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The Joint Airport Weather Studies Project (JAWS) Can Be Mixing Mechanism Proposed by Squires (1958)

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The Joint Airport Weather Studies Project (JAWS) Can Be Mixing Mechanism Proposed by Squires (1958) John McCarthy and James W. Wilson Field Observing Facility National Center for Atmospheric Research2 Boulder, Colo. 80307 The Joint Airport Weather Studies t.t^fX " Department of Geophysical Sciences Project University of Chicago Chicago, 111. 60637 Abstract aircraft performance will be studied, and a number of detection and warning systems will be tested in an active The Joint Airport Weather Studies (JAWS) Project will investigate thunderstorm wind shear environment. JAWS facilities will the microburst event, having 2-10 km spatial and 2-10 min tempo- include three NCAR Doppler radars (two 5 cm and one 10 ral scales, at Denver's Stapleton International Airport during the cm), the Portable Automated Mesonet (PAM), two research summer of 1982. JAWS applications and technology transfer objec- aircraft, three rawinsonde units, and a lightning detection tives include: broadening the data base; providing data for real-time detection of thunderstorm hazards for dissemination to the public system. and avaiation communities; examining aircraft performance in wind JAWS has many applications and technology transfer shear; providing a real-time test for display software; identifying objectives that are related to NOAA's Prototype Regional which scales of atmospheric motion are pertinent to applied objec- Observing and Forecasting Service (PROFS); to the tives; providing a test of optimal Doppler radar placement suitable for metropolitan and airport terminal coverage; and describing in NOAA, Federal Aviation Administration (FAA), Depart- more detail the microburst hazard. ment of Defense (DOD) Next Generation Doppler Radar Program (NEXRAD); and NASA's Office of Aviation Safety Technology (OAST) Program. Consequently, a close working relationship will exist among JAWS, PROFS, 1. Introduction NEXRAD, and OAST. These applied programs will benefit by: Broadening the general weather hazard data base; The Joint Airport Weather Studies (JAWS) Project is a providing data appropriate for real-time detection and warn- joint research and technology transfer effort of the National ing of thunderstorm hazards for dissemination to the public Center for Atmospheric Research (NCAR) and the Univer- and the aviation community; examining aircraft perfor- sity of Chicago. The project began on 1 October 1981 and mance characteristics in wind shear conditions; providing a will continue for three years. The principal focus of JAWS suitable real-time test bed for display software development; will be on the convective microburst event, a small region of providing additional means to identify which scales of atmo- intense downflow and associated outflow that occurs in the spheric motion are pertinent to applied objectives; providing convective boundry layer, usually, but not always, associated an excellent test of optimal Doppler radar placement suit- with thunderstorms. The microburst has a spatial and able for metropolitan and airport terminal coverage; and temporal scale of 1 to 4 km and 2 to 20 min, respectively, and giving a more detailed perspective on the microburst has proved to be a major factor in a number of aircraft hazard. accidents, as reported in Fujita and Caracena (1977), In the sections to follow, a scientific background to the McCarthy et al. (1979, 1980), and Fujita (1980). microburst will be presented, followed by a more detailed JAWS will conduct research on the fine-scale structure of description of the objectives of JAWS. thunderstorm kinematics in the vicinity of Denver's Staple- ton International Airport during the summer of 1982. The effect of thunderstorm-produced, low-level wind shear on 2. The microburst All convective clouds contain updrafts and downdrafts. In "A version of this paper appears in the Preprints of the Confer- cumulus congestus clouds, sailplane (Paluch, 1979) observa- ence on Radar Meterology, American Meterological Society, 30 tions indicate that small, strong downdrafts are common. It November-3 December 1981, Boston, Mass. Information concern- is likely that these downdrafts are caused by the penetrative ing the Joint Airport Weather Studies Project (JAWS) can be mixing mechanism proposed by Squires (1958). This mecha- obtained from one of the following: Dr. John McCarthy, JAWS Project Office, National Center for Atmospheric Research, P.O. nism assumes dry environmental air is entrained near cloud Box 3000, Boulder, Colo. 80307 (Phone: (303) 497-0651 or FTS tops and mixed with the cloud air, producing sufficient 322-7651); Dr. T. Theodore Fujita, JAWS Project Office, Dept. of evaporative cooling to generate downdrafts that penetrate Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., several kilometers into the cloud. Downdrafts also are Chicago, 111. 