VIIRS Cloud Phase Validation

The VIIRS cloud phase algorithm was validated using a 24-hour period on November 10, Validation was performed using collocated CALIPSO data for pixels detected as cloud from both the VIIRS and CALIPSO cloud masks. Validation was performed for the entire dataset but was also separated by day/night, land/water and high latitude/low latitude (60 deg) pixels The validation was also filtered by the optical depth calculated by CALIPSO The initial thresholds for the VIIRS cloud phase algorithm tests were calculated using modeled single layer water and ice clouds These results show that updated thresholds improved the performance of the VIIRS cloud phase algorithm

> 0.0>0.1>0.2>0.3>0.4>0.5>2.0 All data daytime nighttime Over land Over water Lat > 60 deg Lat <60 deg VIIRS Cloud Phase Validation Using the CALIPSO Cloud Phase filtered by Optical Depth November 10, 2012 Optical Depth Filter This chart shows the fraction of the VIIRS cloud phase pixels (filtered by the CALIPSO optical depth) that report a water or ice cloud when the collocated CALIPSO cloud phase agreed CIMSS version of the VIIRS cloud type algorithm with the original threshold function

After looking at individual scenes, it appeared that some opaque ice clouds were being misclassified as supercooled water Looking at the circled area below, the clouds that appear pink in the RGB on the left image are ice clouds The VIIRS cloud type algorithm on the right falsely identifies those clouds as supercooled (green) VIIRS Cloud Phase Validation

Further investigation of the infrared cloud phase discrimination test revealed that the original threshold function was not properly differentiating water and ice clouds The scatter plot was created using the micron BTD from VIIRS and the cloud phase classification from CALIPSO The green line represents the original threshold function calculated using modeled single layer water and ice clouds The red line represents the new threshold function calculated using data from this plot

VIIRS Cloud Phase Validation Previous research has shown that the sensitivity of the micron BTD changes as a function of the satellite viewing angle For this reason offsets to the threshold function were calculated to account for different viewing angles in 10 degree increments The top red line represents the threshold function for viewing angles < 10 degrees The bottom red line represents the threshold function for viewing angles > 70 degrees

After the new threshold functions were applied the VIIRS cloud phase accurately classified the opaque ice clouds shown in the circled area VIIRS Cloud Phase Validation

The updated threshold function improved the performance of the cloud phase algorithm, however, some discontinuity developed at the edge of the pre-defined viewing angle ranges chosen for the threshold function A linear interpolation of the threshold function as a function of the satellite viewing angle was applied to remove the areas of discontinuity shown in the left image below

The next 3 slides show the 24-hour validation results for the following VIIRS cloud phase algorithms: – CIMSS version of the VIIRS cloud phase algorithm with the original infrared cloud phase discrimination threshold function – CIMSS version of the VIIRS cloud phase algorithm with the updated infrared cloud phase discrimination threshold function and viewing angle interpolation – Official VIIRS cloud phase algorithm (version ADL4.2+Mx8.0 - provided by Weizhong Chen) with the updated infrared cloud phase discrimination threshold function and viewing angle interpolation VIIRS Cloud Phase Validation

> 0.0>0.1>0.2>0.3>0.4>0.5>2.0 All data daytime nighttime Over land Over water Lat > 60 deg Lat <60 deg VIIRS Cloud Phase Validation Using the CALIPSO Cloud Phase filtered by Optical Depth November 10, 2012 Optical Depth Filter This chart shows the fraction of the VIIRS cloud phase pixels (filtered by the CALIPSO optical depth) that report a water or ice cloud when the collocated CALIPSO cloud phase agreed CIMSS version of the VIIRS cloud type algorithm with the original threshold function

> 0.0>0.1>0.2>0.3>0.4>0.5>2.0 All data daytime nighttime Over land Over water Lat > 60 deg Lat <60 deg VIIRS Cloud Phase Validation Using the CALIPSO Cloud Phase filtered by Optical Depth November 10, 2012 Optical Depth Filter This chart shows the fraction of the VIIRS cloud phase pixels (filtered by the CALIPSO optical depth) that report a water or ice cloud when the collocated CALIPSO cloud phase agreed CIMSS version of the VIIRS cloud type algorithm with updated threshold function and viewing angle interpolation

> 0.0>0.1>0.2>0.3>0.4>0.5>2.0 All data daytime nighttime Over land Over water Lat > 60 deg Lat <60 deg VIIRS Cloud Phase Validation Using the CALIPSO Cloud Phase filtered by Optical Depth November 10, 2012 Optical Depth Filter This chart shows the fraction of the VIIRS cloud phase pixels (filtered by the CALIPSO optical depth) that report a water or ice cloud when the collocated CALIPSO cloud phase agreed VIIRS cloud type algorithm version ADL4.2+Mx8.0 with updated threshold function and viewing angle interpolation