Meteorology associated with turbulence encounters during NASA's Fall-2000 flight experiments

Abstract

Initial flight experiments have been conducted to investigate convectively induced turbulence and to test technologies for its airborne detection. Turbulence encountered during the experiments is described with sources of data measured from in situ sensors, groundbased and airborne Doppler radars, and aircraft video. Turbulence measurements computed from the in situ system were quantified in terms of RMS normal loads (σ ∆n), where 0.20 g ≤ σ ∆n ≤ 0.30 g is considered moderate and σ ∆n > 0.30 g is severe. During two flights, 18 significant turbulence encounters (σ ∆n ≥ 0.20 g) occurred in the vicinity of deep convection; 14 moderate and 4 severe. In all cases, the encounters with turbulence occurred along the periphery of cumulus convection. These events were associated with relatively low values of radar reflectivity, i.e. RRF < 35 dBz, with most levels being below 20 dBz. The four cases of severe turbulence occurred in precipitation and were centered at the interface between a cumulus updraft turret and a downwind downdraft. Horizontal gradients of vertical velocity at this interface were found to be strongest on the downwind side of the cumulus turrets. Furthermore, the greatest loads to the aircraft occurred while flying along, not orthogonal to, the ambient environmental wind vector. During the two flights, no significant turbulence was encountered in the clear air (visual meteorological conditions), not even in the immediate vicinity of the deep convection.

Numerical study of a convective turbulence encounter

“…Spectral width actually observed during event 191-6 are shown for onboard-turbulence radar and TLH ground-based radar in Figs. 13b and 14b of reference [2]. The peak value in the radar simulation (7 m/s) compares favorably with that observed by TLH radar (7 m/s) and onboard radar (8-9 m/s).…”

An Aircraft Encounter with Turbulence in the Vicinity of a Thunderstorm

“…Metrics for defining the turbulence hazard can be quantified in terms of RMS normal loads (σ ∆n ), where σ ∆n is based on moving 5-second intervals of aircraft normal loads (∆n). 1,2 Using this metric, moderate turbulence is defined for: 0.20 g ≤ σ ∆n < 0.30 g, and severe when σ ∆n > 0.30 g. Bowles 6 has shown that the peak aircraft normal loads, are roughly proportional to σ ∆n , as shown in Figure 1. In no event did |∆n| ever exceed 1 g when σ ∆n < 0.30 g. On the other hand, peak loads may exceed 1 g in the severe turbulence category; thus, turbulence encounter intensities with σ ∆n > 0.30 g may cause passengers and obstacles in the cabin to become weightless, an obviously dangerous situation.…”

“…Recently, an initial flight campaign, that used NASA-Langley's B-757, has tested a Turbulence Prediction And Warning System (TPAWS). 1,2,3,4 Aircraft encounters with turbulence may occur within either clear air or clouds. Turbulence associated with convective clouds, otherwise known as Convectively Induced Turbulence (CIT), is a leading hazard to aviation operations.…”

Large Eddy Simulations of Severe Convection Induced Turbulence

“…In NASA's Fall-2000 flight experiments, it was found that severe turbulence was often within overshooting tops and in growing cumulus turrets. Hamilton and Proctor (2002) reported that such encounters can occur in regions of low radar reflectivity values (peak values ranging from 8dBZ to 33dBZ). These small RRF values are difficult to detect by ground as well as airborne sensors.…”