A Process Model for ITCZ Narrowing under Warming Highlights Clear-Sky Water Vapor Feedbacks and Gross Moist Stability Changes in AMIP Models
A Process Model for ITCZ Narrowing under Warming Highlights Clear-Sky Water Vapor Feedbacks and Gross Moist Stability Changes in AMIP Models
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DOI：
10.1175/jcli-d-22-0689.1
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发表时间：
2023-08-01
影响因子：
4.9
通讯作者：
Su, Hui
中科院分区：
文献类型：
--
作者：
Ahmed, Fiaz;Neelin, J. David;Su, Hui
具有平均上升运动的热带地区--因此纬向平均热带辐合带(ITCZ)--预计将在全球变暖的情况下收缩。为了理解这一过程，基于干静能方程和水汽方程，建立了一个简单的海表面温度驱动的纬向对称倾覆模式。为了深入了解大气模式比对计划(AMIP)的模拟，研究了控制上升面积分数和纬向平均降水的过程。从AMIP系综中估计了SIMPLE模式中控制辐射反馈和湿静态能量输送的总体参数。简单模型中的均匀变暖会导致上升区收缩和降水加剧--这与观测和气候模型相似。贡献效应包括更强的水汽辐射反馈，更弱的云辐射反馈，更强的对流-环流反馈，以及更多向极地的水汽输出。SIMPLE模式确定了AMIP模式间扩散的重要参数；通过扰动控制短波水汽反馈和变暖下总湿稳定度变化的参数产生的集合，用相关系数跟踪AMIP模式间的变化；0.46。SIMPLE模式对热带上升区和降水量的多模式平均变化也有较高的预报精度。此外，该简单模型再现了AMIP模型中观测到的上升区降雨量、上升区强度和上升区百分比之间的关系。AMIP模型间扩散的很大一部分可以追溯到在变暖下湿静态能量和垂直速度剖面如何变化的扩散，这反过来又影响了深对流区的总体湿润稳定性--突出了对这些量进行观测限制的必要性。大多数(但不是全部)气候模型预测，在全球变暖的情况下，这一雨带将缩小；少数模型预测雨带将扩大。为了减少气候模型中的一些不确定性，我们建立了一个更简单的模型，它只包含雨带收窄的基本物理知识。我们发现了几个相互关联的重要过程。对于气候模型来说，最重要的过程是云层将热量和湿度移出雨区的效率。这种效率在不同的气候模型中有所不同，这似乎是气候模型在雨带收窄速度上不一致的主要原因。
Tropical areas with mean upward motion-and as such the zonal-mean intertropical convergence zone (ITCZ)-are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification-similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIPmodel spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ;0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions-highlighting the need for observational constraints on these quantities.SIGNIFICANCE STATEMENT: A large rainband straddles Earth's tropics. Most, but not all, climate models predict that this rainband will shrink under global warming; a few models predict an expansion of the rainband. To mitigate some of this uncertainty among climate models, we build a simpler model that only contains the essential physics of rainband narrowing. We find several interconnected processes that are important. For climate models, the most important process is the efficiency with which clouds move heat and humidity out of rainy regions. This efficiency varies among climate models and appears to be a primary reason for why climate models do not agree on the rate of rainband narrowing.