During JAWS, observed microbursts had a median velocity difference (Δ υ) of 22 m s −1 across an average distance of 3.1 km, with the median time of 5–10 min to reach the maximum Δ υ. 1984 Hjelmfelt 1988) was designed to study microbursts and the associated wind shear that can be hazardous to aircraft arriving and departing from an airport.
Fujita (1981) defined a downburst as “a strong downdraft which induces an outburst of damaging winds on or near the ground,” and a microburst as “a small downburst with…damaging winds extending only 4 km (2.5 mi) or less.” The Joint Airport Weather Studies (JAWS) field project in Colorado (e.g., Wilson et al. Severe winds from convective storms often begin as downbursts.
#Phoenix weather radar in motion series
( Figure 2 shows a photograph taken during the time of peak surface winds associated with one of the downbursts.) The observations facilitate a unique examination of the scales of motion and the life cycle structure of a series of severe downburst pulses as observed by two radars having different spatial and temporal resolutions as well as viewing perspectives. On 27–28 July, a series of severe downbursts occurred south of PHX and were observed at close range by the SR1 and the National Weather Service (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D KIWA). In an effort to better understand and document the occurrence of Sonoran downbursts and their background environments, the National Severe Storms Laboratory (NSSL) deployed a Shared Mobile Atmospheric Research and Teaching Radar near Phoenix (PHX) during the summer of 2004. The severity of wind damage to electrical power transmission lines from an engineering perspective has resulted in speculation of an especially violent class of downbursts that result in velocities and shear exceeding what has been typically observed or documented in the formal literature. Severe winds also pose a threat to aviation and disrupt electrical service by damaging overhead electrical transmission lines. Long-lived dust storms accompanying thunderstorm outflows are referred to as “haboobs” ( Idso et al. The peak period for these storms is the summer monsoon season (e.g., Maddox et al. Sonoran Desert, downbursts from severe storms frequently produce strong outflows that entrain dust and reduce visibility to dangerous levels, particularly hazardous to traffic along the Interstate Highway System in southern Arizona (e.g., Maddox et al. The storm environment displayed characteristics of both moderate-to-high-reflectivity microbursts, typical of the high plains of Colorado.ĭuring the past 10 yr, the frequency of damaging winds from downbursts occurring in the Phoenix, Arizona, metropolitan area and surrounding suburbs has markedly increased, presumably due to population growth and better reporting ( SPC 2007). The SMART-R scans were at 3–5-min intervals and also had difficulty resolving the event. The evolution of the outflow was very rapid with the 5-min KIWA scan intervals being too course to sample the detailed evolution. Asymmetric outflow may have been a factor as well in the different divergence values.
SMART-R wind shear values were 2–3 times greater with the finer resolution of the SMART-R revealing smaller features in the surface outflow wind structure. The radar data illustrate the finescale structure of the microburst pulses, with the SMART-R’s higher-resolution data showing Doppler velocities 3–4 m s −1 greater than the KIWA radar. Several microburst–downburst pulses were observed by radar and a surface wind gust of 67 mi h −1 was reported. During the evening of 27 July, a severe storm formed along the Estrella Mountains south of Phoenix and moved south of the SMART-R as well as the National Weather Service’s (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) in Phoenix (KIWA). The goal was to capture a severe microburst at close range to understand the low-altitude wind structure and evolution. A Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) was deployed near Phoenix, Arizona, during the summer of 2004.
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