Monday, 13 May 2002: 1:45 PM
Numerical simulation of a convective turbulence encounter
A major portion of the accidents from aircraft turbulence encounters are within close proximity of atmospheric convection. NASA, through its Aviation Safety Program, is testing technologies that will reduce the risk of injuries from these type of encounters. Primary focus of the turbulence element within this program is the characterization of turbulence and its environment, as well as the development and testing of hazard-estimation algorithms for both radar and in situ detection. The ultimate goal is to operationally test onboard sensors that will provide ample warning prior to encounters with hazardous turbulence. In support of turbulence characterization, numerical modeling of atmospheric convection is being conducted using a large eddy simulation model. A special need for the modeling is to provide realistic data sets for developing and testing turbulence detection sensors. However, the first step prior to this application must be verification that the numerical model can produce useful and realistic data sets. To meet this goal numerical simulations with NASA's Terminal Area Simulation System (TASS) are being applied to actual cases that have measurements available from radars, satellites, and aircraft penetrations. One particular case targeted for numerical study is an event encountered approximately 40 km ENE of Tallahassee FL, during NASA's fall-2000 flight tests. In this event, hazardous turbulence associated with a narrow line of thunderstorms was encountered by NASA Langley's B-757 on 14 December 2000. Severe intensities of turbulence were measured as the aircraft penetrated updraft plumes near the storm tops. Data from onboard Doppler radar and in situ wind and temperature measurements, supplemented with recorded Nexrad radar data (Tallahassee (TLH)) are compared with the results from the numerical simulation of this case.
The numerical simulation is carried out with TASS, which is a large eddy simulation model developed for simulating convective clouds and atmospheric turbulence. Comparison of the TASS simulated radar echo with that from the onboard radar shows similar scale and intensity, although details in the echo structure differ. The numerical results show buoyant plumes, with simulated radar reflectivities of up to 36 dBz, penetrating the upper level thunderstorm outflow. The simulations produce updraft plumes of similar scale to those encountered during the test flight. The simulated intensity of turbulence compares well with that obtained from the aircraft's in situ system.