Predicting strong local wind with high-resolution nonhydrostatic numerical weather prediction model

A climatological and numerical study of “Karakkaze,” a type of local wind in Japan, was conducted. First, winter days under a winter-type synoptic pressure pattern with daily minimum relative humidity of less than 40% were classified according to strong wind (wind speed ≥ 9 ms-1, Karakkaze day), medium wind (6 ms-1 ≤ wind speed -1), and weak wind (wind speed -1). Secondly, the spatial patterns of the surface wind in each category are confirmed by the Japan Meteorological Agency (JMA)-Automated Meteorological Data Acquisition System (AMeDAS) observation data. In addition, we compared the boundary-layer wind of the three categories using wind speed data from the observation tower of the Meteorological Research Institute (MRI) in Tsukuba and from the JMA wind profiler in Kumagaya. Finally, we performed one-dimensional numerical experiments using a column Planetary Boundary Layer (PBL) model to evaluate the impact of solar radiation and upper-level wind on the formation of the Karakkaze. The results are summarized as follows. On the strong-wind days, strong northwesterly winds appear in the area along the Arakawa River and the Tonegawa River from Maebashi. The surface wind speed has a clear diurnal variation with a peak in the early afternoon. Such a diurnal variation is observed up to a 200 m level, but this diurnal pattern nearly reverses itself between 200 m and 400 m levels. On weak-wind days, the diurnal variation pattern is similar to that in other two categories, but the reversed pattern appears at a 100 m level, not at a 200 m level. Stronger surface winds appear under a clearer winter-type pressure pattern. The correlation coefficient is 0.632 between the daily maximum surface wind speed and the daily mean wind speed at a height of 2759 m, whereas the correlation is 0.284 between the surface wind speed and daily sunshine duration, which, in a previous study, was suggested to be the most significant factor. Numerical experiments indicate that a large amount of solar radiation is a necessary condition for a strong daytime wind, but this is not a sufficient explanation for the difference in the surface wind speed between weak and strong-wind days.

Dust weather conditions with strong wind often occur in Beijing during spring. The number of duststorms in Beijing has declined since 1954. In the last two years, however, the number of duststorms in Beijing has increased drastically due to frequent cold air movements and the deteriorated ecological environment. The heaviest duststorm in the last decade in Beijing occurred on 6 April 2000. It was caused by the development of a high-level small trough along with strong northwest wind behind a large trough that moved eastward and developed a strong cold air system. A non-hydrostatic meso-scale numerical model (MM5) gave a good forecast to the duststorm's major features and movement. Simulated results showed that there was a strong vertical ascending motion with a strong positive vorticity center in mid and low level. The mesoscale structure's formation and development and movement were consistent with the passage and moving direction of the duststorm. At the same time, the trajectories of air particles during this duststorm event and another case (on 27 March 2000) were estimated and analyzed, which had the comparable results with the real situation and the trajectory monitored by meteorological satellite. The trajectories analysis could help us study the transport track and sand source to some extent.

Improving the Turbulence Coupling between High Resolution Numerical Weather Prediction Models and Lagrangian Particle Dispersion Models

Area-averaged estimates of Cn2 from high-resolution numerical weather prediction (NWP) model output are produced from local estimates of the spatial structure functions of refractive index with corrections for the inherent smoothing and filtering effects of the underlying NWP model. The key assumptions are the existence of a universal statistical description of small-scale turbulence and a locally universal spatial filter for the NWP model variables. Under these assumptions, spatial structure functions of the NWP model variables can be related to the structure functions of the atmospheric variables and extended to the smaller underresolved scales. The shape of the universal spatial filter is determined by comparisons of model structure functions with the climatological spatial structure function determined from an archive of aircraft data collected in the upper troposphere and lower stratosphere. This method of computing Cn2 has an important advantage over more traditional methods that are based on vertical differences because the structure function–based estimates avoid reference to the turbulence outer length scale. To evaluate the technique, NWP model–derived structure-function estimates of Cn2 are compared with nighttime profiles of Cn2 derived from temperature structure-function sensors attached to a rawinsonde (thermosonde) near Holloman Air Force Base in the United States.

