Tuesday, 5 October 2004
The relationship of positive and negative (smooth) turbulence pilot reports (PIREPs) to cloud top height and base is examined in an attempt to determine the relative frequency of turbulence in-cloud, above cloud, below cloud, and in clear air. Since PIREPs usually do not include information on whether the turbulence was encountered in or out of cloud, two different methods for determining where a PIREP was in relation to clouds are compared and presented. Three complete years of PIREPs, sounding data, and Current Icing Potential (CIP, formally known as the Integrated Icing Diagnosis Algorithm, IIDA) are included in the evaluation. The data is broken down into three altitude bands: low (surface - 10,000 ft), mid (10,000 ft - 20,000 ft), and high (20,000 ft and above), and comparisons are done seasonally as well as annually. Examination of the entire volume of air space shows that smooth turbulence reports are in clear air 20% of the time and in-cloud 40% of the time, on average. These values decrease to 15% and 25%, respectively, for moderate or greater (MOG) turbulence reports. The vertical distributions show that the majority of in-cloud turbulence occurs at mid-levels while clear air turbulence is more frequent at upper- and low-levels. In addition, the average volume of clouds versus clear air present over the CONUS is also estimated to make the comparisons more meaningful. Using two different methods it is estimated that the annually averaged cloud volume percentage is about 14% to 18%.
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