Application of a new wind gust parameterization: Multiscale case studies performed with the Canadian regional climate model

Abstract

[1] The implementation of a physically based parameterization scheme for computation of wind gusts in a numerical regional climate model (RCM) is described in this paper. The method is based on an innovative approach proposed by Brasseur [2001] that assumes that gusts occurring at the surface result from the deflection of air parcels flowing higher in the boundary layer. Our parameterization scheme is developed so as to use quantities available at each model time step: consequently, the gusts are also computed for each of these time steps. To illustrate the performances of this novel method, gusts simulated for two severe midlatitude windstorms with the Canadian RCM at various resolutions are compared with observed gust speeds. The study is carried out concurrently for the complex terrain of Switzerland and for the smoother topography of Belgium. A preliminary analysis indicates that this parameterization performs equally well over flat and over mountainous regions; it also responds properly in the strengthening as well as the weakening phases of wind storms. The storm-dependent results rely on the model configuration associated with the downscaling procedure, as well as on the accuracy of the simulated flow fields. The model response is dependent on the resolved topography distribution and height and on the types of lower boundary conditions that affect the stability of the boundary layer. The simulated gusts are generally more realistic at higher resolution over the complex topography of Switzerland but are less sensitive to resolution over the flat terrain as in Belgium. On the basis of these two storms, this study also shows that simple scaling coefficients relating gust speeds and resolution are not an appropriate method for addressing such issues.

Assessment of extreme wind speeds from Regional Climate Models – Part 1: Estimation of return values and their evaluation

“…As shown in several studies, highresolution limited area models are able to reproduce reliable wind gusts if a physically based gust wind parameterisation is considered (Goyette et al, 2003;Pinto et al, 2009;Schwierz et al, 2009). The lack of such a parameterisation in the CCLM runs is the main reason for the significant underestimation of the gusts compared to REMO.…”

Relating forest damage data to the wind field from high-resolution RCM simulations: Case study of Anatol striking Sweden in December 1999

“…The modelling approach uses a self-nesting methodology to downscale from the coarse input resolution to the high-resolution output that captures the wind speed and direction (Benoit et al, 1997;Goyette et al, 2001). Recently, a physically-based parameterisation for diagnostic "on-line" computation of wind gusts have been implemented in the Canadian RCM, CRCM hereinafter (Goyette et al, 2003). This parameterisation was recently found to give a good general representation of the wind speed distribution over land (Rockel, 2005;Rockel and Woth, 2007).…”

“…Regional climate models (RCMs) using multiple nesting techniques driven by reanalysis data has shown genuine skill to downscale windstorms over complex terrain such as the February 27, 1990 "Vivian", and the December 26, 1999 "Lothar" storms over Switzerland as well as over the smoother terrain of Belgium (Goyette et al, 2001;Goyette et al, 2003). The modelling approach uses a self-nesting methodology to downscale from the coarse input resolution to the high-resolution output that captures the wind speed and direction (Benoit et al, 1997;Goyette et al, 2001).…”

Climate risks and their impact on agriculture and forests in Switzerland

“…Results from climate models can either be used for numerical downscaling making use of nested RCMs (e.g., Goyette et al 2003), or in combination with local weather data through statistical downscaling to the temporal and spatial resolution of a few kilometers (e.g., Wilby Fig. 2 Relationship between wind speed and storm damage to Swiss forests.…”

“…The so-called 'medium' resolution RCMs (resolution ∼50 km) cannot be used directly as such to infer the change in wind speed at the very fine scales. As a second step, numerical downscaling of re-analysis data using RCMs with a wind gust parameterization is necessary to reproduce the strong winds in a number of documented storms (e.g., Goyette et al 2001Goyette et al , 2003). Simulated hourly means may then be compared with observations if grid spacing is on the order of 1-2 km.…”