The bimodal downslope windstorms at Kvísker

Downslope windstorms at Kvísker in Southeast Iceland are explored using a mesoscale model, observations and numerical analysis of the atmosphere. Two different types of gravity-wave induced windstorms are identified. At the surface, their main difference is in the horizontal extent of the lee-side accelerated flow. Type S (Short) is a westerly windstorm, which is confined to the lee-slopes of Mount Öræfajökull, while a Type E (Extended) windstorm occurs in the northerly flow and is not confined to the lee-slopes but continues some distance downstream of the mountain. The Type S windstorm may be characterized as a more pure gravity-wave generated windstorm than the Type E windstorm which bears a greater resemblance to local flow acceleration described by hydraulic theory. The low-level flow in the Type E windstorm is of arctic origin and close to neutral with an inversion well above the mountain top level. At middle tropospheric levels there is a reverse vertical windshear. The Type S windstorm occurs in airmasses of southerly origin. It also has a well-mixed, but a shallower boundary-layer than the Type E windstorms. Aloft, the winds increase with height and there is an amplified gravity wave. Climate projections indicate a possible decrease in windstorm frequency up to the year 2050.

[1]  D. Durran,et al.  Initial-Condition Sensitivities and the Predictability of Downslope Winds , 2009 .

[2]  Robert M. Banta,et al.  Two- and Three-Dimensional Simulations of the 9 January 1989 Severe Boulder Windstorm: Comparison with Observations , 1994 .

[3]  Fotini V. Katopodes,et al.  The terrain-induced rotor experiment: A field campaign overview including observational highlights , 2008 .

[4]  Hálfdán Ágústsson,et al.  Simulating a severe windstorm in complex terrain , 2007 .

[5]  Branko Grisogono,et al.  A review of recent advances in understanding the meso‐ and microscale properties of the severe Bora wind , 2009 .

[6]  R. Rasmussen,et al.  Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model , 1998 .

[7]  G. Grell,et al.  A description of the fifth-generation Penn State/NCAR Mesoscale Model (MM5) , 1994 .

[8]  James D. Doyle,et al.  Large-Amplitude Mountain Wave Breaking over Greenland , 2005 .

[9]  C MiroslavaPasari´.  Quasi-periodic bora gusts related to the structure of the troposphere , 2004 .

[10]  H. Ólafsson,et al.  Mean gust factors in complex terrain , 2004 .

[11]  Statistics and Dynamics of Aircraft Encounters of Turbulence over Greenland , 2009 .

[12]  Richard Swinbank,et al.  THE GREENLAND FLOW DISTORTION EXPERIMENT , 2008 .

[13]  Chris Hewitt,et al.  Ensembles-based predictions of climate changes and their impacts , 2004 .

[14]  H. Ólafsson,et al.  Gravity wave breaking in easterly flow over Greenland and associated low level barrier- and reverse tip-jets , 2009 .

[15]  Z. Janjic.  The Step-Mountain Eta Coordinate Model: Further Developments of the Convection, Viscous Sublayer, and Turbulence Closure Schemes , 1994 .

[16]  H. Ólafsson,et al.  Downslope windstorm in Iceland - WRF/MM5 model comparison , 2008 .

[17]  J. Otkin,et al.  Understanding Satellite-Observed Mountain-Wave Signatures Using High-Resolution Numerical Model Data , 2009 .

[18]  Rolf F. Hertenstein,et al.  Rotor types associated with steep lee topography: influence of the wind profile , 2005 .

[19]  M. Xue,et al.  An intercomparison of model predicted wave breaking for the 11 January 1972 Boulder windstorm , 2000 .

[20]  Response in extremes of daily precipitation and wind from a downscaled multi-model ensemble of anthropogenic global climate change scenarios , 2008 .

[21]  Dale R. Durran,et al.  Mountain Waves and Downslope Winds , 1990 .

[22]  Precipitation from convective boundary layers in arctic air masses , 1994 .

[23]  Hálfdán Ágústsson,et al.  Forecasting wind gusts in complex terrain , 2009 .

[24]  Hálfdán Ágústsson,et al.  The Freysnes downslope windstorm , 2007 .

[25]  Branko Grisogono,et al.  Numerical simulation of pulsations in the bora wind , 2007 .

[26]  Burghard Brümmer,et al.  Boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice , 1996 .

[27]  Zavisa Janjic,et al.  The Step-Mountain Coordinate: Physical Package , 1990 .

[28]  T. Clark,et al.  Severe Downslope Windstorm Calculations in Two and Three Spatial Dimensions Using Anelastic Interactive Grid Nesting: A Possible Mechanism for Gustiness , 1984 .

[29]  Martin Beniston,et al.  Application of a new wind gust parameterization: Multiscale case studies performed with the Canadian regional climate model , 2003 .

[30]  O. Brasseur.  Development and application of a physical approach to estimating wind gusts , 2001 .