WRF Model Simulations Research Articles

Representation of Ethiopian Wet Spells in Global and Nested Models

Weather forecast and reanalysis models exhibit different performance in daily rainfall estimation over the Ethiopian highlands, 2000–2012, with ECMWF closer to observations than other models. Background is given to illustrate the Hadley circulation and easterly jets over Ethiopia, using sections on 37°E in July–August 2011. ECMWF reanalysis has a narrow band of rainfall >15 mm/day on 10°N, consistent with TRMM satellite estimates, associated with a steep gradient in meridional wind. MERRA and GFS models have a wider band of rainfall and weaker gradients in meridional winds. The contrasting background states influence a nested WRF model simulation of heavy rain in the upper Nile Valley on 29 July, 2011. The GFS (ECMWF) initialization yields stronger northerly (southerly) winds north (south) of Ethiopia, while aircraft observations are southerly at 850 mb and northerly at 700 mb. ECMWF produces heavy and widespread rainfall consistent with observations, with a potentially more realistic simulation of the Hadley circulation.

Evaluation of High-Resolution WRF Model Simulations of Surface Wind over the West Coast of India

这篇论文由大气的数字模型预付研究 WRF (ARW3.3 ) 的 high-resolution-mesoscale 从表面风的 hindcasts 的统计确认论述结果，它被设置在海洋信息服务(INCOIS ) 的印度国家中心强迫运作的沿海的海洋预报系统。评估基于表面风的 day-3 预报的比较被执行与在 situ 和遥远察觉到的数据。结果证明模型在印度的西方海岸上精确地相当预言表面风领域，与在预言在季风前季节期间的表面风的高技巧。模型预言日报有合理精确性的表面风的可变性。模型写实地在沿海的区域模仿陆地海微风周期，它很清楚地在东北季风和季风前季节期间被观察并且在西南季风季节期间是不太突出的。

A WRF Ensemble for Improved Wind Speed Forecasts at Turbine Height

Abstract The Weather Research and Forecasting Model (WRF) with 10-km horizontal grid spacing was used to explore improvements in wind speed forecasts at a typical wind turbine hub height (80 m). An ensemble consisting of WRF model simulations with different planetary boundary layer (PBL) schemes showed little spread among the individual ensemble members for forecasting wind speed. A second configuration using three random perturbations of the Global Forecast System model produced more spread in the wind speed forecasts, but the ensemble mean possessed a higher mean absolute error (MAE). A third ensemble of different initialization times showed larger model spread, but model MAE was not compromised. In addition, postprocessing techniques such as training of the model for the day 2 forecast based on day 1 results and bias correction based on observed wind direction are examined. Ramp event forecasting was also explored. An event was considered to be a ramp event if the change in wind power was 50% or more of total capacity in either 4 or 2 h or less. This was approximated using a typical wind turbine power curve such that any wind speed increase or decrease of more than 3 m s−1 within the 6–12 m s−1 window (where power production varies greatly) in 4 h or less would be considered a ramp. Model MAE, climatology of ramp events, and causes were examined. All PBL schemes examined predicted fewer ramp events compared to the observations, and model forecasts for ramps in general were poor.

WRF Model Simulation of Two Alberta Flooding Events and the Impact of Topography

Abstract This study examines simulations of two flooding events in Alberta, Canada, during June 2005, made using the Weather Research and Forecasting Model (WRF). The model was used in a manner readily accessible to nonmeteorologists (e.g., accepting default choices and parameters) and with a relatively large spatial resolution for rapid model runs. The simulations were skillful: strong storms were developed having the correct timing and location, generating precipitation rates close to observations, and with precipitation amounts near that observed. The model was then used to examine the sensitivity of the two storms to the topography of the Rocky Mountains. Comparing model results using the actual topographic grid with those of a reduced-mountain grid, it is concluded that a reduction in mountain elevation decreases maximum precipitation by roughly 50% over the mountains and foothills. There was little sensitivity to topography in the precipitation outside the mountains.

