events, but only slowly or not at all when the wind speed was kept below the threshold for drifting snow. With drifting, the penetration resistance increased from the original 0.07 N to around 0.3 N in about an hour. Without drifting, a slow, further increase in resistance was observed. In about six hours, the hardness of the top 1-2 cm increased to 0.5 N. During this eight-hour experiment consisting of about two hours with intermittent drifting and six hours without drifting, the density at the surface increased from 66 kg/m3 to around 170 kg/m3. In the unaffected region close to the ground, the density increased from 100 kg/m3 to 110 kg/m3.

The Uncertainty of Seasonal Snow Energy- and Mass Balance Simulations as a Function of Typical Input Uncertainty

Mathias Thierry Pierre Bavay, Charles Fierz, Michael Lehning

The assessment of water resources and their change in time as well as many other applications from hydrology to meteorology require the successful modelling of the dynamics of the seasonal snow cover. Especially if extrapolation in time, e.g. for climate change scenario assessment, is aimed for, the use of energy balance models has been proven to be more trustworthy than simpler temperature index models. The disadvantage of the energy balance method is the requirement of a variety of input quantities, which are not always but become increasingly accessible. What is missing, however, is an in-depth assessment of the influence of typical errors in the individual input quantities on model performance. This contribution presents a sensitivity study, which compares the influence of typical errors and uncertainties in radiation, wind, temperature and humidity input, both measured and modeled, with errors caused by insufficient knowledge of solid precipitation rates. The analysis is carried out with the widely used SNOWPACK model and focuses on the build-up and melt of a mid-latitude seasonal snow cover. While mass input uncertainties still dominate the overall performance especially at high elevations (alpine zone), errors in wind and radiation can cause very significant effects as well. Wind is considered to be more critical in this context, because its spatial variation is both more pronounced and more difficult to estimate than the spatial variation in short- and longwave radiation. In absence of reliable radiation input, its value can successfully be calculated or parameterized if e.g. cloud information is available. Errors in air temperature input or a failure of a correct assessment of its spatial variability can be locally very important because of the feed-back mechanism via atmospheric stability. For the overall mass balance of a seasonal snow cover, humidity errors are less important. Notable exceptions are applications such as the correct assessment of surface hoar formation or prediction of melt rates during rain on snow events, for which good humidity input is required. A main conclusion is that input data through a combination of measurement stations and weather models are now widely available with sufficient quality to even allow a small-scale distributed application of energy balance models.

