the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Understanding the dependence of mean precipitation on convective treatment in tropical aquachannel experiments
Peter Knippertz
Yvonne Ruckstuhl
Robert Redl
Tijana Janjic
Corinna Hoose
Abstract. The intertropical convergence zone (ITCZ) is a key circulation and precipitation feature in the tropics. There has been a large spread in the representation of the ITCZ in global weather and climate models for a long time, the reasons for which remain unclear. This manuscript presents a novel approach with which we disentangle different physical processes responsible for the changeable behavior of the ITCZ in numerical models. The diagnostic tool is based on a conceptual framework developed by Emanuel (2019) and allows for physically consistent estimates of convective mass flux and precipitation efficiency for simulations with explicit and parameterized convection. We apply our diagnostics to a set of tropical aquachannel experiments using the ICOsahedral Nonhydrostatic (ICON) model with horizontal grid resolution of 13 km and with various representations of deep and shallow convection. The channel length corresponds to the Earth's circumference and has rigid walls at 30° N/S. Zonally symmetric sea surface temperatures are prescribed.
All four runs share overall similar rainfall patterns and dynamical structures. They simulate an ITCZ at the equator coinciding with the ascending branch of the Hadley circulation, descending branches at 15° N/S with subtropical jets and easterly trade wind belts straddling the ITCZ. Differences are largest between runs with and without parameterized deep convection. With explicit deep convection, rainfall in the ITCZ increases by 35 % and the Hadley circulation as well as surface winds become stronger. Our diagnostic framework reveals that boundary-layer quasi-equilibrium is a key to physically understanding those differences. The stronger surface horizontal winds with explicit deep convection essentially enhance surface enthalpy fluxes and thus perturb quasi-equilibrium in the boundary layer. This is balanced by increasing convective downdraft mass flux that carries low moist static energy from the lower troposphere into the boundary layer. The downdraft strength is proportional to convective updraft mass flux, which is closely linked to rainfall, since – somewhat surprisingly – the convective treatment does not appear to influence precipitation efficiency significantly. Changes in radiative cooling are largely compensated by changes in dry stability, leading to little impact on rainfall. The results highlight the utility of our diagnostics to pinpoint processes important for rainfall differences between models, suggesting applicability for global climate model intercomparison projects.
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Hyunju Jung et al.
Status: open (until 14 Apr 2023)
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RC1: 'Comment on wcd-2023-7', Anonymous Referee #1, 01 Mar 2023
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This study investigates the impact of convective treatment (particularly, parameterized vs explicit deep convection) on the simulation of mean tropical precipitation (particularly, the ITCZ), using a 13km ICON model in a semi-aquaplanet simulation with walls at 30N/S. They find, with explicit deep convection, rainfall in the ITCZ increases by 35% and the Hadley circulation as well as surface winds become stronger. Based on a diagnostic framework based on Emanuel (2019), they attribute the difference to the stronger surface horizontal winds with explicit deep convection, which modifies the boundary layer equilibrium and consequently the updraft mass flux.ÂI have some concerns about the model setup and the derivation of Eq. 1. I suggest a major revision with the following comments.ÂMajor comments:Â1. Uncertainty due to model setupÂResolution:The effect of explicit versus parameterized deep convection is investigated at a horizontal resolution of 13 km. It is generally believed that the horizontal resolution needed to partially resolve deep convection should be ~1km, the 13km resolution used here is not sufficient to resolve deep convection, so the setup of the S13 experiment would not be recommended. It is unclear how sensitivity is the effect of explicit deep convection to the background horizontal resolution. Will the conclusion be different if a higher horizontal resolution, e.g. 3km, is used?ÂWalls at 30N/S:The aquaplanet simulations has a wall at 30N/S. This setup is likely to effect many spects of the simulations including the ITCZ. It is unclear to me if the conclusion of this study would be different if there is no wall but a global aquaplanet.Â2, The derivation of Eq. A5, which leads to Eq. 1ÂEq. A1 is for the top of the BL (the subsidence is w_e) while the equation at L686 is from the balance between radiative cooling and descending is for free troposphere (i.e., the subsidence is not w_e), then, how could these two equations be combined into Eq. A5.ÂMinor comments:Â
L54: Not sure if "appropriate" is the right word here. Each model center has chosen the model resolution appropriately, according to their needs and computational resources.
ÂL60-70: According to Zhou et al. (2022), the storm-resolving simulation (res ~3km) does not reduce the bias in tropical precipitation characteristics (except for the better representation of strong convection events and tropical cyclones) and is not likely to alleviate the double-ITCZ bias.
Zhou W., L.R. Leung, J. Lu, (2022): Linking large-scale double-ITCZ bias to local-scale drizzling bias in climate models. Journal of Climate 35 (24), 4365-4379.ÂL71: resolving (deep) convectionÂI suggest moving section 4 (description of the diagnostic framework) to section 2.Citation: https://doi.org/10.5194/wcd-2023-7-RC1
Hyunju Jung et al.
Hyunju Jung et al.
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