Ice nucleating particle concentrations in Dust Regional Atmospheric Model (DREAM) – going one step further

<p>Mineral dust particles in the atmosphere have a large influence on the physical properties of clouds and their lifecycle. Findings from field experiments, modeling, and laboratory studies suggest that mineral dust particles are very efficient ice-nucleating particles (INPs) even in regions distant from the desert sources. The major sources of mineral dust present in the Mediterranean basin are located in the Sahara Desert. Understanding the significance of mineral dust in ice initiation led to the development of INPC parameterizations in presence of dust for immersion freezing and deposition nucleation processes. These parameterizations were mineralogically indifferent, estimating the dust ice nucleating particle concentrations (INPCs) based on dust concentration and thermodynamic parameters. In recent studies, feldspar and quartz minerals have shown to be significantly more efficient INPs than other minerals found in dust. These findings led to the development of mineralogy-sensitive immersion freezing parameterizations. In this study, we implement mineralogy-sensitive and mineralogically-indifferent INPC parameterizations into a regional coupled atmosphere-dust numerical model. We use the Dust Regional Atmospheric Model (DREAM) to perform one month of simulations of the atmospheric cycle of dust and its feldspar and quartz fractions during Saharan dust intrusion events in the Mediterranean. EARLINET (European Aerosol Lidar Network) and AERONET (AErosol RObotic NETwork) measurements are used with POLIPHON algorithm (Polarization Lidar Photometer Networking) to derive cloud-relevant dust concentration profiles. We compare DREAM results with lidar-based vertical profiles of dust mass concentration, surface area concentration, number concentration, and INPCs. This analysis is a step towards the systematic analysis of dust concentration and INPC parameterizations performance when compared to lidar derived vertical profiles.</p>

Atmospheric response to SST tendancy in the Eastern topical Atlantic in July-August

<p><span>The objective of this work is to understand how the seasonal tend</span><span>ance</span><span>s of the tropical Atlantic SST influence the migration of the Intertropical Convergence Zone (ITCZ) and the West African precipitation associated with it. For this we carried out different sensitivity tests to the SST, climatological, with the regional atmospheric model WRF-ARW. Our results, based on the July-August period, show a strong influence of SST anomalies in the Dakar Nino (DN) and </span><span>Atlantic </span><span>cold tongue (</span><span>ACT</span><span>) regions on the marine ITCZ and West African precipitation. Above the ocean, the cooling of the tropical northeast Atlantic induces a strong reduction in precipitation north of 10°N, associated with the southward displacement of the ITCZ which is located between 5°-10°</span><span>N </span><span>with a slight increase in rains. On the other hand, the warming of the SST of the tropical south-eastern Atlantic induces an increase in marine precipitation</span><span>s</span><span>, with a maximum centered on 5°N, explained by the location of the ITCZ </span><span>f</span><span>urther south than that associated with the cooling in the region of DN. On the continent, the influence of these SST tend</span><span>ance</span><span>s is characterized by the presence of a zonal dipole of rainfall anomalies over the Sahelian regions. The SST cooling effect in the DN region is more marked in the western Sahel, particularly in Senegal, with a sharp drop in rainfall in this region. While that of warming in the LEF region is more marked in the Sahel, which also induces a strong reduction in the intensity of the rains in this region. However, the combined experience of these two type anomalies shows a dipole of rainfall anomalies over the ocean and over the continent. This dipole is characterized by a decrease (increase) in Sahelian (Guinean) rainfall. Our results also show that, for all simulations, the increase (reduction) in precipitation is more explained by the convective (non-convective) part of the rain. The influence of the SST of DN contributes 40% to 100% on the decrease in rainfall in the West Sahel, while the SST of the </span><span>ACT</span><span> reduces rainfall in the eastern Sahel by 40% to 100%. Thus, this work underlines the importance of taking into account the effect of the seasonal anomaly of the SST of DN on Sahelian precipitation</span><span>s</span><span> in forecasting models.</span></p>

