Atmospheric infrared backscattering profiles: interpretation of statistical and temporal properties

This work describes the design, implementation, and calibration of NOAA's coherent, pulsed, Doppler lidar. This lidar was used to acquire 252 high-quality, independent measurements of atmospheric backscattering profiles from 4 to 30 km altitude over Boulder, Colorado, at a wavelength of 10.6 pm between May 1981 and May 1983, a period that includes the injection and removal of debris from the El Chichon eruptions. Statistical analyses of the data set by computer show that atmospheric backscatter is approximately lognormally distributed for all but the lowest altitudes, and a theoretical explanation is offered for this property. Seasonally averaged profiles and altitudinally stacked, filtered time sequences show the volcanic cloud appearing in the stratosphere and falling through the tropopause into the troposphere at rates far higher than can be explained by gravitational settling alone. The dynamic process of tropopause folding is proposed as the dominant mechanism for the observed exchange of volcanic debris from the stratosphere to the troposphere. This hypothesis is supported by case studies of mid-tropospheric backscatter-enhancing events. Mie calculations and comparisons with other measurements show that vertically integrated backscatter is a good long-term measure of total atmospheric mass loading of volcanic debris. It is found that the time constant that characterizes debris removal is 208 days for the stratosphere and 60 days for the troposphere. No appreciable debris is removed before the volcanic cloud falls to 6 km altitude 420 days after the volcanic eruptions.