![Page 1: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/1.jpg)
Partly Cloudy Pixel Retrievals for 1-km MODIS Observations
J.A. Coakley, W.R. Tahnk, and G. Guo
College of Oceanic and Atmospheric SciencesOregon State University
Goals:1) Implement partly cloudy pixel retrievals for the 1-km
MODIS observations and assess biases in the MOD06 retrievals.
2) Assess feasibility of using shipboard measurements of radiative fluxes to validate CERES estimates of surface fluxes.
![Page 2: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/2.jpg)
The Partly Cloudy Imager Pixel Problem
1-km imager field of view
![Page 3: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/3.jpg)
Assumption:
Z
∆ Z
∆ Z << Z
• Layer is reasonably well-defined
Condition:• No overlying layers
![Page 4: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/4.jpg)
Retrieval MethodRetrieval scheme follows Arking and Childs (1985) and is describedin Coakley et al. (2005).
• For single-layered cloud systems, identify overcast pixels and determine altitude of cloud layer.
• For each pixel, radiances are given by
• For each pixel, adjust so that calculated radiances at 0.64, 1.6, 2.1, 3.7, and 11 µm match those observed.
eC RτA , ,
= Average cloud altitude obtained from nearby overcast pixels.
( )CC zISI
( ) ( )CCCSC zIAIAI +−= 1
= Fractional cloud cover within a pixel= Average cloud-free radiance within a pixel= Average overcast radiance within a pixel
Cz
CA
![Page 5: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/5.jpg)
1-km MODIS ImageryTerra 21 June 2002 1135 Z
True Color
11-µm
Mostly Overcast Pixels
Mostly Partly Cloudy Pixels
![Page 6: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/6.jpg)
Optical Depth and Droplet Effective Radius for Overcast Pixels
0.64-µm Optical Depth Droplet Radius at 2.1 µm
1-km overcast pixels drawn from 50-km scale region containing mostly overcast pixels.
![Page 7: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/7.jpg)
Optical Depth and Droplet Effective Radius for Partly Cloudy Pixels
0.64-µm Optical Depth Droplet Radius at 2.1 µm
1-km partly cloudy pixels drawn from 50-km scale region containing mostly partly cloudy pixels.
![Page 8: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/8.jpg)
Differences in Optical Depth
Overcast Partly Cloudy
Optical depths in partly cloudy pixels severely underestimated.
![Page 9: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/9.jpg)
Differences in Droplet Effective Radius
Overcast Partly Cloudy
Droplet radii in partly cloudy pixels severely overestimated.
![Page 10: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/10.jpg)
Differences in Droplet Effective
Radius
Overcast Partly Cloudy
1.6 µm
3.7 µm
• Retreivals for overcastpixels in goodagreement.
• For partly cloudy pixels,MOD06 underestimatesvisible optical depth andoverestimates dropletradius.
• Discrepancies in dropletradius about the same for 1.6, 2.1, and 3.7-µmretrievals.
![Page 11: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/11.jpg)
Distributions of Optical Depth and Droplet Radius Differences
0.64-µm Optical Depth Droplet Radius at 2.1 µm
Comparisons drawn from several hundred 50-km scale regions containing single-layered, low-level cloud systems with varying fractions of cloud cover.
![Page 12: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/12.jpg)
Distributions of Optical Depth and Droplet Radius Differences Due to Changes in
Atmospheric Profiles0.64-µm Optical Depth Droplet Radius at 2.1 µm
Tropical − Midlatitude winter profiles of temperature, humidity, and ozone.
![Page 13: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/13.jpg)
Shipboard Measurements of Surface Radiative Fluxes
![Page 14: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/14.jpg)
Identification of Cloud-free Scenes
Cloud-Free
Normalized shortwave = shortwave divided by cosine solar zenith angle.
20 May 2001, 44°N 124°W
![Page 15: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/15.jpg)
Surface Shortwave Flux
and Atmospheric Composition
Shortwave flux sensitive to water vapor and aerosol burdens.
![Page 16: Partly Cloudy Pixel Retrievals for 1-km MODIS Observations · Partly Cloudy Pixel Retrievals for 1-km MODIS Observations J.A. Coakley, W.R. Tahnk, and G. Guo College of Oceanic and](https://reader035.vdocument.in/reader035/viewer/2022071007/5fc44540fbae8b7c4a3e7afe/html5/thumbnails/16.jpg)
Calculated and Observed Shortwave
Fluxes• Midlatitude summertime
climatology with 50%moisture used.
• Aerosol optical depth adjusted for best-fit.
• Range of variation ~ 6% and model estimate appears tobe within 2%.
• Difference in slopes of observations suggests need to improve radiative transfercalculations.