Impact of grid spacing, convective parameterization and cloud microphysics in ICON simulations of a warm conveyor belt

Warm conveyor belts are important features of extratropical cyclones and are characterized by active diabatic processes. Previous studies reported that simulations of extratropical cyclones can be strongly impacted by the horizontal grid spacing. Here, we study to what extent and in which manner simulations of warm conveyor belts are impacted by the grid spacing. To this end, we investigate the warm conveyor belt (WCB) of the North Atlantic cyclone Vladiana that occurred around 23 September 2016 and was observed as part of the North Atlantic Waveguide and Downstream Impact Experiment. We analyze a total of 18 limited-area simulations with the ICOsahedral Nonhydrostatic (ICON) model run over the North Atlantic that cover grid spacings from 80 to 2.5 km, including those of current coarse-resolution global climate models with parameterized convection, as well as those of future storm-resolving climate models with explicit convection. The simulations also test the sensitivity with respect to the representation of convection and cloud microphysics. As the grid spacing is decreased, the number of WCB trajectories increases systematically, WCB trajectories ascend faster and higher, and a new class of anticyclonic trajectories emerges that is absent at 80 km. We also diagnose the impact of grid spacing on the ascent velocity and vorticity of WCB air parcels and the diabatic heating that these parcels experience. Ascent velocity increases at all pressure levels by a factor of 3 between the 80 and 2.5 km simulations, and vorticity increases by a factor of 2 in the lower and middle troposphere. We find a corresponding increase in diabatic heating as the grid spacing is decreased, arising mainly from cloud-associated phase changes in water. The treatment of convection has a much stronger impact than the treatment of cloud microphysics. When convection is resolved for grid spacings of 10, 5 and 2.5 km, the above changes to the WCB are amplified but become largely independent of the grid spacing. We find no clear connection across the different grid spacings between the strength of diabatic heating within the WCB and the deepening of cyclone Vladiana measured by its central pressure. An analysis of the pressure tendency equation shows that this is because diabatic heating plays a minor role in the deepening of Vladiana, which is dominated by temperature advection

Impact of grid spacing, convective parameterization and cloud microphysics in ICON simulations of a warm conveyor belt

Warm conveyor belts are important features of extratropical cyclones and are characterized by active diabatic processes. Previous studies reported that simulations of extratropical cyclones can be strongly impacted by the horizontal grid spacing. Here, we study to what extent and in which manner simulations of warm conveyor belts are impacted by the grid spacing. To this end, we investigate the warm conveyor belt (WCB) of the North Atlantic cyclone Vladiana that occurred around 23 September 2016 and was observed as part of the North Atlantic Waveguide and Downstream Impact Experiment. We analyze a total of 18 limited-area simulations with the ICOsahedral Nonhydrostatic (ICON) model run over the North Atlantic that cover grid spacings from 80 to 2.5 km, including those of current coarse-resolution global climate models with parameterized convection, as well as those of future storm-resolving climate models with explicit convection. The simulations also test the sensitivity with respect to the representation of convection and cloud microphysics. As the grid spacing is decreased, the number of WCB trajectories increases systematically, WCB trajectories ascend faster and higher, and a new class of anticyclonic trajectories emerges that is absent at 80 km. We also diagnose the impact of grid spacing on the ascent velocity and vorticity of WCB air parcels and the diabatic heating that these parcels experience. Ascent velocity increases at all pressure levels by a factor of 3 between the 80 and 2.5 km simulations, and vorticity increases by a factor of 2 in the lower and middle troposphere. We find a corresponding increase in diabatic heating as the grid spacing is decreased, arising mainly from cloud-associated phase changes in water. The treatment of convection has a much stronger impact than the treatment of cloud microphysics. When convection is resolved for grid spacings of 10, 5 and 2.5 km, the above changes to the WCB are amplified but become largely independent of the grid spacing. We find no clear connection across the different grid spacings between the strength of diabatic heating within the WCB and the deepening of cyclone Vladiana measured by its central pressure. An analysis of the pressure tendency equation shows that this is because diabatic heating plays a minor role in the deepening of Vladiana, which is dominated by temperature advection.</p

Coulomb gauge studies of SU(3) Yang-Mills theory on the lattice

We study the infrared behaviour of lattice SU(3) Yang-Mills theory in Coulomb gauge in terms of the ghost propagator, the Coulomb potential and the transversal and the time-time component of the equal-time gluon propagator. In particular, we focus on the Gribov problem and its impact on the observables. We observe that the simulated annealing method is advantageous for fixing the Coulomb gauge in large volumes. We study finite size and discretization effects. While finite size effects can be controlled by the cone cut, and the ghost propagator and the Coulomb potential become scaling functions with the cylinder cut, the equal-time gluon propagator does not show scaling in the considered range of the inverse coupling constant. The ghost propagator is infrared enhanced. The Coulomb potential is now extended to considerably lower momenta and shows a more complicated infrared regime. The Coulomb string tension satisfies Zwanziger's inequality, but its estimate can be considered only preliminary because of the systematic Gribov effect that is particularly strong for the Coulomb potential.Comment: 7 pages, 5 pictures, poster presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, Germany; corrected value for fitting parameter

Ice microphysical processes exert a strong control on the simulated radiative energy budget in the tropics

