Development of a Coherent Transilient Turbulence and Convection (CTTC) parameterization scheme for use in numerical weather prediction models

在数值天气预报(NWP)模式中，垂直次网格尺度的运动通常由湍流方案或对流方案来参数化。这一决定通常基于浮力或切变不稳定引起的动力学的预期垂直范围：如果运动是短垂直范围的和/或局限于行星边界层(PBL)，则湍流方案将其视为混合事件。在垂直范围更大的情况下，对流方案将运动视为具有云物理、上升和下降气流以及可能的降水的垂直输送。考虑到这种分类在本质上没有严格的限制，因此需要一个统一或连贯的湍流和对流参数化方案。拟议的项目旨在发展一个统一和无缝的方案，即连贯的湍流和对流参数化方案。为了解决这一问题，我们提出了一种结合了涡扩散系数项(K-Approach)的局部湍流和多种对流羽流的非局部湍流和对流的质量通量项的方法。作为一种直观的形式，我们将应用传递矩阵方法，其中每个矩阵元素量化特定源层和目标层之间的传输。在我们的项目中，经典的质量流项将被允许净质量传输的形式所取代。一个中心问题将是，何时以及在多大程度上在网格尺度上解决湍流/对流环流。换句话说，什么时候以及有多少净质量传输要被参数化。将局地湍流、非局地湍流、浅对流和深对流按其空间尺度进行分类，可以清楚地看到，根据宿主模式的网格尺寸，从无到完全净质量输送可能存在或多或少的平滑过渡。此外，热力学、云形成和降水等复杂的物理过程必须由物理参数控制，而不是模型依赖于网格的决定。根据网格尺寸的不同，在非局地湍流/浅对流的空间区域中将无缝地考虑净质量输送。由于全球高分辨率数值预报模式在计算上过于昂贵，对于业务应用来说，数值预报模式不得不依赖于模型链或在线嵌套的多尺度模式。对于这两类业务数值预报模式，相干和尺度自适应的湍流和对流格式将避免在不同格式之间的人为切换，这取决于预期的动力学空间尺度和当前的网格尺度。在本项目的范围内，我们将使用DWD的ICON模式作为开发环境，但将参数化方案保持为一般形式，以便在其他数值预报模式中实施。

Usually, in numerical weather prediction (NWP) models vertical subgrid scale motions are either parameterized by a turbulence scheme or a convection scheme. The decision is often based on the expected vertical extent of the dynamics resulting from buoyancy or shear instability: If the motion is of short vertical range and/or confined to the planetary boundary layer (PBL), then it is treated as a mixing event by the turbulence scheme. In case of a more extended vertical range, the motion is treated by the convection scheme as a vertical transport with cloud physics, up- and downdrafts and possibly precipitation. Being aware that there are no sharp limits for such classifications in nature, the claim for a unified or coherent turbulence and convection parameterization arises.The proposed project aims at the development of a unified and seamless, i.e. a coherent turbulence and convection parameterization scheme. To solve this problem, we propose an approach that combines eddy diffusivity (K-approach) terms for local turbulence and mass flux terms for a variety of convective plumes for nonlocal turbulence and convection. As an intuitive formalism we will apply the transilient matrix method, in which each matrix element quantifies the transport between a certain source and target layer.In our project, the classical mass flux term will be replaced by a form allowing for net mass transport. A central question will be, when and to which extent the turbulent / convective circulation is resolved on the grid scale. In other words, when and how much net mass transport is to be parameterized. Sorting local turbulence, nonlocal turbulence, shallow and deep convection by their spatial scales, it becomes clear that there may be a more or less smooth transition from no to full net mass transport depending on the grid size of the hosting model. Furtheron, the involved physical processes such as thermodynamics, cloud formation and precipitation have to be controlled by physical arguments rather than model grid dependent decisions. Depending on the grid size, net mass transport will seamlessly taken into account in the spatial regimes of nonlocalturbulence / shallow convection.Because globally high-resolved NWP models are (and still will be) computationally too expensive for operational application, NWP has to rely either on a model chain or a multi scale model with online nesting. For both types of operational NWP models a coherent and scale adaptive turbulence and convection scheme will avoid the artificial switching between different schemes depending on the expected spatial scale of the dynamics and the current grid scale.In the scope of the proposed project, we will use the ICON model of DWD as the development environment but keep the parameterization scheme in a generalized form for implementation in other NWP models as well.