The spatial organization of deep moist convection is known to be an important determinant of the impacts of severe weather, while future changes to convective organization have been linked to various radiative feedbacks under climate warming. Yet there is no unanimously agreed upon definition of convective organization and so there is also no obvious way to objectively define it. In this work, we set out to define a metric for convective organization based on the size and proximity of convectively active regions. The metric is developed based upon tropical radar observations and takes two‐dimensional convective objects, which are predefined in a horizontal plane, as input. We call the metric Radar Organization Metric (ROME). In addition to the proximity of different convective objects, which is used in other organization metrics, ROME is also sensitive to object size. As a result, ROME is also defined for the case of only one convective object. Thus, ROME provides a smoothly evolving measure of the degree of convective organization, which compares well to a visual assessment of the convective objects. ROME is found to be sensitive to different regimes of the North Australian monsoon, and its average diurnal cycle is coherent with the daily evolution of tropical rainfall. Through its dependence on area, ROME adds new capabilities that other metrics lack in measuring the degree of convective organization. In particular, ROME permits quantification of the individual contributions of the object size distribution and the spatial clustering of objects to the overall degree of convective organization.
Earth's equilibrium climate sensitivity (ECS) is the long-term response to doubled atmospheric CO 2 and likely between 1.5 and 4.5 K. Conventional general circulation models do not convincingly narrow down this range, and newly developed nonhydrostatic models with relatively fine horizontal resolutions of a few kilometers have thus far delivered diverse results. Here we use the nonhydrostatic ICON model with the physics package normally used for climate simulations at resolutions as fine as 5 km to study the response to a uniform surface warming in an aquaplanet configuration. We apply the model in two setups: one with convection parametrization employed and one with explicit convection. ICON exhibits a negative total feedback independent of convective representation, thus providing a stable climate with an ECS comparable to other general circulation models, though three interesting new results are found. First, ECS varies little across resolution for both setups but runs with explicit convection have systematically lower ECS than the parametrized case, mainly due to more negative tropical clear-sky longwave feedbacks. These are a consequence of a drier mean state of about 6% relative humidity for explicit convection and less midtropospheric moistening with global warming. Second, shortwave feedbacks switch from positive to negative with increasing resolution, originating foremost in the tropics and high latitudes. Third, the model shows no discernible high cloud area feedback (iris effect) in any configuration. It is possible that ICON's climate model parametrizations applied here are less appropriate for cloud resolving scales, and therefore, ongoing developments aim at implementing a more advanced prognostic cloud microphysics scheme.
The General Circulation Model ECHAM is used to study the effects of three refined vertical resolutions on convection in the tropics and on the structure of the intertropical convergence zone (ITCZ). Additional vertical resolutions have 76, 134, or 192 levels, which is over four times the default resolution of 47 levels. New levels are placed in the troposphere only. The simulations are conducted on an aqua‐planet with equator symmetrical, time and zonal independent sea surface temperature, and without a yearly solar cycle. Whereas the default vertical resolution shows a double ITCZ, refining the vertical resolution yields an equatorward shift of the ITCZ. The ITCZ converges to its equatorial position with 134 levels. The sensitivity of the ITCZ to the vertical resolution is traced back first and foremost to the mixing formulation in the convection scheme. Here a higher number of vertical levels leads to a stronger mixing between the updraft and its environment by design, which favors an equatorward position of the ITCZ. Differences in the relative humidity profiles explain the remaining differences in the ITCZ location. These differences can mostly be eliminated by making clouds transparent.
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