Quantitative infrared imagery of the development of a nocturnal drainage wind during CASES 99 and contrasted with traditional micrometeorological observations

A shallow north/south oriented depression or rill was instrumented with small meteorological towers and a thermistor array for observations of the marked nocturnal drainage flow observed in this rill during the Cooperative Atmosphere-Surface Exchange Study 99 (CASES 99) on the plains just outside of Wichita, Kansas from 1-31 October 1999. The length of this depression and the axially perpendicular sensor array could be observed from the top of the sixty-meter tower facility, 500 m to the north. A cryogenically cooled infrared digital imager (3 - 5 µm wavelengths, 256 x 256 pixels, and 12-bit resolution) thermal imager was mounted at the 56 m level to observe this entire scene with a brightness temperature accuracy of 0.1º K. From these oblique images of the depression, we were able to observe the accelerated development of nocturnal boundary layer cooling. The images of the cooling rill are exceptionally vivid and dramatically illustrate the power of this remote sensing tool. Analyzed radiometric transects along the micrometeorological array show good agreement between the radiometric and the directly sensed air temperature. As this infrared imager can acquire image data at rates up to 60 Hz, additional applications of this technology in micrometeorology seem sure to develop from this CASES 99 exploratory application.