Daan Degrauwe

Coupling the Town Energy Balance (TEB) Scheme to an Operational Limited-Area NWP Model: Evaluation for a Highly Urbanized Area in Belgium

AbstractThe Town Energy Balance (TEB) single-layer scheme is implemented in a numerical weather prediction model running operationally at ~4-km resolution. The primary question addressed is the ability of TEB to function at this relatively coarse resolution and, thus, assessing its potential use in an operational configuration to improve sensible weather performance over Belgium. For this effort, simulations with and without TEB are first evaluated against 2-m observations and wind above the urban canopy for two months (January and July 2010). The results show that promising improvements are achieved by introducing TEB. The 2-m temperature and 2-m relative humidity improve compared to measurements in urban areas. The comparison of wind speed and wind direction above the urban canopy indicates that the structure of the flow in urban areas is better reproduced with TEB. It was found that the implementation of TEB results in an increase in winter precipitation over urban areas and downwind from urban areas, ...

Application of Boyd’s Periodization and Relaxation Method in a Spectral Atmospheric Limited-Area Model. Part I: Implementation and Reproducibility Tests

AbstractThis paper describes the implementation of a proposal of Boyd for the periodization and relaxation of the fields in a full three-dimensional spectral semi-implicit semi-Lagrangian limited-area model structure of an atmospheric modeling system called HARMONIE that is used for numerical weather prediction and regional climate studies. Some first feasibility tests in an operational numerical weather prediction context are presented. They show that, in terms of standard operational forecast scores, Boyd’s windowing-based method provides comparable performance as the old existing spline-based periodization procedure. However, the real improvements of this method should be expected in specific cases of strong dynamical forcings at the lateral boundaries. An extensive demonstration of the superiority of this windowing-based method is provided in an accompanying paper.

Predicting Small-Scale, Short-Lived Downbursts: Case Study with the NWP Limited-Area ALARO Model for the Pukkelpop Thunderstorm

AbstractThe authors consider a thunderstorm event in 2011 during a music festival in Belgium that produced a short-lived downburst of a diameter of less than 100 m. This is far too small to be resolved by the kilometric resolutions of today’s operational numerical weather prediction models. Operational forecast models will not run at hectometric resolutions in the foreseeable future. The storm caused five casualties and raised strong societal questions regarding the predictability of such a traumatic weather event.In this paper it is investigated whether the downdrafts of a parameterization scheme of deep convection can be used as proxies for the unresolved downbursts. To this end the operational model ALARO [a version of the Action de Recherche Petite Echelle Grande Echelle-Aire Limitee Adaptation Dynamique Developpement International (ARPEGE-ALADIN) operational limited area model with a revised and modular structure of the physical parameterizations] of the Royal Meteorological Institute of Belgium is u...

Application of Boyd’s Periodization and Relaxation Method in a Spectral Atmospheric Limited-Area Model. Part II: Accuracy Analysis and Detailed Study of the Operational Impact

AbstractSpectral limited-area models face a particular challenge at their lateral boundaries: the fields need to be made periodic. Boyd proposed a windowing-based method to improve the periodization and relaxation. In a companion paper, the implementation of this windowing method in the operational semi-implicit semi-Lagrangian spectral HARMONIE system was described and some first reproducibility tests, comparing this method to the old existing one, were presented.The present paper provides an in-depth study of the impact of this method for different configurations of the implementation. This is carried out in three steps in well-controlled experimental setups of increasing complexity. First, different aspects of Boyd’s method are analyzed in an idealized perfect-model test using a representative 1D shallow-water model. Second, the implementation is tested in an adiabatic 3D numerical weather prediction (NWP) model with perfect-model experiments. Finally, the impact of using Boyd’s method in a more operat...

Improving the Temporal Resolution Problem by Localized Gridpoint Nudging in Regional Weather and Climate Models

AbstractMost regional numerical models in the atmospheric sciences use temporally interpolated data provided by other low-resolution models, either for a gridpoint coupling at their lateral boundaries or for a spectral nudging of the large scales in the entire domain. In some cases, such as fast-propagating storms, these interpolations can seriously corrupt the meteorological fields.This article shows how to use an operational high-pass filter of the surface pressure field to detect and to localize a propagating storm, and to use this information to locally reinject the available uncorrupted storm in the coupled model. This is achieved by applying a technique of gridpoint nudging in a subarea of the domain, limited to a compact region around the eye of the depression. As an application it is shown that this restores the strength of the storm, while leaving the model state in the rest of the domain quasi intact. It is then discussed how this can improve numerical weather prediction and regional climate models.

Discretization in Numerical Weather Prediction: A Modular Approach to Investigate Spectral and Local SISL Methods

An overview of some spatial and temporal discretization methods used in NWP is given. The authors focus on the spectral semi-implicit semi-Lagrangian scheme, which was and still is one of the most successful schemes. A Z-grid approach with an identical timestep organization as the current semi-implicit semi-Lagrangian schemes is proposed. This provides a testbed to undertake comparison studies between spectral and local spatial discretization schemes.

Coupling the Town Energy Balance Scheme to the High Resolution LAM ALADIN for Belgium

The Town Energy Balance (TEB) single-layer scheme is implemented into a numerical weather prediction model running operationally at ∼4 km resolution. The primary question addressed is the potential use of TEB, at this relatively coarse resolution, in an operational configuration to improve sensible weather performance over Belgium. Simulations with and without TEB are first evaluated against 2 m observations and wind above the urban canopy for 2 months (January and July 2010). The 2 m temperature and 2 m relative humidity improve compared to measurements in urban areas. The comparison of wind speed and wind direction above the urban canopy indicate that the structure of the flow in urban areas is better reproduced with TEB. Results from a 36-h case study during a high heat day (8 July 2010) indicate that even at this coarse resolution, TEB is able to reproduce correctly the intensity of the observed UHI of Brussels.