Effects of Clouds on Availability of Ocean Color Data

• Cloud cover creates a serious limitation to availability of ocean color data. Some areas of the world ocean are especially affected and clear images are very rare. Multiple satellites (sensors) can increase the chances of finding clear skies due to their different orbits and changing cloud patterns.
• A script in Wimsoft (http://wimsoft.com) software was run to evaluate the availability of daily Chl-a data that is affected by mostly the cloud cover and to a lesser extent by the by the orbit characteristics and sensor tilt/no tilt.
• Standard global Level-3 9-km CHLO datasets were used from SeaWiFS and the merged SeaWiFS/Aqua-MODIS.
• The number of days with valid data was estimated for the full time series of daily Level-3 (Count of valid pixels)
• The script wam_composite_month was run separately for each year, e.g.

                    wam_composite_month F:\SeaWIFS\L3\Daily\CHLO_9\S2007*9 count

• Counts of valid data were generated globally but the images of distribution are shown only South-East Asia.
• Time series of the mean counts were generated for several smaller areas in South-East Asia.

Fig. 1. Mean number of days per month of available ocean color data in SE Asia using merged SeaWiFS/Aqua Level-3 data.

The panels show January (upper-left), April (upper-right),

July (lower-left) and October (lower-right). Clicking on the image opens the full image.

Fig. 1 shows that the number of valid ocean color data per month in January changes from 0 (e.g. parts of East China Sea) to above 20 (West of Luzon Island).

Fig. 2. Areas for generating Time Series of monthly statistics:  (1) off Hong Kong, (2) N Vietnam, (3) Central Vietnam, (4) S Vietnam, (5) Eastern Gulf of Thailand, (6) west of Luzon Island of Philippines

Fig. 3. (Left) Number of data days per month for N Vietnam area (area 2 in Fig. 2); (Right) Mean monthly number of data days per month (x-axis has day of the year). SeaWiFS (blue) and merged SeaWiFS/Aqua (red) data are shown.

Fig. 4. (Left) Number of data days per month for Central Vietnam area (area 3 in Fig. 2); (Right) Mean monthly number of data days per month (x-axis has day of the year). SeaWiFS (blue) and merged SeaWiFS/Aqua (red) data are shown.

Fig. 5. (Left) Number of data days per month for Southern Vietnam area (area 4 in Fig. 2); (Right) Mean monthly number of data days per month (x-axis has day of the year). SeaWiFS (blue) and merged SeaWiFS/Aqua (red) data are shown.

Fig. 6. (Left) Number of data days per month for Eastern Gulf of Thailand (area 5 in Fig. 2); (Right) Mean monthly number of data days per month (x-axis has day of the year). SeaWiFS (blue) and merged SeaWiFS/Aqua (red) data are shown.

Fig. 7. (Left) Number of data days per month for West of Luzon Island (area 6 in Fig. 2); (Right) Mean monthly number of data days per month (x-axis has day of the year). SeaWiFS (blue) and merged SeaWiFS/Aqua (red) data are shown.

Figs. 3-7 shows a strong seasonal cycle in availability of ocean color data. In most cases the merged SeaWiFS/Aqua data provide significantly more valid data days than SeaWiFS alone.

For example, Fig. 7 (right panel) shows that the mean number of valid merged SeaWiFS/Aqua data days per month changes from 2 in August to 19 in February. The maximum for SeaWiFS is 12 in February.

Coastal areas off Northern Vietnam are very cloudy in March with less than 1 data day in March. In other areas off Vietnam the number of valid data days per month ranges from about 2 to about 10.

For more information send email to mkahruATucsd.edu (replace AT with @)