Research Page for Phillip B. Chilson

Background

Ph.D. Studies at Clemson

When one thinks of atmospheric radars it is common to imagine the horizontally scanning weather radars that are often featured in TV weather reports. When I began working with atmospheric radars as part of my Ph.D. studies, it was with two of the larger radars in the world. Both of these belong to a class of radar known as wind profilers. These operate at a lower frequency than weather radars and are oriented vertically. They are used to make wind measurements of the atmosphere immediately above the radar site. Depending on the size of the antenna, the amount of transmit power, the frequency of the radar, and other factors, wind profilers can make wind measurements many kilometers above the height of the tropopause.

Part of my Ph.D. research was conducted using the Arecibo Observatory in Puerto Rico (pictured at left). Although the Arecibo Observatory is typically associated with radio astronomy, it has played a significant role in atmospheric investigations. The Arecibo Experiments were primarily concentrated on the study of thunderstorms using dual frequency radar measurements. In particular we were interested in investigating the dynamics of tropical storms, the structure of the resulting precipitation, and the mechanisms associated with the generation of lightning.

The other radar facility was the Middle and Upper Atmosphere (MU) Radar in Japan. It is shown to the right. The MU Radar is a very flexible system in that it can receive the backscattered atmospheric signal on 4 separate channels. (This has recently been upgraded to accomodate 25 separate channels!) This makes it possible to make interferometric observations. During my Ph.D. studies we observed and analysed stratiform precipitation using this radar. The interesting aspect of the data was that they had been collected while the radar was operating in a spaced antenna mode. This application of radar interferometry to precipitation was the first of its kind.

Postdoctoral appointment in Germany

I received a postdoctoral appointment at the Max-Planck-Institut für Aeronomie in Germany. There I worked with the SOUSY group. They were among the first to construct a wind profiler capable of probing the upper atmosphere. The SOUSY Radar located in the Harz mountains is pictured to the left. This radar now operates in Peru.

During my three-year postdoctoral appointment, one of my personal research objectives in Germany was to implement the possibilities of making frequency domain interferometry (FDI) measurements on the SOUSY Radar. This is a technique used to study thin atmospheric layers that requires the radar to transmit and receive on two or more closely spaced frequencies. This was successfully completed and the first publishable data were collected in October of 1994. Several papers have already been completed or are in preparation from the FDI experiments. The most recent was a study of Kelvin-Helmholtz instabilities produced in the shear region of an upper tropospheric jet.

Another series of measurements dealt with the formation of layers at a height of around 85 km that are associated with large radar backscattering cross sections. This phenomenon is most commonly observed near the summer mesopause at arctic latitudes and is referred to as polar mesosphere summer echo (PMSE). Since the SOUSY radar is at a latitude of 52 N, we refer to the echoes that we detected as simply mesosphere summer echo (MSE). Since PMSE and MSE are associated with cold temperatures, we conducted parallel measurements with the SOUSY Rayleigh Lidar. One outcome of these measurements was to show the effects on upward-propagating gravity waves on the mesopause region and their role in the creation of MSE.

Additional measurements made in the upper mesosphere / lower thermosphere involved the study of the interaction of meteors with the Earth's atmosphere. The first of the measurements was a study of ambipolar diffusion coefficients (D_a). The decay rate of the signal for certain types of meteor echoes provide a measure of the local value of D_a. It is also possible to derive this value by knowing the local temperature, density, and type of ion. We made measurements of the atmospheric temperature and density via lidar data and compared the calculated values of D_a for different assumed ion species with the radar observations.

The Sweden Years

After my appointment in Germany, I took up a position at the Swedish Institute of Physics (IRF) where I became a member of the Atmopsheric Research Programme. The IRF is located in the arctic town of Kiruna (68 N). One of the reasons for joining the IRF was the use the Esrange Radar (ESRAD), which is jointly owned and operated by the IRF and the Swedish Space Corporation. ESRAD is VHF MST radar. Usine ESRAD, I continued my studies of PMSE and meteors, but also became interested in the relationships between lower atmospheric dynamics and its relationship to the formation of polar stratospheric clouds (PSC). The occurence of PSCs are important, since their presence affect the stratospheric chemisty and thereby contribute to the destruction of the ozone layer.

I also continued my interests in multiple-frequency interferometry while in Sweden. We implemented FDI on the European Incoherent Scatter (EISCAT) VHF radar for the study of PMSE. Although FDI has been a powerful technique, it has some limiting handicaps. In order to achieve better range resolution using multiple frequencies, my colleagues and I devised a means of using imaging techniques. We were able to implement range imaging (RIM) for the first time on the SOUSY radar in Germany.

Finally, my colleagues at the IRF and I developed a new technique to study PMSE. This was conducted at EISCAT and involved use of ionospheric heating. In 1999 we demonstrated for the first time that the backscattered power from PMSE can be reduced by virtue of raising the electron temperature within these layers. Previously only passive observations could be made. This technique offers exciting opportunities for the study of PMSE, because for the first time we can design active experiments to explore their cause.

Current Research Interests

Primarily my studies are focused on (1) the boundary layer, (2) the free tropospere, and (3) the mesopause region.

Some particular topics of interest are listed below.

• The application of interferometry and imaging to radar systems as means of obtaining higher spatial resolution measurements of the atmosphere.
• Studies of atmospheric dynamics and how it relates to the generation of layered phenomena
• The interaction of radio waves with precipitation
• The abblation of meteors in the Earth's atmosphere and how it relates to atmospheric chemistry
• The study of polar mesosphere summer echoes
• Artificial heating of the Earth's ionosphere