- freely available
Atmosphere 2017, 8(2), 42; https://doi.org/10.3390/atmos8020042
2. Threshold Determination and Detection Process
2.1. Threshold Determination
2.2. Detection Process
- Step 1: detect severe convection center by employing 220 K as the BTIR1 threshold.
- Step 2: detect preliminary convective cloud by employing 240 K as the BTIR1 threshold.
- Step 3: eliminate non-convection area by employing 4 K, 10 K and −16 K as the threshold of BTDIR1–IR2, BTDIR1–IR3 and BTDIR1–IR4, respectively.
- Step 4: matching the convection center with the preliminary convective cloud. The cloud cluster will be retained if it contains a convection center, or it will be labeled as uncertain cloud.
- Step 5: eliminate the small area of broken cloud by employing four pixels as the threshold.
- Step 6: use the same steps above to detect convective cloud an hour before.
- Step 7: tracking and matching the uncertain cloud clusters at the previous time t−1 and the current time t0. Firstly, a grid spacing of and the central position P(i, j) is used to express the space range of the uncertain cloud clusters, where m is the longitude direction length and n is the latitude direction length. We then search the current time cloud cluster in the space range of with the central point P(i, j) at t−1, where and . In the searching area at t−1, we save the cloud clusters whose BTmin increases beyond 8 K and the overlap ratio beyond 50%. Finally, we calculate the cross-correlation coefficient between the satisfying cloud clusters at t−1 and the current uncertain cloud cluster at t0. The cross-correlation coefficient r is defined as:
- The integrated method is effective for convection in different scales and life periods, especially the isolated convective cloud. However, in the large-scale cloud system, it is easy to misinterpret some cirrus clouds as convective cloud as well.
- The BT threshold method was capable of detecting convective cloud with a low BTmin more efficiently, and the tracking methods are more capable of detecting convection which is growing. Also, the combined use of overlap ratio, minimum brightness temperature change and cross-correlation coefficient shows a remarkable effect on the elimination process for non-convection area.
- The statistical result shows that the α-type cloud clusters detected by the integrated method are mostly large-scale cloud systems, and the β- and γ-type cloud clusters have the highest proportion of general convective cloud. However, the proportion of weak convection is higher than that of severe ones in γ-type cloud, but it is the opposite in β-type cloud.
Conflicts of Interest
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|200 < BTmin ≤ 2000||20 < BTmin ≤ 200||0 < BTmin ≤ 20|
|L > 240||α-weak convection (α-SC)||β-weak convection (β-SC)||γ-weak convection (γ-SC)|
|220 < L ≤ 240||α-general convection (α-GC)||β-weak convection (β-GC)||γ-general convection (β-GC)|
|L ≤ 220||α-severe convection (α-WC)||β-severe convection (β-WC)||γ-severe convection (γ-WC)|
|Number||Mean Scale (km)||Mean BTIR1 (K)|
|Severe convection centers||38||20.6||204.8|
|Preliminary detection result||173||23.9||225.2|
|Integrated detection result||80||37.4||225.1|
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