60637 (Phone: (312) 753-8639). initiated and existing ones enhanced by precipitation drag 2NCAR is sponsored by the National Science Foundation. 0003-0007/82/010015-08$06.00 (Clark and List, 1971), and evaporative cooling from rain © 1982 American Meterological Society falling in dry air (Kamburova and Ludlam, 1966). Bulletin American Meterological Society 15 Unauthenticated | Downloaded 10/05/21 06:29 PM UTC 16 Volume 63, Number 1, January 1982 Observations of high plains or desert convective storms mechanisms of microbursts to encourage model modifica- suggest that dry sub-cloud air is particularly important in tions, so that downbursts might be simulated. generating and maintaining strong downdrafts through the The best documented microburst cases to date occurred evaporative cooling process. Although extensive evidence is during the Northern Illinois Meteorlogical Research On lacking, even rather benign-appearing clouds in these Downbursts (NIMROD), which took place near Chicago for regions may produce intense, small scale downdrafts and 45 days in the spring of 1978. In NIMROD, an accurate a corresponding outflow. Lemon and Doswell (1979) have priori estimate of the number of microbursts in and around proposed that downdrafts on the rear flank of severe thun- the network was impossible for lack of data; consequently, derstorms originate at a height of 7 to 10 km. They further Doppler radar baselines of 60 km were chosen in order to propose that downdrafts are dynamically forced by the increase the probability of an observable event. At the nonhydrostatic component of the vertical pressure gradient conclusion of the operations, as many as 10 microbursts, five like those formed on the upwind side of tall buildings in gust fronts, and two supercells were documented. The basic strong winds. Klemp and Wilhelmson (1978) have simulated difficulty encountered was attributable to the ground clutter this process in their three-dimensional numerical model. and the curvature of the earth, which obstructed the detec- Most downdrafts do not reach the ground, since just the tion of the low-level winds at distances in excess of 30 km. right balance of entrainment and evaporation of liquid water Thus, near-ground velocities could only be detected by one must take place to keep pace with adiabatic warming. radar. No dual or triple Doppler measurements of micro- However, when they do reach ground, they spread horizon- bursts at low levels were obtained. Nonetheless, some micro- tally. The stronger the downdraft penetrating the boundary bursts were depicted by single Doppler radar, permitting layer, the stronger the resulting outward burst of horizontal estimations of important characteristics of the low-level winds. Fujita (1978) has defined those downdrafts that wind shear. induce near-surface horizontal maximum winds exceeding The best microburst case, which passed by the Yorkville, 18 m/s (40 m/h) as "downbursts." He further defines a 111. (YKV) NCAR 5 cm Doppler radar on 29 May 1978, downburst having a damage path less than 5 km as a allowed a continuous data acquisition from 15 to 3 km "microburst." When a downdraft reaching the ground distance. The leading edge of the microburst, after passing continues as an expanding outflow, extending over more near the Doppler radar, turned gradually into a weak gust than a few kilometers, a gust front is formed. Thus, a front, which reached O'Hare International Airport one hour microburst can evolve into a gust front. Gust fronts can later. The peak-gust speed, 31 m/s at YKV, decreased to 20 originate from either one or numerous cloud-scale or meso- m/s after travelling 20 km, and to 10 m/s near O'Hare. The scale downdrafts that Zipser (1977) has shown to be present estimated lifetime of the microburst with its peak-gust speed with some squall lines. in excess of 30 m/s was 10 min. The rainfall rate during the Multiple Doppler radar analysis of the JAWS data will peak-gust time at YKV was 30 mm/h (0.02 in/min). Since provide, for the first time, high-resolution data on downdraft no multiple Doppler radar observation of low-level outflow profiles from cloud top to ground, and on the lifetime and was possible because of the obstruction and earth curvature scale of wind shear events given in Table 1. problems discussed earlier, divergence and vertical motion It is unknown what special physical mechanisms cause estimates were obtained as follows: downdrafts to reach downburst intensity; it also is unknown It was assumed that the horizontal airflow of a microburst if these stronger downdrafts have a different origin. Basing is a cylindrically symmetric radial outflow phenomenon his considerations on scale, Emanuel (1981) proposes that superimposed upon the translational velocity. Figure 1 pre- downbursts are caused by
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