The Integrated Terminal Weather System (ITWS), developed for the Federal Aviation Administration by MIT Lincoln Laboratory, has as one of its primary goals the promotion of aircraft safety in the critical phases of take-off and landing. This goal is achieved in part due to the presence of an algorithm designed to detect hazardous microbursts from mature thunderstorms. Aircraft that penetrate these microbursts lose airspeed during take-off or landing operations. This loss of airspeed can cause an incident or accident.
The ITWS Microburst Detection Algorithm has performed well in testing at Memphis, Orlando, and Dallas. However, the current detection algorithm is optimized for a "wet" environment where the microbursts are accompanied by a high amount of precipitation (or reflectivity as seen on weather radar). The algorithm currently uses a "VIL Test" to lower the microburst false alarm rate, whereby divergent regions which are not accompanied by a VIL value of 5 kg/m2 are disregarded. (VIL, or "Vertically Integrated Liquid Water," is the amount of water in a column of air, which is directly related to the degree of precipitation forcing and, therefore, microburst potential.) In locales such as Denver, CO, or other "dry" environments, it is common for microbursts to be accompanied by lower reflectivities and lower VIL values. In fact, areas which can be deemed "wet" environments may occasionally experience conditions suitable for the occurrence of low-reflectivity microbursts. A case study from Memphis with low-reflectivity microbursts will be used to illustrate the impact the current algorithm-specified VIL value would have on the detection performance.
The parameters necessary for low-reflectivity microbursts (which can be determined from temperature and moisture profiles) will be discussed based on a review of the literature, as well as the possibility of integrating an "adaptive VIL threshold" into the algorithm. An adaptive threshold, based on temperature and moisture conditions in the boundary layer, would allow the algorithm to selectively lower the VIL threshold when the atmosphere is conducive to low-reflectivity microbursts so that fewer events are missed. This adaptive threshold would increase the probability of detection (POD) for low-reflectivity events while maintaining its current POD under typical conditions. It is hoped that lowering the VIL value under conditions conducive to the formation of low-reflectivity microbursts will not adversely affect the low false-alarm rate. This technique would allow the algorithm to achieve its optimum performance in both types of microburst environments. It will be modeled after the ITWS Microburst Prediction Algorithm logic for dealing with thermodynamic forcing within different boundary layer regimes.
* This work was sponsored by the Federal Aviation Administration. The views expressed are those of the authors and do not reflect the official policy or position of the U.S. Government.
+ Opinions, interpretations, conclusions and recommendations
are those of the authors and are not necessarily endorsed by the United
States Air Force
The 8th Conference on Aviation, Range, and Aerospace Meteorology