Alex Deckmyn

A Wavelet Approach to Representing Background Error Covariances in a Limited-Area Model

The use of orthogonal wavelets for the representation of background error covariances over a limited area is studied. Each wavelet function contains both information on position and information on scale: using a diagonal correlation matrix in wavelet space thus gives the possibility of representing the local variations of correlation scale. To this end, a generalized family of orthogonal Meyer wavelets that are not restricted to dyadic domains (i.e., powers of 2) is introduced. A three-bases approach is used, which allows one to take advantage of the respective properties of the spectral, wavelet, and gridpoint spaces. While the implied local anisotropies are relatively small, the local changes in the two-dimensional length scale are rather well represented.

Effects of Historical Urbanization in the Brussels Capital Region on Surface Air Temperature Time Series: A Model Study

Abstract The authors examine the local impact of change in impervious surfaces in the Brussels capital region (BCR), Belgium, on trends in maximum, minimum, and mean temperatures between 1960 and 1999. Specifically, data are combined from remote sensing imagery and a land surface model including state-of-the-art urban parameterization—the Town Energy Balance scheme. To (i) isolate effects of urban growth on near-surface temperature independent of atmospheric circulations and (ii) be able to run the model over a very long period without any computational cost restrictions, the land surface model is run in a stand-alone mode coupled to downscaled 40-yr ECMWF reanalysis data. BCR was considered a lumped urban volume and the rate of urbanization was assessed by estimating the percentage of impervious surfaces from Landsat images acquired for various years. Model simulations show that (i) the annual mean urban bias (AMUB) on minimum temperature is rising at a higher rate (almost 3 times more) than on maximum t...

Study of the Lateral Boundary Condition Temporal Resolution Problem and a Proposed Solution by Means of Boundary Error Restarts

To properly utilize coupled limited-area models (LAMs), the time scales of the cross-boundary fluxes in the available lateral boundary data must be assessed. In current operational practice, the update frequencies of these data are usually determined by common sense guesswork and by technical constraints. This paper quantifies the required temporal resolution of the lateral boundary conditions. For a mesoscale LAM it is concluded that in standard forecast cases, coupling updates of about 3 h are sufficient. However, in rare cases of severe storms, this can lead to errors in the coupling data of about 10 hPa. To avoid such errors, it is found that one should update the coupling fields with the period given by the time step of the model that provides the coupling data. However, in most existing operational applications this is not feasible. For those cases, it is shown that the forecast can be substantially improved by restarting the model run at a forecast range when the storm has entered the domain. The proper restart time can be detected in an operational suite by an existing strategy of monitoring the coupling update frequency. Additionally, it is argued that the forecast should then be initialized by a scale-selective digital filter.

Model-Inspired Predictors for Model Output Statistics (MOS)*

This article addresses the problem of the choice of the predictors for the multiple linear regression in model output statistics. Rather than devising a selection procedure directly aimed at the minimization of the final scores, it is examined whether taking the model equations as a guidance may render the process more rational. To this end a notion of constant fractional errors is introduced. Experimental evidence is provided that they are approximately present in the model and that their impact is sufficiently linear to be corrected by a linear regression. Of particular interest are the forcing terms in the coupling of the physics parameterization to the dynamics of the model. Because such parameterizations are estimates of subgrid processes, they are expected to represent degrees of freedom that are independent of the resolved-scale model variables. To illustrate the value of this approach, it is shown that the temporal accumulation of sensible and latent heat fluxes and net solar and thermal radiation utilized as predictors add a statistically significant improvement to the 2-m temperature scores.

Coupling of the CTM CHIMERE to the High Resolution LAM ALADIN for Belgium

At the Royal Meteorological Institute of Belgium we use the chemical transport model (CTM) CHIMERE. In a first phase it has been coupled to meteo fields from ECMWF. The emission database used is EMEP. The simulation domain covers Western Europe with a spatial resolution of 50 km. RMI also runs the Limited Area NWP model ALADIN operationally four times a day at a spatial resolution of 7 km. The meteorological fields from ALADIN have been used to apply a one way nesting to the CTM model running at a spatial resolution of 7 km on a domain of about 680 × 680 km2 covering Belgium and The Netherlands. The emission database used for this high resolution is the TNO/GEMS emission database. We present the first results of this coupling for ozone and PM10 for two different episodes. Also the influence of running ALADIN and its updated version ALARO with improved physical parameterizations will be investigated.

