High-resolution Limited-area Models Research Articles

Global response of parameterised convective cloud fields to anthropogenic aerosol forcing

Abstract. The interactions between aerosols and convective clouds represent some of the greatest uncertainties in the climate impact of aerosols in the atmosphere. A wide variety of mechanisms have been proposed by which aerosols may invigorate, suppress or change the properties of individual convective clouds, some of which can be reproduced in high-resolution limited-area models. However, there may also be mesoscale, regional or global adjustments which modulate or dampen such impacts which cannot be captured in the limited domain of such models. The Convective Cloud Field Model (CCFM) provides a mechanism to simulate a population of convective clouds, complete with microphysics and interactions between clouds, within each grid column at resolutions used for global climate modelling, so that a representation of the microphysical aerosol response within each parameterised cloud type is possible. Using CCFM within the global aerosol–climate model ECHAM–HAM, we demonstrate how the parameterised cloud field responds to the present-day anthropogenic aerosol perturbation in different regions. In particular, we show that in regions with strongly forced deep convection and/or significant aerosol effects via large-scale processes, the changes in the convective cloud field due to microphysical effects are rather small; however in a more weakly forced regime such as the Caribbean, where large-scale aerosol effects are small, a signature of convective invigoration does become apparent.

An NWP Model Intercomparison of Surface Weather Parameters in the European Arctic during the Year of Polar Prediction Special Observing Period Northern Hemisphere 1

AbstractIncreased human activity in the Arctic calls for accurate and reliable weather predictions. This study presents an intercomparison of operational and/or high-resolution models in an attempt to establish a baseline for present-day Arctic short-range forecast capabilities for near-surface weather (pressure, wind speed, temperature, precipitation, and total cloud cover) during winter. One global model [the high-resolution version of the ECMWF Integrated Forecasting System (IFS-HRES)], and three high-resolution, limited-area models [Applications of Research to Operations at Mesoscale (AROME)-Arctic, Canadian Arctic Prediction System (CAPS), and AROME with Météo-France setup (MF-AROME)] are evaluated. As part of the model intercomparison, several aspects of the impact of observation errors and representativeness on the verification are discussed. The results show how the forecasts differ in their spatial details and how forecast accuracy varies with region, parameter, lead time, weather, and forecast system, and they confirm many findings from mid- or lower latitudes. While some weaknesses are unique or more pronounced in some of the systems, several common model deficiencies are found, such as forecasting temperature during cloud-free, calm weather; a cold bias in windy conditions; the distinction between freezing and melting conditions; underestimation of solid precipitation; less skillful wind speed forecasts over land than over ocean; and difficulties with small-scale spatial variability. The added value of high-resolution limited area models is most pronounced for wind speed and temperature in regions with complex terrain and coastlines. However, forecast errors grow faster in the high-resolution models. This study also shows that observation errors and representativeness can account for a substantial part of the difference between forecast and observations in standard verification.

Intense convection over West Africa during AMMA SOP3 experiment

ERA-Interim product from the European Center for Medium-Range Weather Forecast (ECMWF) assimilation of African Monsoon Multidisciplinary Analysis (AMMA) resources, Meteosat satellite images, and synoptic observations were used to study local- and regional-scale environments associated with intense convective systems during the AMMA-SOP3 experiment over West Africa in the Northern Hemisphere of summer 2006. The convective system, from the 21st to 23rd of August 2006, was more active at 0000 and 1800UTC showing diurnal cycle of deep convection over West Africa where the African easterly waves (AEWs) are developed downstream. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. From a dynamical viewpoint, the low-level vorticity presents strong positive values over the sea and Sahara zone, indicating that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. The 925-hPa equivalent potential temperature structure show a maximum over the Sahara which corresponds to the depression observed in this region. A mosaic of three hourly infrared (IR) satellite images, depicts a very distinct signal from an initial region of convection, developing through several stages and moving off the African coast. These observations, along with those available from the World Weather Watch, provide an opportunity to carry out numerical weather prediction (NWP) studies over West Africa utilizing high resolution limited area models.

