LES4CCFM: Using LES to characterize and parameterize the convective cloud field

该项目的目的是使用大涡模拟（LES）来表征和量化对流的关键要素。这项工作将在一个新的参数化框架内进行，对流云场模型（CCFM）耦合到英国气象局统一模型（UM）。这项工作的结果不仅会导致一个更好的CCFM，但更重要的是，将通知任何新的或现有的对流参数化的UM。在前三年的主要重点，探索阶段，是改进对流的物理理解及其在CCFM的代表性。CCFM是一种多羽流对流方案，并在瓦格纳和格拉夫（2010年）中进行了描述。该方案使用一个卷吸块模式，计算了给定环境和大尺度强迫下不同初始半径的可能云。CCFM的显着特点是云谱计算，确定可能的云的实际数量。谱计算由多变量Lotka-Volterra系统描述，其中云通过它们的云功函数竞争CAPE（对流可用势能）。从单独的云和它们的数量，质量通量，从而对流加热和增湿可以推导出来。在ECHAM气候模式内进行了CCFM的初步试验。在单柱模式和降水时间和强度方面，ECHAM-CCFM表现出显着优于标准ECHAM。虽然没有调整已经应用，但ECHAM-CCFM改善了许多已知的降水偏差在全球模拟大气与规定的海面温度。作为拟议项目的一部分，我们将实施CCFM作为对流参数化的附加选项。UM-CCFM将用于翻译其他工作包中LES研究的结果，并评估对流参数化变化对大尺度动力学的影响。由于观测结果往往是稀疏和不完整的，由观测驱动的LES为CCFM的评估提供了最好的工具。迄今为止，高分辨率的模拟主要集中在描述积云集合。尽管取得了进展，但真实的突破还不可能，因为积云系综是非常不同的实体的平均值，这些实体相互作用很强，很难直接参数化。拟议的项目LES 4CCFM将克服以前的高分辨率研究的局限性，明确调查个别云和由此产生的云光谱分开。我们将使用比许多以前的研究更高的空间分辨率（为了decametres），以确保云和它们的环境之间的混合，通过夹带和detrainment的重要部分明确解决。LES研究将在三个连续的工作包中进行。首先，我们通过比较LES输出与CCFM中夹带包裹模型的预测来改进对流参数化中单个云的表示。大涡模拟研究卷吸、消散将从一个已知的、因此规定的云底开始。大气概况将以实地活动和密集的实地观察为基础。在第二个工作包将集中在对流云触发执行LES与完全互动的边界层，包括表面通量，云微物理和辐射。这项工作将取代目前相当简单和特设对流云触发CCFM与一个更复杂和物理基础。作为第三个工作包的一部分，我们将描述对流云谱随着时间的推移和达到平衡后的演变。我们将评估CCFM谱计算中的捕食者-猎物假设。结果将不仅是一个新的对流计划内的UM更广泛的物理基础，但此外将提供新的定量洞察对流过程，是至关重要的任何对流参数化的发展。

The aim of this project is to use Large Eddy Simulations (LES) to characterize and quantify key elements of convection. The work will be performed within the framework of a new parameterization, the Convective Cloud Field Model (CCFM) coupled to the UK Met Office Unified Model (UM). Outcomes from this work will not only lead to a better CCFM but more crucially, will inform any new or existing convection parameterization for the UM. The main focus during the first three years, the exploratory phase, is the improved physical understanding of convection and its representation in CCFM. CCFM is a multi-plume convection scheme and described in Wagner and Graf (2010). Using an entraining parcel model the scheme calculates possible clouds of different initial radii for a given environment and large scale forcing. The distinctive feature of CCFM is the cloud spectrum calculation which determines the actual number of possible clouds. The spectrum calculation is described by a multivariate Lotka-Volterra system in which clouds compete for CAPE (convective available potential energy) through their cloud work function. From the individual clouds and their number, mass fluxes and thus convective heating and moistening can be derived. Initial tests of CCFM have been performed within the ECHAM climate model. In single column mode and in terms of precipitation timing and intensity ECHAM-CCFM performs significantly better than the standard ECHAM. Although no tuning has been applied yet ECHAM-CCFM improves many of the known precipitation biases in global simulations of the atmosphere with prescribed sea surface temperatures. As part of the proposed project we will implement CCFM as an additional option for the parameterization of convection. UM-CCFM will used to translate findings from the LES studies in other work packages and to evaluate the impact of changes in the convection parameterization on large scale dynamics. Since observations are often sparse and incomplete, LES driven by observations offer the best tool for the evaluation of CCFM. High-resolution modelling so far has focused on describing the cumulus ensemble. Despite progress a real breakthrough has not been possible since the cumulus ensemble is an average over very different entities that strongly interact and that is difficult parameterize directly. The proposed project LES4CCFM will overcome limitations of previous high resolution studies by explicitly investigating individual clouds and the resulting cloud spectrum separately. We will use higher spatial resolution (order decametres) than many previous studies to ensure that important parts of the mixing between clouds and their environment through entrainment and detrainment are explicitly resolved. LES studies will be performed in three consecutive work packages. First, we improve the representation of individual clouds in convection parameterizations by comparing LES output with prediction from the entraining parcel model in CCFM. LES to study entrainment, detrainment will start from a known, thus prescribed cloud base. Atmospheric profiles will be based on field campaigns and intensive field observations. In a second work package will focus on the convective cloud trigger by performing LES with a fully interactive boundary layer including surface fluxes, cloud microphysics and radiation. This work will replace with currently rather simple and ad hoc convective cloud trigger in CCFM with one that is more sophisticated and physical based. As part of a third work package we will characterize the convective cloud spectrum as it evolves over time and after it reaches equilibrium. We will evaluate the predator-prey assumption in the CCFM spectrum calculation. The outcome will not only be a new convection scheme within the UM with much broader physical basis but in addition will deliver new quantitative insight into convective processes that is crucial for any convection parameterization development.