June 29, 2012

High Resolution Radar can track Individual Raindrops from over 1 mile away

US Naval Research Laboratory (NRL) scientists are leading a multi-agency study which reveals that a very high-resolution Doppler radar has the unique capacity to detect individual cloud hydrometeors in the free atmosphere. This study will improve scientists' understanding of the dynamics and structure of cloud systems.

This Doppler radar was previously used to track small debris shed from the NASA space shuttle missions during launch. Similar to the traces left behind on film by sub-atomic particles, researchers observed larger cloud particles leaving well-defined, nearly linear, radar reflectivity "streaks" which could be analyzed to infer their underlying properties. Scientists could detect the individual particles because of a combination of the radar's 3 Megawatt power, narrow 0.22 degree beamwidth, and an unprecedented range resolution as fine as 0.5 meters. This combination of radar attributes allows researchers to sample a volume of cloud about the size of a small bus (roughly 14 m3) when operating at a range of 2 km. With such small pulse volumes, it becomes possible to measure the properties of individual raindrops greater than 0.5mm in diameter due to the low concentration of such drops in naturally occurring cloud systems and the overwhelming dominance such drops have on the measured radar reflectivity when present in a field comprised of smaller particles.


The image was obtained as a deep convective cloud system passed over the vertically pointed radar on August 27, 2010. The image displays a time-height plot of both the bulk radar structure obtained from the MCR's lower resolution (37m range resolution) waveform (inset) as well as the peculiar, nearly linear sloping radar reflectivity features (or "streaks") determined to have been generated by individual raindrops as they traversed the higher resolution (0.5m range resolution) radar beam. The bulk radar structure revealed in the inset shows that the higher reflectivity values (warmer colors) resulted from a high concentration of larger ice particles generated aloft (top of the image) which then settled through the melting layer and contributed to the population of individual raindrops observed in the lower portion of the cloud by the MCR's high-resolution waveform. The location of the higher-resolution radar streak observations is denoted by the small orange box depicted in the lower-left portion of the inset. The small colored circles within the inset show the locations of the research aircraft while it was obtaining in situ cloud measurements over the radar just below the melting level. The left-to-right downward sloping streaks evident in the outer image indicate the movement of particles toward the earth's surface. The along-streak reflectivity values, measured Doppler phase shift, and slope can be exploited to determine the concentration, size, velocity (or other properties) of the individual cloud particles, as depicted graphically for an isolated streak with the labeled white lines. Note that the vertical scale of the inset (ordinate) represents a range of approximately 5.5 km while that of the streak image is 63m. The time scale of the inset (abscissa) represents approximately 21 minutes of elapsed time while that of the streak image is roughly 25 seconds. This image was created from the raw MCR data by Dr. Jerome Schmidt and placed in final form using Adobe Photoshop with the assistance of Ms. Cynthia Karengin (NRL)




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