Abrupt changes in wind direction and speed can dramatically impact wildfire development and spread, endangering firefighters. A frequent cause of such wind shifts is outflow from thunderstorms and organised convective systems; thus, their identification and prediction present critical challenges for fire weather forecasters. Here, we develop a methodology and implement it in a software tool that can identify and depict convective outflow boundaries in high-resolution numerical weather prediction (NWP) models to provide guidance for fire weather forecasting. The tool can process model output, objectively identify gust fronts, and graphically display detected gust fronts and similar boundaries in NWP model forecasts. The tool is demonstrated with output from the Weather Research and Forecasting (WRF) model from the operational High-Resolution Rapid Refresh (HRRR) forecasting system and from a WRF ensemble run at the National Center for Atmospheric Research that can provide probabilistic information about model-predicted gust fronts. The tool can identify outflow boundaries in model forecasts of convective events occurring in both simple and complex terrain, both with and without concurrent wildfire activity. With accurate underlying model forecast output, the tool can reliably reveal areas of potential gust front activity and thus provide valuable guidance to incident meteorologists and command personnel.

Radar ducting is caused by sharp vertical changes in temperature and, especially, water vapor at the top of the atmospheric boundary layer, both of which are sensitive to variations in the underlying surface conditions, local mesoscale weather, and synoptic weather patterns. High-resolution numerical weather prediction (NWP) models offer an alternative to observation to determine boundary layer (BL) structure and to assess the spatial variability of radar ducts. The benefit of using NWP models for simulating ducting conditions very much depends on the initial state of sea surface temperature (SST) and the model spinup time, both of which have a great impact on BL structure. This study investigates the effects of variation of NWP-model initial conditions and simulation length on the accuracy of simulating the atmosphere’s refractive index over the Wallops Island, Virginia, region, which has pronounced SST variability and complex BL structure. The Met Office Unified Model (MetUM) with horizontal resolution of 4 km (4-km model) was used to simulate the atmospheric environment when observations were made during the Wallops-2000 experiment. Sensitivity tests were conducted in terms of the variability of SST and model spinup time. The evaluation of the model results was focused on low-level moisture, temperature, and surface ducting characteristics including the frequency, strength, and the height of the ducting layer. When provided with more accurate SST and adequate simulation length, the MetUM 4-km model demonstrated an improved ability to predict the observed ducting.

Influence of dust accumulation on building roof thermal performance and radiant heat gain in hot-dry climates

NUMERICAL MODELS | Regional Prediction Models

A diagnostic study of a summer convective precipitation event in the Czech Republic using a nonhydrostatic NWP model

The limited atmospheric predictability has been addressed by the development of ensemble prediction systems (EPS) that are now routinely applied for medium-range synoptic-scale numerical weather prediction (NWP). With the increase of computational power, interest is growing in the design of high-resolution (cloud resolving) NWP models and their associated short-range EPS. This development raises a series of fundamental questions, especially concerning the type of error growth and the validity of the tangent-linear approximation. To address these issues, a comparison between perturbed medium-range (10 day) synoptic-scale integrations (taken from the operational ECMWF EPS with a horizontal resolution of about 80 km) and short-range (1 day) high-resolution simulations (based on the Lokal Modell of the Consortium for Small-Scale Modeling with a grid spacing of 2.2 km) is conducted. The differences between the two systems are interpreted in a nondimensional sense and illustrated with the help of the Lorenz att...

Calibrated high-temporal-resolution precipitation forecasts are desirable for a range of applications, for example, flood prediction in fast-rising rivers. However, high-temporal-resolution precipitation observations may not be available to support the establishment of calibration methods, particularly in regions with low population density or in developing countries. We present a new method to produce calibrated hourly precipitation ensemble forecasts from daily observations. Precipitation forecasts are taken from a high-resolution convective-scale numerical weather prediction (NWP) model run at the hourly time step. We conduct three experiments to develop the new calibration method: (i) calibrate daily precipitation totals and disaggregate daily forecasts to hourly; (ii) generate pseudohourly observations from daily precipitation observations, and use these to calibrate hourly precipitation forecasts; and (iii) combine aspects of (i) and (ii). In all experiments, we use the existing Bayesian joint probability model to calibrate the forecasts and the well-known Schaake shuffle technique to instill realistic spatial and temporal correlations in the ensembles. As hourly observations are not available, we use hourly patterns from the NWP as the template for the Schaake shuffle. The daily member matching method (DMM), method (iii), produces the best-performing ensemble precipitation forecasts over a range of metrics for forecast accuracy, bias, and reliability. The DMM method performs very similarly to the ideal case where hourly observations are available to calibrate forecasts. Overall, valuable spatial and temporal information from the forecast can be extracted for calibration with daily data, with a slight trade-off between forecast bias and reliability.

