The CIRA layer precipitable water product from 3-4 December 2015 is shown below:
Note the black region across South Dakota, Minnesota and portions of Nebraska / Iowa. There are also black regions in portions of the Rockies and Canada. Why is data missing over these regions?
The visible imagery from 1745 UTC on 4 December:
Depicts what appears to be a snow field over South Dakota, Minnesota and portions of Iowa and Nebraska. Snow fields will reveal rivers and lakes, unlike a cloud field such as that over Indiana / Ohio / Illinois. Another method to discriminate snow vs cloudy regions is the CIRA shortwave albedo (also known as low cloud) product:
This product consists of the 10.7 and 3.9 micrometer channels. Low clouds show up as white /bright while snow cover shows up as black / dark since snow cover has low reflectivity in the 3.9 micrometer channel.
The Microwave Integrated Retrieval System (MIRS) uses passive microwave microwave radiances from polar orbiting satellites to solve for the temperature and moisture profile in the atmosphere. Snow cover on the ground changes the microwave land surface emissivity, which determines how much microwave radiation is received by the sensor. As snow is complex with varying depth, grain size and water content, this impact is complex. In order to eliminate erroneous retrievals over snow and ice covered surfaces, these regions are flagged for no retrieval in the MIRS system and appear black in the CIRA blended product. Snow emissivity modeling work continues, and improved atmospheric retrievals over snow and ice could be realized in the future.
Further details on and examples of the CIRA blended Layer Precipitable Water Vapor product is available in the article “A Multisensor, Blended, Layered Water Vapor Product for Weather Analysis and Forecasting”, available at http://www.nwas.org/jom/abstracts/2015/2015-JOM5/abstract.php.
Real-time imagery for the CIRA layer precipitable water product can be found here: