The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations
<p>Clouds present many challenges to climate modelling. To develop and verify the parameterisations needed to allow climate models to represent cloud structure and processes, there is a need for high-quality observations of cloud optical depth from locations around the world. Retrievals of clo...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-09-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/12/5087/2019/amt-12-5087-2019.pdf |
| _version_ | 1830357728882589696 |
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| author | J. K. P. Shonk J.-Y. C. Chiu A. Marshak D. M. Giles D. M. Giles C.-H. Huang G. G. Mace S. Benson I. Slutsker I. Slutsker B. N. Holben |
| author_facet | J. K. P. Shonk J.-Y. C. Chiu A. Marshak D. M. Giles D. M. Giles C.-H. Huang G. G. Mace S. Benson I. Slutsker I. Slutsker B. N. Holben |
| author_sort | J. K. P. Shonk |
| collection | DOAJ |
| description | <p>Clouds present many challenges to climate modelling. To
develop and verify the parameterisations needed to allow climate models to
represent cloud structure and processes, there is a need for high-quality
observations of cloud optical depth from locations around the world.
Retrievals of cloud optical depth are obtainable from radiances measured by
Aerosol Robotic Network (AERONET) radiometers in “cloud mode” using a
two-wavelength retrieval method. However, the method is unable to detect
cloud phase, and hence assumes that all of the cloud in a profile is liquid.
This assumption has the potential to introduce errors into long-term
statistics of retrieved optical depth for clouds that also contain ice.
Using a set of idealised cloud profiles we find that, for optical depths
above 20, the fractional error in retrieved optical depth is a linear
function of the fraction of the optical depth that is due to the presence of
ice cloud (“ice fraction”). Clouds that are entirely ice have positive
errors with magnitudes of the order of 55 % to 70 %. We derive a simple linear
equation that can be used as a correction at AERONET sites where ice
fraction can be independently estimated.</p>
<p>Using this linear equation, we estimate the magnitude of the error for a set
of cloud profiles from five sites of the Atmospheric Radiation Measurement
programme. The dataset contains separate retrievals of ice and liquid
retrievals; hence ice fraction can be estimated. The magnitude of the error
at each location was related to the relative frequencies of occurrence in
thick frontal cloud at the mid-latitude sites and of deep convection at the
tropical sites – that is, of deep cloud containing both ice and liquid
particles. The long-term mean optical depth error at the five locations
spans the range 2–4, which we show to be small enough to allow calculation
of top-of-atmosphere flux to within 10 % and surface flux to about
15 %.</p> |
| first_indexed | 2024-12-20T02:28:51Z |
| format | Article |
| id | doaj.art-368cb9fcd652483e8c677d96368813d0 |
| institution | Directory Open Access Journal |
| issn | 1867-1381 1867-8548 |
| language | English |
| last_indexed | 2024-12-20T02:28:51Z |
| publishDate | 2019-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Measurement Techniques |
| spelling | doaj.art-368cb9fcd652483e8c677d96368813d02022-12-21T19:56:37ZengCopernicus PublicationsAtmospheric Measurement Techniques1867-13811867-85482019-09-01125087509910.5194/amt-12-5087-2019The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observationsJ. K. P. Shonk0J.-Y. C. Chiu1A. Marshak2D. M. Giles3D. M. Giles4C.-H. Huang5G. G. Mace6S. Benson7I. Slutsker8I. Slutsker9B. N. Holben10National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, UKDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USANASA Goddard Space Flight Center, Greenbelt, Maryland, USANASA Goddard Space Flight Center, Greenbelt, Maryland, USAScience Systems and Applications, Inc., Lanham, Maryland, USACenter for Environmental Monitoring and Technology, National Central University, Taoyuan, TaiwanDepartment of Atmospheric Sciences, University of Utah, Salt Lake City, Utah, USADepartment of Atmospheric Sciences, University of Utah, Salt Lake City, Utah, USANASA Goddard Space Flight Center, Greenbelt, Maryland, USAScience Systems and Applications, Inc., Lanham, Maryland, USANASA Goddard Space Flight Center, Greenbelt, Maryland, USA<p>Clouds present many challenges to climate modelling. To develop and verify the parameterisations needed to allow climate models to represent cloud structure and processes, there is a need for high-quality observations of cloud optical depth from locations around the world. Retrievals of cloud optical depth are obtainable from radiances measured by Aerosol Robotic Network (AERONET) radiometers in “cloud mode” using a two-wavelength retrieval method. However, the method is unable to detect cloud phase, and hence assumes that all of the cloud in a profile is liquid. This assumption has the potential to introduce errors into long-term statistics of retrieved optical depth for clouds that also contain ice. Using a set of idealised cloud profiles we find that, for optical depths above 20, the fractional error in retrieved optical depth is a linear function of the fraction of the optical depth that is due to the presence of ice cloud (“ice fraction”). Clouds that are entirely ice have positive errors with magnitudes of the order of 55 % to 70 %. We derive a simple linear equation that can be used as a correction at AERONET sites where ice fraction can be independently estimated.</p> <p>Using this linear equation, we estimate the magnitude of the error for a set of cloud profiles from five sites of the Atmospheric Radiation Measurement programme. The dataset contains separate retrievals of ice and liquid retrievals; hence ice fraction can be estimated. The magnitude of the error at each location was related to the relative frequencies of occurrence in thick frontal cloud at the mid-latitude sites and of deep convection at the tropical sites – that is, of deep cloud containing both ice and liquid particles. The long-term mean optical depth error at the five locations spans the range 2–4, which we show to be small enough to allow calculation of top-of-atmosphere flux to within 10 % and surface flux to about 15 %.</p>https://www.atmos-meas-tech.net/12/5087/2019/amt-12-5087-2019.pdf |
| spellingShingle | J. K. P. Shonk J.-Y. C. Chiu A. Marshak D. M. Giles D. M. Giles C.-H. Huang G. G. Mace S. Benson I. Slutsker I. Slutsker B. N. Holben The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations Atmospheric Measurement Techniques |
| title | The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations |
| title_full | The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations |
| title_fullStr | The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations |
| title_full_unstemmed | The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations |
| title_short | The impact of neglecting ice phase on cloud optical depth retrievals from AERONET cloud mode observations |
| title_sort | impact of neglecting ice phase on cloud optical depth retrievals from aeronet cloud mode observations |
| url | https://www.atmos-meas-tech.net/12/5087/2019/amt-12-5087-2019.pdf |
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