Numerical vs. turbulent diffusion in geophysical flow modelling

Numerical advection schemes induce the spreading of passive tracers from localized sources. The effects of changing resolution and Courant number are investigated using theWAF advection scheme, which leads to a sub-diffusive process. The spreading rate from an instantaneous source is compared with the physical diffusion necessary to simulate unresolved turbulent motions. The time at which the physical diffusion process overpowers the numerical spreading is estimated, and is shown to reduce as the resolution increases, and to increase as the wind velocity increases

Nudging technique for scale bridging in air quality/climate atmospheric composition modelling

Abstract. The interaction between air quality and climate involves dynamical scales that cover a very wide range. Bridging these scales in numerical simulations is fundamental in studies devoted to megacity/hot-spot impacts on larger scales. A technique based on nudging is proposed as a bridging method that can couple different models at different scales. Here, nudging is used to force low resolution chemical composition models with a run of a high resolution model on a critical area. A one-year numerical experiment focused on the Po Valley hot spot is performed using the BOLCHEM model to asses the method. The results show that the model response is stable to perturbation induced by the nudging and that, taking the high resolution run as a reference, performances of the nudged run increase with respect to the non-forced run. The effect outside the forcing area depends on transport and is significant in a relevant number of events although it becomes weak on seasonal or yearly basis

Assessment of the numerical diffusion effect in the advection of a passive tracer in BOLCHEM

The effects of the numericalsc heme implemented in the advection equation of BOLCHEM have been quantified with reference to the diffusion of a passive tracer. An equivalent horizontal diffusion coefficient has been measured and is found to be dependent on wind field and resolution

Corrigendum to "Effects of resolution on the relative importance of numerical and physical horizontal diffusion in atmospheric composition modelling" published in Atmos. Chem. Phys., 10, 2737–2743, doi:10.5194/acp-10-2737-2010, 2010

No abstract available

A mesoscale model intercomparison: A case of explosive development of a tropical cyclone (COMPARE III)

The performance of current mesoscale numerical models is evaluated in a case of model intercomparison project (COMPARE III). Explosive development of Typhoon Flo (9019) occurred in the case in September 1990 during the cooperative three field experiments, ESCAP/WMO-led SPECTRUM, US-led TCM-90, and former USSR-led TYPHOON-90 in the western North Pacific. Sensitivity to initial fields as well as impact of enhanced horizontal resolution are examined in the model intercomparison. Both track and intensity predictions are very sensitive to the choice of initial fields prepared with different data assimilation systems and the use of a particular synthetic tropical cyclone vortex. Horizontal resolution enhanced from 50km through 20km down to a 10km grid has a large impact on intensity prediction. This is presumably due to a better presentation of inner structure with higher resolution. There is little impact on track prediction in this target period when the typhoon was in its before-recurvature stage. While most models show large biases in underestimating central pressure deepening, some of the participating models with a particular initial field succeed in reproducing qualitatively the time evolution of central pressure, including slow deepening in the first half and rapid deepening in the second half of the simulation period of 72 hours. However, differences leading to different intensity predictions among models have yet to be identified. Intercomparison of the simulation results shows that wind field has a close relationship with precipitation distribution. This suggests that better prediction of precipitation distribution is crucial for better prediction of wind field, and vice versa. Through the COMPARE III experiments, it has become clear that precise simulation of tropical cyclone structure, especially in the inner-core region, is very important for accurate intensity prediction. Consideration, therefore, should be given to this point, when improvements in resolution, initialization, and physics of numerical models for tropical cyclone intensity prediction are reviewed

Effects of resolution on the relative importance of numerical and physical horizontal diffusion in atmospheric composition modelling

Numerical diffusion induced by advection has a large influence on concentration of substances in atmospheric composition models. At coarse resolution numerical effects dominate, whereas at increasing model resolution a description of physical diffusion is needed. A method to investigate the effects of changing resolution and Courant number is defined here and is applied to the WAF advection scheme (used in BOLCHEM), evidencing a sub-diffusive process. The spread rate from an instantaneous source caused by numerical diffusion is compared to that produced by the physical diffusion necessary to simulate unresolved turbulent motions. The time at which the physical diffusion process overpowers the numerical spread is estimated, and it is shown to reduce as the resolution increases, and to increase with wind velocity

