VIIRS Cloud-Free Compositing For Nighttime Lights

Transcription

1 VIIRS Cloud-Free Compositing For Nighttime Lights Kimberly Baugh, CIRES University of Colorado Feng Chi Hsu, CIRES University of Colorado Mikhail Zhizhin, CIRES University of Colorado Tilottama Ghosh, CIRES University of Colorado Chris Elvidge, NOAA National Geophysical Data Center (NGDC)

2 Average VIIRS DNB Composite 2012/10/ /10/23 version created on Nov. 11, 2012 NGDC s Average DNB product has average DNB radiance values in units nanowatts/(cm 2.sr) Available online at NGDC web site

3 VIIRS Day-Night Band (DNB) Data The VIIRS DNB sensor has the capability to collect low-light imagery. Polar orbiting 3000 km swath 742m spatial resolution Daily global coverage Wavelength: um 14-bit data avoids saturation Aggregate SVDNB_npp_d _t _e _b05051_c _noaa_ops.h5

4 Generation of Average VIIRS DNB Composite 1) DNB, M15 and geolocation data files are obtained from CLASS archive covering desired date range. 2) VIIRS flag images are created with bit-codes designating quality of the DNB data. 3) VIIRS DNB and flag images are reprojected into 15-arc second grids. The highest quality data is kept for inclusion into the composite product. 4) 15-arc second DNB grids are composited, creating a suite of files including an average DNB image, number of cloud-free observations used, and standard deviation of DNB for each grid cell.

5 Processing VIIRS DNB Aggregates: Flag Bands DNB DNB FLAG Red: Daytime Green: Terminator zone Blue: Nighttime Aggregate SVDNB_npp_d _t _e _b05050_c _noaa_ops.h5 For each nighttime VIIRS DNB aggregate, a companion flag band is generated with bit-codes designating: Daytime (pixel solar zenith angle < 96) Day-Night terminator zone (96 < pixel solar zenith < 101) Zero lunar Illuminance (< lux) Stray light region (90 < nadir solar zenith < 118.5) Lunar illuminance is a function of lunar phase, azimuth, and elevation, which are also based on lat, lon, and time of each DNB pixel. This entire aggregate was flagged as having zero lunar illuminance and being affected by stray light.

6 Processing VIIRS DNB Aggregates: Flag Bands Aggregate SVDNB_npp_d _t _e _b05050_c _noaa_ops.h5 DNB Increased noise at edge of scan DNB FLAG For each nighttime VIIRS DNB aggregate, edge pixels are discarded due to increased noise (DNB aggregation zones 29-32). Blue: Stray light region Orange: Edge of Scan (DNB aggregation zones 29-32). Black: This area is considered high quality nighttime data. This entire aggregate was flagged as having zero lunar illuminance.

7 Processing VIIRS DNB Aggregates: Cloud Mask Clouds obscure the nighttime lights data. To avoid using data that has been impacted by cloud-cover, a cloud mask is needed. Cloud masks are generated using an NGDC algorithm comparing the VIIRS M15 band (10.763um) with modeled surface temperature. DNB M15 Brightness Temperature Note: Black areas at edges of scans in M15 data are no-data values due to onboard bow-tie deletions. They do not affect the reprojected image. M15 Cloud Mask Clouds are impacting nighttime lights of Chicago by reducing the intensity of the lights and by blurring the spatial detail of the light features. Aggregate npp_d _t _e _b05050

8 Processing VIIRS DNB Aggregates: Reprojection DNB and flag data is reprojected into 15-arc second grids using terrain-corrected position information VIIRS M-bands and DNB have different sensor and aggregation characteristics which require them to be reprojected independently. The cloud mask generated from M15 is merged with the flags generated from the DNB after reprojection. Flag from DNB Combined VIIRS flag Flag from M15 (cloud mask) DNB Aggregate SVDNB_npp_d _t _e _b05050_c _noaa_ops.h5

9 DNB Average Processing: Compositing The 15-arc second DNB grids are masked to areas of highestquality nighttime data using the flag band. Highest quality means: cloud-free, zero lunar illuminance, dark nightttime, outside stray-light region, and middle of swath (aggregation zones 1-28). The masked grids are then composited, creating a suite of files including an average DNB image, number of cloud-free observations used, and standard deviation of DNB for each grid cell. Average DNB N Cloud-free Observations

10 Average VIIRS DNB 2012/10/ /10/23 version created on Nov. 11, 2012 NGDC s Average DNB product has average DNB radiance values in units nanowatts/(cm 2.sr) April and October 2012 monthly composites are available online at NGDC website

11 Next Steps for VIIRS DNB Composites Filter out lights from aurora Filter out lights due to South Atlantic Anomoly Normalize data from region impacted by stray-light so its usable in the composites Improve geolocation, or orbit-toorbit alignment, to keep lighting features sharp Improve cloud-detection algorithm over mountainous areas and along coastlines Filter out ephemeral lights from boats and fires to create a VIIRS DNB Stable Lights product Nile Delta version created on Nov. 11, 2012

12 Close-Ups from the VIIRS DNB Composite Nile Delta Hawaiian Islands Lights from volcanic activity version created on Nov. 11, 2012

13 Close-Ups from the VIIRS DNB Composite Nile Delta Lights from gas flares version created on Nov. 11, 2012 United Arab Emirates

14 Close-Ups from the VIIRS DNB Composite Nile Delta Lights from fishing boats Korea Strait and Sea of Japan version created on Nov. 11, 2012

15 Close-Ups from the VIIRS DNB Composite Lights from fires in Western Australia Nile Delta version created on Nov. 11, 2012