Claire Louise Vincent

Resolving Nonstationary Spectral Information in Wind Speed Time Series Using the Hilbert-Huang Transform

Abstract This work is motivated by the observation that large-amplitude wind fluctuations on temporal scales of 1–10 h present challenges for the power management of large offshore wind farms. Wind fluctuations on these scales are analyzed at a meteorological measurement mast in the Danish North Sea using a 4-yr time series of 10-min wind speed observations. An adaptive spectral analysis method called the Hilbert–Huang transform is chosen for the analysis, because the nonstationarity of time series of wind speed observations means that they are not well described by a global spectral analysis method such as the Fourier transform. The Hilbert–Huang transform is a local method based on a nonparametric and empirical decomposition of the data followed by calculation of instantaneous amplitudes and frequencies using the Hilbert transform. The Hilbert–Huang transformed 4-yr time series is averaged and summarized to show climatological patterns in the relationship between wind variability and time of day. First,...

The Wind Profile in the Coastal Boundary Layer: Wind Lidar Measurements and Numerical Modelling

Traditionally it has been difficult to verify mesoscale model wind predictions against observations in the planetary boundary layer (PBL). Here we used measurements from a wind lidar to study the PBL up to 800 m above the surface at a flat coastal site in Denmark during a one month period in autumn. We ran the Weather Research and Forecasting numerical model with two different roughness descriptions over land, two different synoptic forcings and two different PBL schemes at two vertical resolutions and evaluated the wind profile against observations from the wind lidar. The simulated wind profile did not have enough vertical shear in the lower part of the PBL and also had a negative bias higher up in the boundary layer. Near the surface the internal boundary layer and the surface roughness influenced the wind speed, while higher up it was only influenced by the choice of PBL scheme and the synoptic forcing. By replacing the roughness value for the land-use category in the model with a more representative mesoscale roughness, the observed bias in friction velocity was reduced. A higher-order PBL scheme simulated the wind profile from the west with a lower wind-speed bias at the top of the PBL. For easterly winds low-level jets contributed to a negative wind-speed bias around 300 m and were better simulated by the first-order scheme. In all simulations, the wind-profile shape, wind speed and turbulent fluxes were not improved when a higher vertical resolution or different synoptic forcing were used.

Evolution of the Diurnal Precipitation Cycle with the Passage of a Madden-Julian Oscillation Event through the Maritime Continent

AbstractChanges in the diurnal precipitation cycle as the Madden–Julian oscillation (MJO) propagates through the Maritime Continent are investigated to explore the processes behind seaward-propagating precipitation northeast of New Guinea. Satellite rainfall estimates from TRMM 3B42 and the Climate Prediction Center morphing technique (CMORPH) are combined with simulations from the Weather Research and Forecasting (WRF) Model with a horizontal resolution of 4 km.Comparison with 24-h rain gauge measurements indicates that both satellite estimates and the WRF Model exhibit systematic biases. Despite these biases, the changing patterns of offshore precipitation with the passage of the MJO show good consistency between satellite estimates and the WRF Model. In the few days prior to the main MJO envelope, light background wind, relatively clear skies, and an increasingly moist environment promote favorable conditions for the diurnal precipitation cycle.Two distinct processes are identified: 100–200 km from the...

The Impact of Grid and Spectral Nudging on the Variance of the Near-Surface Wind Speed

AbstractGrid and spectral nudging are effective ways of preventing drift from large-scale weather patterns in regional climate models. However, the effect of nudging on the wind speed variance is unclear. In this study, the impact of grid and spectral nudging on near-surface and upper boundary layer wind variance in the Weather Research and Forecasting Model is analyzed. Simulations are run on nested domains with horizontal grid spacing of 15 and 5 km over the Baltic Sea region. For the 15-km domain, 36-h simulations initialized each day are compared with 11-day simulations with either grid or spectral nudging at and above 1150 m above ground level (AGL). Nested 5-km simulations are not nudged directly but inherit boundary conditions from the 15-km experiments. Spatial and temporal spectra show that grid nudging causes smoothing of the wind in the 15-km domain at all wavenumbers, both at 1150 m AGL and near the surface where nudging is not applied directly, while spectral nudging mainly affects longer wav...

