Convective Turbulence. The appearance of cumulus clouds indicates a great deal of tur-bulence. In general, the taller the cloud, the more turbulence there is. The initiating mechanism in convective-type turbulence is thermal instability. The roughness is caused by the large wind shears between the updrafts and downdrafts. The intermingling of the downdrafts and the updrafts results in moderate to severe turbulence, particularly at the boundary between regions of precipitation and regions having no precipitation. This result was confirmed by radar studies of thunderstorms that were made in this country and in England. Severe turbulence occurs in regions of strong echoes, and particularly in regions of sharp boundaries of echoes. The intensity of turbulence in thunderstorms seems to increase with height, well past the middle of the clouds. The actual turbulent velocities may increase to even greater elevations but have less effect on aircraft, because the air density is lower at higher levels. The air within the cloud subsides in the last stage of the thunderstorm and the storm dissipates, leaving no indication of having severe turbulence.

The present methods of estimating the tur-bulence expected in cumuliform clouds before the clouds have formed generally make use of the temperature differences between the interior of the cloud and the surrounding air. Refer to Unit 6, Lesson 2 for a method used to calculate convective turbulence using the Skew T, Log P Diagram.

MECHANICAL TURBULENCE. Mechan-ical turbulence is the effect of wind shear (changes in wind speed and/or direction) caused by physical features. These physical features may be geo-graphic features, such as Earths surface (ranging from flat ground to high mountains); or they may be physical features of the atmosphere, such as jet streams, frontal surfaces, or inversions. The wind shear that causes mechanical turbulence may be an abrupt change in the speed or direction of wind in either the horizontal or the vertical or both. In this section we will discuss the different mechanisms that produce mechanical turbulence, and their effect on aircraft.

JET STREAM TURBULENCE. Jet stream turbulence is produced by both strong horizontal and vertical wind-speed shears in the vicinity of the jet streams. Since the jet streams are usually located well above all but the cirrus clouds, and the location of jet stream turbulence is not directly associated with cirrus cloudiness, jet stream tur-bulence is often briefed as CAT.

Jet stream turbulence effects many commer-cial and military aircraft. The usual location of the jet streams is from 30,000 to 40,000 feet, which is also the optimum flying altitude for most jet aircraft. Of all reports of turbulence received, only 20 percent of these reports were related to the jet streams. However, over two thirds of all reports of severe turbulence were associated with the jet stream.

Turbulence occurs all around the axis of the jet stream core because of the large increase in wind speed encountered while progressing into the region of the core. The least turbulence is usually encountered at the level of the jet core on the equatorial side. Figure 6-1-3 shows cross-sectional areas around the jet stream where patches of tur-bulence are usually reported. It is a distribution diagram showing areas where turbulence is most often located and the strength of the turbulence most often reported in those areas. Do not imply that the areas shown are one solid area of turbulence.

Where the jet stream is in a relatively straight line, the turbulence tends to be concentrated both above and below the jet core on the polar side (fig. 6-1-3, part A). Little or no turbulence occurs within the jet stream core. In areas where the jet stream follows a trough and is curved cyclonically, the strongest turbulence tends to be located below the jet core toward the colder air advection, as in part B. In part C, we see that when the jet stream is curved anti-cyclonically, the area of

turbulence tends to be most concentrated at the tropopause above the jet core on the polar side. Data also indicated that CAT is less frequently encountered over ocean areas than over con-tinents, although the CAT patches over the oceans seem to be larger than those overland. The average thickness of a turbulent layer is about 2,000 feet.

In the horizontal, the turbulent patches often are from 10 to 40 nmi wide (across the direction of the wind flow) and usually extend about 50 nmi above land and 100 nmi over water in the direction of the wind flow.

When jet streams appear to merge on a constant pressure chart, such as the 300-millibar analysis, the colder jet stream core actually lies under the warmer jet core. The area of the intersection and westward of the intersection are typically favorable areas for strong CAT occurrence because of the directional shear of the underlying northwesterly jet and the overlying southwesterly jet.

Calculation of the horizontal and vertical speed shears will yield the strength of the turbulence that may be expected. The critical values of the speed differences and the associated turbulence intensities are listed in table 6-1-2. In cases where evaluation of directional shear must be made, turbulence within the shear zone can be calculated by taking the vector difference of the vertical winds in the column being evaluated. Compare this difference to the tur-bulence criteria in the vertical wind shear column of table 6-1-2. Review AG2, Volume 1, Unit 2, Lesson 1 if you need to refresh yourself on vector addition and subtraction.

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