The Winding Path

By: Stephen Cohn, '85

Maryland Physics as an undergraduate was great! It was hard and fun at the time, but also a great foundation to build on. The department then (and now) has such a good reputation that I could choose among some of the top graduate schools. One school I visited was MIT. There the physics Department was in a dark foreboding corridor. I wanted to get a look at Cambridge and Boston so I went to the top of the tallest building on campus to get a view of the city. This turned out to house the Center for Meteorology and Physical Oceanography. Some of the grad students invited me to look out their window, and we got to talking. That fortuitous view started me on the path to a Ph.D. in Atmospheric Science. Hey, a lot of meteorology is fluid dynamics and that's PHYSICS, right? My graduate research at MIT took me into the world of Doppler radar, both to observe the Ionosphere and also much lower in the atmosphere. The first boundary layer (lowest few km above the surface) wind profiling radars were being developed then. These use Bragg scatter to bounce energy from refractive index gradients (caused by turbulence and variations in water vapor, pressure, and temperature). The great thing about these radars is that they can measure the wind above the ground even in "clear air". The weather radars we see on the TV news need rain or snow to scatter energy, but Bragg scatter doesn't - so they greatly expand options for measuring atmospheric phenomena. Today the U.S. and several other countries operate networks of wind profilers.

Next stop was a post-doctoral job at McGill University. There the Department of Atmospheric and Oceanic Sciences was just getting a boundary layer radar, so it was a natural fit. McGill has an impressive reputation too, in many fields. There were two great aspects of the two years I spent there. The first was working with Prof. Roddy Rogers. He's a very nice guy, and he also wrote the book on cloud physics - literally ("A Short Course in Cloud Physics"). Cloud physics has aspects of thermodynamics, fluid dynamics, molecular physics, and just plain dynamics. Incidentally, I had met Prof. Rogers at a radar meteorology conference in Paris. He had a poster presentation next to mine, so we had a chance to talk a lot. I suspect that chance meeting had a lot to do with my getting chosen for this post-doc. The second great part of McGill was its location - downtown Montreal, Canada. Montreal has an unbelievable mix of cultures. The dominant French Canadian culture makes it like living in Europe (but only a 12 hour drive from MD), but the western part of the island of Montreal is more English. There are also many, many other cultures and recent immigrant groups. In summer there is a festival just about every weekend (Jazz, Caribbean, Comedy, etc.), the suburbs are great for cycling, the Laurentian Mountains are an hour away, and the community is friendly. Then again, after two Canadian winters I was ready to move south again.

This time it was to Boulder, Colorado and the National Center for Atmospheric Research (NCAR). Finding that job was also serendipity. I made a trip to Boulder to meet with the group at NOAA who had pioneered wind profilers, but also stopped in at NCAR to visit a colleague I knew from a workshop. It turned out he was leaving NCAR to return to his home in Belgium. His departure created a job opening, and I was hired a few months later. That move was almost 10 years ago and I'm still there. At NCAR I'm a scientist in the Atmospheric Technology Division, and still work with wind profilers, but also with other radars, lidars, and other instruments. Over the years we've improved the capabilities, and developed measurement and signal processing techniques that have increased the time and space resolution of these instruments. This makes them more powerful tools to study atmospheric phenomena. My group also collects data in support of many research projects. Using wind profilers in conjunction with many other measurement devices, we've studied Lake Effect snowstorms, atmospheric gravity waves, temperature inversions that can trap pollution close to the surface for many days, modification of storms passing over mountain chains, and so many other topics. Most recently, I've been the lead scientist on a project to develop a safety system to warn airplanes of turbulence and wind shear near the Juneau Alaska airport.

So at NCAR it's been a mix of science and engineering, and always with an emphasis on studying problems that affect our lives. Ultimately, I think that's one reason I choose meteorology over a more fundamental area of physics - there are so many opportunities to have an immediate impact on people's lives.

Boulder is also a great place to live. I met my wife Jennifer here and we now have a 3-year old daughter and a new baby boy born this September. It's been 18 years(!) since I got that B.S. at UM Physics. Its served me well - especially the hands-on laboratory exercises. Hey, do you guys still use Bevington's book on Data Analysis techniques? I do.