Linking Lagrangian model simulations with stable water isotope measurements in Arctic weather systems.

<p>Transport of water from an evaporation source towards a precipitation sink is the essence of the atmospheric water cycle. However, there are significant challenges with the representation of the atmospheric water cycle in models. For example, incomplete representation of sub-grid scale processes like evaporation, mixing or precipitation can lead to substantial model errors. Here we investigate the combined use of Lagrangian and Eulerian models and in-situ observations of stable water isotopes to reduce such sources of model error. The atmospheric water cycle in the Nordic Seas during cold air outbreaks (CAOs) is confined to a limited area, and thus may be used as a natural laboratory for hydrometeorological studies. We apply Lagrangian and Eulerian models together with observations taken during the ISLAS2020 field campaign in the Arctic in spring 2020 for characterising source-sink relationships in the water cycle. During the field campaign, we observed an alternating sequence of cold air outbreaks (CAO) and warm air intrusions (WAI) over the key measurement sites of Svalbard and northern Norway. Thereby, meteorological and stable water isotope measurements have been performed at multiple sites both upstream and downstream of the CAOs and WAIs. The Lagrangian model FLEXPART has been run with the input data from the regional convection-permitting numerical weather prediction model AROME Arctic at 2.5 km resolution to investigate transport patterns. The combination of observations and model simulations allows us to quantify the connection between source and sink for different weather systems, as well as the link between large-scale transport and stable water isotopes. Findings will lead to a better understanding of processes in the water cycle and the degree of conservation of isotopic signals during transport. This study may also serve as a guideline on how to evaluate the performance of Lagrangian transport models using stable water isotope measurements, and on how to detect constraints for quantifying the transport route and evaporation source from stable water isotope measurements for future work, including an aircraft campaign planned in 2021.</p>

Using an antidiffusive transport scheme for vertical transport: a promising path towards solving the long-standing problem of excessive vertical diffusion in Eulerian models

<p>A well-known drawback of Eulerian models is excessive numerical diffusion of the transported species. This applies to chemical species but also to water vapor. We present a new way of dealing with this problem in the vertical direction by using the Després and Lagoutière (1999) scheme – hereinafter DL99, a simple 1<sup>st</sup>-order advection scheme with antidiffusive properties. These authors have only studied this scheme in the context of 1d transport. Here we test the applicability of this scheme in atmospheric modelling by applying it to tracer transport in the vertical direction in idealized 2d (zonal-vertical) atmospheric circulations and quantify the gain compared to classical advection schemes. <br><br>In this idealized framework, we have tested the efficiency of DL99 in two cases representative of important situations for atmospheric transport : <br>- Formation of a thin plume from an initially thick column of tracer (e.g. volcanic plume, biomass burning plume etc.) under the effect of zonal wind shear in presence of large-scale variations in vertical wind <br>- Long-range advection of a thin polluted layer under the action of zonal wind in presence of large scale oscillations in the vertical wind. <br><br>In these idealized case studies, we show that using DL99 in the vertical direction yields dramatically improved performance compared to any other scheme we have tested, including the 3<sup>rd</sup>-Order Piecewise Parabolic Method (PPM). As an example, in a simulation with a vertical resolution of 500m and a zonal resolution of 25 km initialized by a 1000m-thick, zonally uniform layer of tracer, after being transported horizontally over 2000km over 48 hours by a uniform zonal wind ~11.5ms<sup>-1</sup> together with an oscillating vertical wind of +-0.05ms<sup>-1</sup>, maximal tracer concentration at the end of the simulation with DL99 is 94% of its initial value instead of 51% with PPM, l<sup>2</sup> relative error is 10% instead of 61%, and 92% of the tracer mass is still confined in the correct 1000m-thick envelope instead of 50%.<br><br>This and other numerical experiments shows that, by design, DL99 reduces numerical diffusion, but it also proves it to be able to preserve the areas of uniform tracer concentration even if these areas cover only a very small number of cells in the vertical direction. We argue that this unique set of properties, along with the simplicity of its formulation and its minimal computational cost make the DL99 an extremely attractive candidate for a robust and non-diffusive representation of vertical advection in Eulerian meteorological and chemistry-transport models. This scheme has been implemented in the state-of-the-art CHIMERE chemistry-transport model (Mailler et al., 2017), and we have shown that it brings a clear improvement in the representation of the structure of a volcanic plume from the Etna volcano (Lachâtre et al., 2020, Atmos. Chem. Phys.)</p><p>Main references:</p><p>Després, B., and F. Lagoutière, 1999, Un schéma non linéaire anti-dissipatif pour l'équation d'advection linéaire, <em>Comptes Rendus de l’Académie des Sciences</em></p>

The firn meltwater Retention Model Intercomparison Project (RetMIP): Evaluation of nine firn models at four weather station sites on the Greenland ice sheet

Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain part of the surface meltwater, buffering the ice sheet’s contribution to sea level. Multi-layer firn models are traditionally used to simulate the firn processes and estimate meltwater retention. We present the output from nine firn models, forced by weather-station-derived mass and energy fluxes at four sites representative of the dry snow, percolation, ice slab and firn aquifer areas. We compare the model outputs and evaluate them against in situ observations. Models that explicitly account for deep meltwater percolation overestimate percolation depth and consequently firn temperature at the percolation and ice slab sites although they accurately simulate the recharge of the firn aquifer. Models using Darcy's law and a bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models, that transfer firn through fixed layers, smooth sharp gradients in firn temperature and density over time. From the model spread, we find that simulated densities (respectively temperature) have an uncertainty envelope of ±60 kg m−3 (resp. ±14 °C) in the dry snow area and up to ±280 kg m−3 (resp. ±15–18 °C) at warmer sites.

Thermo-Fluidic Characteristics of Two-Phase Ice Slurry Flows Based on Comparative Numerical Methods

Ice slurry is a potential secondary refrigerant for commercial refrigeration systems because of its remarkable thermal properties. It is necessary to optimize the heat transfer process of ice slurry to reduce the energy consumption of the refrigeration system. Thus, this study investigates the heat transfer performance of single-phase (aqueous solution) and two-phase (ice slurry) refrigerants in a straight horizontal tube. The numerical simulations for ice slurry were performed with ice mass fraction ranging from 5% to 20%. The effects of flow velocity and ice concentration on the heat transfer coefficient were examined. The results showed that heat transfer coefficient of ice slurry is considerably higher than those of single-phase flow, particularly at high flow velocity and ice content, where increase in heat transfer with a factor of two was observed. The present results confirmed that ice slurry heat transfer ability is considerably affected by flow velocity and ice concentration in laminar range. Moreover, the second part of this paper reports on the credibility three distinct two-phase Eulerian–Eulerian models (volume of fluid (VOF), mixture, and Eulerian) for the experimental conditions reported in the literature. All two-phase models accurately predict the thermal field at low ice mass fraction but underestimate that at high ice mass fractions. Regardless of the thermal discrepancies, the Eulerian–Eulerian models provide quite reasonable estimation of pressure drop with reference to experimental data. The numerical predictions from the VOF model are more accordant with the experimental results and the maximum percentage error is limited to ~20% and ~13% for thermal and pressure drop predictions, respectively.