KMANMSC

Improvement of Cloud Detection with COMS in the Day-Night Transition Area

Byung-il Lee1, Hyungmin Park2, Sung-Rae Chung1

1 : National Meteorological Satellite Center / Korea Meteorological Administration (NMSC/KMA), bilee01@korea.kr 2 : Department of Atmospheric Science, Pusan National University

2015 NOAA Satellite ConferenceApril 27 ~ May 1, 2015

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 algo-

rithm improvement for reducing discontinuity using ‘normalized reflectance test’ and ‘brightness temperature difference

(BTD 3.7-11) test’. The results of new approaches compare to the operational looks like to improve low-level clouds detec-

tion 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 test REFVIS, REFSWIR

2. Dual channel reflectance ratio test RatioVIS/SWIR

3. Single channel temperature test BTSWIR, BTIR1, BTIR2

4. Dual channel temperature difference test BTDIR1-SWIR, BTDIR1-WV, BTDIR1-IR2, BTDIR2-SWIR, BTDIR2-SWIR

5. Homogeneity test of 3x3 pixels STDDEVVIS 3x3, STDDEVSWIR 3x3, STDDEVIR1 3x3, STDDEVIR2 3x3

6. Dual channel temperature difference test (Sunglint) BTDSWIR-IR1

• Spatial Resolution 1x1 pixel2 (4 x 4 km2)• Time zone Separation Day SZA < 85˚, Day+Sunglint SZA < 85˚ and Sunglint

Night 95 ˚ < SZA, Twilight 85˚ < 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.

2015 NOAA Satellite Conference April 27 ~ May 1, 2015

(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)

< Result >

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) BTD11-3.7 Test• BTD3.7-11 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 3.7-11 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 dif-

ference (BTD 3.7-11) 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 BTD3.7-11 and normalized reflectance in

transition region.

• And this technique will be applied to the next Korean geostationary meteorological satellite Geo-KOMPSAT-2A CLD al-

gorithm(See the poster #3-42).

• KMA Satellite Conference will be held in South Korea, on September, 2015

ReferencesChung, 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, 367-372.

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, 1765-1780.

Li, J. and Kiyotaka Shibata, 2006, On the Effective Solar Pathlength, Journal of the Atmospheric Sciences, Vol. 63,1365-1373.

CO

MS

Clo

ud

M

ask

2015-04-302000 UTC (0500

KST)

2015-04-302045 UTC (0545

KST)

2015-04-302100 UTC (0600

KST)

2015-04-302245 UTC (0745

KST)

2015-04-302345 UTC (0845

KST)

CO

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SW

IR

Imag

eC

OM

S V

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m-

ag

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(Top) Operational COMS Cloud Mask in the East Asia on 2000 ~ 2345 UTC(0500 ~0845 KST) April 30, 2014. 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 reflectanceGOODVisible reflectance is an eidetic way to distinguish between cloud and clear sky.

BADVisible reflectance is not used at twi-light and dawn area because it rapidly shrinks to high solar zenith angle.

BTD11-3.7

GOODIt is very useful to detect low clouds at nighttime.

BADIt doesn’t use at twilight/dawn region in the COMS cloud detection algo-rithm.

Reducing solar angle dependency! Normalization VIS test

Set dynamic threshold values! BTD11-3.7 test (at Twilight, Day-

time)

CO

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Clo

ud

M

ask

2000 UTC (0500 KST)

2045 UTC (0545 KST)

2100 UTC (0600 KST)

2245 UTC (0745 KST)

2345 UTC (0845 KST)

BTD

11

-3.7

Red LineBlue Line

Norm

. V

IS im

ag

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pro

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lou

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ask

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 co-sine 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.

𝑁𝑜𝑟𝑚 .𝐹𝑎𝑐𝑡𝑜𝑟=24.35

2cos (𝜃𝑠𝑜𝑙 )+√498.5255cos2 (𝜃𝑠𝑜𝑙 )+1

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 3.7-11, 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 Resolu-tion Half-Ampli-

tude(IFOV in μrad)

(km)

Required Range

of Measure-mentLower Upper

VIS 0.55 0.80 28 (1km)0-115%(Albedo)

SWIR 3.5 4.0 112 (4km) 4-350K

WV 6.5 7.0 112 (4km) 4-330K

IR1 10.3 11.3 112 (4km) 4-330K

IR2 11.5 12.5 112 (4km) 4-330K