Publications

The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem for scientific and operational wind shear measurement system This paper considers an improved version of the technique used by Fujita and Wakimoto for compensating the obstruction effects by the use of mathematical models relating the unobstructed wind speed to the measured wind speed and the observed obstructions at each site. Over eight million wind speed measurements gathered over 197 days (15 February–31 August) were used. The effects of obstructions at a particular site were evidenced by a strong negative correlation between the observed wind speed transmission factors and the obstruction angles as measured from panoramic photographs taken of the horizon around each station. The functional relationship between them was modeled as a decaying exponential plus a constant, and an iterative least squares regression technique was used on data from all of the stations at once in deriving the three parameters of the equation. It was found that the first 8° of obstruction have the greatest blockage effects, and that even a 2° or 3° high isolated clump of trees can have a pronounced effect on the measured wind speeds from that direction. The possibility that the transmission factors are scale dependent and time dependent is explored.

The FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project is developing methods for automatically detecting and warning against aviation weather hazards, such as low-altitude wind shear, in airport terminal areas using NEXRAD-like Doppler weather radars. Currently, the FAA uses the Low Level Wind Shear Alert System (LLWAS), an anemometer array situated within and around an airport terminal area, for real-time detection of wind shear events. Even with the installation of Terminal Doppler Weather Radars (TDWRs) at some airports, the LLWAS systems there could still play an important role in the accurate detection of wind shear events, and at airports without TDWRs, the LLWAS will remain the primary detection system. The slowing or obstruction of wind by local obstacles is a well know n problem to those wishing to make accurate wind speed measurements. Anemometers should always be located where there will be, as nearly as passible, an unobstructed wind flow free from turbulent eddies in all directions. Because of the fairly precise required sensor configuration of the anemometers in an LLWAS system, it can occasionally be difficult or impossible to find sites with good exposure in all directions. The FLOWS project is interested in the unobstructed wind speed measurements for two main reasons. First, when analyzing a snapshot of the wind field over a mesonet (or LLWAS) for horizontal wind shear and/or for comparison with Doppler radar data, use of the measured, uncorrected winds would reveal spurious patterns of divergence or vorticity that depend little on time but greatly on the prevailing wind direction and that would, in some cases, obscure the true wind shear pattern. Second, when using surface wind measurements to estimate winds aloft that might be encountered by an aircraft on take-off or landing, an· appropriate power law can be accurately used if the original surface wind speed measurements are representative of the unobstructed flow.

A unique meteorological field experiment (COHMEX) is scheduled to be conducted from March-July 1986 with a core period of operation in June and July. It is taking place in the region covering northern Alabama and the adjoining portion of central Tennessee. The experiment's uniqueness derives from the fact that it is actually composed of three distinct experiments sponsored by National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the Federal Aviation Administration (FAA), respectively, with extensive sharing of resources and data. A diagram of the experiment's domain with observational coverage is included in Fig. 1.

As part of the 1984-85 FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project, mesonet and Doppler radar data are being collected on rain and thunderstorms in the Memphis, TN area. One of the key goals of the FLOWS Project is to characterize and evaluate the various form of potentially aviation-hazardous low-altitude wind shear in parts of the country where this type of high spatial and temporal resolution meteorological data have not previously been collected. The 1982 JAWS (Joint Airport Weather Studies) Project revealed that the "microburst", a small scale, intense downdraft which hits the surface and causes a strong divergent outflow of wind, has been the source of much of the hazardous wind shear in the Denver area. The 1978 NIMROD (Northern Illinois Meteorological Research on Downbursts) Project revealed that microbursts occur there on convectively unstable days along with gust fronts and "macrobursts" (scale 4-40 km). Other experiments have largely failed to detect microbursts because their observational networks have not been dense enough to resolve this small scale. A compilation of pioneering studies of microburst-related aircraft accidents around the world by Fujita (1985) illustrates clearly the inherent danger of the microburst wind pattern to jet aircraft, wherever it occurs. In developing ways to best meet the goal of providing warning and protection from low-altitude wind shear in the airport terminal areas, the FAA will need to characterize the problem in different parts of the country. It may be misleading, for example, to use the results on wind shear in the Denver area, or any other single geographical locale, to typify the requirements for microburst warnings at all airports in the country. An important region in terms of its frequency of commercial air traffic control and of thunderstorms, in which high resolution measurements capable of revealing microburts have never before been collected, is the southeastern part of the United States (excluding Florida). During 1984 Lincoln Laboratory continuously collected surface meteorological data from 25-30 mesonet stations and FAA Low Level Wind Shear Alert System (LLWAS) data from the 6 anemometers at the Memphis International Airport from May through November (212 days total). Presented here are preliminary results on the characteristics of wind shear events in the Memphis area. Microburst statistics for Memphis are contrasted with those computed by Fujita and Wakimoto (1983) for the Denver area during JAWS and the Chicago area during NUMROD. A detailed analysis of a microburst that occurred on August 11, 1984 is also presented.