Abstract Three idealized high‐resolution simulations of tropical storm formation from a weak vortex are analyzed. The three simulations include a case using warm rain microphysics, a similar case in which surface friction is omitted, and a case in which ice microphysics is used. The goal is to understand the mechanisms controlling the intensity and distribution of convection in the formation process in each of these cases. Simulations of convection in weak temperature gradient convective models show that a combination of low to middle tropospheric moist convective instability, the saturation fraction or column relative humidity, and the surface moist entropy flux explain a high percentage of the variance in precipitation and lower tropospheric vertical mass flux. Tropical cyclones differ from other convective environments in that intense frictional convergence occurs in the boundary layer. Adding a measure of convective inhibition to account for this process enables the lower tropospheric mass flux to be predicted even in the core regions of the simulated tropical cyclones. These results are pertinent to the development of more accurate convective parameterizations for large‐scale models.
Abstract We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of boundary‐layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm‐cored disturbances such as tropical depressions, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle‐latitude ‘supercell’ storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but to a reversal also in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.
Abstract An idealized, three‐dimensional, 1 km horizontal grid spacing numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin‐up process within the inner‐core boundary layer and eyewall regions of the system‐scale vortex. Unbalanced and non‐axisymmetric processes are prominent features of the rapid spin‐up process. In particular, the wind asymmetries, associated in part with the asymmetric deep convection, make a substantive contribution ( ∼ 30%) to the maximum wind speed inside the radius of this maximum. The analysis provides a novel explanation for inflow jets sandwiching the upper‐tropospheric outflow layer which are frequently found in numerical model simulations. In addition, it provides an opportunity to assess the applicability of generalized Ekman balance during rapid vortex spin‐up. The maximum tangential wind occurs within and near the top of the frictional inflow layer and as much as 10 km inside the maximum gradient wind. Spin‐up in the friction layer is accompanied by supergradient winds that exceed the gradient wind by up to 20%. Overall, the results affirm prior work pointing to significant limitations of a purely axisymmetric balance description, for example, gradient balance/Ekman balance, when applied to a rapidly intensifying tropical cyclone.
Abstract. Providing observed date- and site-specific turbulent inflow fields for Large-eddy simulations (LES) of the flow through wind turbines becomes more and more important for realistic estimates of power production. In this study, data assimilation techniques are used to adapt the atmospheric inflow field towards measurement data. A Newtonian relaxation technique and a vibration assimilation method are implemented in the geophysical flow solver EULAG. Their capability of adapting mean wind profiles towards field measurements while maintaining the atmospheric turbulence of an idealized LES is investigated. The sensitivity of the methods to grid refinement and to parameter changes is analysed. The performance of the vibration assimilation technique is better suited for fine grids (dx=dy=dz=5 m) because of smaller damping effects on the atmospheric turbulence. Furthermore, the vibration method is used to nudge the inflow field of an idealized atmospheric simulation towards velocity profiles measured at the wind-farm site WiValdi at Krummendeich. A near neutral stratification is chosen from the measurements to test the assimilation technique. With the vibration assimilation method it is possible to adapt the zonal and meridional velocity components of an atmospheric flow. The LESs applying data assimilation are compared with the measurements and independent mesoscale simulations. A good accordance is found for the mean inflow velocity profiles and the turbulence intensities. In a final step, the assimilated flow field is taken as inflow for a wind-turbine simulation. The windturbine simulation shows characteristic structures of a wake in the atmospheric boundary layer. This study demonstrates that an efficient computing of different and realistic inflow fields for wind-turbine simulations is possible applying the vibration assimilation method.
Abstract We present idealized numerical model experiments to investigate the convective generation of vertical vorticity in a tropical depression. The ambient vertical vorticity is represented by a uniform solid‐body rotation. The calculations are motivated by observations made during the Pre‐Depression Investigation of Cloud‐systems in the Tropics (PREDICT) experiment. A specific aim is to isolate and quantify the effects of low‐ to mid‐level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. The results do not support a common perception that dry air aloft produces stronger convective downdraughts and more intense, cold‐air outflows therefrom. Indeed, we find that dry air aloft weakens both updraughts and downdraughts, corroborating the recent results of James and Markowski. As in the recent calculations of Wissmeier and Smith, the growing convective cells locally amplify the ambient rotation at low levels by more than an order of magnitude and this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Moreover, significant amplification of vorticity occurs even for clouds of only moderate vertical extent. The maximum amplification of vorticity is relatively insensitive to the maximum updraught strength, or the height at which it occurs, and it is not unduly affected by the presence of dry air aloft. Thus the presence of dry air is not detrimental to the amplification of low‐level vorticity, although it reduces the depth through which ambient vorticity is enhanced. Results for a limited number of different environmental soundings indicate that the maximum amplification of vorticity increases monotonically with the strength of the thermal perturbation that initiates the convection, but the amount of increase depends also on the thermodynamic structure of the sounding.