Simulating Weather Events with a Linked Atmosphere-Hydrology Model

This study aims at assess the importance of a conceptual representation of hydrological processes when modelling atmospheric circulation. It compares results from a regional atmospheric model that interprets land surface hydrological processes based on parameterizations with results from a two-way coupled atmosphere-hydrological model that has a process-based approach to the land surface hydrological cycle. These numerical models were applied to a region covering the Rio Grande basin, Brazil. The same input data, initial and boundary conditions were used on a 31-day simulation period. Results obtained from these simulations were compared to visible satellite images and gauging rainfall stations for three case studies that included a cold front, deep convective clouds and stable atmospheric conditions. Both models could reproduce regional patterns of air circulation and rainfall influenced by the orography of the basin. However, atmospheric processes driven by spatial gradients of land surface temperature or local surface heating were spatially better represented by the atmospheric-hydrological modelling system rather than the regional atmospheric model. Since areas characterized by spatial gradients of land surface temperature and local surface heating were closely associated with convergent air flows near land surface and strong vertical motion in the mid troposphere, this finding enhanced the role of a good representation of land surface hydrological processes for a better modelling the atmospheric dynamics.

Transition of low clouds in the East China Sea and Kuroshio region in winter: A regional atmospheric model study

<p>Bias in simulating the stratocumulus-to-cumulus transition remains a main source of uncertainties in regional climate projection and can significantly affect the energy budget in climate models. To gain insights into the transition, this study investigates the cloud transition forced by the sea surface temperature (SST) front and synoptic disturbances in the East China Sea and Kuroshio region in winter based on both observations and regional atmospheric model simulations. The Kuroshio SST front greatly accelerates cloud transition by enhancing surface turbulent heat flux, marine atmospheric boundary layer (MABL) dynamical adjustment and cloud-top entrainment. With the sharp SST increase from the cold flank to the Kuroshio SST warm tongue (KWT), surface wind convergence (SWC) over the KWT induced by the SST front and synoptic disturbances[Office1]  enhances the coupling between the cloud layer and subcloud layer. An underlying positive feedback between the SWC and latent heating in the cloud layer can enhance abrupt change in cloud properties and maintain cloud band over the KWT against the decoupling through the so-called “Deepening-Warming” mechanism induced by latent heating. From the KWT downwind southward, the surface layer turbulent mixing weakens, while latent heating in the cloud layer and cloud-top longwave radiative cooling enhance buoyancy and vertical mixing in the cloud layer. This difference in vertical mixing between the cloud layer and subcloud layer facilitates the MABL decoupling and impedes upward moisture transport. Meanwhile, decreasing lower tropospheric stability is conducive to the entrainment of drier and warmer air from above into the cloud layer, strengthening cloud evaporation.</p>

The role of spatial and temporal model resolution in a flood event storyline approach in Western Norway

<p><span>A physical climate storyline approach is applied to an autumn flood event caused by an atmospheric river in the West Coast of Norway. The aim is to demonstrate the value and challenges of higher spatial and temporal resolution in simulating impacts. The modelling chain used is the same as the one used operationally, to issue flood warnings for example. Its output is therefore familiar to many users, which we expect will facilitate stakeholder engagement. Two different versions of a hydrological model are run to show that on the one hand, the higher spatial resolution between the global and regional model is necessary to realistically simulate the high spatial variability of precipitation in such a mountainous region. On the other hand we also show that the intensity of the peak streamflow is only captured realistically with hourly data. The higher resolution regional atmospheric model is able to simulate the fact that with the passage of an atmospheric river, some valleys receive high amounts of precipitation and others not. However, the coarser resolution global model shows uniform precipitation in the whole region. Translating the event into the future leads to similar results: while in some catchments, a future flood might be 50% larger than a present one, in others no event occurs because the atmospheric river does not hit that catchment.</span></p>