Subgrid-Scale Variability for Thermodynamic Variables in an Offline Land Surface Prediction System

Abstract This study presents a numerical analysis of the impact of the horizontal resolution on the forecast capability of the Canadian offline land surface prediction system (SPS; formerly known as GEM-Surf) forced by the 15-km Global Environmental Multiscale (GEM) atmospheric model. This system is used to quantify on a statistical basis the subgrid-scale variability of (near-)surface variables for 25-km grid spacing based on the 2.5- or 10-km SPS run at regional scale over the 2012 summer season. The model bias and the distributions characterizing the subgrid-scale variability drastically depend on the geographic areas as well as on the diurnal cycle. These results show the benefits of high-resolution land surface simulations to account for length scales that are more consistent with the scales at which the actual land surface balance is affected by the heterogeneous geophysical fields (i.e., roughness length, land–water mask, glacier mask, and soil texture). The model bias results highlight the potential of an SPS–GEM two-way coupling strategy for refining predictions near the surface through the upscaling of high-resolution surface heat fluxes to the coarser atmospheric grid spacing, with these fluxes being significantly different from those explicitly resolved at 25 km and featuring nonlinear behavior with respect to the horizontal resolution. Since the computational power of meteorological operational centers progressively increases, making it possible to run high-resolution limited-area models, solving the surface at high resolution in a surface–atmosphere fully coupled system becomes a key aspect for improving numerical weather and environmental forecast performance.

Simulating weather regimes: impact of model resolution and stochastic parameterization

The simulation of quasi-persistent regime structures in an atmospheric model with horizontal resolution typical of the Intergovernmental Panel on Climate Change fifth assessment report simulations, is shown to be unrealistic. A higher resolution configuration of the same model, with horizontal resolution typical of that used in operational numerical weather prediction, is able to simulate these regime structures realistically. The spatial patterns of the simulated regimes are remarkably accurate at high resolution. A model configuration at intermediate resolution shows a marked improvement over the low-resolution configuration, particularly in terms of the temporal characteristics of the regimes, but does not produce a simulation as accurate as the very-high-resolution configuration. It is demonstrated that the simulation of regimes can be significantly improved, even at low resolution, by the introduction of a stochastic physics scheme. At low resolution the stochastic physics scheme drastically improves both the spatial and temporal aspects of the regimes simulation. These results highlight the importance of small-scale processes on large-scale climate variability, and indicate that although simulating variability at small scales is a necessity, it may not be necessary to represent the small-scales accurately, or even explicitly, in order to improve the simulation of large-scale climate. It is argued that these results could have important implications for improving both global climate simulations, and the ability of high-resolution limited-area models, forced by low-resolution global models, to reliably simulate regional climate change signals.

Overlapping Interests: The Impact of Geographic Coordinate Assumptions on Limited-Area Atmospheric Model Simulations

AbstractThere is growing use of limited-area models (LAMs) for high-resolution (<10 km) applications, for which consistent mapping of input terrestrial and meteorological datasets is critical for accurate simulations. The geographic coordinate systems of most input datasets are based on spheroid-shaped (i.e., elliptical) Earth models, while LAMs generally assume a perfectly sphere-shaped Earth. This distinction is often neglected during preprocessing, when input data are remapped to LAM domains, leading to geolocation discrepancies that can exceed 20 km at midlatitudes.A variety of terrestrial (topography and land use) input dataset configurations is employed to explore the impact of Earth model assumptions on a series of 1-km LAM simulations over Colorado. For the same terrestrial datasets, the ~20-km geolocation discrepancy between spheroidal-versus-spherical Earth models over the domain leads to simulated differences in near-surface and midtropospheric air temperature, humidity, and wind speed that are larger and more widespread than those due to using different topography and land use datasets altogether but not changing the Earth model. Simulated differences are caused by the shift of static fields with respect to boundary conditions, and altered Coriolis forcing and topographic gradients.The sensitivity of high-resolution LAM simulations to Earth model assumptions emphasizes the importance for users to ensure terrestrial and meteorological input data are consistently mapped during preprocessing (i.e., datasets share a common geographic coordinate system before remapping to the LAM domain). Concurrently, the modeling community should update preprocessing systems to make sure input data are correctly mapped for all global and limited-area simulation domains.