Convective cold pools routinely pass over the dense network of wind turbines in northern Germany, causing short-term changes in boundary-layer wind speeds (i.e., wind ramp events) and atmospheric stability. These large, rapid, and more-localized variations in the low-level kinematic and thermodynamic structure are difficult for numerical weather prediction models to forecast with sufficient spatial and temporal accuracy for utilization by wind turbine operators. As boundary-layer stability and winds strongly influence wind turbine structural loads, downstream turbulent wake behavior, and power generation, it is important to better understand how rapid changes in dynamic processes evolve within the vertical layer of wind turbine rotor blades (~50 - 150 meters altitude). Using in-situ observations and high-resolution modeling focused on the WiValdi research wind park in Krummendeich, Germany, we examine how convective cold pool passages during July 2023 impact the inflow and turbulent wakes for two installed turbines with a hub height of 92 meters. Meteorological mast, Doppler wind lidar, and microwave radiometer observations provide upstream and downstream measurements of stability, vertical shear, and turbulence variations at ~1-minute resolution. While this measurement coverage adequately captures the cold pool evolution relative to each turbine, we remain somewhat limited by the fixed instrument locations for measuring upstream conditions and the three-dimensional turbulent wake structure. Therefore, we also utilize the mesoscale model WRF in large-eddy-simulation mode, with inserted generalized actuator disks acting as proxy wind turbines, to analyze far-upstream inflow conditions and three-dimensional wake characteristics during cold pool passages. The proposed work will provide a foundation for future analysis which will more robustly verify WRF output using additional WiValdi instrumentation.
An announcement of a seminar hosted by NPS Department of Meteorology, and presented by Gerard Kilroy of the University of Munich, LMU; Host, Professor Michael T. Montgomery.
The formation of tropical cyclones within a few degrees latitude of the Equator is investigated using European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses of some prominent cyclogenesis events there. The possibility of formation at the Equator is demonstrated also using idealized model simulations, starting from a prescribed, weak (maximum wind speed 5 m/s) initial counter‐clockwise vortex in an otherwise quiescent, non‐rotating environment. In the real events investigated, vortex formation occurred within a broadscale counter‐clockwise flow that encompasses a region of predominantly positive absolute vertical vorticity typically extending more than 5° south of the Equator. Patches of enhanced vertical vorticity form within this region as a result of vorticity stretching by deep convection. These vorticity patches are organized by the convection, the collective effects of which produce an overturning circulation that fluxes vorticity at low levels towards some centre within the convective region. By Stokes' theorem, the tangential circulation about circles of fixed radius around this centre increase and the vortex spins up. This process of spin‐up is the same as that which occurs away from the Equator.
Abstract. Providing observed date- and site-specific turbulent inflow fields for Large-eddy simulations (LES) of the flow through wind turbines becomes more and more important for realistic estimates of power production. In this study, data assimilation techniques are used to adapt the atmospheric inflow field towards measurement data. A Newtonian relaxation technique and a vibration assimilation method are implemented in the geophysical flow solver EULAG. Their capability of adapting mean wind profiles towards field measurements while maintaining the atmospheric turbulence of an idealized LES is investigated. The sensitivity of the methods to grid refinement and to parameter changes is analysed. The performance of the vibration assimilation technique is better suited for fine grids (dx=dy=dz=5 m) because of smaller damping effects on the atmospheric turbulence. Furthermore, the vibration method is used to nudge the inflow field of an idealized atmospheric simulation towards velocity profiles measured at the wind-farm site WiValdi at Krummendeich. A near neutral stratification is chosen from the measurements to test the assimilation technique. With the vibration assimilation method it is possible to adapt the zonal and meridional velocity components of an atmospheric flow. The LESs applying data assimilation are compared with the measurements and independent mesoscale simulations. A good accordance is found for the mean inflow velocity profiles and the turbulence intensities. In a final step, the assimilated flow field is taken as inflow for a wind-turbine simulation. The windturbine simulation shows characteristic structures of a wake in the atmospheric boundary layer. This study demonstrates that an efficient computing of different and realistic inflow fields for wind-turbine simulations is possible applying the vibration assimilation method.
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