Four-dimensional variational assimilation in the unstable subspace and the optimal subspace dimension

International audienceKey a priori information used in 4D-Var is the knowledge of the system's evolution equations. In this article we propose a method for taking full advantage of the knowledge of the system's dynamical instabilities in order to improve the quality of the analysis. We present an algorithm for four-dimensional variational assimilation in the unstable subspace (4D-Var - AUS), which consists of confining in this subspace the increment of the control variable. The existence of an optimal subspace dimension for this confinement is hypothesized. Theoretical arguments in favour of the present approach are supported by numerical experiments in a simple perfect nonlinear model scenario. It is found that the RMS analysis error is a function of the dimension N of the subspace where the analysis is confined and is a minimum for N approximately equal to the dimension of the unstable and neutral manifold. For all assimilation windows, from 1 to 5 d, 4D-Var - AUS performs better than standard 4D-Var. In the presence of observational noise, the 4D-Var solution, while being closer to the observations, is farther away from the truth. The implementation of 4D-Var - AUS does not require the adjoint integration. Copyright © 2010 Royal Meteorological Society

Nudging technique for scale bridging in air quality/climate atmospheric composition modelling

The interaction between air quality and climate involves dynamical scales that cover a very wide range. Bridging these scales in numerical simulations is fundamental in studies devoted to megacity/hot-spot impacts on larger scales. A technique based on nudging is proposed as a bridging method that can couple different models at different scales. <br></br> Here, nudging is used to force low resolution chemical composition models with a run of a high resolution model on a critical area. A one-year numerical experiment focused on the Po Valley hot spot is performed using the BOLCHEM model to asses the method. <br></br> The results show that the model response is stable to perturbation induced by the nudging and that, taking the high resolution run as a reference, performances of the nudged run increase with respect to the non-forced run. The effect outside the forcing area depends on transport and is significant in a relevant number of events although it becomes weak on seasonal or yearly basis

Nudging technique for scale bridging in air quality/climate atmospheric composition modelling

The interaction between air quality and climate involves dynamical scales that cover a very wide range. Bridging these scales in numerical simulations is fundamental in studies devoted to megacity/hot-spot impacts on larger scales. A technique based on nudging is proposed as a bridging method that can couple different models at different scales. <br></br> Here, nudging is used to force low resolution chemical composition models with a run of a high resolution model on a critical area. A one-year numerical experiment focused on the Po Valley hot spot is performed using the BOLCHEM model to asses the method. <br></br> The results show that the model response is stable to perturbation induced by the nudging and that, taking the high resolution run as a reference, performances of the nudged run increase with respect to the non-forced run. The effect outside the forcing area depends on transport and is significant in a relevant number of events although it becomes weak on seasonal or yearly basis

Transport of volcanic aerosol in the troposphere: The case study of the 2002 Etna plume

The transport of aerosol in the troposphere during the 2002 eruption of Mount Etna was investigated integrating lidar observations and numerical simulations. The case study concentrates on the period 30 October to 2 November. The lidar observations performed in Potenza, Italy, reveal the presence of aerosol layers made up of young sulfate particles and a low soot content, characteristic of the volcano's emission. Downward large-scale motion was measured, with a velocity larger than that due to gravitational sedimentation. Forward trajectories from the volcano simulated from 27 October to 4 November show that particles released at the beginning of the period reached Potenza after traveling over the southern and western Mediterranean basin (partially including the Sahara region); direct northward transport occurred on 31 October and 1 November. The main result of this study is to highlight how the integrated use of observations and model simulations leads to understanding the main features of transport in this case study. Furthermore, some specific points are outlined. There is fair agreement between the simulated and observed presence of particles over Potenza. The vertical structure of the aerosol layers and the downward motion are also well evidenced. Time variations of the particle concentration deduced from measurements and approximately estimated from the numerical simulations also show qualitative agreement