S. Harvey Melfi

Professor

Ph.D. College of William and Mary 1970

Research Interests

Our group develops LIDAR instruments for the remote study of the earth's atmosphere. LIDAR is an acronym for LIght Detection And Ranging, and it is the optical analogue of Radar. A LIDAR consists of a high powered pulsed laser and an optical telescope. The two are pointed in the same direction so that the scattered light from the laser pulse, as it propagates through the atmosphere, can be collected by the telescope. Recording the scattered energy as a function of time, from when the laser is fired, gives us a range resolved measure of the interaction of the laser pulse with the atmosphere. The laser energy is scattered, absorbed and attenuated by the constituents of the atmosphere. It is these interactions which we study in an attempt to increase our understanding of various atmospheric processes. We are developing LIDAR instruments to study the dynamics of the planetary boundary layer (PBL), the lowest layer of the atmosphere; and the micro-structure of clouds.
The PBL LIDAR will be designed to measure profiles of temperature, aerosol scattering, and aerosol attenuation in the first several kilometers of the atmosphere. Changes in temperature and aerosol scattering with altitude can be used to measure the structure and evolution of the PBL. The PBL is important to understand since it is the layer through which heat, moisture and momentum from the earth's surface must pass to drive atmospheric circulation. It is also important in understanding air pollution episodes since pollutants from the surface tend to be trapped in this layer.
The cloud LIDAR will be designed to measure cloud particle size distributions and to determine the amount of liquid water in clouds. These characteristics will be studied by observing attenuation, multiple-scattering and possibly Raman shifted scattering of the laser pulse as it moves through a cloud. The micro-structure of clouds is important in understanding radiative transfer through the Earth-atmosphere system. Clouds consisting of a large number of small particles have higher reflectivities in the visible than clouds with a smaller number of large particles even when the total liquid water in the clouds remains constant. Cloud reflectivity is one of the largest unknowns in predicting the possible future change of the Earth's climate.
This research is performed in collaboration with researchers in the Joint Center for Earth Systems Technology (JCET), a cooperative venture between UMBC and the NASA Goddard Space Flight Center in Greenbelt, MD.

Figure legend: A lidar acquired profile, as a function of altitude, of the aerosol- scattering-ratio (denoted as aerosol - top scale) and water-vapor-mixing-ratio (denoted as water vapor - bottom scale). The data shows the relationship between two clouds at an altitude of 400-700m and at 2.75km (seen as increases in the aerosol data) and the water-vapor distribution. Also shown in the figure are balloon-sonde acquired profiles of water-vapor-mixing-ratio (denoted as 0233 vai) and the saturated water- vapor-mixing-ratio (denoted as 100% RH vai).