Effects of artificial local compensation of convective mass flux in the cumulus parameterization

The resolution of the European Centre for Medium‐range Weather Forecast (ECMWF) integrated forecast system (IFS) is expected to reach 5 km in the coming decade. Assumptions in the parametrization of deep convection, such as that all of the compensating environmental flow occurs in the grid column, i.e. the convective and environmental mass fluxes cancel each other in term of mass transport, have to be challenged. In this paper, we further develop the original concept of separating the convective updraught from the subsiding branch of the overturning convective circulation and apply it to the global hydrostatic equations of the IFS. In practice, this constitutes a revised convection–dynamics coupling where the mass flux subsidence of the dynamical variables is not computed locally by the convection scheme, but instead is recomputed from the revised continuity equation and is effective through the semi‐Lagrangian advection of the dynamical core. Therefore horizontal divergence/convergence is also generated in the dynamics at the top/bottom of the convective columns, thus adding to the representation of deep convection a three‐dimensional character which is not present in traditional schemes. The proposed physics–dynamics coupling is intended to be applicable to any mass flux convection scheme and within any regional or global model. We first demonstrate the accuracy of the revised physics–dynamics coupling in terms of global temperature and moisture budgets. The potential impact of the coupling on the convective organization is demonstrated for an idealized squall line case at high horizontal resolution using the small planet testbed. Model reforecasts at 9 km and 5 km resolution confirm the viability of the method in terms of forecast skill and model climate. However, the model impacts are limited as the main factor that still determines the convective stabilization and organization is the current conceptual model of subgrid mass flux which, actually, remains unchanged.

“…() and Ong et al . (). Attempts to implement such an approach in a global NWP model have also been made (e.g., by Cullen et al .…”

Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

The coupling of deep convection with the resolved flow via the divergence of mass flux in the IFS

Malardel

Bechtold

2019

“…Ong et al . () modified the Kain–Fritsch convection scheme (Kain and Fritsch, ; ; Kain, ) to allow a net convective mass transport and conducted idealized tropical cyclone experiments with the Weather Research and Forecasting (WRF) model. They find that the hybrid approach is less sensitive to changes in the grid spacing and has potential benefits on tropical cyclone dynamics.…”

Section: Introductionmentioning

confidence: 99%

Implementing the HYbrid MAss flux Convection Scheme (HYMACS) in ICON – First idealized tests and adaptions to the dynamical core for local mass sources

Langguth

Kuell

Bott

2020

In this study, the Hybrid MAss flux Convection Scheme (HYMACS) is implemented in the ICOsahedral Non‐hydrostatic (ICON) weather prediction model. In contrast to conventional convection parametrization schemes, the convective up‐ and downdraughts are solely treated as subgrid‐scale processes in HYMACS, whereas the environmental subsidence is passed to the grid‐scale dynamics of the hosting model. It is shown that the operational anisotropic divergence damping in ICON distorts the grid‐scale dynamical response on the net mass transport parametrized by HYMACS. Thus, a revised numerical filter configuration is developed which focuses on both the compatibility to local mass sources (sinks) and the effective suppression of numerical modes inherent from the model's triangular grid. Evaluation of Jablonowski–Williamson dynamical core experiments reveal that the combination of an isotropic second‐order divergence damping with a modified version of the fourth‐order divergence damping outperforms against numerical filters based on diffusion. The obtained results are similar to the operational set‐up indicating just a minor effect on the properties of the dynamical core. Moreover, a series of dry mass lifting experiments with the revised numerical filter confirms its compatability with HYMACS. The distortion of the grid‐scale circulation is removed while gravity waves are still retained despite the potentially degenerative effect of the fourth‐order divergence damping. Analyses of kinetic energy spectra confirm the effective suppression of checkerboard noise for a wide range of different situations. The present study may be understood as a base for future applications of HYMACS with a full cloud model in real‐case studies.

“…Furthermore, real deep convection also occurs in highly curved geometries, and there are a number of interesting studies tackling aspects of this problem by utilizing fully three‐dimensional numerical simulations with complex physics. In such simulations, more realistic physical features, such as typhoon‐generated gravity waves (Kim and Chun, ; Kim et al ; Ong et al ), mesoscale circulation around squall lines (Pandya et al ), and gravity waves generated by deep convection (Piani et al ; Lane and Reeder, ) can be modelled. Two‐dimensional planar geometry in the absence of shear (which achieves wave reflection/refraction through a change in stratification) is chosen here as the simplest model with which we can confront the above questions.…”

Section: Introductionmentioning

confidence: 99%

Forced gravity waves and the tropospheric response to convection

Halliday

Griffiths

Parker

et al. 2018

We present theoretical work directed toward improving our understanding of the mesoscale influence of deep convection on its tropospheric environment through forced gravity waves. From the linear, hydrostatic, non‐rotating, incompressible equations, we find a two‐dimensional analytical solution to prescribed heating in a stratified atmosphere, which is upwardly radiating from the troposphere when the domain lid is sufficiently high. We interrogate the spatial and temporal sensitivity of both the vertical velocity and potential temperature to different heating functions, considering both the near‐field and remote responses to steady and pulsed heating. We find that the mesoscale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and the assumed stratification. We find a 50% reduction in tropospherically averaged vertical velocity when moving from a trapped (i.e. low lid) to upwardly radiating (i.e. high lid) solution but, even with maximal upward radiation, we still observe significant tropospheric vertical velocities in the far‐field 4 h after heating ends. We quantify the errors associated with coarsening a 10 km‐wide heating to a 100 km grid (in the way a general circulation model (GCM) would), observing a 20% reduction in vertical velocity. The implications of these results for the parametrization of convection in low‐resolution numerical models are quantified, and it is shown that the smoothing of heating over a grid box leads to significant in‐grid‐box tendencies, due to the erroneous rate of transfer of compensating subsidence to neighbouring regions. Further, we explore a simple time‐dependent heating parametrization that minimizes error in a parent GCM grid box, albeit at the expense of increased error in the neighbourhood.