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Variations in the Ratio of IR Window Radiance to Microwave Water Path Observed Under Cloudless Convection

Platt, C.M.(a) and Austin, R.T.(b), Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
Thirteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting

The measurement of the radiance of cirrus (and other) clouds at atmospheric window 8-13 micron wavelengths requires a correction for the water vapor radiance and transmittance below the clouds. Calculating radiances at the times of routine radiosonde ascents and interpolating the radiance/water path ratio between ascents can achieve this. However it has been observed experimentally that IR radiance/water path ratios appear to vary between radiosonde ascents away from the interpolated values. This occurs particularly under warm convective conditions in the tropics and during summer afternoon periods at the SGP. The observed variations in water vapor path, routinely measured by a microwave radiometer at ARM CART sites, can be used as a surrogate for variations in the IR radiance between radiosonde ascents. However, ratios of radiance/water path during periods of variation in both radiance and water path were also observed to vary from the calibrated values obtained from the radiosonde ascents. These variations were observed under dry convective conditions during observations at the SGP in the summer of 2001 and at a tropical site during the MCTEX project in 1995. The ratio of radiance to water path under different temperature and humidity profiles is investigated for model atmospheres to obtain a quantitative estimate of variations of the radiance/water path ratio. These calculations are compared with the observed changes in radiance water path at the SGP and during MCTEX. Variations in humidity and temperature profiles measured under convective conditions at the SGP were estimated from Raman lidar and AERI data respectively.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).