Evaluation of WRF model performance in different European regions with the DELTA-FAIRMODE evaluation tool

One-year (2006) WRF model simulations performed at a European scale and ECMWF-IFS forecasts are compared with 10 m wind speed and 2 m temperature observations from around 1,200 surface stations. A statistical evaluation on the modelled meteorological fields is performed using the DELTA software, developed in the framework of FAIRMODE, the forum for air quality modellers relevant to the application of the European Air Quality Directive. In terms of wind speed, ECMWF forecasts are pretty good over most of the domain, while WRF model simulations are less skillful, e.g., they show a larger bias and RMSE. Regarding 2 m temperature, performance criteria are better satisfied by both modelling systems. Finally, the models’ statistics are exemplified in a couple of specific areas: near Berlin, one of the urban areas showing a better model performance, and in the Alpine region, where the model skill is very poor.

The Comparison Study of the Initiation Mechanism of two Convective Cases

Base the NCEP data and numerical simulation way the initiations of two different convective weather scale were comparison studied. The results showed that not only the cold dry air invasion at middle level led by the front shear moving southward but also the uplifting caused by air flow convergence led by the strengthen westward of the subtropical High can arouse wide range of convective weather in Guangxi. The study of the ω equation showed that the contribution of to the vertical velocity is different. The contribution to vertical velocity led by deviation of vorticity advection is more than that from the temperature advection in cold front shear convective style course. The contribution to vertical velocity led by the temperature advection is more than deviation of vorticity advection of the subtropical high style. The WRF model simulation results showed that the vertical integrated liquid reflectivity which occurred in April is large, so it can lead hail. The water vapor which occurred in July was small, so it led damage wind.

Neighborhood-Based Verification of Precipitation Forecasts from Convection-Allowing NCAR WRF Model Simulations and the Operational NAM

Abstract Since 2003 the National Center for Atmospheric Research (NCAR) has been running various experimental convection-allowing configurations of the Weather Research and Forecasting Model (WRF) for domains covering a large portion of the central United States during the warm season (April–July). In this study, the skill of 3-hourly accumulated precipitation forecasts from a large sample of these convection-allowing simulations conducted during 2004–05 and 2007–08 is compared to that from operational North American Mesoscale (NAM) model forecasts using a neighborhood-based equitable threat score (ETS). Separate analyses were conducted for simulations run before and after the implementation in 2007 of positive-definite (PD) moisture transport for the NCAR-WRF simulations. The neighborhood-based ETS (denoted 〈ETS〉r) relaxes the criteria for “hits” (i.e., correct forecasts) by considering grid points within a specified radius r. It is shown that 〈ETS〉r is more useful than the traditional ETS because 〈ETS〉r can be used to diagnose differences in precipitation forecast skill between different models as a function of spatial scale, whereas the traditional ETS only considers the spatial scale of the verification grid. It was found that differences in 〈ETS〉r between NCAR-WRF and NAM generally increased with increasing r, with NCAR-WRF having higher scores. Examining time series of 〈ETS〉r for r = 100 and r = 0 km (which simply reduces to the “traditional” ETS), statistically significant differences between NCAR-WRF and NAM were found at many forecast lead times for 〈ETS〉100 but only a few times for 〈ETS〉0. Larger and more statistically significant differences occurred with the 2007–08 cases relative to the 2004–05 cases. Because of differences in model configurations and dominant large-scale weather regimes, a more controlled experiment would have been needed to diagnose the reason for the larger differences that occurred with the 2007–08 cases. Finally, a compositing technique was used to diagnose the differences in the spatial distribution of the forecasts. This technique implied westward displacement errors for NAM model forecasts in both sets of cases and in NCAR-WRF model forecasts for the 2007–08 cases. Generally, the results are encouraging because they imply that advantages in convection-allowing relative to convection-parameterizing simulations noted in recent studies are reflected in an objective neighborhood-based metric.

