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One of the primary reasons is that in many places there will be a diurnal heating cycle due to warming of the surface skin layer by sunlight. It is also dependent on wind speed, as the heating effect of sunlight will be affected by wave action and mixing with the cooler waters below the surface skin layer. The temperature of the layer below the surface skin layer is often referred to as the "bulk" SST.
by febryanto25 » Tue Mar 30, 2021 11:35 am America/New_York
Thank you for your answer. My further question is, if MODIS Aqua provides night and daytime SST products then if we want to take daily SST, which product should we select? night or daytime? or can we just consider an average of night and daytime SST products as the daily SST?
The "best" data product for your requirements is a more complex question. I am providing a couple of references (even though one is several years old) that discuss the issue. Because there are several causes of differences between daytime and nighttime SST, I do not think an averages would be recommended, but that answer is certainly not definitive.
I will also ask colleagues from the OB.DAAC to examine your question and determine if they can provide additional guidance. Furthermore, the Physical Oceanography DAAC (PODAAC), which has its own user forum, is the primary site for NASA SST data products, so you may wish to consult their documentation and expertise as well.
Daily SST can be viewed/ordered from the Ocean Color browse tools. You can choose day or night or both for your needs. The most popular SST dataset among users over the past year was daytime 11um SST L2 product. Here are the links to the Ocean Color site's browse tools:
by febryanto25 » Fri Apr 02, 2021 4:00 am America/New_York
Thank you for the answer. Following your answer, how can the SST product level 3 of MODIS Terra/Aqua have a SST value while there are lots of cloud cover especially in the tropical area?
What kind of cloud-removal algorithm is applied in the SST product level 3 of MODIS Terra/Aqua?
by gnwiii » Fri Apr 02, 2021 2:57 pm America/New_York
Regarding "SST can provide data with some cloud cover where there will be no visible range data". SST uses wavelengths outside the visible range. There are also microwave SST sensors that can provide data in cloudy conditions. For some use cases (e.g., using chlor_a and SST together to impute values for parameters not directly measured by optical means or to locate dynamic boundaries for Longhurst's biogeochemical provinces) we would like to measurements of both quantities from the same sensor. In practice, however, pixels with daytime SST and chlor_a both valid can be a fraction of the total numbers of valid pixels for the two products due different sensitivities to cloud, sun glint, and atmospheric path length.
In areas with high cloudiness, even monthly data may only represent a few days, and you can encounter cases where features such as eddies are visible in both SST and chlor_a, but not at the same location. There's "rocket science", and then there's sensor science.