The 1998 Sydney-Hobart yacht race was affected by storm-force winds on the afternoon of 27 December 1998. Of the 115 yachts that contested the race, 71 retired, six were abandoned, five sank, 55 crew were rescued, and six lives were lost. This band of storm-force winds was associated with a secondary cyclone that developed on the western side of an occluding deep-tropospheric low-pressure system. A high-resolution (approximately 5 km) horizontal resolution mesoscale numerical weather prediction model is used to model this storm, and is shown to reproduce many of its observed features. These model fields are then used to both diagnose the processes that led to the development of the low and to describe its mesoscale structure and evolution. The synoptic environment prior to the development of the storm shows an amplifying mid-latitude trough that negatively tilted and cut off as an equatorward-directed jet stream on its western flank moved to the apex of the trough. An unusual feature of this case was the interaction of this developing cut-off low with the circulation of a lower-latitude extratropical cyclone in the northern Tasman Sea. This interaction led to the development of a strong thermal gradient through Bass Strait prior to the development of the secondary low, with the direction of the gradient directed opposite to the direction of the pressure gradient – a reversed shear structure. Warm, moist air moving over Tasmania from the northeast also provided a deep layer of weak static stability in the lower troposphere over Tasmania and southern Bass Strait. As the upper-tropospheric low cut off, a filament of cyclonic Isentropic Potential Vorticity (IPV) air was advected to the rear poleward side of this low. The development of the secondary low is hypothesised to be associated with the vertical circulation induced by the advection of this IPV filament over the weak static stability air-mass over Bass Strait. The evolution of this IPV filament in the forecast model is supported by the water vapour imagery from the Japanese Geostationary Meteorological Satellite (GMS-5), which shows dark (dry) features in close agreement with the IPV patterns. The secondary low had a warm-core structure in its lowest levels, but this was overlain by a cold-core in the middle troposphere. The reversed shear environment on the northern flank of the low led to an extreme low-level jet.

Two types of vortex disturbance over the Japan Sea in winter are well simulated in the case of 23-24 January 1990, with a high-resolution (6.0km grid at 36° N) one-way triple-nested hydrostatic numerical weather prediction model. They take different tracks with a very short interval. One is generated over the Continent of Asia and advances toward the east-northeast over the Japan Sea. Its vorticity center is found at a middle layer (650-700hPa) at first, and then shifts slowly down to a lower layer (∼950hPa). It has a multi-scale structure in which a meso-β-scale vortex is embedded in a meso-α-scale one. The other type of disturbance is originally generated over the northwestern part of the Japan Sea and advances toward the southeast along the topographically-induced horizontal shear line associated with the Japan-Sea Polar-airmass Convergence Zone (JPCZ) in the lower layer. The disturbance consists of several meso-β-scale vortices in line with their vorticity maxima in the lower layer. These characteristics of each disturbance are related to its own synoptic conditions, the key features of which are summarized in a scenario of evolution of the vortex disturbances.

A numerical simulation of the strong southeasterly (SE) wind in Yamagata, Japan on August 30, 2004, is examined and compared to the Coherent Doppler Lidar (CDL) observation. Three-dimensional numerical simulations were performed using a non-hydrostatic meso-scale model developed by the Japan Meteorological Agency. The sensitivity of the numerical simulation was examined with respect to the resolution of the horizontal and vertical grid, surface roughness, and ground surface temperature. For the case of a 1-km grid with 85 vertical layers including realistic surface settings, the observed characteristics of Kiyokawa-dashi were well reproduced: the strong SE wind (10 m s -1 ) was extremely low, about 0.1-0.5 km AGL, and the maximum wind speed over 12 m s -1 was observed at 0.1-0.2 km, AGL, under the low-level stable layer. Strong winds appeared at the foot of the final lee slope facing the Shonai Plain and 1 km south of the valley; this result was in good agreement with the CDL observation. On the basis of the stream line analysis, Kiyokawa-dashi was strongly affected by the upper SE wind, i.e., upper air on the eastern side of the mountains blew down to the middle and lower layer on the western side. The strongest wind field located 1 km south of the valley was affected by the mountain at the southwestern end of the valley. It was observed from the sensitivity experiments that the height of the jet-like flow was found to be closely related to the low-level stable layer, and the critical layer was not very important in reinforcing the low-level SE wind. This case of Kiyokawa-dashi was explained on the basis of the hydraulic theory.

A wind-driven circulation model of the Tyrrhenian Sea area