Experimental Tropical Cyclone Prediction Using the GFDL 25-km-Resolution Global Atmospheric Model

Abstract This paper describes a forecasting configuration of the Geophysical Fluid Dynamics Laboratory (GFDL) High-resolution Atmospheric Model (HiRAM). HiRAM represents an early attempt in unifying, within a global modeling framework, the capabilities of GFDL’s low-resolution climate models for Intergovernmental Panel on Climate Change (IPCC) type climate change assessments and high-resolution limited-area models for hurricane predictions. In this study, the potential of HiRAM as a forecasting tool is investigated by applying the model to the near-term and intraseasonal hindcasting of tropical cyclones (TCs) in the Atlantic basin from 2006 to 2009. Results demonstrate that HiRAM provides skillful near-term forecasts of TC track and intensity relative to their respective benchmarks from t = 48 h through t = 144 h. At the intraseasonal time scale, a simple HiRAM ensemble provides skillful forecasts of 21-day Atlantic basin TC activity at a 2-day lead time. It should be noted that the methodology used to produce these hindcasts is applicable in a real-time forecasting scenario. While the initial experimental results appear promising, the HiRAM forecasting system requires various improvements in order to be useful in an operational setting. These modifications are currently under development and include a data assimilation system for forecast initialization, increased horizontal resolution to better resolve the vortex structure, 3D ocean model coupling, and wave model coupling. An overview of these ongoing developments is provided, and the specifics of each will be described in subsequent papers.

Weather forecasts obtained with a Multimodel SuperEnsemble technique in a complex orography region

The Multimodel SuperEnsemble technique (KRISHNAMURTI et al., 2000a) is a new powerful post-processing method for the estimation of weather forecast parameters. Several model outputs are combined, using weights calculated during a training period. Piedmont region is characterised by complex mountainous orography and direct model outputs, even from high-resolution limited area models, show many strong systematic and random errors in the forecast, compared to the values observed by our high-density non-GTS network. This is one of the first applications of this technique in a narrow mountain area and combines both global and limited-area models. Our results show a good improvement of meteorological parameter forecasts such as temperature, humidity, wind speed and precipitation.

Impact of nesting strategies in dynamical downscaling of reanalysis data

Coarse–grid global numerical weather simulations or analysis data have to be downscaled, e.g., with nested limited–area models (LAMs), for regional interpretation. Here, the impact of different one–way nesting strategies on precipitation simulations over the European Alps with the LAM ALADIN is studied. The LAM is forced by initial and lateral boundary data derived from ERA40 reanalyses with 120 km horizontal gridspacing and 6 h update interval. The nesting strategies considered include relaxation–based techniques with direct nesting of the high–resolution LAM (horizontal gridspacing Δx = 12 km; domain size 2800 × 2500 km2) or double nesting with an intermediate–resolution nest (Δx = 50 km). Additionally, the impact of a spectral initialization technique is investigated. Results indicate that the considered nesting strategies are comparably successful in terms of precipitation simulation, despite the large resolution jump (120 to 12 km) involved. Thus, the cheapest method in terms of computational resources, i.e., direct nesting, seems to be the most adequate for dynamical downscaling of reanalysis data over complex terrain.

Distribution-Oriented Verification of Limited-Area Model Forecasts in a Perfect-Model Framework

Abstract Nested limited-area models (LAMs) have been used by the scientific community for a long time, with the implicit assumption that they are able to generate meaningful small-scale features that were absent in the lateral boundary conditions and sometimes even in the initial conditions. This hypothesis has never been seriously challenged in spite of reservations expressed by part of the scientific community. In order to study this hypothesis, a perfect-model approach is followed. A high-resolution LAM driven by global analyses is used over a large domain to generate a “reference run.” These fields are filtered afterward to remove small scales in order to mimic low-resolution nesting data. The same high-resolution LAM, but over a small domain, is nested with these filtered fields and run for several days. The ability of the LAM to regenerate the small scales that were absent in the initial and lateral boundary conditions is estimated by comparing both runs over the same region. The simulations are ana...

Forecasting Skill Limits of Nested, Limited-Area Models: A Perfect-Model Approach

The fundamental hypothesis underlying the use of limited-area models (LAMs) is their ability to generate meaningful small-scale features from low-resolution information, provided as initial conditions and at their lateral boundaries by a model or by objective analyses. This hypothesis has never been seriously challenged in spite of some reservations expressed by the scientific community. In order to study this hypothesis, a perfectmodel approach is followed. A high-resolution large-domain LAM driven by global analyses is used to generate a ‘‘reference run.’’ These fields are filtered afterward to remove small scales in order to mimic a low-resolution run. The same high-resolution LAM, but in a small-domain grid, is nested within these filtered fields and run for several days. Comparison of both runs over the same region allows for the estimation of the ability of the LAM to regenerate the removed small scales. Results show that the small-domain LAM recreates the right amount of small-scale variability but is incapable of reproducing it with the precision required by a root-mean-square (rms) measure of error. Some success is attained, however, during the first hours of integration. This suggests that LAMs are not very efficient in accurately downscaling information, even in a perfect-model context. On the other hand, when the initial conditions used in the small-domain LAM include the small-scale features that are still absent in the lateral boundary conditions, results improve dramatically. This suggests that lack of high-resolution information in the boundary conditions has a small impact on the performance. Results of this study also show that predictability timescales of different wavelengths exhibit a behavior similar to those of a global autonomous model.

