In Memory of radar pioneer Eugene A. Mueller

Dr. Eugene A. Mueller, one of the original engineers of the CHILL research weather radar system, passed away on 14 February, 2016. He made significant contributions to the field of radar meteorology over a span of four decades. Gene’s technical education began in the U. S. Navy during the final stages of World War II. Due to his rapid absorption of this subject, the Navy designated him an instructor in their radar school. One of the radars that he taught was the 3 cm wavelength APS-15 airborne search system. After discharge from the Navy, he completed both his BS and MS degrees in Electrical Engineering at the University of Illinois. While at the U of I, he came across a job announcement from the Illinois State Water Survey (ISWS) regarding a project to map rainfall using a surplus APS-15 radar. Gene was hired based on his Navy experience with this radar. At that time, radar data was displayed on an analogue cathode ray tube (CRT), with stronger echoes producing brighter areas on the phosphorescent screen. The data was archived by photographing the CRT. To better resolve the recorded signal intensity levels, Gene developed hardware that made a pre-programmed step change to the receiver sensitivity at the end of each antenna revolution. The ISWS APS-15 operating in this stepped attenuator mode captured the first observations of a tornadic hook echo on 9 April, 1953.

As the ISWS continued to experiment with radar-based rainfall mapping, it became apparent that lack of knowledge of the distribution of rain drop diameters introduced major uncertainty into the interpretation of the radar data. To address this problem, Gene designed and built a camera system that could automatically collect images of raindrops with diameters between 0.5 and 5 mm within a one cubic meter sample volume. Gene’s development of the drop camera equipment, along with the microwave backscattering and absorption calculations from the drop measurements, became the basis of his 1966 University of Illinois Ph.D. thesis. The camera system design was sufficiently robust to support the collection of drop size distribution data in climatic regions ranging from Alaska to Indonesia.

In the mid-1960s the National Science Foundation announced the desire to fund the development of a 10 cm wavelength research weather radar for use in the National Hail Research Experiment (NHRE). Two proposals to develop such a radar were independently submitted by both the ISWS and the University of Chicago. NSF was willing to provide funding for a single radar jointly developed by the two institutions. The resultant radar was named CHILL to recognize both the university of CHicago and the university of ILLinois / ISWS aspects. Much of the theoretical support for the radar’s required performance was provided by University of Chicago Prof. Dave Atlas while Gene lead the engineering effort to construct the radar at the University of Illinois Willard Airport at Champaign. After the NHRE program ended in the early 1970s, the CHILL radar went on a series of over ten remote deployments to support atmospheric science field projects at sites ranging from Oklahoma to Montana.

In the late 1970s Gene adapted the CHILL to make exploratory dual polarization radar measurements. Prof. Tom Seliga of Ohio State University had proposed that the preferential oblate shape assumed by larger diameter raindrops should provide a small, but measurable increase in the received echo strength when a radar was operating at horizontal vs. vertical polarization. To test this theory, Gene adapted a motorized switch to the CHILL waveguide system. This arrangement allowed the transmitter power to be selectively routed to either the horizontal or vertical polarization ports on the antenna. Prof. Seliga’s student, V. N. Bringi, processed the CHILL data received in the alternating horizontal and vertical polarization blocks. His results confirmed the existence of the expected enhancement to the horizontally-polarized signal level when oblate raindrops were present. The utility of dual polarization weather radar measurements has become well established since Gene’s pioneering demonstration data; this technology has now been adopted for operational use in the NWS radar network.

In 1990, the NSF transferred the CHILL radar’s funding support from the University of Illinois to Colorado State University. Gene accompanied the radar on its relocation to an operating site next to the Greeley Weld County Airport. At CSU Gene oversaw the continuing technical advancement of the radar (now known as CSU-CHILL). Notable radar improvements made at CSU included the adaptation of a dual transmitter configuration in which separate, identical transmitters were used to generate the horizontally and vertically polarized signals. Gene also developed the specifications for the custom-built dual polarization antenna that was acquired in 1994. He retired from his position as CSU-CHILL radar senior engineer in July 1995 and continued to live at his home in Loveland, Colorado.

Gene will be missed as both a mentor as well as a friend.

—Pat Kennedy and Steven Rutledge