Simulations of the global climate system at storm-resolving resolutions of 2 km are now becoming feasible and show promising realism in clouds and precipitation. However, shortcomings in their representation of microscale processes, like the interaction of cloud droplets and ice crystals with radiation, can still restrict their utility. Here, we illustrate how changes to the ice microphysics scheme dramatically alter both the vertical profile of cloud-radiative heating and top-of-atmosphere outgoing longwave radiation (terrestrial infrared cooling) in storm-resolving simulations over the Asian monsoon region. Poorly-constrained parameters in the ice nucleation scheme, overactive conversion of ice to snow, and inconsistent treatment of ice crystal effective radius between microphysics and radiation alter cloud-radiative heating by a factor of four and domain-mean infrared cooling by 30 W m−2. Vertical resolution, on the other hand, has a very limited impact. Even in state-of-the-art models then, uncertainties in microscale cloud properties exert a strong control on the radiative budget that propagates to both atmospheric circulation and regional climate. These uncertainties need to be reduced to realize the full potential of storm-resolving models

Tug‐Of‐War on Idealized Midlatitude Cyclones Between Radiative Heating From Low‐Level and High‐Level Clouds

We present baroclinic life-cycle simulations with two versions of the atmosphere model ICON to understand how cloud-radiative heating and cooling affect an idealized midlatitude cyclone. Both versions simulate the same cyclone when run without radiation, but disagree when cloud-radiation-interaction is taken into account. The radiative effects of clouds weaken the cyclone in ICON2.1 but strengthen it in ICON2.6. We attribute the disagreement to low-level clouds, which in ICON2.1 are more abundant and show stronger radiative cooling of the boundary layer. We argue that radiative cooling from low-level cloud tops weakens the cyclone by increasing boundary-layer static stability, and that radiative cooling from high-level cloud tops strengthens the cyclone by decreasing static stability in the upper troposphere and sharpening the tropopause. Our results indicate that clouds and the vertical distribution of their radiative heating and cooling can influence the dynamics of midlatitude cyclones

Tropical cloud-radiative changes contribute to robust climate change-induced jet exit strengthening over Europe during boreal winter

The North Atlantic jet stream is projected to extend eastward towards Europe in boreal winter in response to climate change. We show that this response is robust across a hierarchy of climate models and climate change scenarios. We further show that cloud-radiative changes contribute robustly to the eastward extension of the jet stream in three atmosphere models, but lead to model uncertainties in the jet stream response over the North Atlantic. The magnitude of the cloud contribution depends on the model, consistent with differences in the magnitude of changes in upper-tropospheric cloud-radiative heating. We further study the role of regional cloud changes in one of the three atmosphere models, i.e. the ICON model. Tropical cloud-radiative changes dominate the cloud impact on the eastward extension of the jet stream in ICON. Cloud-radiative changes over the Indian Ocean, western tropical Pacific, and eastern tropical Pacific contribute to this response, while tropical Atlantic cloud changes have a minor impact. Our results highlight the importance of upper-tropospheric tropical clouds for the regional circulation response to climate change over the North Atlantic-European region and uncertainty therein

Cloud‐Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming

Previous work demonstrated the strong radiative coupling between clouds and the mid‐latitude circulation. Here, we investigate the impact of cloud‐radiative changes on the global warming response of the mid‐latitude jet streams and storm tracks in the North Atlantic, North Pacific and Southern Hemisphere. To this end, we use the ICON global atmosphere model in present‐day setup and with the cloud‐locking method. Sea surface temperatures (SST) are prescribed to isolate the circulation response to atmospheric cloud‐radiative heating. In the annual mean, cloud‐radiative changes contribute one‐ to two‐thirds to the poleward jet shift in all three ocean basins, and support the jet strengthening in the North Atlantic and Southern Hemisphere. Cloud‐radiative changes also impact the storm track, but the impact is more diverse across the three ocean basins. The cloud‐radiative impact on the North Atlantic and North Pacific jets varies little from season to season in absolute terms, whereas its relative importance changes over the course of the year. In the Southern Hemisphere, cloud‐radiative changes strengthen the jet in all seasons, whereas their impact on the jet shift is limited to austral summer and fall. The cloud‐radiative impact is largely zonally‐symmetric and independent of whether global warming is mimicked by a uniform 4 K or spatially‐varying SST increase. Our results emphasize the importance of cloud‐radiative changes for the response of the mid‐latitude circulation to global warming, indicating that clouds can contribute to uncertainty in model projections of future circulations

A climate network perspective on the intertropical convergence zone

The intertropical convergence zone (ITCZ) is an important component of the tropical rain belt. Climate models continue to struggle to adequately represent the ITCZ and differ substantially in its simulated response to climate change. Here we employ complex network approaches, which extract spatiotemporal variability patterns from climate data, to better understand differences in the dynamics of the ITCZ in state-of-the-art global circulation models (GCMs). For this purpose, we study simulations with 14 GCMs in an idealized slab-ocean aquaplanet setup from TRACMIP – the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project. We construct network representations based on the spatial correlation patterns of monthly surface temperature anomalies and study the zonal-mean patterns of different topological and spatial network characteristics. Specifically, we cluster the GCMs by means of the distributions of their zonal network measures utilizing hierarchical clustering. We find that in the control simulation, the distributions of the zonal network measures are able to pick up model differences in the tropical sea surface temperature (SST) contrast, the ITCZ position, and the strength of the Southern Hemisphere Hadley cell. Although we do not find evidence for consistent modifications in the network structure tracing the response of the ITCZ to global warming in the considered model ensemble, our analysis demonstrates that coherent variations of the global SST field are linked to ITCZ dynamics. This suggests that climate networks can provide a new perspective on ITCZ dynamics and model differences therein