KMA NMSC Abstract Operational COMS(Communication, Ocean and Meteorological Satellite) Cloud Detection(CLD) algorithm shows that fog and low-level clouds are often undetected in the day-night transition areas. In this study, we introduce COMS CLD algorithm improvement for reducing discontinuity using ‘normalized reflectance test’ and ‘brightness temperature difference (BTD ) test’. The results of new approaches compare to the operational looks like to improve low-level clouds detection including sea fog around the Korean Peninsula. And this technique will be applied to the next Korean geostationary meteorological satellite Geo-KOMPSAT-2A CLD algorithm. COMS COMS is the first multi-purpose geostationary satellite of Korea for Meteorology, Ocean and Communication.  Meteorological Imager  Multispectral imaging radiometer : 1 visible and 4 infrared channels  Mission: ① Continuous monitoring of imagery and extracting of meteorological products ② Early detection of severe weather phenomena ③ Monitoring of climate change and atmospheric environment Operational COMS Cloud Detection Algorithm  COMS CLD Algorithm consists of 6 major tests (16 minor tests). 1.Single channel reflectance testREF VIS, REF SWIR 2.Dual channel reflectance ratio testRatio VIS/SWIR 3.Single channel temperature testBT SWIR, BT IR1, BT IR2 4.Dual channel temperature difference testBTD IR1-SWIR, BTD IR1-WV, BTD IR1-IR2, BTD IR2-SWIR, BTD IR2-SWIR 5.Homogeneity test of 3x3 pixels STDDEV VIS 3x3, STDDEV SWIR 3x3, STDDEV IR1 3x3, STDDEV IR2 3x3 6.Dual channel temperature difference test (Sunglint) BTD SWIR-IR1 Spatial Resolution1x1 pixel 2 (4 x 4 km 2 ) Time zone SeparationDaySZA < 85˚,Day+SunglintSZA < 85˚ and Sunglint Night95 ˚ < SZA, Twilight85˚ < SZA < 95˚ Discontinuity of the Cloud Mask in Time  Discontinuity of operational COMS CLD There are clouds in the SWIR and VIS images(red circle and square) continuously but COMS CLD algorithm can’t detect on 2045 and 2100 UTC. This discontinuity due to different detection test for different time zone. (1) Normalized Visible Test Normalized VIS reflectance test at dwan and daytime (60 < SZA and 55 < VZA) Helpful to detect cloud at high SZA Li and Shibata’ (2006) normalization factor for neglecting solar angle dependency -Stable near 90˚ (SZA) than inverse cosine -Stable normalization reflectance until 93˚ (SZA) Very dark and hard to distinguish cloud and clear pixel without normalization. Using normalization factor of Li and Shibata represents stable normalization approach to 90˚ (SZA). (2) BTD Test BTD is useful to detect fog and low cloud. Because, the 3.7μm is more sensitive to water droplet than 11μm However, brightness temperature at 3.7μm is rapidly increase or decrease near day/night transition zone by solar reflection component For that reason, ’BTD test’ has been only used at nighttime in operational COMS cloud detection algorithm Although the week point of this test, we used it at day/night transition with its threshold value of the first order function of SZA Results The improved CLD algorithm is reduced discontinuity using ‘normalized reflectance test’ and ‘brightness temperature difference (BTD ) test’. The results of new approaches compare to the operational looks like to improve low-level clouds detection including sea fog around the Korean Peninsula. Plans We will improve the operational COMS CLD algorithm to solve discontinuity using BTD and normalized reflectance in transition region. And this technique will be applied to the next Korean geostationary meteorological satellite Geo-KOMPSAT-2A CLD algorithm(See the poster #3-42). KMA Satellite Conference will be held in South Korea, on September, 2015 References Chung, C. Y., H. K. Lee, H. J. Ahn, M. H. Ahn, and S. N. Oh, 2006, Developing Cloud Detection Algorithm for COMS Meteorological Data Processing System, Korean Journal of Remote Sensing, Vol. 22, Derrien, M. and H. Le Gleau, 2010, Improvement of cloud detection near sunrise and sunset by temporal-differencing and region-growing techniques with real-time SEVIRI, International Journal of Remote Sensing, Vol. 31, Li, J. and Kiyotaka Shibata, 2006, On the Effective Solar Pathlength, Journal of the Atmospheric Sciences, Vol. 63, COMS Cloud Mask UTC (0500 KST) UTC (0545 KST) UTC (0600 KST) UTC (0745 KST) UTC (0845 KST) COMS SWIR Image COMS VIS Image (Top) Operational COMS Cloud Mask in the East Asia on 2000 ~ 2345 UTC(0500 ~0845 KST) April 30, White, blue, green pixel represents cloudy, clear land, clear sea, respectively. (Middle) SWIR (3.75μm) images at the same time (Bottom) Same as middle except for VIS (0.68μm) VIS reflectance GOOD Visible reflectance is an eidetic way to distinguish between cloud and clear sky. BAD Visible reflectance is not used at twilight and dawn area because it rapidly shrinks to high solar zenith angle. BTD GOOD It is very useful to detect low clouds at nighttime. BAD It doesn’t use at twilight/dawn region in the COMS cloud detection algorithm. Reducing solar angle dependency!  Normalization VIS test Set dynamic threshold values!  BTD test (at Twilight, Daytime) COMS Cloud Mask 2000 UTC (0500 KST)2045 UTC (0545 KST)2100 UTC (0600 KST)2245 UTC (0745 KST)2345 UTC (0845 KST) BTD Red Line Blue Line Norm. VIS image Improvement Cloud Mask Red Line : SZA = 95˚ Blue Line : SZA = 85˚ Flow chart for the COMS Cloud Detection. (Chung et al., 2006) List of the COMS cloud detection tests are used in STEP 6 (Fig. 2). Shading box means that test is used applicable time. Motivation Comparison of normalized factor of Li and Shibata and the inverse cosine factor. Dashed line represents the inverse cosine method, and solid line represents Li and Shibata’ method. List of the COMS cloud detection tests are used in STEP 6 threshold test. Shading box means that test is used applicable time. Comparison of normalized factor of Li and Shibata and the inverse cosine factor. Dashed line represents the inverse cosine method, and solid line represents Li and Shibata’ method. The same figure except for BTD , Norm. VIS and improvement CLD. MI (Meteorological Imager) GOCI (Geostationary Ocean, Color Imager) Communication Antenna (Ka-band) Solar Array Channel Number Channel Full Width at Half Maximum (μm) Spatial Resolution Half- Amplitude (IFOV in μrad) (km) Required Range of Measurement LowerUpper VIS (1km) %(Albedo) SWIR (4km) 4-350K WV (4km)4-330K IR (4km)4-330K IR (4km)4-330K