2013

Atmospheric heterogeneous reactions of chlorine and bromine containing molecules

Arnaud Aguzzi

This thesis deals with a laboratory study of heterogeneous reactions involving bromine and chlorine containing reservoir species on alkali salt substrates which are relevant to the marine boundary layer, and on ice substrates which are relevant to the lower stratosphere. The aim is to obtain reliable data on uptake coefficients γ and reaction mechanisms for the sake of extrapolation of results obtained in the laboratory to atmospheric conditions. The experiments have been performed in a Teflon coated Knudsen flow reactor equipped with a quadrupole mass spectrometer. The reactions of BrONO2 with solid alkali halides have been studied at ambient temperature. The initial uptake coefficients γ0 of BrONO2 on NaCl and KBr substrates are γ0 = 0.31 ± 0.10 and γ0 = 0.32 ± 0.09, respectively. For NaCl substrates BrCl, Br2 and HCl and for KBr both Br2 and HBr are observed as products. HCl and HBr result from the interaction of NaCl and KBr, respectively, with HNO3 generated in the hydrolysis of BrONO2 with H2O condensed on the salt sample. Using NaCl we observed Br2 which is formed from heterogeneous BrONO2 decomposition occurring on solid KBr samples. Halogen exchange reactions are competing with hydrolysis and decomposition which also take place on non-reactive salts (NaNO3, Na2SO4). A high rate of initial uptake (0.2 ≤ γ0 ≤ 0.4) and a Br2 yield on the order of 50 % are observed on non-reactive salts with a closed mass balance for Br2. The ClONO2-salt system has been reinvestigated. Experiments on NaCl, KBr (reactive salts) and on NaNO3 and Na2SO4 (non-reactive salts) have been performed. The hydrolysis of ClONO2 was not observable in contrast to heterogeneous decomposition which occurred on non-reactive salts with a Cl2 yield of (25 ± 5) %. Calculations have been performed using a 0-D box model in order to observe the effects of heterogeneous chemistry on sea salt in the marine boundary layer. Chlorine and bromine chemistry is activated by heterogeneous reactions occurring on NaCl and KBr which are the sole sources of chlorine and bromine in the model. Simulations performed with chlorine heterogeneous chemistry and gas phase chemistry indicate: 1) Denitrification of the troposphere (NOy converted to solid nitrates). 2) Increase of the oxidative capacity of the troposphere by the release of Cl atoms. When bromine chemistry is added to the model, the recombination reaction of NO2 with BrO leads to an increase of tropospheric denitrification through formation of BrONO2. Calculations show that at high [NOx], the most important bromine reservoir is BrONO2, while at low [NOx] HOBr is the most important bromine reservoir. The uptake of HNO3 on ice, solid sulfuric acid solutions and solid ternary solutions (STS) has been investigated at 180 to 211 K. The uptake coefficient γ decreases from 0.30 at 180 K to 0.06 at 211 K. The Arrhenius representation shows two distinct regimes. The first at low temperatures (180-190 K) shows a constant value γ of 0.3, whereas the second regime at T > 195 K corresponds to an activation energy of Ea = -7 ± 1 kcal/mol. At a fixed temperature, γ is independent of [HNO3] thus confirming a rate law first order in [HNO3]. The γ values of HNO3 on H2SO4/H2O solid solutions linearly decreases from 0.20 (0.10) at 10 wt % H2SO4 to 0.05 (0.03) at 98 wt % H2SO4 at 180 K (200K). The γ values of HNO3 with STS of H2SO4/HNO3/H2O is equal to 0.10 ± 0.03 in the temperature range 185-195 K at conditions where the composition of the interface was held constant at the given temperature by adding an external flow of H2O. Br2O hydrolysis on ice surfaces occurs rapidly in the chosen temperature range of 180 to 210 K. At a fixed temperature the uptake kinetics follow an apparent first order rate law in [Br2O]. The observed negative temperature dependence leads to an activation energy Ea for heterogeneous hydrolysis of -2.3 ± 0.6 kcal/mol. These facts point towards a complex reaction mechanism implying that the interaction of Br2O with ice is not an elementary reaction. BrONO2 hydrolysis has been measured on bulk (B), condensed (C) and single crystal (SC) ice in the temperature range 180-210 K. On all types of ice, HOBr and Br2O are observed as products. At a fixed temperature the rate law is first order in [BrONO2] with γ ≈ 0.3 at 180 K. The observed negative temperature dependence leads to an activation energy Ea for the hydrolysis of BrONO2 on pure ice of -2.0 ± 0.2, -2.1 ± 0.2 and -6.6 ± 0.3 kcal/mol on (C), (B) and (SC) ice, respectively, pointing towards different kinetics of BrONO2 on these different types of ice. Doping the ice samples with HBr leads to the formation of Br2, preceded by BrONO2 hydrolysis. The Arrhenius representation corresponds to an activation energy of Ea = -1.2 ± 0.2 kcal/mol. We investigated the structural properties of the near-surface region of various types of HX (X = Cl or Br) doped ice. Basically, we studied the time dependent [HX] in the interface region using the fast reaction titration XONO2 + HX → X2 + HNO3. These experiments reveal that HX is located near the surface of the substrate in a well defined region of thickness I. The HX molecules composing the interface are immediately available for the titration reaction, whatever the flow of XONO2. In the temperature range 190-200 K, we measured a value of IHCl = 43 ± 16, 250 ± 50 and 444 ± 120 nm for (SC), (C) and (B) ices, respectively and IHBr = 110 ± 15 nm on (C) ice at 205K. Finally, we assessed the bulk diffusion coefficient for HX, DHX by modeling our results according to Fick's laws of diffusion. We obtained values of DHCl = 4.5·10-15 - 1.0·10-12 cm2/s at 190 K and DHBr = (6.5 ± 3.0)·10-15 cm2/s at 205 K.

EPFL2001

Large-Eddy Simulation of the Convective Atmospheric Boundary Layer over Heterogeneous Land Surfaces

Chad Higgins, Daniel Nadeau, Eric Richard Pardyjak, Marc Parlange

The parameterization of the atmospheric boundary layer is essential for accurate numerical weather predictions. The near-surface values of air temperature or wind speed for instance are highly dependent on the complex land – atmosphere interactions over heterogeneous terrain. Over such surfaces, several open challenges remain regarding the growth of internal boundary layers, the determination of mixing layer heights and the spatial distribution of heat and momentum fluxes. The large-eddy simulation (LES) code we apply is based on the robust Lagrangian scale-dependent dynamic subgrid-scale model (Bou-Zeid et al., 2005, Physics of Fluids). The flow is driven by a mean pressure gradient expressed in terms of horizontal geostrophic winds. The code is pseudo-spectral with spectral decomposition in the horizontal dimensions. Land surface heterogeneities take the form of spatially distributed patches of surface temperature and roughness derived from satellite imagery and land use analysis. The simulation of the complete diurnal cycle is possible but we restrict our discussion to fully convective conditions. Data from LITFASS – 2003 (Lindenberg Inhomogeneous Terrain Fluxes between Atmosphere and Surface: a long-term Study) are used to validate the simulation results. The LES domain covers a 99-m meteorological tower with turbulent measurements and five surface micrometeorological stations. We investigate the presence of large-scale turbulent structures that are typical for daytime conditions with a particular interest in the blending height. Overall, this study illustrates how important is the accurate parameterization of heterogeneous terrain in studies of the atmospheric boundary layer.