CLDCOMP

General Description:

The most basic cloud "measurement" is whether there are clouds present, and if so to what amount. Various means have been used to infer a "cloud amount" on various time scales. At the same time, questions of what spatial scales these various means represent at what time scales remain ill defined. In addition, retrieval of night time cloud amounts is limited by our currently fielded instrumentation.

The Cloudiness Intercomparison IOP is designed to address some of these fundamental questions, and to field test and compare some recently developed ideas for measuring cloudiness, particularly at night.

Scientific Requirement:

One of the primary purposes of the IOP is to assess the performance and capabilities of the Nephelo, ICI, and scanning IRT systems, particularly with respect to nighttime retrievals. However, this is not necessarily a straightforward endeavor. One fundamental reason is the recognition that there is not any available "truth" on what is and is not a cloud, and therefore what is the "true" cloud amount. Thus we are left with comparing between the various methodologies for detecting cloud presence and inferral of cloudiness to see how well they agree and the conditions related to that agreement. Another question is do the retrievals based on IR radiation, which is highly sensitive to any condensed water in the column, well relate to the WSI/TSI retrievals; and can they detect the high, thin cirrus?

Thus, in essence, the IOP will field various instruments using various means for detecting the presence of "clouds", and then compare the results in various ways. Some of the particular focii of the IOP will be to address questions of

1. spatial and temporal representativeness
2. differences in what is and is not considered (detected) as cloud by the various methodologies
3. how well can we measure or infer cloudiness at night, including what is the relationship between the ARM WSI day and night sky cover retrievals
4. can we measure cloudiness with low enough uncertainty to study diurnal differences, and particularly the sunrise/sunset transition period where our current capabilities are most limited?

Activity Summary

The Cloudiness Inter-Comparison IOP, conducted operations February 21 through April 21, 2003. During this period, the skies over the SGP Central Facility exhibited a wide variety of cloud types and amounts: exactly what the IOP participants were hoping for. For example, the daylight periods were nearly clear (sky cover <10%) about 25% of the time, mostly cloudy (sky cover >90%) about 44% of the time, and varying degrees of partly cloudy the remaining 31% of the time. The cloud conditions ranged from episodes of overcast and rain, to cumulus cloud fields, through high cirrus and multiple layer cases.

Both the ARM standard and IOP guest instruments critical to the IOP remained operational for the most part during the two month period. Each guest instrument managed a fairly high amount of operational time for the IOP. This was largely accomplished with the diligent daily care and feeding provided by SGP Operations, which also on occasion included support of a more technical nature. In addition, the ARM spares which were "borrowed" for the IOP were fortunately not needed elsewhere during the two months of the IOP.

Timeline

Campaign Data Sets