Predictability of a Nested Limited-Area Model

Abstract This note investigates the nature of the extended predictability commonly attributed to high-resolution limited-area models (LAM) nested with low-resolution data at their lateral boundaries. LAM simulations are performed with two different sets of initial, nesting, and verification data: one is a set of regional objective analyses and the other is a synthetic high-resolution model-generated dataset. The simulation differences (equivalent to forecast errors in an operational framework) are studied in terms of their horizontal scale distribution normalized by the natural variability in each scale, as a measure of predictability, which constitutes an original contribution of this note. The results suggest that the extended predictability in LAM is confined to those scales that are present both in the initial condition and lateral boundary conditions (LBCs). No evidence is found for extended predictability of scales that are not forced through the LBCs. Instead, these smaller scales exhibit predictiv...

Perspektief vir genestelde klimaatmodellering oor suidelike Afrika

The climate of southern Africa is fundamentally affected by mesoscale circulation patterns that are not adequately simulated by global atmospheric general circulation models (AGCMs). The technique of nested climate modelling (NCM) utilises high-resolution limited area models (LAMs) to obtain climate simulations of the mesoscale from essentially synoptical scale AGCM results.

Development and validation of a three-dimensional curvilinear model for the study of fluxes through the North Channel of the Irish Sea

A three-dimensional non-orthogonal curvilinear model has been developed for the Celtic Sea, Irish Sea and North Channel. With a maximum resolution of approximately 1 km in the North Channel, the model resolves the complex topography and horizontal flow variability in this region, whilst the large geographical domain enables the inclusion of far field forcing. Predicted co-tidal charts of the region are in good agreement with published charts based on observed tidal elevations and from previous modelling studies. Comparisons of observed and predicted M 2 and S 2 tidal elevations and currents suggested that in general, this model is considerably more accurate than a previous large area, coarse resolution model of the region, and comparable with previous high-resolution limited area models. The predicted tidal residual flow through the North Channel of 0.023 Sv is in good agreement with previous modelling studies. These residual currents showed considerable spatial variability in the North Channel, clearly demonstrating the importance of a high resolution grid in this region. Of particular interest are the predictions of a strong cyclonic gyre at the mouth of Belfast Lough, and a strong southwards coastal current along the Mull of Galloway. The latter may in part explain the observations of fresher water in this region.

Deterministic methods for prediction of tropical cyclone tracks

ABSTRACT. The paper presents a state-of-art review of different objective techniques available for tropical cyclone track prediction. A brief description of current theories of tropical cyclone motion is given. Deterministic models with statistical and dynamical methods have been discussed. Recent advances in the understanding of cyclone structure and motion aspects have led to improved prediction of tropical cyclones. There has been considerable progress in the field of prediction by dynamical methods. High resolution Limited Area Models (LAM) as well as Global Circulation Models (GCM) are now being used extensively by most of the leading operational numerical weather prediction (NWP) centres in the world The major achievements towards improvement of such models have come from improved horizontal resolution of the models, inclusion of physical processes, use of synthetic and other non-conventional data in the data assimilation schemes and nudging method for initial matching of analysed cyclone centres with corresponding observations.
 A brief description of further improvement in deterministic approach for prediction of tropical cyclone tracks is outlined.

Data Assimilation for High-Resolution Limited-Area Models (gtSpecial IssueltData Assimilation in Meteology and Oceanography: Theory and Practice)

The present status of development of data assimilation techniques for high resolution limited area models is reviewed and various candidates for a new generation of a data assimilation system are compared. It is concluded that data assimilation based on 3-dimensional or 4-dimensional variational techniques is the most promising approach. Further investigations are needed, however, to find out whether variational data assimilation is feasible over "limited" model integration areas or whether global/hemispheric model integration areas have to be introduced for high resolution limited area data assimilation purposes. The main problem in this connection is the rapid propagation of forecast errors on a global or at least hemispheric atmospheric scale within a time-scale of a few days.