Improving the Mellor–Yamada–Janjić Parameterization for wind conditions in the marine planetary boundary layer

The structure of the lower part of the marine planetary boundary-layer (PBL) is relevant not only for climate and numerical weather prediction simulations but also for commercial applications such as offshore wind energy harvesting. A proper description of turbulence might have an important influence on the wind field properties such as the mean wind speed, turbulent fluxes and especially the vertical wind profile. In this study, the Mellor–Yamada–Janjic boundary-layer and surface-layer parameterizations in the Weather Research and Forecasting Model (WRF) were improved by redefining the master length scale (MLS), which controls the diffusion and dissipation of the turbulent fluxes as well as the pressure– temperature and pressure–strain covariances. In the surface layer, the modified MLS is dependent on the surface stability. In the PBL, the surface stability correction of the MLS is included, which has the strongest influence close to the surface. The non-local effects in the stable boundary layer based on surface heat forcing are also included. WRF model simulations with the original and the new PBL parameterization were compared with measurements. Improvements in the wind-shear simulations in the lower part of the boundary layer (up to around 30 m) with the new parameterization have been found, while its impact higher in the PBL is less pronounced. The simulated wind speed is however only slightly dependent on the boundary layer parameterization.

Structure and Evolution of Precipitation along a Cold Front in the Northeastern United States

Abstract Heavy precipitation in the northeastern United States is examined through observational and numerical modeling analyses for a weather system that produced extreme rainfall rates and urban flash flooding over the New York–New Jersey region on 4–5 October 2006. Hydrometeorological analyses combine observations from Weather Surveillance Radar-1988 Doppler (WSR-88D) weather radars, the National Lightning Detection Network, surface observing stations in the northeastern United States, a vertically pointing lidar system, and a Joss–Waldvogel disdrometer with simulations from the Weather Research and Forecasting Model (WRF). Rainfall analyses from the Hydro-Next Generation Weather Radar (NEXRAD) system, based on observations from WSR-88D radars in State College, Pennsylvania, and Fort Dix, New Jersey, and WRF model simulations show that heavy rainfall was organized into long-lived lines of convective precipitation, with associated regions of stratiform precipitation, that develop along a frontal zone. Structure and evolution of convective storm elements that produced extreme rainfall rates over the New York–New Jersey urban corridor were influenced by the complex terrain of the central Appalachians, the diurnal cycle of convection, and the history of convective evolution in the frontal zone. Extreme rainfall rates and flash flooding were produced by a “leading line–trailing stratiform” system that was rapidly dissipating as it passed over the New York–New Jersey region. Radar, disdrometer, and lidar observations are used in combination with model analyses to examine the dynamical and cloud microphysical processes that control the spatial and temporal structure of heavy rainfall. The study illustrates key elements of the spatial and temporal distribution of rainfall that can be used to characterize flash flood hazards in the urban corridor of the northeastern United States.

Mechanisms Supporting Long-Lived Episodes of Propagating Nocturnal Convection within a 7-Day WRF Model Simulation

Abstract A large-domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one-week period of successive zonally propagating heavy precipitation episodes occurring over the central United States. Similar to climatological studies of phase-coherent warm-season convection, the longest-lived precipitation episodes initiate during the late afternoon over the western Great Plains (105°–100°W), reach their greatest intensity at night over the central Great Plains (100°–95°W), and typically weaken around or slightly after sunrise over the Midwest (95°–85°W). The longest-lived episodes exhibit average zonal phase speeds of ∼20 m s−1, consistent with radar observations during the period. Composite analysis of the life cycle of five long-lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east–west-oriented lower-tropospheric frontal zone. An elevated ∼2-km-deep layer of high-θe air helps sustain convection during its period of greatest organization overnight. Trajectory analysis for individual episodes reveals that the high-θe air originates both from within the frontal zone and to its south where, in this latter case, it is transported northward by the nocturnal low-level jet (LLJ). The mature (nocturnal) stage composite evinces a thermally direct cross-frontal circulation, within which the trajectories ascend 0.5–2 km to produce the elevated conditionally unstable layer. This transverse vertical circulation is forced by deformation frontogenesis, which itself is supported by the intensification of the nocturnal LLJ. The frontal zone also provides an environment of strong vertical shear, which helps organize the zonally propagating component of convection. Overnight the convection exhibits squall-line characteristics, where its phase speed is typically consistent with that which arises from deep convectively induced buoyancy perturbations combined with the opposing environmental surface flow. In a large majority of cases convection weakens as it reaches the Midwest around sunrise, where environmental thermodynamic stability is greater, and environmental vertical shear, frontogenesis, and vertical motions are weaker than those located farther west overnight.