07 Feb 2023
 | 07 Feb 2023
Status: this preprint is open for discussion.

The “Hydro-ABC model” (Vn 2.0): a simplified convective-scale model with moist dynamics

Jiangshan Zhu and Ross Noel Bannister

Abstract. The prediction of convection (in terms of position, timing, and strength) is important to achieve in high-resolution weather forecasting. This problem not only requires good convective-scale models, but also data assimilation systems to give initial conditions which neither improperly hinders nor hastens convection in the ensuing forecasts. Solving this problem is difficult and expensive using operational-scale numerical weather prediction systems, and so a simplified model of convective-scale flow is under development. This paper extends the “ABC model” of dry convective-scale flow to include mixing ratios of vapour and condensate phases of water. This revised model is called “Hydro-ABC”.

Hydro ABC includes transport of the vapour and condensate mixing ratios within a dynamical core, and transitions between these two phases via a micro-physics scheme. A saturated mixing ratio is derived from model quantities, which helps determine whether evaporation or condensation happens. Latent heat is exchanged with the buoyancy variable (ABC's potential temperature-like variable) in such a way to conserve total energy, where total energy is the sum of dry energy and latent heat. The model equations are designed to conserve the domain-total mass, water, and energy.

An example numerical model integration is performed and analysed, which shows the development of a realistic looking anvil cloud, and excitation of inertio-gravity and acoustic modes over a wide range of frequencies. This behaviour means that Hydro-ABC is a challenging model to allow experimentation with innovative data assimilation strategies in the next stage of work. Further, an ensemble of Hydro-ABC integrations is also performed in order to study the possible forecast error covariance statistics (necessary for data assimilation). These show patterns that are dependent on the presence of convective activity (at any model's vertical column), thus giving a taste of flow-dependent error statistics. Candidate indicators/harbingers of convection are also evaluated (namely relative humidity, hydrostatic imbalance, horizontal divergence, convective available potential energy, and convective inhibition), which appear to be reliable diagnostics concerning the presence of convection. These diagnostics will be useful in the selection of the relevant forecast error covariance statistics when data assimilation for Hydro-ABC is developed.

Jiangshan Zhu and Ross Noel Bannister

Status: open (until 07 May 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Jiangshan Zhu and Ross Noel Bannister

Model code and software

Hydro ABC Vn 2.0 model code and user documentation Jiangshan Zhu, Ross Bannister, and Ruth Petrie

Jiangshan Zhu and Ross Noel Bannister


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Short summary
We describe how condensation and evaporation are included in the existing (otherwise dry) simplified ABC model. The new model (Hydro-ABC) includes transport of vapour and condensate within a dynamical core, and transitions between these two phases via a micro-physics scheme.

The model shows the development of an anvil cloud, and excitation of atmospheric waves over many frequencies. The covariances that develop between variables are also studied, together with indicators of convective motion.