Regional climate modelling

Regional climate modelling is becoming increasingly popular. The most common technique employs high resolution limited-area models to economically produce detaited climatologies for selected regions. A short review is presented of the underlying principles, recent simulations limitations of the method and future prospects.

The Sydney Australia Wildfires of January 1994 - Meteorological Conditions and High Resolution Numerical Modeling Experiments

Southeastern Australia is particularly vulnerable to wildfires during the spring and summer months, and the threat of devastation is present most years. In January 1994, the most populous city in Australia, Sydney, was ringed by wildfires, some of which penetrated well into suburban areas and there were many other serious fires in coastal areas of New South Wales (NSW). In recent years much research activity in Australia has focussed on the development of high resolution limited area models, for eventual operational prediction of meteorological conditions associated with high levels of wildfire risk. In this study, the period January 7-8, 1994 was chosen for detailed examination, as it was the most critical period during late December 1993/early January 1994 for the greater Sydney area. Routine forecast guidance from the Australian Bureau of Meteorology's operational numerical weather prediction (NWP) models was very useful in that both the medium and short range models predicted synoptic patterns suggesting extreme fire weather conditions up to several days in advance. However, vital information of a detailed nature was lacking. A new high resolution model was run at the operational resolution of 150 km and the much higher resolutions of 25 km and 5 km. The new model showed statistically significant greater skill in predicting details of wind, relative humidity and temperature patterns both near the surface and above the boundary layer. It also produced skilful predictions of the Forest Fire Danger Index.

日本の南岸で山岳の影響で発生した中規模雲システムと、それに続く低気圧形成過程の数値シミュレーション

In cold seasons, a low-level mesoscale cloud system associated with an orographically-induced shear line often develops off the south coast of central Japan. Numerical experiments using high resolution limited-area models based on Japan Spectral Model (JSM) were performed for the 14 October 1991 case, where active development of the cloud system and associated cyclogenesis occurred. The model reproduced the evolution of the disturbance well, which is described as follows. In the initial stage of the cloud formation, a strong low-level northerly wind in the upwind side exhibited a splitting pattern around the Chubu Mountains, while the wind in the lee side was weak. High Be air that was formerly brought by a typhoon was trapped in the south coast area. Then an easterly wind gradually intensified along the south coast of Kanto, which caused the westward migration of the cloud system, and a NW-SE oriented shear line with strong positive vorticity in the lee of the Chubu Mts. A band-shaped precipitation area was simulated along the northeast side of the shear line, whereas on the southwest side, there was a dry air mass that descended the south slope. At the 18-h forecast, a shallow, mesoscale cyclone appeared on the shear line, which was nearly in geostrophic balance with the strong cyclonic circulation. The mesoscale cyclone then combined with the middle-level trough that progressed along the south coast and developed further as a synoptic-scale, extratropical cyclone. Simulated results show that the initial growth of the disturbance is triggered by the orographic effects, and that the later development is strongly influenced by the approach of the middle-level trough.

Prediction of African waves and specification of squall lines

This paper is part of a sequence of numerical weather prediction efforts utilizing the special observations from the GARP Atlantic Tropical Experiment (GATE). These observations, along with those available from the World Weather Watch, provide a unique opportunity to carry out numerical weather prediction (NWP) studies over portions of West Africa and the eastern Atlantic ocean utilizing high resolution limited area models. The present study reports on the results of 34 short-range numerical weather prediction experiments, with two simple models, designed to study the westward propagation of African waves. In the time frame of 24 to 48 hours, predictions at 700 mb (the level of maximum amplitude of these waves, and the level of non-divergence) show extremely promising results. These results are comparable to fine-mesh predictions in higher latitudes. Errors in phase speed and vector root mean square (RMS) wind errors are summarized for the third phase of GATE. A second part of this study deals with the problem of specification of the West African squall line with respect to the position of the predicted trough line of the African wave. A body of recent literature provides useful observational statistics relating the position of African waves, certain geographical regions, the time of day and the formation of line squall systems. These studies also provide information on the relative speeds of westward motion of the squall line and the wave trough. The decay of the squall lines during their westward propagation away from the trough is also a part of these observational statistics. Based on these observations, an empirical set of selection rules may be defined to specify the squall lines with respect to the predicted motion field. Successful results were obtained in the time frame of 24 to 48 hours for these experiments, the results of which are summarized below. DOI: 10.1111/j.2153-3490.1980.tb00949.x