Variability within cold air outbreaks and implications for parametrization

The changing structure of the marginal sea-ice zone (MIZ), together with high temperature gradients between the cold Arctic air and relatively warm sea water, contribute to uncertainty in numerical weather prediction (NWP). Since cold-air outbreaks (CAO) over the MIZ contribute strongly to heat transfer in the polar areas, assessment of variability in them is of a great importance. This thesis deals with extending our understanding of the variability within CAO by means of large eddy simulations, performed in The Met Office Large Eddy Model (LEM). The novel approach of this study lies within: Firstly, introducing three different patterns of heterogeneity in surface temperatures that represent conditions in MIZs; secondly, investigating both the spatial and temporal variability in the developing boundary-layer convection. A set of idealised scenarios and a case study are analysed. The case study is performed for a weak CAO event observed during ACCACIA field campaign on 21 March 2013. The study shows a profound impact of the inhomogeneous surface on both the spatial organisation of the boundary-layer convection and the latent heat flux at the surface and the boundary layer. The effect of heterogeneities depends strongly on the wind shear, the size and the orientation of the heterogeneity, and the initial stratification. When active cumuli clouds form, the effect of heterogeneities quickly diminish due to a top-driven mixing. In a stronger stratification that inhibits cumulus formation, the effect of heterogeneity is maintained. Stripes of temperature anomalies parallel to mean wind direction drive the formation of forced convective rolls and facilitate significantly higher latent heat flux. The impact of this heterogeneity usually increases with increased wind-shear, while the impact of other heterogeneities generally decreases. The impact of temperature heterogeneity is generally stronger than the impact of varying ice roughness. The implications for the parametrization of convective boundary layer are discussed

Aerosol impacts on the entrainment efficiency of Arctic mixed-phase convection in a simulated air mass over open water

International audienceAbstract. Springtime Arctic mixed-phase convection over open water in the Fram Strait as observed during the recent ACLOUD (Arctic CLoud Observations Using airborne measurements during polar Day) field campaign is simulated at turbulence-resolving resolutions. The first objective is to assess the skill of large-eddy simulation (LES) in reproducing the observed mixed-phase convection. The second goal is to then use the model to investigate how aerosol modulates the way in which turbulent mixing and clouds transform the low-level air mass. The focus lies on the low-level thermal structure and lapse rate, the heating efficiency of turbulent entrainment, and the low-level energy budget. A composite case is constructed based on data collected by two research aircraft on 18 June 2017. Simulations are evaluated against independent datasets, showing that the observed thermodynamic, cloudy, and turbulent states are well reproduced. Sensitivity tests on cloud condensation nuclei (CCN) concentration are then performed, covering a broad range between pristine polar and polluted continental values. We find a significant response in the resolved mixed-phase convection, which is in line with previous LES studies. An increased CCN substantially enhances the depth of convection and liquid cloud amount, accompanied by reduced surface precipitation. Initializing with the in situ CCN data yields the best agreement with the cloud and turbulence observations, a result that prioritizes its measurement during field campaigns for supporting high-resolution modeling efforts. A deeper analysis reveals that CCN significantly increases the efficiency of radiatively driven entrainment in warming the boundary layer. The marked strengthening of the thermal inversion plays a key role in this effect. The low-level heat budget shifts from surface driven to radiatively driven. This response is accompanied by a substantial reduction in the surface energy budget, featuring a weakened flow of solar radiation into the ocean. Results are interpreted in the context of air–sea interactions, air mass transformations, and climate feedbacks at high latitudes

Surface impacts and associated mechanisms of a moisture intrusion into the Arctic observed in mid-April 2020 during MOSAiC

