Observations and simulations of gravity waves and turbulence during the Atlantic THORPEX Regional Campaign

On 6 December 2003 the NOAA Gulfstream-IV weather reconnaissance aircraft experienced severe clear-air turbulence during the Atlantic THORPEX Regional Campaign (ATReC). The aircraft was above a region of active moist convection at the time, to the east of a surface low pressure system off the north-east coast of the United States. In-situ measurements of this encounter are presented, along with turbulence parameters derived from spectral analysis of the measurements. Results from a multiply-nested numerical simulation of the event are also presented. These simulations show that the turbulence occurred in a region of reduced background Richardson number associated with an area of large scale reduced stability and enhanced vertical wind shears. This region of low Richardson number is perturbed by vertically propagating gravity waves generated from the underlying convection. The perturbations are substantial enough to initiate Kelvin-Helmholtz instabilities leading to turbulence.