A One Year Evaluation of the CTM CHIMERE Using SURFEX/TEB Within the High Resolution NWP Models ALARO and ALADIN for Belgium

At the Royal Meteorological Institute of Belgium the chemical transport model (CTM) CHIMERE runs at a spatial resolution of 7 × 7 km on a domain, covering Belgium and The Netherlands (680 × 680 km). The CTM is first run on a coarse resolution of 50 km covering Western Europe, using the meteorological fields of ECMWF. The emission database is provided by EMEP. The output of this model run is used as input for the boundary conditions for the coupled high resolution chemical transport model run.

Added economic value of limited area multi-EPS weather forecasting applications

ABSTRACT We compare the GLAMEPS system, a pan-European limited area ensemble prediction system, with ECMWFs EPS over Belgium for an extended period from March 2010 until the end of December 2010. In agreement with a previous study, we find GLAMEPS scores considerably better than ECMWFs EPS. To compute the economic value, we introduce a new relative economic value score for continuous forecasts. The added value of combining the GLAMEPS system with the LAEF system over Belgium is studied. We conclude that adding LAEF to GLAMEPS increases the value, although the increase is small compared to the improvement of GLAMEPS to ECMWFs EPS. As an added benefit we find that the combined GLAMEPS-LAEF multi-EPS system is more robust, that is, it is less vulnerable to the (accidental) removal of one of its components.

The Impact of Heat Waves and Urban Heat Island on the Production of Ozone Concentrations Under Present and Future Climate Conditions for the Belgian Domain

Due to a strong urbanization in Belgium, a lot of areas can be considered as particularly vulnerable to heat waves due to the urban heat island (UHI) effect. However, little information exists on the interaction between the urban heat island effects during heat waves and their interactions under present and future conditions. The heat wave definition and heat stress index chosen in this study are based upon the warnings issued by the Public Health of Belgium for the Brussels Capital Region. For this study, regional simulations were performed using the limited area model ALARO, coupled with the Town Energy Balance scheme. The offline air quality simulations are calculated using the CHIMERE model. Results from our observations and climate simulations indicate that for the present climate conditions night time UHI is enhanced during heat waves which affects also urban and rural surface energy balance differently. The projected climate change under scenario A1B for 2050 leads to an increase of the number and duration of heat waves. More specifically, for rural (urban) areas, climate change increases the intensity of heat waves more during the day (night). We will also look more closely to the effect these changes have on air quality when taking the present and future climate scenarios under consideration. There is a significant increase in the number of days for which ozone concentrations exceed the warning threshold during heat waves. Besides the urban scale we will also investigate the impact of this configuration on air quality for the rural scale under present and future climate conditions.

Future Climate and Air Quality of the Brussels Capital Region for the 2050s Under A1B Scenario

Within the framework of the ACCEPTED project (an Assessment of Changing Conditions, Environmental Policies, Time-activities, Exposure and Disease), a high-resolution urban dynamical downscaling technique has been applied for the Brussels Capital Region. First, regional climate simulations were performed with a new version of the limited-area model of the ARPEGE-IFS system running at 4-km resolution called ALARO coupled with the Town Energy Balance scheme (TEB). Then, to further downscale the regional climate projections to an urban scale, at 1 km resolution, a stand-alone surface scheme was employed in offline mode. Downscaling simulations of present and future urban climate of the Brussels areas are conducted. The downscaling strategy was first evaluated for a 10-years period [2001–2010] using ERA-INTERIM re-analysis data. In a next step, a downscaling simulation for the period 2046−2055 under the A1B scenario was performed. Results from our simulations indicate that while both urban and rural areas warm substantially for the 2050s horizon (1.5 °C), climate change will have a neutral impact on annual mean urban heat island (UHI) intensity. The most important increase is noted for the nocturnal UHI during the winter (+15 %) and the most important decrease is noted for the daytime UHI during the summer (−43 %). However, the model projects an increase of stable situations in the lower atmosphere during winter which may tend to keep pollutants concentrated over urban areas, with the associated health effects. Two approaches have been used to examine meteorological conditions that are unfavorable for the dispersion of air pollution under climate change conditions: (i) a transport index, based on the wind speed and Brunt-Vaisala frequency, that characterizes a typical length of horizontal and vertical transport, (ii) state-of-the-art chemistry transport models CHIMERE coupled to the 4 km regional climate simulations. The results from both approaches will be compared to assess future concentrations at the urban scale.

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.