Distinct events of warm and moist air intrusions (WAIs) from mid-latitudes have pronounced impacts on the Arctic climate system. We present a detailed analysis of a record-breaking WAI observed during the MOSAiC expedition in mid-April 2020. By combining Eulerian with Lagrangian frameworks and using simulations across different scales, we investigate aspects of air mass transformations via cloud processes and quantify related surface impacts. The WAI is characterized by two distinct pathways, Siberian and Atlantic. A moist static energy transport across the Arctic Circle above the climatological 90th percentile is found. Observations at research vessel Polarstern show a transition from radiatively clear to cloudy state with significant precipitation and a positive surface energy balance (SEB), i.e., surface warming. WAI air parcels reach Polarstern first near the tropopause, and only 1–2 days later at lower altitudes. In the 5 days prior to the event, latent heat release during cloud formation triggers maximum diabatic heating rates in excess of 20 K d-1. For some poleward drifting air parcels, this facilitates strong ascent by up to 9 km. Based on model experiments, we explore the role of two key cloud-determining factors. First, we test the role moisture availability by reducing lateral moisture inflow during the WAI by 30%. This does not significantly affect the liquid water path, and therefore the SEB, in the central Arctic. The cause are counteracting mechanisms of cloud formation and precipitation along the trajectory. Second, we test the impact of increasing Cloud Condensation Nuclei concentrations from 10 to 1,000 cm-3 (pristine Arctic to highly polluted), which enhances cloud water content. Resulting stronger longwave cooling at cloud top makes entrainment more efficient and deepens the atmospheric boundary layer. Finally, we show the strongly positive effect of the WAI on the SEB. This is mainly driven by turbulent heat fluxes over the ocean, but radiation over sea ice. The WAI also contributes a large fraction to precipitation in the Arctic, reaching 30% of total precipitation in a 9-day period at the MOSAiC site. However, measured precipitation varies substantially between different platforms. Therefore, estimates of total precipitation are subject to considerable observational uncertainty

DALES - Dutch Atmospheric Large Eddy Simulation

The version of DALES used on Fugaku for the Cloud Botany dataset

Constraints on simulated past Arctic amplification and lapse rate feedback from observations

Abstract. The Arctic has warmed more rapidly than the global mean during the past few decades. The lapse rate feedback (LRF) has been identified as being a large contributor to the Arctic amplification (AA) of climate change. This particular feedback arises from the vertically non-uniform warming of the troposphere, which in the Arctic emerges as strong near-surface and muted free-tropospheric warming. Stable stratification and meridional energy transport are two characteristic processes that are evoked as causes for this vertical warming structure. Our aim is to constrain these governing processes by making use of detailed observations in combination with the large climate model ensemble of the sixth Coupled Model Intercomparison Project (CMIP6). We build on the result that CMIP6 models show a large spread in AA and Arctic LRF, which are positively correlated for the historical period of 1951–2014. Thus, we present process-oriented constraints by linking characteristics of the current climate to historical climate simulations. In particular, we compare a large consortium of present-day observations to co-located model data from subsets that show a weak and strong simulated AA and Arctic LRF in the past. Our analyses suggest that the vertical temperature structure of the Arctic boundary layer is more realistically depicted in climate models with weak (w) AA and Arctic LRF (CMIP6/w) in the past. In particular, CMIP6/w models show stronger inversions in the present climate for boreal autumn and winter and over sea ice, which is more consistent with the observations. These results are based on observations from the year-long Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic, long-term measurements at the Utqiaġvik site in Alaska, USA, and dropsonde temperature profiling from aircraft campaigns in the Fram Strait. In addition, the atmospheric energy transport from lower latitudes that can further mediate the warming structure in the free troposphere is more realistically represented by CMIP6/w models. In particular, CMIP6/w models systemically simulate a weaker Arctic atmospheric energy transport convergence in the present climate for boreal autumn and winter, which is more consistent with fifth generation reanalysis of the European Centre for Medium-Range Weather Forecasts (ERA5). We further show a positive relationship between the magnitude of the present-day transport convergence and the strength of past AA. With respect to the Arctic LRF, we find links between the changes in transport pathways that drive vertical warming structures and local differences in the LRF. This highlights the mediating influence of advection on the Arctic LRF and motivates deeper studies to explicitly link spatial patterns of Arctic feedbacks to changes in the large-scale circulation. </jats:p

dalesteam/dales: v4.3 for Cloud Botany on Fugaku

The version of DALES used on Fugaku for the Cloud Botany dataset. Adds support for the Fujitsu compiler for running on Fugaku, and several optimizations to version 4.3: single precision support better vectorization faster thermodynamics Most of these changes have subsequently been merged into the official v4.4 release.</p

franslql/dales: DALES 4.4 with open boundary conditions

&lt;p&gt;DALES 4.4 code with open boundary conditions and scripts for publication &quot;Open Boundary Conditions for Atmospheric Large Eddy Simulations and their implementation in DALES4.4&quot;.&lt;/p&gt

DALES - Dutch Atmospheric Large Eddy Simulation

The version of DALES used on Fugaku for the Cloud Botany dataset