Long-term radiance trends at the ARM Southern Great Plains Site

 

Authors

Jonathan Gero — University of Wisconsin
David D. Turner — NOAA- Global Systems Laboratory

Category

Radiation

Description

Radiance trends at 11.4 µm in all-sky scenes for different seasons (left); for different sky conditions in summer and winter (center); and for clear-sky and thin-cloud conditions in the winter for day and night (right). A black "X" on the symbol indicates that the trend is not statistically significant.

Over 14 years of downwelling spectral infrared radiation data have been collected by the atmospheric emitted radiance interferometer (AERI) at the Department of Energy's Atmospheric Radiation Measurement (ARM) site in north-central Oklahoma. The absolute calibration of each AERI spectrum is ensured to be better than 1% of the ambient radiance by the design of the instrument and its calibration subsystem. The measured infrared spectra, numbering more than 800,000, were classified as clear-sky, thin cloud, and thick cloud scenes using a neural network method and were further sorted by diurnal and seasonal cycles.

Many significant long-term trends are obtained from the AERI radiance data set when looking at the data on annual, seasonal, and diurnal time scales. For example, the downwelling radiance is decreasing over this 14-year period in the winter, summer, and autumn seasons, but it is increasing in the spring; these trends are statistically significant and are primarily due to long-term change in the cloudiness above the site. The trend spectra reveal changes in cloud characteristics that may be attributed to changes in cloud height, temperature, and particle size.

Given the decadal time span of the data set, natural variability needs to be considered when drawing broad conclusions. Nonetheless, these accurate spectral observations have high value since they can be used to infer possible mechanisms for any trends from the observations themselves, and they can be compared with global and regional climate models to evaluate their performance. Furthermore, this work provides a foundation for using global infrared radiance measurements from satellite instruments to ascertain climate trends and test general circulation models.