Researching Microbursts

© 2012 by  Fernando Caracena

Growing up in the Desert Southwest

Just before my starting grammar school, my parents moved us to Highland Park, a neighborhood which was located on a mesa-like apron of the south end of the Franklin Mountains in El Paso, Texas. Our house sat on a bluff on the edge of the southeast side of the mesa, from where the terrain fell of abruptly to the east and south. From my home, I could see vast distances to mountains in northern Mexico, to Sierra Blanca near Van Horne, Texas, and various mountains in New Mexico to the east and northeast. To the west the views were cut off by the hogback, limestone ridge at the end of the Franklin Mountains.

During the summer rainy season, I had a great vantage point for watching thunderstorms developing over the mountains, which later spread into the valleys. A still better vantage point for distant viewing them was but a short hike away on the hog-back ridge. After a short walk, I could be at the mountainside, and in less than an hour, at the top of the ridge. From there, I could look very far out in every direction, except due north, where the view was blocked by the peaks of the Franklin Mountains. To the west, I could see more of New Mexico, Arizona and Mexico. Looking down at El Paso, Ithe human effort down there looked like the activity in an ant bed. The experience gave me a mountain high.

Weather watching was one major interest, which I expanded on by reading. I could see different kinds of thunderstorms. An extremely dry type, produced some lightning, but no more than sprinkles and strong wind gusts. A particularly spectacular type of dry thunderstorm happened from completely overcast skies of high based clouds that developed in the late afternoon from horizon to horizon. The bottom of the cloud cover, fuzzy from ice crystals, had some prominent shafts of virga, which is rain descending to the ground. Under dry conditions, the rain does not reach the ground in any strength and the virga column never becomes a visible rain shaft. These dry storms produced occasional bolts of lightning. At night, the lightning turned into spectacular cloud to cloud displays, with occasional cloud to ground strikes.

One feature of  the dry-type of storms caught my attention. Sometimes, looking out across the fuzzy base of the extensive cloud cover, I would see a hard base form in the clouds that marked an up-draft. Had I been in an aircraft flying above the clouds, I no doubt would have seen a new cumulus tower over such a hard base. After about fifteen or twenty minutes, I would often see a virga shaft moving down from where the hard base had been. Its motion was rapid enough to be visible to the unaided eye, which was followed shortly afterwards by the development of a ring of dust on the desert floor. When the descending virga was directly overhead, we would get a few sprinkles, blast of strong winds that kicked up a lot of dust, and occasional lightning. Later, I saw that the same phenomena of childhood experience would figure prominently in the adult world of air safety.

Teaching Physics at Metro

I landed in the Denver area during the time of the PhD glut in physics. After an intial series of part-time jobs in the Denver Area, I finally settled on a "permanent" job teaching physics at a new start up college, Metropolitan State College (Metro), which was located in downtown Denver. This college has now grown into a university, MSU. During a tranquil period at this job, I gradually gave up the variety of part-time ventures that had sustained me during my period of "unemployment", which was then my idea of work that was not a position in academia.

When the affable president, Dr.Philips, of Metro left for some reason or other, and a new "dynamic" president took his place, things took a sharp turn for the worse for the faculty. Some of my colleagues were heading for the exits. After a few years of constant harassment, some of my colleagues had left the state going on to higher-level jobs elsewhere.

I recognized that teaching physics at Metro was becoming a dead-end job. I started looking for something to do during my summers, when the college was partially shut down. In Science Magazine, I saw an advertisement for summer post-doctoral positions avaliable at the National Center For Atmospneric Research (NCAR) at Boulder. Colorado

Post-Doc at NCAR

I applied for a position with the Advanced Studies Program (ASP) and was accepted for the summer of 1975.

As a result of working at NCAR in Boulder, and still living in the Denver Area, I had about a 55 minute commute, which fortunately was then in the opposite sense of normal traffic. My daily travel route took me past Stapleton International Airport in Denver. On August 7, 1975 on the way home, I heard the news on the radio that a crash at Stapleton had happened that afternoon. I wondered what had caused the crash. The weather had been fair that afternoon in Boulder. I looked at the sky above, which was cloudless, but I noticed a few storm clouds on the north-east horizon.

