Sea surface temperature estimation by AVHRR spilt window function - A case studies by using Mutsu Bay
Setting up of the match ups.
As shown in figure 1 Mutsu Bay is situated in the northern end of Honshu Japan The Mutsu Bay automatic buoy system is composed of six fixed buoys have been measuring marine environmental items at each every hour on the hour . The temperatures at 1 m depth were used as the sea truth SST in the analysis .Its accuracy is claimed to be ± 0.1.
OC in the design specification of the buoy system .
Out of the archived AVHRR data of NOAA-9 received in 1985-1988 at the institute of industrial Science 78 scenes listed in table 2 were picked up as they had cloud free around the bay after applying due their latitude and longitude coordinates The errors were evaluated within one pixel resolution which is about 1.1 Km x 1.1 km at the nadir.
In order to insure the quality of the brightness temperatures in the match up data set we applied several checks as follows
- Careful visual check for cloud free and noise free buoy position .
- Restriction of the buoy position within 40 degree of the satellite zenith angle.
- Homogeneity check of brightness temperatures the standard deviation of the brightness temperatures in 3x3 pixels of a neighborhood buoy position should not exceed 0.2OC,
Finally total of 309 buoy positions were screened out the brightness temperature of ch.4 and ch 5 say X
4 and X
5 were specified to be mean values in the 3x3 pixel window The recovering procedures of the brightness temperatures from the ch.4 and the ch. 5 AVHRR data were completely followed to those described in Lauritson et. Al (1979) and brown et al (1985).
To the pair of X
4 and X
5 at each buoy position the buoy SST (y) measured at the nearest time to its overpass was combined to set up a match by using a regression analysis an outlier test was applied to the det of screened match ups there appeared some outliers then the circumstances at their collections were examined by referring to various meteorological observation records Two abnormal cases were found as follows.
- Contamination by forecast fire smoke in the summer of 1988 large forecast broke out continuously in eastern Siberia some match ups under the smoke were outliers since the smoke was thin and homogeneous over the bay it was not detected in the former tests.
- Strong radioactive cooling condition some match-ups under strong radioactive cooling condition in midnight scenes in autumn were outliers . There might were have been a steep change in the vertical temperature profile between the sea surface of the AVHRR and the 1m depth of the buoy measurement.
The causes of the above outliers were evident we included the all scenes to which the outliers belonged and finally got 276 selected match ups .
Form the preparation of the match-ups. The temporal coincidence in each match-up is within 30 minutes since the measurement interval of the buoys is none hour. The spatial coincidence is within one pixel.
