P4.17
Polar cloud-detection algorithms for a real-time analysis and forecasting model
Poster PDF (1.1 MB)
The algorithms fall into two fundamental classes: “static” spectral tests and “temporal-difference” tests. Additionally there are two hybrid spectral-temporal tests that identify inversion fogs and moving/forming cloud features both day and night. In all there are 7 multispectral tests: 3 daytime-only, 2 day/night, and 2 nighttime-only algorithms. Daytime tests include obviously bright clouds, snow-cloud discrimination, and highly reflective clouds at ~3.7-micron wavelengths. Nighttime modules test for fog and thin cirrus. In both day and night checks are made for obviously cold clouds, and thinner-cirrus and low-cloud edges.
There are three temporal differencing tests, each of which operate both day and night. A “traditional” single-channel infrared temporal differencing test flags forming and moving cloud systems. This is akin to the techniques used operationally by the US Air Force global cloud model with geostationary data. A second temporal differencing test identifies inversion fogs in the polar night, when such clouds are much warmer than the underlying surface. A third experimental test checks for midwave-longwave temporal signatures.
These polar cloud detection algorithms will be transitioned to the US Air Force's Cloud Depiction and Forecast System (CDFS) operational global cloud models during calendar year 2006.