At NCAR the day after the crash at Stapleton, we received a phone call from Dr. Raymond Jordon, retired dean from the Colorado School of Mines in Golden, Colorado. He operated a network of micro-barographs (note: link1-link2) at his home a few miles south of Stapleton in Aurora, Colorado. He had recorded a series of strong pressure signals on his array of pressure sensors  around the time of the accident and wondered if that information might be important for determining the cause of the accident. A few anemometers located near his pressure sensors also recorded sharp wind shifts.

At that time I began to suspect a thunderstorms as the possible cause of the accident. Through Dr. Jordon, I met a number of airline pilots from the Airline Pilots Association (ALPA) who were also interested in what caused the crash. Over the next few days, I spent time on the phone calling various people who had access to wind and pressure data for various sensors and arrays of sensors near the airport. The wind data available were from anemometers mounted on top of ten meter wind towers.

I got lucky. I received records form 14 wind towers located in the vicinity of Stapleton, nine of those, northeast of the runway 35L, near where the crash occurred.  The center field wind tower was near the point where the aircraft became airborne. Another wind tower, located a short distance due west of the center field  anemometer, was under the updraft of the storm that caused the accident. In addition to these data, I ordered radar data from a National Weather Site operated in Limon, Colorado, which depicted the radar echo of the storm itself.

NTSB testimony

At NCAR, I had been doing research on hail-producing thunderstorms characterized by strong outflows of rain-cooled air, called gust fronts that are driven by rain-shaft downdrafts. I had read Fujita's articles with great interest, and learned how to apply his time-space conversion techniques to the analysis of gust fronts. Fujita had developed these time-space analysis technique in analysing the effects of atomic bomb destruction in Japan during World War II, which he later applied to the analysis of storms. This technique is a methodology that takes advantage of the high density of temporal observations to fill in details of flow that are below the sampling resolution of spatially sparse weather networks.

The wind-shear event at Stapleton was a smaller scale feature of downdraft phenomena, but was still resolvable by the time-space technique. Using Fujita's time-space conversion techniques I was able to analyse an outflow pattern that corresponded to the base of a strong downdraft over the departure end of runway 35L at the time of the accident.

I attended the NTSB hearings at Stapleton the Fall of 1975 at the invitation of ALPA. At that time the board was leaning toward a decision to blame the accident on "pilot error." My analysis of the weather suggested that cause to be "windshear." Perhaps, "microburst" would have been a better term, but Fujita had not yet invented the name. Captain William Melvin who was the aircaft safety expert from ALPA kept pushing the NTSB to let me testify at the hearing. Almost at the end of the day, the NTSB agreed to allow my testimony. After my presentation, the NTSB agreed with my analysis, and stated: "The National Transportation Safety Board determines that the probable cause of the accident was the aircraft's encounter, immediately following takeoff with severe wind shear at an altitude and airspeed which precluded recovery to level filght; the wind shear caused the aircraft to descend at a rate which could not be overcome even though the aircraft was flown at or near its maximum lift capability throughout the encounter. The wind shear was generated by the outflow from a thunderstorm which was over the aricraft's departure path" .

At NOAA's Atmospheric and Chemistry Laboratory

Dr. Peter Gilman, head of the ASP, had convinced me to apply for another term with the ASP at NCAR, so that I did not go back to teaching at Metro the end of the Summer of 1975. A was appointed a post-doctoral fellow at NCAR for another, longer term, during which I was on a leave of absence from Metro. Meanwhile, that Winter of 1976, I was offered a research position at NOAA's, Atmospheric Physics and Chemistry Laboratory (APCL), where I began working in April 1976.  I was drawn in two directions when working there: Pete Kuhn was doing work on remote sensing using IR radiometry, and there was a mesoscale meteorology group headed by Charlie Chappell. To this mix, I brought my own interests in convective downdrafts and time-space analysis techniquues.



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