Analysis of Satellite Signatures, Pattern Recognition, and Eddy Dissipation Rate In Determining Potential Areas of Convectively Induced Turbulence

This study assists and improves the procedures of aircraft avoidance of turbulence near thunderstorms. Convectively induced turbulence (CIT) and near cloud turbulence (NCT) represents a significant hazard for the aviation industry and has shown to be responsible for over 60% of turbulence-related aircraft accidents (Cornman and Carmichael 1993). Current Federal Aviation Administration (FAA) avoidance guidelines do not properly address any formal tactical procedures when aircraft are more than 20 miles from a thunderstorm. However, CIT and NCT, often occur 200-300 miles downwind of large mesoscale convective systems (MCS). Using pattern recognition associated with the development of MCSs, satellite imagery of transverse bands within the outflow cloud shield, pilot reports (PIREPS) and aircraft eddy dissipation rate (EDR) data, this study shows that certain sub-synoptic scale environments are more conducive to moderate or severe turbulence. By examining several case studies, it is hopeful that operational aviation forecasters, airline flight dispatchers, pilots and air traffic controllers can utilize these results by updating current procedures as well as those to be implemented into the FAA next-generation air transportation system (NextGen).

D. Data Analysis MCS and Short-Wave Troughs Two cases of turbulence, 23 May and 16 June 2011, associated with a short-wave trough adjacent to the northern outflow portion of an MCS are examined. Each MCS existed during the time frame 12 - 21 UTC with initial development between 12 – 14 UTC. The precipitation core in both cases (not shown) exhibited a slightly east to southeast movement while the anvil and associated cirrus outflow developed an east to northeastward movement. Both MCSs developed within a westerly to west-northwesterly flow of 30 – 50kts from 500 to 300 hPa. In both cases, a well defined 500 hPa short-wave trough was progressively moving through the flow approximately 300 - 400 miles north and east of the track of the MCS and 200 – 300 miles from the edge of the cirrus outflow. Tropopause heights were generally 30-50 hPa lower in the vicinity of the short-wave trough in each case.

a) 23 May 2011 GOES Visible satellite imagery for 1445 UTC, 23 May 2011, shows a developing anvil spreading northward towards the MO/IA border associated with an MCS in southwest Missouri (Figure 7). No reported turbulence was associated with this system at this time. Note the well-defined cyclonic circulation over Minnesota and Wisconsin. At 1645 UTC (Figure 8), anvil cloud producing cirrus blow-off with an area of weakly developing transverse bands begins to move eastward into central Illinois. 9 PIREPS and 3 EDR reports of moderate turbulence between FL 310 – 390 from 1600 to 1730 UTC are evident in the northern flank of the expanding outflow at the same time transverse bands begin to multiply. Note the lower level cumulus cloud field in central Iowa is directly beneath the most northern turbulence reports. This is an example of two distinct streams of wind flow; a lower level wind flow related to the short-wave trough moving through the upper Midwest and the newly developing northern flank outflow associated with the developing MCS. At 1845 UTC visible satellite imagery shows the larger MCS sinking southward towards northern Arkansas while a second cluster of convective activity develops over western Illinois (Figure 9). Additional streaks of transverse bands now develop further northward through Illinois. 12 PIREPS and 7 EDR reports of moderate turbulence occur between FL 240 – 390 at 1800 – 1945 UTC from central IA into northern IL and southern WI. Turbulence closer to the short-wave trough tends to be at a lower altitude, FL 240 – 330, while closer to the anvil, turbulence is at a higher altitude, FL 330 – 390. Single reports of moderate to severe and severe turbulence occurred at FL 270 and FL 290, respectively, between 1914 and 1928 UTC over southern Wisconsin where it appears the transverse bands were directly overhead the strongly cyclonic lower level flow. EDR reports of 0.15 to 0.35 indicating moderate or greater turbulence from 14 – 19 UTC from FL 240 – 390 shows an excellent spatial relationship with transverse bands and lower level cyclonic flow during this time frame (Figure 10). Several reports of 0.35 to 0.45 were within 50 km of the two reports indicating moderate to severe and severe turbulence. Note the spatial relationship of the EDR within the inner circle with the transverse bands overlaying the lower level cumulus cloud field.

In the AMDAR wind plot at FL 180 – 290 from 1800 – 2000 UTC (Figure 11), northwesterly flow of 50 kts depicts the western edge of the short-wave trough. Two reports of moderate-severe and severe turbulence occurred during this time frame at FL 270 – 290. The enhanced southwesterly jetstream due to the outflow from the MCS can clearly be seen from IA into WI and IL (Figure 12). Not only does the wind velocity increase to nearly 110kts but also the wind direction in this layer varies by almost 50 degrees from the lower layer, FL 180 – 290. Comparison of the AMDAR wind data from FL 180 - 290 to earlier 1200 UTC 500 hPa analysis shows that wind velocity in the lower layers did not increase more than 10 kts (Figure 13). Evidence suggests, that the enhanced jetstream due to the developing MCS did not have any effect on this layer. However, wind velocity did increase by almost 50 kts from FL 290 - 390 in 6 to 8 hours compared to earlier 1200 UTC 250 hPa winds (Figure 14).