an automated, dynamic threshold cloud detection …...data set fy-2c data vis visible 0.55-0.90ir4...
TRANSCRIPT
An Automated, Dynamic Threshold Cloud Detection
Algorithm Jian Liu and Jianmin Xu
National Satellite Meteorological CenterChina Meteorological Administration
An Automated, Dynamic Threshold Cloud Detection Algorithm
1. Data Set2. Cloud detection method3. Discussion
Data Set
FY-2C data
0.55-0.90Visible VIS
3.5-4.0Near infraredIr4
6.5-7.3Water vaporIR3
11.5-12.5Infrared splitIR2
10.3-11.3Far infrared IR1
WAVELENGTH(μm)
CHANNEL NAME
CHANNEL ID
Cloud Detection Method
1. Getting dynamic cloud detection threshold for 32*32 pixels area by histogram analysisDifferent surface typeDEM modify
2. Cloud detection threshold validationCurve fit to check threshold
3. Cloud detectionDynamic thresholdMulti-channel data
Dynamic Threshold
450 500 550 600 650 700 7500
20
40
60
80
100
120
140
Num
ber o
f Pix
els
(331-BT)*5
C
400 450 500 550 600 650 700 750 800-10
0
10
20
30
40
50
60
70
80
Num
ber o
f Pix
els
(3310-bt)*5
Peak
Dynamic Threshold
original histogram analysis smoothed histogram
area average processing
Land
Mask
Using DEM data to modify dynamic threshold
threshold validation_update dynamic threshold
0 2 4 6 8 10 12 14 16 18 20 22
265
270
275
280
285
BT
Hour
Land Gauss fit
0 2 4 6 8 10 12 14 16 18 20 22250
255
260
265
270
275
280
285
290
BT
Hour
Desert Gauss fit
* Dynamic Cloud detection threshold
Cloud Detection
1. Dynamic threshold method 2. Visible channel data 3. Deviation analysis4. Relationship analysis _Brightness difference
infrared & water vapor channelinfrared split channel
4. Multi-day composed brightness temperature
Case study
CLD image
Discussion
1. The algorithm performs well for most area.2. In high latitude regions, the cloud detecting
methods failed sometimes due to strong surface temperature inversions.
3. Some surface conditions may make this approach inappropriate, most notably over snow and ice condition.
4. Some cloud types such as thin cirrus, low stratus at night, and small cumulus are difficult to detect because of insufficient contrast with the surface radiance.
Thanks