Toward Eliminating the Decades-Old "Too Zonal and Too Equatorward" Storm-Track Bias in Climate Models.

Generations of climate models exhibit biases in their representation of North Atlantic storm tracks, which tend to be too far near the equator and too zonal. Additionally, models have difficulties simulating explosive cyclone growth. These biases are one of the reasons for the uncertainties in projections of future climate over Europe, and the underlying causes have yet to be determined. All three biases are shown to be related, and diabatic processes are pointed to as a likely cause. To demonstrate this, two hemispherically symmetric storm tracks forming downstream of an idealized sea surface temperature (SST) front on an aquaplanet are examined using the seamless ICOsahedral Non-hydrostatic weather and climate model (ICON) and its grid refinement capabilities. The analyzed perpetual boreal winter has a global grid spacing of 20 km, two bi-directionally interacting grid nests over the Northern Hemisphere that refine the grid to 10-km spacing over much of the stormtrack and further to 5-km spacing near the SST front. In contrast, no grid refinement is performed for the Southern Hemisphere. Feature-based cyclone tracking shows that the poleward propagation in the NH is enhanced, so the high-resolution storm track is less equatorward and less zonal; explosive deepening rates are more frequent and precipitation rates are amplified. The implication is that resolving diabatic processes on the storm scale improves all three intersecting biases in the representation of storm tracks. While new challenges arise at cloud resolving scales, much improvement for the representation of storm tracks will be gained once climate models resolve the meso-γ scale.

Global forestation and deforestation affect remote climate via adjusted atmosphere and ocean circulation.

Forests can store large amounts of carbon and provide essential ecosystem services. Massive tree planting is thus sometimes portrayed as a panacea to mitigate climate change and related impacts. Recent controversies about the potential benefits and drawbacks of forestation have centered on the carbon storage potential of forests and the local or global thermodynamic impacts. Here we discuss how global-scale forestation and deforestation change the Earth's energy balance, thereby affect the global atmospheric circulation and even have profound effects on the ocean circulation. We perform multicentury coupled climate model simulations in which preindustrial vegetation cover is either completely forested or deforested and carbon dioxide mixing ratio is kept constant. We show that global-scale forestation leads to a weakening and poleward shift of the Northern mid-latitude circulation, slows-down the Atlantic meridional overturning circulation, and affects the strength of the Hadley cell, whereas deforestation leads to reversed changes. Consequently, both land surface changes substantially affect regional precipitation, temperature, clouds, and surface wind patterns across the globe. The design process of large-scale forestation projects thus needs to take into account global circulation adjustments and their influence on remote climate.

Quantifying the circulation induced by convective clouds in kilometer-scale simulations.

The complex coupling between the large-scale atmospheric circulation, which is explicitly resolved in modern numerical weather and climate models, and cloud-related diabatic processes, which are parameterized, is an important source of error in weather predictions and climate projections. To quantify the interactions between clouds and the large-scale circulation, a method is employed that attributes a far- and near-field circulation to the cloud system. The method reconstructs the cloud-induced flow based on estimates of vorticity and divergence over a limited domain and does not require the definition of a background flow. It is subsequently applied to 12- and 2-km simulations of convective clouds, which form within the large-scale cloud band ahead of the upper-level jet associated with an extratropical cyclone over the North Atlantic. The cloud-induced circulation is directed against the jet, reaches up to 10 m·s-1, and compares well between both simulations. The flow direction is in agreement with what can be expected from a vorticity dipole that forms in the vicinity of the clouds. Hence, in the presence of embedded convection, the wind speed does not steadily decrease away from the jet, as it does in cloud-free regions, but exhibits a pronounced negative anomaly, which can now be explained by the cloud-induced circulation. Furthermore, the direction of the reconstructed circulation suggests that the cloud induces a flow that counteracts its advection by the jet. Convective clouds therefore propagate more slowly than their surroundings, which may affect the distribution of precipitation. The method could be used to compare cloud-induced flow at different resolutions and between different parameterizations.