Idealized Mesoscale Model Simulations of Open Cellular Convection Over the Sea

The atmospheric conditions during an observed case of open cellular convection over the North Sea were simulated using the Weather Research and Forecasting (WRF) numerical model. Wind, temperature and water vapour mixing ratio profiles from the WRF simulation were used to initialize an idealized version of the model, which excluded the effects of topography, surface inhomogeneities and large-scale weather forcing. Cells with an average diameter of 17.4 km developed. Simulations both with and without a capping inversion were made, and the cell-scale kinetic energy budget was calculated for each case. By considering all sources of explicit diffusion in the model, the budgets were balanced. In comparison with previous work based on observational studies, the use of three-dimensional, gridded model data afforded the possibility of calculating all terms in the budgets, which showed that the important terms in the budgets were buoyancy, pressure balance and inter-scale transfer to subgrid scales. Cells were also composited to calculate the average cell-scale flow and each of the budget terms on two-dimensional cross-sections through the cells, parallel and perpendicular to the mean wind direction.

A 10-Year Austral Summer Climatology of Observed and Modeled Intraseasonal, Mesoscale, and Diurnal Variations over the Maritime Continent

AbstractThe Maritime Continent is one of the wettest regions on the planet and has been shown to be important for global budgets of heat and moisture. Convection in the region, however, varies on several interrelated scales, making it difficult to quantify the precipitation climate and understand the key processes. For example, the diurnal cycle in precipitation over the land varies substantially according to the phase of the Madden–Julian oscillation (MJO), and the diurnal precipitation cycle over the water is coupled to that over the land, in some cases for distances of over 1000 km from the coast.Here, a 10-yr austral summer climatology of diurnal and MJO-scale variations in rain rate over the land and sea over the Maritime Continent is presented. The climatology is based on mesoscale model simulations with a horizontal grid length of 4 km and satellite precipitation estimates. The amplitude of the observed diurnal precipitation cycle is shown to reach a maximum just prior to the MJO active phase, with...

A local index of Maritime Continent intraseasonal variability based on rain rates over the land and sea

A local index for describing intraseasonal variability over the Maritime Continent is developed. The index is based on the ratio of area-averaged rain-rate over the land to that over the sea. It takes advantage of the fact that the main convective envelope of intraseasonal variability events tends to modulate the diurnal precipitation cycle over the land over the entire Maritime Continent. Lagged analysis is used to create composite intraseasonal variability events, where ‘day 0’ is chosen according to when the normalized rain-rate over the sea becomes greater than that over the land. The index identifies intraseasonal variability events associated with the MJO as well as equatorial Kelvin waves and westward propagating equatorial Rossby waves. The results suggest a similar local impact of all such events in suppressing the rain-rate over land relative to that over the sea when the main convective envelope approaches.

Forecasting intrahourly variability of wind generation

Intrahourly fluctuations in wind power generation cause various technical and economic challenges for wind farm integration. With increasing levels of wind energy uptake, managing these fluctuations is likely to require further attention. Intrahourly fluctuations in wind power are caused by fluctuations in wind speed that can be linked with a range of atmospheric phenomena. Explicitly forecasting intrahourly wind variability requires both statistical and physical modelling. Predictors of wind and power variability include large-scale weather patterns, the wind speed timeseries itself, and information about approaching weather regimes. Numerical weather prediction models have been shown to be a useful tool for day-ahead forecasts of intrahourly variability. A promising application of intrahourly variability forecasts is the management of variable wind farm output using a flexible dispatch margin.

Measurements and Modeling of the Wind Profile Up to 600 meters at a Flat Coastal Site

This study shows long-term ABL wind profile features by comparing long-range wind lidar measurements and the output from a mesoscale model. The study is based on 1-year pulsed lidar (Wind Cube 70) measurements of wind speed and direction from 100 to 600 m with vertical resolution of 50 m and time resolution of 10 min at a coastal site on the West coast of Denmark and WRF ARW (NCAR) simulations for the same period. The model evaluation is performed based on wind speed, wind direction, as well as statistical parameters of the Weibull distribution of the wind speed time series as function of height. It is found that (1) WRF is generally under predicting both the profiles of the measured wind speed, direction and power density, (2) the scatter of observations to model results of the wind speed does not change significantly with height between 100 and 600 m, and (3) the scale (A) and shape (k) parameters of the Weibull distribution above 100 m. The latter signifies that the model suggests a wider distribution in the wind speed compared to measurements.