Analyzing and Forecasting Clear - Air Turbulence Probabilities over the United States

Pilot reports from special turbulence-reporting periods were used t o investigate methods of analyzing and forecasting clear-air turbulence over the United States. Meteorological analyses for the special reporting periods were made objectively by computer using only standard upper air rawinsonde measurements. The wind analyses were built upward from the 400-mb level to the 200-mb level using thermal wind shears to compensate for missing wind data in high-speed portions of the flow. The best meteorological indicators of turbulence were found to be the vertical vector wind shea.r and the product of wind shear and horizontal deformation. T o a somewhat lesser extent, large gradients of relative humidity (at the 400-mb level) and large magnitudes of divergence also tended to be associated with turbulent regions. Turbulence analyses based on both meteorological relationships and pilot reports were made. These analyses are in terms of the probability of encountering significant turbulence (moderate or severe) within a 100-n.mi. flight sector. Advection forecasts of the turbulence probabilities were made, and these showed reasonable skill for periods out to 12 hr in advance. 1, INTRODUCTION Clear-air turbulence is an atmospheric phenomenon consisting of random three-dimensional eddies that cause aircraft to experience appreciable high-frequency accelerations. Turbulence that occurs in cloudless regions or within thin clouds such as cirrus is included in the clear-air category, while turbulence that occurs in thunderstorms and other rain-producing clouds is not. The presence or absence of turbulence in the atmosphere is not measured directly by conventional radiosonde instruments; therefore, indirect analysis methods are required. Theoretical considerations indicate that certain combinations of vertical wind shear and temperature lapse rate provide a criterion for turbulence; that is, Richardson's number. Also, it appears that atmospheric conditions must be favorable for the growth of wave motions (such as shear-gravity waves), which can eventually break into turbulent eddies (Kuettner, 1952; Reiter and Burns, 1966). I n general, turbulence tends to occur in certain portions of upper fronts and the tropopause particularly when associated with strong. jet streams and sharp troughs or ridges. ,Also, mountain ranges are well known as being favorable locations for encountering turbulence, particularly where the lee sides are steep. Close agreement is generally found between turbulence and meteorological factors when measurements are made of the atmospheric mesostructure using practical turbulence analysis and forecasting must be based on standard rawinsonde measurements. The upper air data routinely available for the United States only permit analyses representative of …