How Much Condensed Water Does It Take to Make "Cloud?"
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Long, C. N., Pacific Northwest National Laboratory
Cloud Distributions/Characterizations
Radiative Processes
Dupont J.C., M. Haeffelin, and C.N. Long. 2008. Evaluation of cloudless-sky periods detected by shortwave and longwave algorithms using lidar measurements, Geophysical Research Letters 35(10), doi:10.1029/2008GL033658.
Long, C. N., and T. P. Ackerman. 2000. Identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects, J. Geophys. Res. 105, No. D12, 15609-15626.
Studies of cloud radiative forcing, cloud effects, and cloud feedbacks all inherently include some form of definition of what is and is not a "cloud." In other words, how much and/or how concentrated must condensed water in the atmospheric column be before it is considered to be "cloudy" rather than "cloud-free?" With respect to aerosol forcing, precise methods to distinguish condensed water from other aerosols (e.g., mineral or moist hydrophilic aerosols) is needed. Lidars are known to be particularly sensitive to the presence of condensed water and aerosols in the column, where any signal detectable above the instrument noise level is labeled either a cloud or aerosol la
Using the Radiative Flux Analysis methodology developed under the ARM Program, we compare cloud-free periods detected by an analysis of shortwave (SW) and longwave (LW) irradiance time series with corresponding lidar detections of condensed water or ice in the column. We find that situations classified as cloud-free by analysis of SW (LW) measurements are also classified as cloud-free (no discernable lidar significant level of return) by the lidar in more than 60% (50%) of the cases. The remaining 40% (50%) of the cases are classified as "condensed water detected in the column" by the lidar, and are hence considered as hazy. These hazy situations are predominantly (90%) composed of high-altitude (average 9.5 km height) cirrus clouds, partitioned equally between subvisible and semi-transparent optical thickness classes. We find that, in hazy situations, the average cloud radiative forcing on surface downwelling SW is on average 5 Wm-2 out of 375 Wm-2 total SW irradiance, but can reach values of 15 Wm-2. The SW algorithm effectively produces a "cloud-free" definition for visible optical depths of 0.15 and less.
This study now establishes a definition of the delineation between cloud-free and cloudy by the Long and Ackerman (2000), and cloud fraction estimations by the Long et al. (2006), SW analysis techniques in terms of visible optical depth. Since these SW techniques were developed using corresponding sky imager and human sky observations, the same cloud/no cloud optical depth limit is also largely applicable to these more traditional sky observations.