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

Spectral 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 operational-like NWP context is investigated as well. The presented tests show that, while the implementation of Boyd’s method is neutral in terms of scores, it is superior to the existing spline method in the case of strong dynamical forcings at the lateral boundaries

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

Predicting small-scale, short-lived downbursts : case study with the NWP limited-area ALARO model for the Pukkelpop thunderstorm

The 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 used. While the model in its operational configuration at the time of the event did not give a clear hint of a downburst event, it has been found that (i) the use of unsaturated downdrafts and (ii) some adaptations of the features of this downdraft parameterization scheme, specifically the sensitivity to the entrainment and friction, can make the downdrafts sensitive enough to the surrounding resolved-scale conditions to make them useful as indicators of the possibility of such downbursts

Numerical simulations of the impacts of mountain on oasis effects in arid Central Asia

The oases in the mountain-basin systems of Central Asia are extremely fragile. Investigating oasis effects and oasis-desert interactions is important for understanding the ecological stability of oases. However, previous studies have been performed only in oasis-desert environments and have not considered the impacts of mountains. In this study, oasis effects were explored in the context of mountain effects in the northern Tianshan Mountains (NTM) using the Weather Research and Forecasting (WRF) model. Four numerical simulations are performed. The def simulation uses the default terrestrial datasets provided by the WRF model. The mod simulation uses actual terrestrial datasets from satellite products. The non-oasis simulation is a scenario simulation in which oasis areas are replaced by desert conditions, while all other conditions are the same as the mod simulation. Finally, the non-mountain simulation is a scenario simulation in which the elevation values of all grids are set to a constant value of 300 m, while all other conditions are the same as in the mod simulation. The mod simulation agrees well with near-surface measurements of temperature, relative humidity and latent heat flux. The Tianshan Mountains exert a cooling and wetting effects in the NTM region. The oasis breeze circulation (OBC) between oases and the deserts is counteracted by the stronger background circulation. Thus, the self-supporting mechanism of oases originating from the OBC plays a limited role in maintaining the ecological stability of oases in this mountain-basin system. However, the mountain wind causes the cold-wet'' island effects of the oases to extend into the oasis-desert transition zone at night, which is beneficial for plants in the transition region

Time tracking of different cropping patterns using Landsat images under different agricultural systems during 1990-2050 in Cold China

Rapid cropland reclamation is underway in Cold China in response to increases in food demand, while the lack analyses of time series cropping pattern mappings limits our understanding of the acute transformation process of cropland structure and associated environmental effects. The Cold China contains different agricultural systems (state and private farming), and such systems could lead to different cropping patterns. So far, such changes have not been revealed yet. Based on the Landsat images, this study tracked cropping information in five-year increments (1990-1995, 1995-2000, 2000-2005, 2005-2010, and 2010-2015) and predicted future patterns for the period of 2020-2050 under different agricultural systems using developed method for determining cropland patterns. The following results were obtained: The available time series of Landsat images in Cold China met the requirements for long-term cropping pattern studies, and the developed method exhibited high accuracy (over 91%) and obtained precise spatial information. A new satellite evidence was observed that cropping patterns significantly differed between the two farm types, with paddy field in state farming expanding at a faster rate (from 2.66 to 68.56%) than those in private farming (from 10.12 to 34.98%). More than 70% of paddy expansion was attributed to the transformation of upland crop in each period at the pixel level, which led to a greater loss of upland crop in state farming than private farming (9505.66 km(2) vs. 2840.29 km(2)) during 1990-2015. Rapid cropland reclamation is projected to stagnate in 2020, while paddy expansion will continue until 2040 primarily in private farming in Cold China. This study provides new evidence for different land use change pattern mechanisms between different agricultural systems, and the results have significant implications for understanding and guiding agricultural system development

Climate sensitivity to land use changes over the City of Brussels

Prompted with the ongoing and projected climate change, a wide range of cities have committed, not only to mitigate greenhouse gas emissions but also to implement different climate change adaptation measures. These measures serve to ensure the wellbeing of the urban population. In practice, however, the planning of realistic adaptation measures is a complex process. Prior to starting such endeavor, it may therefore be useful to explore the maximum potential benefit that can be gained through adaptation measures. In this work, simple, extreme yet realistic adaptation measures are proposed in terms of changes in albedo and vegetation fraction. The impact of these land-use scenarios is explored by use of the land surface model SURFEX on the summer climate in terms of heat waves and the urban heat island for the city of Brussels. This is done for different periods in the future using the greenhouse gas scenario RCP8.5

Application of Boyd’s periodization and relaxation method in a spectral atmospheric limited-area model, part I: implementation and reproducibility tests

This 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